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Dentron MLA-2500 Question
Hi Group,
I have a 2500 that had one bad 8875, and as-is EBay buy, and I am in the process of a conversion to a single 8877. This may not be the best choice of a tube for the application, but I had a good one in a box for nearly 20 years, so I decided to give it a try. Before the conversion I powered it up, and with one tube I could get 450 watts on 80M with 50 watts drive. I did notice however that the variable loading capacitor had to be at full mesh on any band I tried. I blew off that issue because with only one tube the PI-Net was not seeing the as-designed input impedance. My question is from my blurry schematic and parts list. The fixed Centralab loading capacitors in my rig are all 1000pf, and the fixed plate tuning side capacitors are all Centralab 100pf units. The parts list does not show any 1000pf HV Centralab capacitors. Is there a typo in the parts list, or does my amplifier have the wrong capacitors installed in the loading area? I powered up the amplifier last night, and I can get 600 watts RF out on 80 meters with 50 watts drive. The cathode current reaches 0.8 ampere at 1700 volts Plate. So that is 1360 watts in for 600 watts output. That is about 44% efficiency. The variable loading control always wants to be at maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading variable capacitor must also be at maximum mesh. This makes me think my PI-Network is not correct for the impedances involved, and then it sure appears that the Dentron documentation does not match my rig. I might have to reverse engineer that PI-Net. So does anyone have a better set of drawings to see if their MLA-2500 PI-Net parts match the drawings? Regards, Jim WD5JKO |
Dentron MLA-2500 Question
Update:
The fixed loading capacitors on the MLA-2500B for 75, 160 meters are both 1000pf per Dentron parts list whereas they are 100pf on the MLA-2500 parts list. It looks like a typo in the documentation. Then I measured my 1000pf capacitors in my amp, these both measured about 820pf on my capacitance bridge at 4 Mhz. For sanity sake, I also measured a 820 pf silver mica capacitor, and the bridge read 850pf @ 4 Mhz. So it seems that I uncovered two problems: * Dentron MLA-2500 parts list has typos in it * My two loading capacitors for 75 and 160 are out of tolerence on the low side. This explains why my variable loading capacitor needs to be at full mesh. Anybody have a 8877 Teflon chimney they want to get rid of? Jim WD5JKO "JC" wrote in message . com... Hi Group, I have a 2500 that had one bad 8875, and as-is EBay buy, and I am in the process of a conversion to a single 8877. This may not be the best choice of a tube for the application, but I had a good one in a box for nearly 20 years, so I decided to give it a try. Before the conversion I powered it up, and with one tube I could get 450 watts on 80M with 50 watts drive. I did notice however that the variable loading capacitor had to be at full mesh on any band I tried. I blew off that issue because with only one tube the PI-Net was not seeing the as-designed input impedance. My question is from my blurry schematic and parts list. The fixed Centralab loading capacitors in my rig are all 1000pf, and the fixed plate tuning side capacitors are all Centralab 100pf units. The parts list does not show any 1000pf HV Centralab capacitors. Is there a typo in the parts list, or does my amplifier have the wrong capacitors installed in the loading area? I powered up the amplifier last night, and I can get 600 watts RF out on 80 meters with 50 watts drive. The cathode current reaches 0.8 ampere at 1700 volts Plate. So that is 1360 watts in for 600 watts output. That is about 44% efficiency. The variable loading control always wants to be at maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading variable capacitor must also be at maximum mesh. This makes me think my PI-Network is not correct for the impedances involved, and then it sure appears that the Dentron documentation does not match my rig. I might have to reverse engineer that PI-Net. So does anyone have a better set of drawings to see if their MLA-2500 PI-Net parts match the drawings? Regards, Jim WD5JKO |
Dentron MLA-2500 Question
It is unlikely that the fixed loading capacitor is as high as 1000 pf, except for 160m. However if Dentron has really used a Centralab 1000-pf capacitor as fixed loading capacitor, that is a sign of bad design, because those capacitors are not suitable for that purpose.
As a matter of fact the Centralab 1000-pF capacitors use a different ceramic material than the 100 pF units, and they therefore have a very different behavior, as explained below: - the 1000-pF units can withstand high RF current, but cannot withstand high RF voltage at the same time (DC voltage does not matter). A more rigorous way to express that concept is to say that the 1000-pf units cannot withstand high "reactive power" (reactive power = RF current times RF voltage). In practice they are only suitable for bypassing or plate-to-pi coupling (high RF current but very little RF voltage). - on the contrary, the 100-pF units are suitable for operation in resonating circuits, where both the RF voltage and the RF current is high (= high reactive power). So they are suitable as fixed loading capacitors. If one uses a Centralab 1000-pF capacitor as the fixed loading capacitor of a high-power linear, the capacitor will heat up and change its capacitance, so detuning the circuit. Prolonged high-power operation may result in capacitor failure. 73 Tony I0JX .. "JC" ha scritto nel messaggio . com... Hi Group, I have a 2500 that had one bad 8875, and as-is EBay buy, and I am in the process of a conversion to a single 8877. This may not be the best choice of a tube for the application, but I had a good one in a box for nearly 20 years, so I decided to give it a try. Before the conversion I powered it up, and with one tube I could get 450 watts on 80M with 50 watts drive. I did notice however that the variable loading capacitor had to be at full mesh on any band I tried. I blew off that issue because with only one tube the PI-Net was not seeing the as-designed input impedance. My question is from my blurry schematic and parts list. The fixed Centralab loading capacitors in my rig are all 1000pf, and the fixed plate tuning side capacitors are all Centralab 100pf units. The parts list does not show any 1000pf HV Centralab capacitors. Is there a typo in the parts list, or does my amplifier have the wrong capacitors installed in the loading area? I powered up the amplifier last night, and I can get 600 watts RF out on 80 meters with 50 watts drive. The cathode current reaches 0.8 ampere at 1700 volts Plate. So that is 1360 watts in for 600 watts output. That is about 44% efficiency. The variable loading control always wants to be at maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading variable capacitor must also be at maximum mesh. This makes me think my PI-Network is not correct for the impedances involved, and then it sure appears that the Dentron documentation does not match my rig. I might have to reverse engineer that PI-Net. So does anyone have a better set of drawings to see if their MLA-2500 PI-Net parts match the drawings? Regards, Jim WD5JKO |
Dentron MLA-2500 Question
Tony,
I appreciate the comments. For sure there is something awry in this pi-net. I looked at your tutorial on your web page, and wow, you know your stuff! The schematics of the 2500, and 2500B are at the following link: ftp://bama.sbc.edu/downloads/dentron/mla2500/ Another aspect of this pi-net is that the tank coil consists of tight wound silver plated stranded Teflon insulated wire of about 10 awg. Users of healthy 2500's report 1100 to 1300 watts key down (output), and 1500 watts PEP out with a pulser. This is very good for the size and weight of this amplifier. The loading variable cap is about 50-500pf with a 100pf Centralab in parallel with that. The bandswitch is not a shorting type, so it adds 200pf on 40m, 1000pf on 75, and 1000pf on 160 (all on 2500B schematic and parts list). These do not add up due to the type of rotary switch used. I'd prefer a shorting switch arrangement where on 160m, all the fixed loading caps are in parallel and each unit has less capacitance. I will look more carefully at the capacitors used to see what the part number is. Maybe the correct values are in there, but of the wrong type. I plan to use my impedance bridge from the 50 ohm amp output looking into the amp, and replace the 8877 with the calculated plate load RL going into the pi-net. Hopefully a 1/2 watt carbon comp resistor around 2K (a guess) will represent the plate side impedance. My hope here is that I can evaluate the pi-net 160-10m at a low level (0dbm?) to see what the impedance matching capability is. Maybe the values used are correct, but with high power, the capacitors get lossy (hot), drift down in value, and cause the efficiency of the amp to really stink. I'll let you'all know what I find. Thanks, Jim WD5JKO "Antonio Vernucci" wrote in message ... It is unlikely that the fixed loading capacitor is as high as 1000 pf, except for 160m. However if Dentron has really used a Centralab 1000-pf capacitor as fixed loading capacitor, that is a sign of bad design, because those capacitors are not suitable for that purpose. As a matter of fact the Centralab 1000-pF capacitors use a different ceramic material than the 100 pF units, and they therefore have a very different behavior, as explained below: - the 1000-pF units can withstand high RF current, but cannot withstand high RF voltage at the same time (DC voltage does not matter). A more rigorous way to express that concept is to say that the 1000-pf units cannot withstand high "reactive power" (reactive power = RF current times RF voltage). In practice they are only suitable for bypassing or plate-to-pi coupling (high RF current but very little RF voltage). - on the contrary, the 100-pF units are suitable for operation in resonating circuits, where both the RF voltage and the RF current is high (= high reactive power). So they are suitable as fixed loading capacitors. If one uses a Centralab 1000-pF capacitor as the fixed loading capacitor of a high-power linear, the capacitor will heat up and change its capacitance, so detuning the circuit. Prolonged high-power operation may result in capacitor failure. 73 Tony I0JX .. "JC" ha scritto nel messaggio . com... Hi Group, I have a 2500 that had one bad 8875, and as-is EBay buy, and I am in the process of a conversion to a single 8877. This may not be the best choice of a tube for the application, but I had a good one in a box for nearly 20 years, so I decided to give it a try. Before the conversion I powered it up, and with one tube I could get 450 watts on 80M with 50 watts drive. I did notice however that the variable loading capacitor had to be at full mesh on any band I tried. I blew off that issue because with only one tube the PI-Net was not seeing the as-designed input impedance. My question is from my blurry schematic and parts list. The fixed Centralab loading capacitors in my rig are all 1000pf, and the fixed plate tuning side capacitors are all Centralab 100pf units. The parts list does not show any 1000pf HV Centralab capacitors. Is there a typo in the parts list, or does my amplifier have the wrong capacitors installed in the loading area? I powered up the amplifier last night, and I can get 600 watts RF out on 80 meters with 50 watts drive. The cathode current reaches 0.8 ampere at 1700 volts Plate. So that is 1360 watts in for 600 watts output. That is about 44% efficiency. The variable loading control always wants to be at maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading variable capacitor must also be at maximum mesh. This makes me think my PI-Network is not correct for the impedances involved, and then it sure appears that the Dentron documentation does not match my rig. I might have to reverse engineer that PI-Net. So does anyone have a better set of drawings to see if their MLA-2500 PI-Net parts match the drawings? Regards, Jim WD5JKO |
Dentron MLA-2500 Question
"JC" wrote in message
. com... Hi Group, I have a 2500 that had one bad 8875, and as-is EBay buy, and I am in the process of a conversion to a single 8877. This may not be the best choice of a tube for the application, but I had a good one in a box for nearly 20 years, so I decided to give it a try. Before the conversion I powered it up, and with one tube I could get 450 watts on 80M with 50 watts drive. I did notice however that the variable loading capacitor had to be at full mesh on any band I tried. I blew off that issue because with only one tube the PI-Net was not seeing the as-designed input impedance. My question is from my blurry schematic and parts list. The fixed Centralab loading capacitors in my rig are all 1000pf, and the fixed plate tuning side capacitors are all Centralab 100pf units. The parts list does not show any 1000pf HV Centralab capacitors. Is there a typo in the parts list, or does my amplifier have the wrong capacitors installed in the loading area? I powered up the amplifier last night, and I can get 600 watts RF out on 80 meters with 50 watts drive. The cathode current reaches 0.8 ampere at 1700 volts Plate. So that is 1360 watts in for 600 watts output. That is about 44% efficiency. The variable loading control always wants to be at maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading variable capacitor must also be at maximum mesh. This makes me think my PI-Network is not correct for the impedances involved, and then it sure appears that the Dentron documentation does not match my rig. I might have to reverse engineer that PI-Net. So does anyone have a better set of drawings to see if their MLA-2500 PI-Net parts match the drawings? Regards, Jim WD5JKO Jim - For reference purposes, here are the Communication Quarterly and KU7G articles (Adobe Acrobat) regarding 4CX400a or 4CX800a conversions http://www.tomstubes.com/images/mla_2500_4cx400a.pdf KU7G http://www.tomstubes.com/images/mla2500_4cx800a.pdf w9gb |
Dentron MLA-2500 Question
For reference purposes, here are the Communication Quarterly and KU7G articles (Adobe Acrobat) regarding 4CX400a or 4CX800a conversions http://www.tomstubes.com/images/mla_2500_4cx400a.pdf KU7G http://www.tomstubes.com/images/mla2500_4cx800a.pdf w9gb Hi, Thanks for the excellent links. I had seen the first one before. In looking at the article about the 2 times 4cx400a, it jumps out at me that this conversion leads to about 350-500 watts less RF output than original (1100-1300 versus 750-800) even when using tubes with more capability. I attribute this to running Tetrodes at low plate voltage. In this application Hi-Mu Triodes are a better choice. Also the low RF drive need for the Tetrodes make them attractive for low power exciters, and a problem for us with a 100 watt exciter. My conversion is similar to the 4cx800 article except I am using a 8877. I use the same external box to house the socket, and mount the fan. The 8877 may not be designed for use at 1700 volts, but what the heck, I only want about 1000 watt RF output. I need a chimney like what is described for tthe PI-Network he 4cx800. Any idea where I can get a block of Teflon, and then get it machined? That article about the 4cx800 has some good visuals of the pi-net that is being a problem with my 2500. Thanks again, Jim WD5JKO |
Dentron MLA-2500 Question
"JC" wrote in message
et... For reference purposes, here are the Communication Quarterly and KU7G articles (Adobe Acrobat) regarding 4CX400a or 4CX800a conversions http://www.tomstubes.com/images/mla_2500_4cx400a.pdf KU7G http://www.tomstubes.com/images/mla2500_4cx800a.pdf w9gb Hi, Thanks for the excellent links. I had seen the first one before. In looking at the article about the 2 times 4cx400a, it jumps out at me that this conversion leads to about 350-500 watts less RF output than original (1100-1300 versus 750-800) even when using tubes with more capability. I attribute this to running Tetrodes at low plate voltage. In this application Hi-Mu Triodes are a better choice. Also the low RF drive need for the Tetrodes make them attractive for low power exciters, and a problem for us with a 100 watt exciter. My conversion is similar to the 4cx800 article except I am using a 8877. I use the same external box to house the socket, and mount the fan. The 8877 may not be designed for use at 1700 volts, but what the heck, I only want about 1000 watt RF output. I need a chimney like what is described for tthe PI-Network he 4cx800. Any idea where I can get a block of Teflon, and then get it machined? That article about the 4cx800 has some good visuals of the pi-net that is being a problem with my 2500. Thanks again, Jim WD5JKO Jim, Bob Schetgen, KU7G machined that Teflon block in the 4CX800 article . Unfortunately, Bob passed away on December 5, 2005. Bob worked at the ARRL Headquarters for nearly 23 years. He was a member of the editorial staff for the past 16 years, he was perhaps best known as the editor of the "Hints & Kinks" column in QST. He also served as the managing editor of the League's technical and experimenter's journal QEX. w9gb |
Dentron MLA-2500 Question
On Sun, 21 May 2006 09:16:28 -0500, "g. beat" @spam protected
wrote: "JC" wrote in message .com... Hi Group, snip So does anyone have a better set of drawings to see if their MLA-2500 PI-Net parts match the drawings? Regards, Jim WD5JKO Jim - For reference purposes, here are the Communication Quarterly and KU7G articles (Adobe Acrobat) regarding 4CX400a or 4CX800a conversions http://www.tomstubes.com/images/mla_2500_4cx400a.pdf KU7G http://www.tomstubes.com/images/mla2500_4cx800a.pdf I would suggest an alternate chimney though using Silastic rubber sheet wound into a tube shape as is done in the Alpha series amps. Relatively inexpensive, no machining, and easy to install. One other comment. The MLA 2500 suffers from poor voltage regulation as did most of the Dentron amps. It does after all use a doubler in the power supply. So when tuning be sure to use the "tune" position and then switch to operate. Tuning in the operate position drags the plate voltage down so far that the amp will not be properly tuned for the PEP on SSB. With my voice characteristics and under the old rules of 1 KW average input mine would put out about 2200 PEP. Good Luck, Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com w9gb |
Dentron MLA-2500 Question
On Sun, 21 May 2006 15:50:55 GMT, "JC" wrote:
For reference purposes, here are the Communication Quarterly and KU7G articles (Adobe Acrobat) regarding 4CX400a or 4CX800a conversions http://www.tomstubes.com/images/mla_2500_4cx400a.pdf KU7G http://www.tomstubes.com/images/mla2500_4cx800a.pdf w9gb Hi, Thanks for the excellent links. I had seen the first one before. In looking at the article about the 2 times 4cx400a, it jumps out at me that this conversion leads to about 350-500 watts less RF output than original (1100-1300 versus 750-800) even when using tubes with more capability. I attribute this to running Tetrodes at low plate voltage. In this application Hi-Mu Triodes are a better choice. Also the low RF drive need for the Tetrodes make them attractive for low power exciters, and a problem for us with a 100 watt exciter. My conversion is similar to the 4cx800 article except I am using a 8877. I use the same external box to house the socket, and mount the fan. The 8877 may not be designed for use at 1700 volts, but what the heck, I only want As I recall the MLA-2500 ran 2200 volts. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com about 1000 watt RF output. I need a chimney like what is described for tthe PI-Network he 4cx800. Any idea where I can get a block of Teflon, and then get it machined? That article about the 4cx800 has some good visuals of the pi-net that is being a problem with my 2500. Thanks again, Jim WD5JKO |
Dentron MLA-2500 Question
"Roger" wrote in message
... On Sun, 21 May 2006 09:16:28 -0500, "g. beat" @spam protected wrote: I would suggest an alternate chimney though using Silastic rubber sheet wound into a tube shape as is done in the Alpha series amps. Relatively inexpensive, no machining, and easy to install. You can purchase the Silastic rubber from Alpha directly. It is no longer red, but black -- supplier to Alpha changed this ! w9gb Good Luck, Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com |
Dentron MLA-2500 Question
As I recall the MLA-2500 ran 2200 volts. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com Roger: Yes that ia about correct for no load. I am running mine off 120 vac, and the B+ drags down to about 1600 volts at 0.8 amp load. I've seen other conversions that run off 220 vac, and they say about 1700 volts for the same load. The voltage doubler capacitors if increased in value, say 330 uf instead of 125 uf would be better. This however would require adding a soft-start turn on modification to avoid welding the contacts of the power switch, or blowing the diode stack. Jim WD5JKO |
Dentron MLA-2500 Question
On Mon, 22 May 2006 01:44:31 GMT, "JC" wrote:
As I recall the MLA-2500 ran 2200 volts. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com Roger: Yes that ia about correct for no load. I am running mine off 120 vac, and the B+ drags down to about 1600 volts at 0.8 amp load. I've seen other I'm assuming that is in the tune position? 800 mills sounds kinda high in the tune/CW position. Also I was able to get well over 1500 watts PEP out of a pair of 8873s without driving them hard. conversions that run off 220 vac, and they say about 1700 volts for the same It should be just a transformer tap change for 220. load. The voltage doubler capacitors if increased in value, say 330 uf It sounds like they are tuning in the SSB mode. The voltage stays right up there in SSB so if tuned up with carrier in that mode the tuning will not be correct for SSB. instead of 125 uf would be better. This however would require adding a soft-start turn on modification to avoid welding the contacts of the power switch, or blowing the diode stack. That lack of regulation is why they run a lot more output PEP when tuned in the tune/CW position instead of the SSB position. Years ago I sold and worked on a lot of the Dentron Amps. Good Luck, Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com Jim WD5JKO |
Dentron MLA-2500 Question
I'm assuming that is in the tune position? 800 mills sounds kinda high in the tune/CW position. The MLA-2500 has no tune / cw switch like some other Dentron's do. I guess the amps you are referring to must have a tap in the HV transformer to run 1KW DC input on CW at lower plate voltage. Is that correct? I'm sure glad this DC input stuff is past tense, and 1500 watts PEP output is simpler. Of cource the AM'ers that ran 1Kw dc input with plate modulation got their power cut back because it was legal for them to run ~ 3kw PEP output with a 750 watt carrier. I was one who did that back in the 1980's. That was a blast! My MLA-2500 has a "Cont Duty" switch that forces the fan on high speed whenever the amp is keyed. Also I was able to get well over 1500 watts PEP out of a pair of 8873s without driving them hard. Aren't the 8873's a conduction cooled version of the 8873,8874,8875 family? Those have reduced plate dissipation, but no need for a fan. conversions that run off 220 vac, and they say about 1700 volts for the same It should be just a transformer tap change for 220. Yea, that asummes there is a 220 vac outlet in your ham shack. :-) I still need a verification of them pi-net capacitors in the MLA-2500. It appears that the original documentation is incorrect, and so is a redrawn schematic on Bama. Jim |
Dentron MLA-2500 Question
On Mon, 22 May 2006 23:05:09 GMT, "JC" wrote:
I'm assuming that is in the tune position? 800 mills sounds kinda high in the tune/CW position. The MLA-2500 has no tune / cw switch like some other Dentron's do. I guess the amps you are referring to must have a tap in the HV transformer to run It has been along time, but there was a way to tune them to get full power on SSB. I was remembering a CW/SSB switch which aparently isn't correct. 1KW DC input on CW at lower plate voltage. Is that correct? I'm sure glad this DC input stuff is past tense, and 1500 watts PEP output is simpler. Of cource the AM'ers that ran 1Kw dc input with plate modulation got their power cut back because it was legal for them to run ~ 3kw PEP output with a 750 watt carrier. I was one who did that back in the 1980's. That was a blast! Most of the legal limit amps I ran at 1KW average including exciter would hit about 2200 PEP out. My MLA-2500 has a "Cont Duty" switch that forces the fan on high speed whenever the amp is keyed. Also I was able to get well over 1500 watts PEP out of a pair of 8873s without driving them hard. Aren't the 8873's a conduction cooled version of the 8873,8874,8875 family? Those have reduced plate dissipation, but no need for a fan. Oops! I'm talking about a new, stock, MLA-2500. Make that a pair of 8875s, The transverse cooled ones that are nigh on to impossible to find. I did have an amp that used the 8873s, but if you did much operating you put a couple of fans on the heat sink. The 8873, 74, and 75 and the same tube with different cooling. conversions that run off 220 vac, and they say about 1700 volts for the same It should be just a transformer tap change for 220. Yea, that asummes there is a 220 vac outlet in your ham shack. :-) There wasn't here, but there soon was:-)) It's quite easy to add a circuit breaker, some wire, and a couple of mutiple outlet boxes IF you own your own home and your wife is also a ham. :-)) I still need a verification of them pi-net capacitors in the MLA-2500. It appears that the original documentation is incorrect, and so is a redrawn schematic on Bama. All I remember is they switched in extra capacitance on the lower bands. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com Jim |
Dentron MLA-2500 Question
Yea, that asummes there is a 220 vac outlet in your ham shack. :-) There wasn't here, but there soon was:-)) It's quite easy to add a circuit breaker, some wire, and a couple of mutiple outlet boxes IF you own your own home and your wife is also a ham. :-)) Roger, It aint ever easy! I'm in a ranch style house with two breaker boxes. Both boxes are full of breakers! I do have breakers for an electric stove and I have gas, and I have a breaker for a sauna, and I have none. Those come from the far away box. The stove outlet is opposite a wall to the ham shack, so that is my best chance to use that, and change the breaker to 15 amps. Still need to pull the stove, and put a hole in the wall. The XYL is not a ham! Regards, Jim |
Dentron MLA-2500 Question
JC wrote: I have a 2500 that had one bad 8875, and as-is EBay buy, and I am in the process of a conversion to a single 8877. This may not be the best choice of a tube for the application, but I had a good one in a box for nearly 20 years, so I decided to give it a try. Good luck on that tube. The early to mid 80's were very bad years for 8877 production. If you get a good tube it will last many years. If you get a bad one it might fail in hours. Before the conversion I powered it up, and with one tube I could get 450 watts on 80M with 50 watts drive. I did notice however that the variable loading capacitor had to be at full mesh on any band I tried. I blew off that issue because with only one tube the PI-Net was not seeing the as-designed input impedance. Dentron always had messed up tank values. Just look at the values they picked. They typically ran excessive Q on 160 meters and 80 meters. As I recall Q was in the 20's on 160 in many amps they made. They had the idea "one tank coil fits all". The only thing that hurts is circulating currents in the tank are a bit high and the loading control is swamped out by too much shunting capacioatnce so it has very little range. My question is from my blurry schematic and parts list. The fixed Centralab loading capacitors in my rig are all 1000pf, and the fixed plate tuning side capacitors are all Centralab 100pf units. The parts list does not show any 1000pf HV Centralab capacitors. Is there a typo in the parts list, or does my amplifier have the wrong capacitors installed in the loading area? Dentron used what they could buy. How do I know? I worked there for a short period and watched them do that. They would often make parts runs to surplus stores in Cleveland for components. I powered up the amplifier last night, and I can get 600 watts RF out on 80 meters with 50 watts drive. The cathode current reaches 0.8 ampere at 1700 volts Plate. So that is 1360 watts in for 600 watts output. That is about 44% efficiency. The variable loading control always wants to be at maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading variable capacitor must also be at maximum mesh. This makes me think my PI-Network is not correct for the impedances involved, and then it sure appears that the Dentron documentation does not match my rig. I might have to reverse engineer that PI-Net. It was never engineered in the first place. It is seriously lacking on inductance. Be aware also the larger ceramic caps have a high temperature coefficient. While they will marginally handle the current, any temperature change seriously affects tuning. If you really want to fix it, start from scratch. While you are at it get rid of the carbon resistors across the filter caps. Get rid of the carbon resistors across the diodes also. Any high dissipation critical resistor should be a metal component, not carbon. If you don't change the carbons across the filter caps to metal films or wire wound they will eventually fail and take out the electrolytics. If that particular amp has small carbons across the diodes they need to be removed. 73 Tom |
Dentron MLA-2500 Question
Jim Said:
maximum mesh for maximum RF output. On 160, 40, and 20m meters the loading variable capacitor must also be at maximum mesh. This makes me think my PI-Network is not correct for the impedances involved, and then it sure appears that the Dentron documentation does not match my rig. I might have to reverse engineer that PI-Net. Tom's reply: It was never engineered in the first place. It is seriously lacking on inductance. Jim's reply: Tom, Thanks for putting this all in perspective for me. Your description of the high Q, and resultant heating of the loading capacitors, and detuning makes sense. Users have reported much higher peak power than RMS with these amps. I always attributed that to the voltage doubler, and B+ sag under load. That is probably the dominate factor, but apparently RF losses are significant as well after a sustained key down at 1 KW input. So what is your opinion about the tank coil being wound of stranded and insulated wire where the wire strands are silver plated, and Teflon insulated? They are also close wound where the turns touch each other? I have never seen that done before. It sounds like I need to hit the charts for pi-networks, and pick the Q, and I/O impedances, and then make it happen. The inductance of that orange 160-80-40m coil with Teflon wire will be hard to model. I will try to measure it with my RF bridge, and see how it comes out (uh / 10 turns). I have 2 large 400pf 2500v silver micas, and a strange 4 section variable mica variable where each section is 300pf. This item has a common stud, and each section is about 1" diameter, and 1/4" thick. I might be wrong, but I recall someone saying these were used in the ART-13. These capacitors should handle lots of RF current, but I'm not sure of the voltage rating of that 4 section mica variable capacitor. The Dentron 1000pf loading capacitors are CRL type 858S, 5KV, X5U. These are best for bypassing use, and have high dissipation factor, and high capacitance drift as it heats up. Regards, Jim |
Dentron MLA-2500 Question
JC wrote: Thanks for putting this all in perspective for me. Your description of the high Q, and resultant heating of the loading capacitors, and detuning makes sense. Users have reported much higher peak power than RMS with these amps. I always attributed that to the voltage doubler, and B+ sag under load. That is probably the dominate factor, but apparently RF losses are significant as well after a sustained key down at 1 KW input. First, power you read on RF power meters is RMS power. Peak envelope power is the peak value of short term RMS power, average power is the short term average of RMS power. This is why for CW PEP and average power is the same, while for SSB and AM it is not. The RF losses are the same between the two. A weak power supply can cause a disparity between peak and average power, but that has little to do with the supply being a doubler. It mostly has to do with the size and quality of components. Remember when Dentron's were made the legal limit was 1000 watts DC plate input. So what is your opinion about the tank coil being wound of stranded and insulated wire where the wire strands are silver plated, and Teflon insulated? That's OK. The big problem is the inductance is too low. They are also close wound where the turns touch each other? I have never seen that done before. Cost cutting. Efficiency is determined by the ratio of unloaded Q to loaded Q, and certainly stranded wire, insulated wire, and close spacing reduces unloaded Q. Losses in the tank are reduced by half if you double overall unloaded Q, or if you simply cut operating Q in half. It would be difficult to double unloaded Q, but it is easy to cut loaded or operating Q in half. It sounds like I need to hit the charts for pi-networks, and pick the Q, and I/O impedances, and then make it happen. The inductance of that orange 160-80-40m coil with Teflon wire will be hard to model. I will try to measure it with my RF bridge, and see how it comes out (uh / 10 turns). You need to measure it in place with the lid on. The chassis and lid in the Dentron, since they are right across the coil axis, greatly reduce inductance. Whatever you measure outside the amp, you will have a lot less with the coil inside and the cover on. I have 2 large 400pf 2500v silver micas, and a strange 4 section variable mica variable where each section is 300pf. This item has a common stud, and each section is about 1" diameter, and 1/4" thick. I might be wrong, but I recall someone saying these were used in the ART-13. These capacitors should handle lots of RF current, but I'm not sure of the voltage rating of that 4 section mica variable capacitor. The Dentron 1000pf loading capacitors are CRL type 858S, 5KV, X5U. These are best for bypassing use, and have high dissipation factor, and high capacitance drift as it heats up. You will find some people complain about current rating, but in truth current rating is the least of your worries. The real problem with the caps is temperature coefficient and the fact Dentron's have terrible tank Q. The operating Q is far too high on many bands. I suggest you get all the components correct, and get a tank operating Q between 10 and 15 on bands where you can. Ten meters will be higher than that because of stray C, so just use the lowest possible value there. Then you can get NP0 (N P zero) caps of smaller value and parallel them for loading. Ameritron uses some ceramic chips that are high current temperature stable NP0, or you could use multiple N750 or less ceramic caps in parallel. You will not find high value doorknobs in stable temperature coefficients. For the anode padding caps, you will probably need to use small value HV ceramics in parallel. You'll see Dentron, in MOST amps, has far too much capacitance and too little inductance. Remember to measure that tank coil with the metal in place at the coil axis. You also must measure the coil at radio frequencies. One cheap common tool is a grid dip meter with a few fixed caps of known value... although a small light bulb, some fixed capacitors, and a transceiver will work every bit as well. 73 Tom |
Dentron MLA-2500 Question
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Dentron MLA-2500 Question
Gary Schafer wrote: On 26 May 2006 03:09:34 -0700, wrote: First, power you read on RF power meters is RMS power. Peak envelope power is the peak value of short term RMS power, average power is the short term average of RMS power. This is why for CW PEP and average power is the same, while for SSB and AM it is not. If you sustitute the word AVARAGE in place of RMS this will be correct. There is no such thing as RMS power. :) Better tell these people that.... http://en.wikipedia.org/wiki/Root_mean_square and this person... http://www.eznec.com/Amateur/RMS_Power.pdf 73 Tom |
Dentron MLA-2500 Question
Gary Schafer wrote: On 26 May 2006 03:09:34 -0700, wrote: First, power you read on RF power meters is RMS power. Peak envelope power is the peak value of short term RMS power, average power is the short term average of RMS power. This is why for CW PEP and average power is the same, while for SSB and AM it is not. If you sustitute the word AVARAGE in place of RMS this will be correct. There is no such thing as RMS power. :) By RMS power, I mean the power derived from RMS voltage or current as opposed to using the PEAK value of voltage and/or current to determine power. I suppose I could have said effective or heating power, or power derived from RMS values of voltage or current. It gets too confusing when we say average power is the average of the average power. By the way, there is no such thing as avarage power. :-) 73 Tom |
Dentron MLA-2500 Question
Update, My Dentron MLA-2500 has taken a back seat to my sons HS graduation, and the 20th anniversary to my XYL. That said, I did sneak in a little time for a few things: 1.) On 160 meters the 1000pf fixed loading capacitor is placed in parallel with the 80 meter 1000 pf fixed loading capacitor. These are then in parallel with the variable 500 pf (max) load control. Not much tuning range here. The Dentron schematics are clearly wrong by not showing this. Sure could use a bigger loading variable, and less fixed C for more tuning range. 2.) When replacing the RF PA plate load impedance with a carbon comp resistor between 1500-2500 ohms and hooking my RF impedance bridge (Autek) to the linear's 50 ohm output, I get a SWR null on all bands 160-10 meters by tuning the pi-net for each band. This makes me want to reverse engineer that pi net to figure out whatever the loaded Q may be, and to start optimizing for my use with a 8877. I still agree with Tony that those ceramic loading capacitors may be the crux of the problem because they are not made of the right stuff for carrying hi RF currents; instead these are made for bypass applications. 3.) The Alpha folks are trying to build a chimney for me with their Silastic RTV material. I think they are either going to charge me an arm and a leg, or kick the order back. This will be a custom design, and not an off the shelf part. I may go back to the solid Teflon block idea, and try to find a place that will machine it. With SSB, or AM, doesn't a wattmeter show the average (over several sylables) reading of the RMS power? :-) Jim WD5JKO |
Dentron MLA-2500 Question
On Sun, 28 May 2006 13:47:35 GMT, "JC" wrote:
Update, snip 3.) The Alpha folks are trying to build a chimney for me with their Silastic RTV material. I think they are either going to charge me an arm and a leg, or kick the order back. This will be a custom design, and not an off the Ahhhh... That is just a pice of Silastic (TM) sheet wide enough and long enough to form a chimney when wrapped around the tube. I can't imagine it'd take much engineering. It's a very crude, but effective design and easy to make if you have the material. shelf part. I may go back to the solid Teflon block idea, and try to find a place that will machine it. With SSB, or AM, doesn't a wattmeter show the average (over several sylables) reading of the RMS power? :-) Some do and some don't. There are a number of watt meters including at least one Bird model that are capable of measuring PEP. Even the MFJ tuners have that capabililty but I don't know how accurage they'd be. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com Jim WD5JKO |
Dentron MLA-2500 Question
"Roger" wrote in message ... On Sun, 28 May 2006 13:47:35 GMT, "JC" wrote: Ahhhh... That is just a pice of Silastic (TM) sheet wide enough and long enough to form a chimney when wrapped around the tube. I can't imagine it'd take much engineering. It's a very crude, but effective design and easy to make if you have the material. Roger, I proposed a single rectangular piece to wrap like they do on their 8874 amps. I even gave them dimensions, and a good picture of my 8874 mounted in the amp. A round continuous piece would also work. So far two business days have elapsed without a reply from Alpha. I hope to hear from them on Tuesday. I'll keep you posted. For my brief tests I used a 10" long X 2" wide X .02" strip of Mylar wrapped around the tube. It worked fairly well, but got de-formed from the heat. I later jumpered the fan resistor to speed up the cooling fan. For some reason the MLA-2500 fan speed relay was not kicking in the high speed mode when keying the amp. I need to look further into that wiring, and see what is going on there. There is a button to enable the high speed fan option, but it does nothing until the thermal Klixon OT-switch clicks on, and then the fan goes to hi speed. Jim |
Dentron MLA-2500 Question
On Sun, 28 May 2006 19:41:21 GMT, "JC" wrote:
"Roger" wrote in message .. . On Sun, 28 May 2006 13:47:35 GMT, "JC" wrote: Ahhhh... That is just a pice of Silastic (TM) sheet wide enough and long enough to form a chimney when wrapped around the tube. I can't imagine it'd take much engineering. It's a very crude, but effective design and easy to make if you have the material. Roger, I proposed a single rectangular piece to wrap like they do on their 8874 amps. I even gave them dimensions, and a good picture of my 8874 mounted in the amp. A round continuous piece would also work. So far two business days have elapsed without a reply from Alpha. I hope to hear from them on Tuesday. I'll keep you posted. For my brief tests I used a 10" long X 2" wide X .02" strip of Mylar wrapped around the tube. It worked fairly well, but got de-formed from the heat. I later jumpered the fan resistor to speed up the cooling fan. For some reason the MLA-2500 fan speed relay was not kicking in the high speed I'm trying to remember how the fan was set up in the 2500. I think you are going to need to pressurize the chassis to get enough air flow through the fins of the 8874s and they do take more air with a higher pressure drop than the originals. Those little suckers are expensive too. Good Luck, Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com mode when keying the amp. I need to look further into that wiring, and see what is going on there. There is a button to enable the high speed fan option, but it does nothing until the thermal Klixon OT-switch clicks on, and then the fan goes to hi speed. Jim |
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