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

"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

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

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

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

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. :)

73
Gary K4FMX

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

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