"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

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

"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

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

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

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

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

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

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