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

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

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