The AM Forum

Hello again.  Now that I've nailed down the plate supplies for the 4-65A transmitter project, I'm looking at the screen supply.  I have a UTC VM-3 modulation iron and am not sure if it has a separate winding for modulation of the screen.  I'd like to avoid building a separate screen supply and have some Ohmite adjustable resistors 75K and 100K at 100 watts.  My no-load plate voltage will be around 2KV and I'd like to just pick my 250 volt screen voltage off that.  Question is will I need an additional RF choke between the dropping resistor and the screen?  What value?  This makes way more sense than building a separate supply.  Mike - W9ZSL

  Hey Mike,

     I think your going to need to tag that conn. Labeled "N/C"
to ground. otherwise you will dump 2kv on the screen.

/Dan

Try a Handbook either east or west coast in the 40's - 50's era.  I've never seen a RF choke in the screen supply of a tetrode..  

al

Right about grounding " NA" or providing another power resistor to ground right off the screen, say 25k.
Because you Need a dump for reverse current screen swings.

Also right about not needing a choke if the screen is fed off the same modulated plate supply.  You do need a choke if feeding screen off a separate supply.

Hey Rick,

   The part of the "R" between the SCR conn and ground should fill the bill.
Hook the Iscr meter before the big "R". lets it read "upscale" a bit anyway.

   I forgot about the reverse screen current. good point. shouldn't be an
issue with a Big Arse WW R with a slider though.

/Dan

After reading this thread, I downloaded an Eimac datasheet for the 4-65A and I was surprised at some of the circuit details.  Here is the link to the downloaded document:

http://frank.pocnet.net/sheets/088/4/4-65A.pdf

The document provides sample schematics for both single-ended and push-pull 4-65As modulated by a pair.

The document shows the single-ended final modulated by a class AB1 pair of 4-65A tubes, and the push-pull final modulated by class AB2 4-65A tubes. In both cases, a single power supply is used. 

I found it interesting that in both cases, the screen supply is devoid of any modulation.  A screen dropping resistor is used, and an RF choke is placed in series with the resistor and the final screen.  I would assume the RF choke is just there to keep the RF from getting back to the power supply, probably for TVI reduction.  Certainly it does not provide any ability to self-modulate the screen, because it has insufficient reactance at audio frequencies.  This implementation certainly cannot produce 100 percent modulation, but it probably gives a good account of some of the circuit values and voltages and currents needed to implement an amplitude modulated 4-65 final. 

I am not sure how robust the screen is in the 4-65A, but I would think some means of protecting it from excessive voltage and dissipation would be prudent in the implementation of a 4-65A plate modulated final. A voltage divider instead of just a series resistor would help, and feeding the screen from the modulated, rather than the unmodulated plate supply, is obviously preferred.

Some reading that may be of interest:

http://www.bruhns.us/T-P_Mod_Lin/T-P_Mod_Lin.html

Notice it says to get the screen voltage from the UNMODULATED B+ voltage through a dropping resistor, or with a choke.
The screen will SELF modulate with a dropping resistor, as when the plate voltage goes UP, the screen current goes DOWN, lowering the voltage drop in the resistor.

For very good modulation, its common to get the voltage from some combo of the unmodulated and modulated voltage, not all one or the other. Different tubes call for different amounts of swing.

And really, it makes very little difference in the real world if you was to ask me, and you did'nt.
The 4x150/4x250 has warnings in the RCA tube manual that the screens need to be modulated from a tap on the mod transformer, as the screen current curve must look odd.
I built my 4x150 deck as an experimental screen mod setup, and when that did not work out, I tried plate modulation and it works fine.
Distortion products are as low as anything else I run, and positive modulation is fine.

On my 32V3, I had to change the screen setup to about 50/50% to get good positive modulation.

But its a moot point, if you do 100% in both directions instead of 120% positive at 100% negitive, no one is going to be able to tell at the far end.
I would say most people running AM run things well below 100% modulation anyway, or they have no real idea of what the modulation actually is.

RF chokes in the screens is something you might have to do at VHF and UHF frequencies, for HF, a bypass cap seems to work fine.

Other Rick and friends,

Boy I like those Eimac schematics showing shielding and recommended ground points. Don't think I've ever seen such detailed schematic drawings. They had amateurs in mind for a lot of their popular tubes culminating in the ceramic, high mu, focused anode series of 88xx's.

 Also noted taking the screen dropping resistor from unmodulated B+   Several threads on this in the past with what Collins and others did or didn't do. I see Johnson in the Ranger took screen voltage, clamper tube and all right off the modulated side of HV..  I've done both ways, one time simply because running out of HV wire, I didn't want to make an extra HV run to outboard power supply.  Audio Sounded about the same as noted.  We may be chasing points of little return (pun.). On my 32v2 ,stock except for AF stages and hooked up as is, I get way more than 100% mod running a d-104 one phase way and about 100% the other.  Approx. 100% easily determined when beads just start to appear on the scope. The AF gain pot more than covers a multitude of sins other than somewhat flat and dynamic peaks missing if in wrong phase.

I have the spec sheet for the 4-65A and am basing my build on their diagram.  I was mainly concerned about the screen modulation bit also about the possibility of getting its voltage from the plate supply.  My original plan was to use a separate supply so I think I'm going to stick with that.  Thanks!  Mike - W9ZSL

Hey Bacon!

There's a screen concept that has always bothered me. Maybe someone can explain it without referring to tube transfer curves...

In Linear Mode:

It's a fact that when using a tetrode in linear mode, (RF linear or as modulators) the screen voltage needs to be well regulated. The screen voltage has a large effect on plate current. An electronic regulator is often used to keep the screen voltage stiff.  I know this is true cuz I've seen unregulated screens sag just a few volts and produce THD/ IMD problems.

Now, if the linear operation of a tetrode is so sensitive to screen regulation, how can it be that we can actually tie the screen to the grid OR to the plate  and have the screen voltage  swing all over the place, and STILL the tube acts linear?   If just a few volts of screen sag/swing will deteriorate a linear stage, what would hundreds of volts swing do to it?  I realize it now becomes a "triode" but what happened to that "big" influence from the screen?


In  class C operation:

We have the choice of placing a  15H choke or resistor for "self modulation" of the screen circuit. This all seems quite unprecise and needing careful tuning, considering that just a few volts of sag will ruin the linearity of a stage.


It's like saying the screen has a huge influence on the stage, but don't worry, we can do about anything we want with it and it works OK... ;)  I just don't get it.

T

What has worked for me was to build a screen supply with a small variac, then put a big wire wound pot in series.
That allows a few good things to happen:
The screen self modulates,
The screen is somewhat self protecting.
It works with any tube or number of tubes if the voltage is high enough.
You can dial out the resistance and use the rf deck as an amplifier.

With the addition of an adjustable overload relay (easy to make circuit) it makes the rig fool proof, very important in my shack to have things fool proof....

 

A plate modulated RF final only needs to be linaer under modulation, not with the grid input signal.
For the best modulation, the grid is (has to be) driven into saturation, more grid drive does NOT give more output.

But increasing the plate voltage does.

If the screen voltage is fixed, it will limit the modulation, since plate current is supposed to follow the plate voltage.

 

Here is a circuit I originally designed for a modulated 4-1000 but modified for the 4-65.

It is an adaptation of an RCA circuit.

It uses a separate screen supply, but can supplied with any other supply by changing the "tap slider" of R1.

It is both self modulated by L2 with an "assist" by C4.

Screen voltage is limited by the zener diodes.

Phil - AC0OB

I like the looks of that DMOD.  Is this the diagram with the connections shown correct?  I basically copied it from the link and am double checking.  I do have a couple of questions.

I came up with a nifty supply that will give me the right voltage.  First, considering the choke 6-10 Hy following the slider and feeding the screen, why should this be rated at 250 ma when according to Eimac the maximum current draw for AM would be only 40 ma?  I have a 200 ma/10 Hy but that seems like overkill.  Eliminating all the extra components, I have a 15 Hy @ 90 ma that can be used straight off the screen supply. 

Second, I've done some checking and haven't found any Zeners with with those specs.  I have a bunch of power supply diodes in every voltage and current value.  Must zeners be used?

Tom....I'm working on a Mcmartian Ba1K....It has separate screen supplies for the finals and modultors....4-400's all.....Mcmartin lowers the plate voltage to get lower output for nite time use but the screen supplies stay the  same voltage all the time..(700 volts)...It works but I'm not convinced this is the way to go....I'm getting some modulation patterns that aren't right...


Mike...I have built plate modulated rigs that put modulated B+ on the screens thru a dropping resistor...also tried "self modulating" the screens with a choke....In actual operation, no noticeable difference in received audio...When using a scope to check the screens, the Modulated B+ method shows more audio on the screens than the choke method...I havent checked out the audio on the screens when using a separate screen supply...Good luk...

I see that the RCA tube manual (1956) calls for the same screen voltage on 4-250's (@600 volts)
for all plate voltages from 1500 to 4000.....4-65's call for 250 volts on the screens for all plate voltages from 600 to 2500 volts...I remain confused..

That does not seem right, but I know some tubes work over a wide range.
My 4cx250b/4x150a mod deck calls for the same bias and screen voltage from 1000 volts on the plate to 2000 volts on the plate, and adjusting the plate voltage does not change the resting plate current.
I do not monitor the screen current, so I do not know how much that changes.
With an RF deck, I always monitor the screen current and voltage, and its easy to exceed the recommended screen current.

You can run whatever screen voltage and current you want, as long as the screen does not melt.
Its a good idea to protect the screen, and measure the cleanest operating points.

The bottom Zener should be 25V.

If you already have a well filtered 300V supply, you don't need L1, C1.
The R1 tap would then need to be set to about 2.8k.




That L2 choke value was for the 4-1000A. You could use anything from 50 to 100 mA, depending on the amount of breathing room you want.



Yes. On the modified schematic below the zeners are: D3 is NTE5199A, D2, D1 are NTE5226A.

With this zener string the screen voltage is clamped to 325V which should rarely happen but acts as a safety to limit plate and SG current if that should occur.

I am just a believer in stiff supplies and a bit of safety circuitry.

Phil

The AH HA moment.

I tracked down some of those diodes at Arcade Electronics and have some questions.  Given the package, the anode is threaded and evidently bolts to the chassis...or something...so how can they be effectively connected in series?  If they don't heat up I suppose they could be mounted on a piece of plastic or perf board.  I've never used them so am not familiar.  As for the power supply, I have a transformer salvaged from a receiver.  Using a 6X4 rectifier (it has no 5 volt winding), a pair of 20 mfd caps with a 10Hy @ 90 ma choke in a capacitive input filter and 35K bleeder, I get 370 volts.  I have another 10 Hy or a 15 Hy choke I can use for L2.  As for R1, I would substitute a 15K adjustable which I already have on hand.  I'm going with Eimac's recommended 250 volts on the screen. This would be after eliminating L1 and C1. Of course, I could always use a choke input filter, but then I get only 280 VDC no load. Seems to me plan "A" would work just fine.

I would like to use those Zeners as a safety factor, but on the other hand, maybe just using a 30K or 50K adjustable bleeder as a voltage divider and taking the screen voltage from the slider into the choke for self-modulation would work just fine...with just a bypass cap. Judging from the comments so far, the advantages of adding any influence from the modulation transformer through C4 would probably be minor so I'm not sure the extra cost would be worth it.  Those diodes aren't cheap.

Use whatever you brung to the party to get it working.  ;D

As for the zeners, you could use a total of six 5W zeners, with two of each type in parallel.

With the circuit shown, R1 was calculated from 35 mA as the design current: 2mA from the R2 bleeder, and 33 mA max screen current.

so R1 = (Supply Voltage - Screen Voltage)/0.035

I have always felt that a separate screen supply with self modulation reduces the mod transformer current burden.

Once you get it working please post the schematics and any comments on the trials and tribulations you had.  :)

Phil

I'll start with the basics and modify as needed.  So Phil, that works for me.  Do these things generate any heat?  Seems to me I can mount them on some perf board.

Will keep everyone posted.  Remember, this started as a 4-125A project and I've downsized to the 4-65A because that little bottle seems to be like an empty canvas, LOL!  ;)

Hmm, I haven't seen any comments on the use of C4.

The theory behind C4 was to add some higher frequency audio voltages as the AF frequency increases, since the inductor's reactance increases with frequency.

C4's reactance decreases with higher AF frequencies allowing the higher frequency audio components to reach the screen grid.

Is it absolutely necessary, probably not.

Phil - AC0OB

Didn't think so Phil.