This could be a great next project!! Plate Modulated amp

[KJ4OLL]

Hi,
Now that the "training wheels" first GG homebrew attempt is 95% complete, thinking about what my Elmer has been saying for years:
"You need to build a Plate Modulated amp for AM"

Googled around for a while to learn what I could about the basics.

Seems like the Modulation Transformer availability is the issue.
It took years to collect the iron the the GG amp project, expect the same for the Mod Transformer.

Looked through the "pile'o transformers" that is securely holding down the concrete floor of the shack, found this to be used for HV (plate) and screen:



Is it a good candidate for Class C, plate & screen for a pair of 813 or 833 modulating a 4-1000A?

What label numbers should I look for in a Modulation Transformer that would fit this model?

73
Frank
KJ4OLL

[w4bfs]

the ratios between primary and main secondary are too high to use pp tube drive as a mod xfmr because the impedance transformation follows the square of the turns ratio .... it looks like a decent hv xfmr but some of the info did not come across in the picture

this could change if you were planning the use of a solid state amp as the audio power source ... since the power xfmr is not gapped then you will still need to use a modified Heising arrangement .... leakage reactance will rear it's ugly head and affect audio high frequency response ... if it is low enuff might still work fb ... may require 2nd xfmr for additional Z lowering

take some time to look thru the tech forum past threads ....lotsa discussions on this subject

[KA2DZT]

The xfmr pictured is a plate xfmr.  I think he is looking for a mod xfmr that would match the plate xfmr.

[KJ4OLL]

Correct, I have a plate transformer, need Modulation transformer to match it's capabilities.

The plate transformer shown is for a Collins 30S-1.
8168- 4CX1000A

Collins ran the screen in this AB amp @ 200 volts, even though the 4CX1000A will operate at 325.

So possibly this plate transformer is not a good match for an 8166- 4-1000A, which can have a screen voltage of  1000.

Might be able to increase the screen voltage so as to work w/ 8166 - 4-1000A  w/ external step-up transformer?

[K1JJ]

Collins ran the screen in this AB amp @ 200 volts, even though the 4CX1000A will operate at 325.

So possibly this plate transformer is not a good match for an 8166- 4-1000A, which can have a screen voltage of  1000.

Might be able to increase the screen voltage so as to work w/ 8166 - 4-1000A  w/ external step-up transformer?

Hi Frank,

You will definitely want the screen voltage to be a separate supply with a small Variac to control its voltage. Varying the screen voltage independently in a class C plate modulated stage to get the proper screen current under various plate load conditions is important.  400-500 volts  works well at 140 mA for a normally loaded class C   plate modulated 4-1000A.

I'll bet that 51 Watt Fred has the ideal 500 volt, 200 ma transformer in his stash for you... :-)

Your plate transformer in full wave  choke input will do a nice  3100 V DC  out for the plate supply. Great voltage for a 4X1.  Though small, and you will have to see how it holds up under full carrier and modulation service. If loaded lightly, it may work OK depending on its core and how long you talk.


T

[KJ4OLL]

T,
Thanks very much for the suggestion!
There is an HP 6521A in the rack that would be perfect for the screen supply.
0-1000vdc, 0-200ma.
Not much use over the years, except for hi-pot and testing for breakdown/arcing (HP is current limited).

So far, the pile'o parts for this project is coming together, and also using up some items that have collected around the shack.
Better to use them for a project, than drag to the hamfest to sell.

- HV plate - Transformer from Collins 30S-1
- Rectifier - still have piles of spare diodes and resistors from the last one, just need to find another sheet of nice green fiberglass.
- Screen supply - HP 6521A
- Filament transformer for 4-1000A  - use this spare from 3CX3000A7 project.



Not sure what to do about the PS filtering on this one.

Last supply I used fixed chokes from a Gates, 20H, and 12mfd of 5kv oil filled caps.
However, big chokes are becoming scarce.
Oil-filled and the more modern plastic HV caps are still easy to find.

Frank

[K1JJ]

Hi Frank,

You axed:  "What label numbers should I look for in a Modulation Transformer that would fit this model?"


I would put some "wanted" ads out in all the popular ham web site listings looking for a broadcast 1 KW or larger mod transformer along with a matching reactor.   If you go with a 4X1  X 833As  or even triode-connected 4-1000A modulators,  most of the popular BC transformer ratios available out there should work OK.  

Stick with broadcast iron since a lot of other wimpy commercial  iron will either pop or have a restricted audio range like the older WWII stuff.  There are probably plenty of aging hams these days that have these gems in their cellars and have finally realized they will never use them.  Or, maybe someone here has what you need.


Wanted ads have brought me about everything I have ever axed for, parts-wise. 
Looks like you have a plan for the fil xfmr and PS.  You can find your missing PS filter choke or filter caps the same way.

T

[Opcom]

350mA seems a bit light for both the final and a modulator. You need another one of those. A separate plate supply for RF and modulator is the way to go because the widely varying modulator plate current won't pull at the RF amp plate voltage and reduce your peak modulation ability.

Individual variacs for each plate supply is another way to "adjust" the modulation transformer ratio.

Explanation: if the ratio of half the modulator primary to the RF side secondary is, for example 1:1.05, and each plate supply is 2500V, then increasing the modulator plate supply to 3000V gives more modulator output voltage swing, as if you had a 1:1.26 step up ratio.
1:1.05 * (3000/2500) = 1:1.26. Not that it's needed but just a nice byproduct. Also lets you run quite low power too.

COL. Tucker is precisely a modulated RF amp. Continuously adjustable for class C through A in the final so it could be used as a linear amp if the modulator is not needed, such as with a plastic radio that already makes its own modulation.

99% of the time it is used as a modulated class C RF amp, driven by a speech amplifier and the plastic radio set to CW mode.

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