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Author Topic: Screen Supply For 4-65A Amp  (Read 3921 times)
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W9ZSL
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« on: March 15, 2015, 08:11:52 PM »

I intend to build this 813 amp and modulate it with a pair of 4-65A's.  Problem is, the screen voltage for the 813 is derived from the plate supply which is rated at a stout 300 MA but adding and extra load for the screen is out of the question.  The 813 needs 350 VDC at 40 MA regulated if I want to use the amp in an SSB mode.  No problem.  The 4-65A's in the modulator need 250 volts at 85 MA max when the plate supply for both is 2KV.  Since current limitations dictate a separate screen supply, can I use one supply for both RF and Mod provided the RF voltage is well regulated?

I don't want to take the screen supply off the HV to the 813 plate as shown in the diagram because I'm pushing the envelope as it is regarding current draw.  Previous comments have shown that the iron on hand will work for all three plates.  Since the screen supply will be solid state the same as the HV, applied voltages will be simultaneous to avoid voltage to the screens before the plates.

I guess the point of all this is, what considerations, if any, should I take in isolation of the screen supply voltages from one supply to the PA and Modulator?  Al, VE3AJM emailed this same circuit to me.  I already had a copy of the original which is shown.  I've scored all the coils to build the input turret.

I guess the major difference would be elimination of the screen VR's taken from the plate supply and substituting a separate supply for all the screens.  In addition, the fixed caps in the output tank would be replaced with a variable.


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N2DTS
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« Reply #1 on: March 15, 2015, 08:51:52 PM »

I am not sure what the question is, but..
For the RF deck, I like a variac screen supply with a big wirewound pot as a dropping resistor.
Say your 813 screen wants 400 volts at 30ma, a resistor in series with the screen supply can be adjusted to drop 100 volts in the resistor, that helps protect against screen damage, more current drops more voltage in the resistor.
It also self modulates the screen in AM service.
For amplifier service, the resistor can be adjusted to zero ohms, the voltage set on the variac to the correct voltage for AB operation.
You should always have a screen current overload circuit to trip at a maximum screen current.

The same needs to be done with the grid bias if you want to use it in plate modulated service, it needs fixed and grid leak bias, and in amplifier service it needs fixed bias and no grid leak bias.

Running tetrodes as amplifiers in AF or RF service needs a stiff or regulated screen supply.
In plate modulated service, they need very unregulated supplies.

I like having things metered and adjustable, you can operate things at a wide range of voltages and powers, and use other tubes/rf decks, but it makes things very complicated and big.

It makes things a lot simpler to build something for AM only and one band.

Oh, and if you do step start on the HV, you need to key the screen voltage AFTER the step start is done.
I use the step start relay to key the screen supply.

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W9ZSL
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« Reply #2 on: March 15, 2015, 09:26:06 PM »

The question is "Can I use one power supply for the screens of both the PA and modulator"  If so, do I have to use any special techniques to separate the divided voltages?  I can build two screen supplies but would rather use just one.  And yes, screen voltage for both the PA and Mod will NOT be applied before the plate voltage. This will be AM only.
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N2DTS
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« Reply #3 on: March 15, 2015, 10:26:10 PM »

I have never seen it done.
Most times the voltage will be much different, the modulator supply needs to be stiff/regulated and the RF deck needs to have a resistor and/or choke in it, and some sort of current limiting.

The screen must self modulate or be modulated off a mod transformer winding.
I do not think you want any interaction between the two at all.
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