Not Biased, Yet

[Barrie]

The ongoing saga of the push-pull VT-127As continues.

I've got most of the chassis metalwork done.  All major components are mounted. 

Now, I'm looking at the bias supply.  Several sources indicate that the VT-127A requires -340 volts bias for class C CW.  No mention is made as to what the bias should be for plate modulation.

If the bias requirements are similar to the 100TH (which I think they are, judging by the rather high voltage involved), then I think I could use the same factor to determine the AM bias for the VT-127A.

The 100TH calls for -225 volts bias for CW and -400 volts for AM.

The -400 volts is 1.77 times the CW voltage.

If I multiply the CW bias for the VT-1127A by 1.77 I get -600 volts.

Even to the unwashed and uneducated (me), that seems like a lot of minus volts.

Any washed and educated thoughts on this subject?

73, Barrie, W7ALW

[Opcom]

Could it be more like the 100TL? Check the data for it here. The AM PL data and class C data sort of reflects what you have posted. But I am not washed enough to really answer. If it were me and there was no data I would equip a temporary bias supply and test, then build the proper one. All my stuff has variac controls, so either way it would be OK.

[WD5JKO]

I saw a reference to the VT-127A having a Mu of 15.5. I also see a condensed data sheet here:

http://www.nj7p.org/Tube4.php?tube=VT-127A

Class C Amplifier
Plate Voltage ................................. 2000 V
Grid No. 1 Voltage ............................ -340 V
Grid No. 1 Current ............................ 67 mA
Plate Current ................................. 210 mA
Driving Power ................................. 25 W
Power Output (approx) ......................... 315 W

Doesn't say anything about AM though, but the above conditions should be a good place to start.

You can look at 100TL (Mu-14), and 100TH (Mu=38) at this web site:
http://www.scottbecker.net/tube/sheets1.html

Jim
WD5JKO

[Barrie]

I've gone ahead and built a variac controlled bias supply, so I have 0 to 680 volts.

My only problem now is how to attach it to the tubes.  I see a method in a 1955 handbook that looks pretty simple.  I've enclosed a rough schematic.

In this circuit, it looks as if the grid resistor will also act as a bleeder for the bias supply?

What value should the RFC be?

Should there be a by-pass capacitor on the bottom of the RFC?

Any help appreciated.

73, Barrie, W7ALW