The AM Forum

Does anyone know of a published table or chart, with the resultant power, and P to P load impedance for a pair of 811A's running in various classes?  I am only interested in AB or AB2 I suppose.

I am looking for something like: 2x 811A's, Plate Voltage 1250, Plate Current .250 = 312 watts input 70% = 218 watts.
 P to P load 12,500 ohms, or something like that.

Thanks and
HAPPY THANKSGIVING to everyone.

https://frank.pocnet.net/sheets/049/8/811A.pdf (https://frank.pocnet.net/sheets/049/8/811A.pdf)

Gives various voltages currents and loads for class B. 

Ed

Attached is a table from The Radio Handbook, Thirteen Edition, which has several operating points for 811As.  I believe they are all class B, however.

Dean

Class AB- operation is really hard on 811A's.

Class B is the default for a reason.

Don't cheap out on the driver design.

73DG

Thanks everyone and Happy Thanksgiving.  This is why I love this forum. good solid answers.

Now I will return with a Class C question.

BM W4RFM

Hi Bob,
Here are some things I have learned about speech amps and modulators. I am not an engineer, so this is all anecdotal, but it has worked for me. I get very good audio reports when I am on my homebrew transmitter.

The speech amp should start with a high gain pentode. I use a 6SJ7, but 6AU6 would work if you prefer miniature tubes. The second tube should be a phase inverter so that you can drive PP driver tubes. The phase inverter can be a single triode such as a 6C4 or a twin triode such as a 6N7. Phase inverter circuits are fairly straightforward and can be found in any old handbook. The phase inverter should drive PP low mu triodes such as 2A3, 6A3, 6A5, or 6B4. Low mu triodes are the best drivers since they handle the variable impedance of the modulator grids better than other types of tubes. However they are getting more scarce and more expensive. You can also use tetrodes as drivers if you use negative feedback. Use a regulated plate voltage power supply on the speech amp if you can. I use a lambda 300VDC regulated power supply. The driver tubes will need to deliver some power, and the tubes listed above can deliver up to 15W or so depending on plate voltage and type of bias. Use a high quality driver transformer that is designed to be used as a driver transformer. I use a UTC S-9. Stancor and Thordarson also made good driver transformers.

At 1250V, the 811A’s can be used without bias, and 8-10W of audio from the drivers will be plenty. The plate to plate impedance presented to the modulator tubes will be determined by the plate impedance of the final amplifier and the ratio of the mod transformer. I would recommend that you don’t get too wrapped up in getting the plate to plate impedance perfect. Anything close will work well. If you can get ahold of a good mod reactor, I would use it to get the final amplifier plate current off of the mod transformer secondary. I can use three different RF decks with my modulator. Depending on which one I am using, I run the final at anywhere from 1400-2000VDC and from 150-250mA. The modulator operating parameters are not changed, and it works well with any of the RF decks. I do monitor my audio continuously with a trapezoidal pattern on an oscilloscope.

Hope this was helpful. I would be happy to post the schematics of my speech amp and modulator if you’d like to see them.

Ron

Excellent advice Ron!

Joe-GMS