Russian 310A + 300B

[yangshau]

I recently completed a 310A+300B Amp. The schematics is attached. After assembly, I first used Russian's 310A equivalent 10*12C for test. The voltage at various nodes are shown.
I am concerned with the plate of 10*12C is only about 90V now (I was originally thinking it should be around 190V). Apparently the plate current is larger than I had expected as 2.8mA instead of 2mA.
The Amp still produce good sound. What should I do? Should I adjust the screen voltage down to 100V or 90V? Will that help to make the plate back to above 150V? Or should I reduce the plate load resistor?
Thanks for your suggestions and comments.

 

[yangshau]

The schematics.

The schematics.

 

[JimW]

Not familiar with the Russian equivalent, but I do have a couple of amps using 6sj7, and the voltages work out correctly in these amps. the screen voltage will affect the plate current, as a rule, I try to get the screen voltage to be about 1/2 of the plate voltage. Lowering the screen (from your diagram, it looks like it would actually be running around 125 volts, based on the resistor divider network coming off the 410v supply) to 90-100 would be a good start, should allow the plate voltage to rise a bit, may need to lower the value of the 27k used to decouple the plate from the main b+ .

 

[yangshau]

Thanks JimW. I tried both lowering the screen voltage by changing the 33K to 22K, and also reduce the 27K by paralleling a 33K with it. Now the plate sits at 182V and the screen is at 78V. So everything looks perfect. I attached the schematics again with all the node voltage measured for anyone who might be interested in building a similar one.

 

[yangshau]

The final schematics and measured node voltage

The final schematics and measured node voltage. Some 10J12S will have slightly different characteristics thus resulting some minor shift of the voltage.

 

[v4lve lover]

for a tube that old, its fine. older tubes especially Russian ones made on out of date American equipment tend to show abit more deviation in characteristics .

tubes of different make´s Can show somewhat different parameters. nothing to worry about as most circuits whit these are not critical.


by the way, you could try a zener diode to stabilize G2 of the tubes, this might change the `sound perception`

cheers.

V4lve