Another voltage too high question ( bias too hot )

[zapped]

I'm looking for help understanding the power transformer used in RCA intercom amp I have been converting to an Ampeg J12-B using Sluckey's version of this amp. The starting base is a RCA MI-14388 has 10W, 2 x 6V6, 1 X 6SJ7, 1 x 6SJ7 and 5Y3. Seems perfect for the task ?? Amp was all original. Problem is PT puts out about 785 vac no load, 525 vdc from 5Y3 with no other tubes installed and over 440 to the plates. All much higher than Sluckeys schematic. I cant seem to get the plate dissipation below 14w. My fault for not checking the voltages before removing old circuit. I assumed the iron would be very close. I have tried to adjust the bias resistor from 250 to 500 but still the plate dissipation is at 12+ for each 6V6. I have never been able to find a RCA schem. for the original circuit to see what the voltages should have been. How could only 10 watts come from such high voltage ?? Thanks if you can help.  Ed

[sluckey]

I'm surprised the PT puts out that much voltage for that tube line-up. What does the filament winding put out?

[tubenit]

Make sure your voltmeter has a fresh battery & you are not getting false readings.  JJ 6V6's are rated for 500v, I think?


With respect, Tubenit

[Tony Bones]

If your problem is high line voltage (like 125VAC) then a 12VAC bucking transformer would probably bring the 440VDC on the 6V6 plates down to something around 400 or less.

http://www.angelfire.com/electronic/funwithtubes/Buck_Boost.html

[terminalgs]

525 vdc from 5Y3 with no other tubes installed

What's your 6v6 plate voltage with all tubes installed?

[zapped]

A brief follow up and correction: the original tube line up had a 6sl7 and a 6sj7 for preamp tubes I swapped the 6sj7 for a second 6sl7 to be closer to the ampeg circuit. The latest voltage checks taken @ 121volts at the wall are 759 vac in and 413 vdc out at the 5Y3GT. Plate 402, screen 395, heaters 6.62 v. All tubes were in place. I don't know why the plates are not showing 440 tonight ???? They were 437 to 442 at last check. Thanks for all the replies. Regards Ed

[zapped]

Is it true that cathode bias power tubes can safely idle at 100%plate dissipation ? Also if you choose to have separate bias resistors for each tube the ohms would stay the same as single bias resistor but 1/2 the watt value?  thanks Ed

[sluckey]

Is it true that cathode bias power tubes can safely idle at 100%plate dissipation ?

yes

Also if you choose to have separate bias resistors for each tube the ohms would stay the same as single bias resistor but 1/2 the watt value?  thanks Ed

Half right. You could use half the wattage rating but the resistance must be twice the resistance of a single resistor.

[zapped]

Thanks again Steve.

[frankenxtein]

Voltage here in Memphis , varies with the time of day.

 I use a 12VCT (4A sec) bucking transformer but , 6VCT would work for our 122.5VAC so far max here.

 Half the 12VCT drops me to 114.9VAC , 108VAC with the whole thing. 

It's all about lowering the filament voltage for me  , a little lower B+ is a bonus.