• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Power supply problem


2004-08-22 9:46 am
Im building my first tube amp (boozhound 6v6) and suprise; i have a problem.
I started to test this amp by measuring power transformers voltages.. they seemed to be ok. Then i put 5Y3 in place and measured DC voltage over the first 20uf capasitor and saw 400V in my multimeters screen. Same 400V appeared even after the choce, which i think is weird.

I have already triple checked everything.. ;)

power supplys schematic:


Please help me out!
Hi Niila

The problem you have is probably the same problem of tens of other people that are building their first amp and, being overwhelmed by lots of new things and with 10 eyes open to detect smoke or whatever, forget simple things as ohm's law.

What I think is happening is that you do not have the load (amp) attached, so there is no current draw from the power supply. As soon as you draw current the internal resistances of the circuit (transformer, rectifier, choke) will cause voltage drop due to V = I*R. At least you have a 1M resistor there that is acting as a bleeder (though 1M is quite a high value for this duty, but I could not tell you why one should use less).

Good luck with the further experimentation



diyAudio Moderator Emeritus
2004-01-27 8:55 am
What happens is, the first cap charges to the peak voltage from the transformer, which is 1.414 x the RMS voltage = 275 x 1.414 = 388.85v. If you have a load connected, i.e. the amplifier, the current demanded by the load will cause the voltage at the first cap to fall until it is charged up again on the following peak, which happens half a cycle later. The net DC voltage will depend on the load, the capacitance of the first cap (20uF) and the internal impedance of the power supply (including the DC resistance of the transformer, the rectifier tube and the choke). It will be significantly less than 388.85v.
Niila said:
Thanks for explaining it down for me:) The voltage after the choke was now about 320V, and after the dropper resistor 300V. Amplifier works like a dream but im worried my 6V6's cannot really take that much. Something needs to be done here....

I don't know that amp specifically, but I suspect that the output tubes are cathode biased. If that's the case, you have to subtract the cathode voltage from the B+ to get the net plate voltage. If you still want to drop B+, you can reduce the size of you input cap to adjust it. A smaller cap will increase the ripple some.



2008-04-10 3:52 pm
According to my reference the 6V6 has a design-max plate voltage of 350V, so you are running pretty close to max. You could also try a choke input power supply. That would decrease the plate voltage some (1.1 x RMS vs. 1.4 x RMS) while providing a bit better B+ regulation. I think that if this were my amp I would just let it run and see how long the 6V6's last before they need replacement. Enjoy your work and creation.