Jumping voltages on 5F2 build

[mfreqmaster]

I am in the process of building a 5F2 style amp and I have run into a problem I just can't seem to figure out. When measuring the DC voltage from the rectifier only, meaning not hooked up to anything else, I get 330-345 V. Fine. Then I attach that the rest of the B+ components, choke, caps and resistors. I then get 485V. I have double checked and triple and quadruple checked my wiring to the standard 5F2 schematic and it is wired correctly. I have tried two different rectifier tubes, same issue. Another thing to note is that there is no voltage drop across the choke or the 22k resistor. The choke measures 100 ohms or so and the 22k resistor is within tolerance. Is this a cap problem? Any other ideas?

[dwhutchens]

If the only tube you have in the amp is the rectifier then there is no load on the power supply. With no load there is no current drawn and no voltage drop. When you put the other tubes in the voltage will drop quite a bit.
David

[Enzo]

He's got it the other way.

When you have ONLY the rectifier tube, then the resulting DC is unfiltered, and pulsing 120 times a second. Your meter will give an average reading of in this case 340v more or less. I bet that is not far from the AC volts on the plates of the rectifier tube.

When you add the filter cap, then it charges up to the peak and stays there. If I start with 340VAC and calculate the peak, I get about 480VDC. ABout what you get.

When you then add the rest of the tubes, they all start drawing current, and the voltage will drop down to something more reasonable.

VOltage readings around an amp are taken when teh amp is complete and in the operating condition. Voltage readings taken with all the tubes removed or the filter caps not installed will be meaningless - well other than to just show it is partially working.

[mfreqmaster]

Jumping voltages on 5F2 build

Thanks, I finally figured what was going on now. Sorry it took me so long to reply-I couldn't get away from playing my newly built amp!