Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ernie_jr on November 20, 2017, 01:43:20 pm

Title: is this normal?
Post by: ernie_jr on November 20, 2017, 01:43:20 pm

Hello all,
It has been awhile. Started rebuilding a SUNN 200s. Using sand rectofiers instead of 5ar4. The question I have is,
the tranny puts out 875 volts ac across the 2 red wires which is about right. Off the diodes I only get 400 volts dc.
There are no filter caps installed yet. I was expecting some where aroud 570 volts dc. Is this because the filter caps
are not yethooked up?
Thanks,
Ernie

Title: Re: is this normal?
Post by: sluckey on November 20, 2017, 01:46:39 pm

Quote

Is this because the filter caps are not yet hooked up?

yes

Title: Re: is this normal?
Post by: ernie_jr on November 20, 2017, 01:50:28 pm

Thanks for the quick reply. Time to keep on building and no need to trouble shoot yet
Ernie

Title: Re: is this normal?
Post by: PRR on November 20, 2017, 04:27:30 pm

1.414 applies WITH cap.

No-cap, the Average DC will be 0.9 times the AC RMS.

875/2 is 437V each side.

0.9 times 437V is 394V average (very lumpy) DC.

Title: Re: is this normal?
Post by: 92Volts on November 20, 2017, 07:22:14 pm

Quote from: PRR on November 20, 2017, 04:27:30 pm

1.414 applies WITH cap.

No-cap, the Average DC will be 0.9 times the AC RMS.

875/2 is 437V each side.

0.9 times 437V is 394V average (very lumpy) DC.

Where does 0.9 come from? It seems like average and RMS mean the same thing and on average the DC is simply the AC, rectified. There will be diode losses but surely this can't be standardized across different voltages (and would generally be low in the context of a tube amp's hundreds of volts)?

Title: Re: is this normal?
Post by: sluckey on November 20, 2017, 08:35:25 pm

V_p = 1.414V_rms

V_avg = .637V_p

V_avg = .637 x 1.414V_rms

V_avg = .9V_rms

http://www.rfcafe.com/references/electrical/sinewave-voltage-conversion.htm

Title: Re: is this normal?
Post by: PRR on November 21, 2017, 06:56:10 pm

> It seems like average and RMS mean the same thing

No.

Homework: show I am wrong or right.