33volts a/c in p.i.

[islandamp]

I can't seem to get an answer for this question so I thought someone here might know. My 1987 Plexi clone has harsh treble, muddy bass, and a hum with gain and volume up. I have also turned the el34 bias up to 110 ma each tube across an accurate 1 ohm resistor and had no redplating. The amp has a Larmar type ppimv. Master is wired correctly. I am finding 33 v a/c at the p.i. cap +,, at the junction of the 2- 10k 3 watt dropping resistors and 18v a/c at the p.i . My question is,, should I be seeing 33v a/c in these places? I wouldn't think a/c should be anywhere in the circuit.

[dobbhill]

#1. What's your B+ voltage @ the EL34's?
#2. Is there ac voltage on the EL34's?
D

[islandamp]

B+ is 420v at the el34's,, I haven't checked for any a/c at the tube, but I can tonight. Looks like the ac doesn't start showing until the p.i cap then its at the junction of the 2-10k 3watt step down resistors. After the 1st 10k it drops to 19 vac. Bias voltage is right,,about 34 as I recall, and bias voltage is adjusted now to 35-40 ma. I can't see a reason why 33 vac is in the p.i and cap area.

[Andy Le Blanc]

@ 420 v and 110ma (.11A) thats 46w static dissapation
each tube would last longer at 60ma or 25w
40ma. at 420v is only 17w

it might be an issue in you power supply thats injecting some ripple
or it might be an occilation building at that stage

if you can try and find the frequency
and see if it diminishes if you ground the input to the phase inverter

could just be the leads to the master are picking it up

[islandamp]

Could be something in that master. I removed preamp tubes 1 and 2 and still had the 33 v, but after removing the p.i. tube it vanished. I may remove that master and try a Kevin O Conner version. See what happens.

[islandamp]

Could be something in that master. I removed preamp tubes 1 and 2 and still had the 33 v, but after removing the p.i. tube it vanished. I may remove that master and try a Kevin O Conner version. See what happens.