Sunn Sceptre

[AdamN]

Hey there. First time poster.

I've got a Sunn Sceptre which is a bit noisy. I've (I'm pretty sure) tracked down the noise to the first gain stage. The plate voltages on the first and second stages are pretty different when they should be the same. Here's the voltages:


Actual Schematic
Recto 556 500
A 551 485
B 436 385
C 339.5 295
D 297.3 260
pin 1 192 165
pin 6 235 165

http://sunn.ampage.org/site/schematics/sceptre.gif

I understand it's common for all voltages to be high given the higher wall voltages today, but I've replaced both 100K plate resistors and it has made no difference. Whom else should I suspect in here?

Any help is greatly appreciated.

[52 Bill]

Did you measure the voltages on the cathodes (Pin 3 and 8)?

What kind of noise problem?
Did you try a different tube?

[AdamN]

pin 3 1.5V
pin 8 2.2V

Tried a different tube, cathode voltages changed slightly but pin 8 was still 2.1V. The noise is like a white noise. I currently have removed all 4 grid stoppers from pin 6, just leaving the grid-to-ground resistor. This pin 8 voltage seems high, no?

[AdamN]

Ok, I can't believe I overlooked this, but someone put a 2.2K in series with the 1.5K cathode resistor off of pin 8, the first gain stage. Going to remove it, but confused about ramifications gain/tone-wise. Looking at load line, at .6mA, it seems to kind of skim the bottom edge of these bias curves.