Sunn 100S: odd voltages at 6AN8

[Ryan Phelps]

Hi guys! I need some help with my 1967 Sunn 100S. I recently re-capped it with discrete caps on terminal strips inside the chassis, replaced bias and cathode caps, replaced all of the signal caps and all drifted resistors, installed new matched Svetlana 6550s and a new 6AN8. The existing 12AX7 and 5AR4 were OK.

Here are the power supply voltages:

A reads 532V vs. 480V schematic
B reads 438V vs. 425V schematic
C reads 337V vs. 320V schematic

The 12AX7A voltages are:

Pin1 reads 213V vs. 195V schematic
Pin 3 reads 1.6V vs. 1.5V schematic
Pin 6 reads 213V vs. 195V schematic
Pin 8 reads 1.6V vs 1.5V schematic

The 6AN8 voltages are:

Pin 1 reads 283V vs. 340V schematic
Pin 3 reads 152V vs. 90V schematic
Pin 6 reads 143V vs. 75V schematic
Pin 7 reads 25V vs. 35V schematic
Pin 9 reads .61V vs. .75V schematic

Bias voltage set to -67V for a current draw of 48ma per 6550 tube.

The symptoms are......past 3 on the volume pot the sound becomes very loud and very distorted. If I back down the guitar's volume control, I can turn the amp volume up further before the amp begins distorting. The voltages at the 6AN8 seem really out of whack. I've checked all the surrounding parts and swapped out all suspicious and drifted ones. What do you guys suggest I look at? I've attached the schematic.

[loudthud]

I would try lowering the 1.5Meg resistor to get the pin 3 voltage closer to 90V to 100V. Try 1.2Meg or 1 Meg. Another thing to try would be to lower the 680 ohm or install a trim pot.

[Ryan Phelps]

Thanks loudthud! I thought I was missing something and that the amp should be made to work correctly with the parts values as originally designed. But, your idea is worth a try!

[Isaac]

I think pin 3 is high and pin 1 is low because the triode side is conducting heavily when it shouldn't be. That's because pin 6 (and therefore pin 2) voltage is too high, indicating that the tetrode side isn't drawing enough current. I can't figure out why that might be, unless the tube is bad. The C voltage is high, so it seems to me that the pin 7 voltage ought to be high as well, but it's low.

Anyway, in my experience, the most likely cause of problems in tube amps are tubes. Before changing components, I'd try a known good 6AN8. I have a similar amp. I'll take some voltage measurements. If I get around to it. If you haven't solved the problem before then...

[Ryan Phelps]

Thanks Isaac! I did install a NOS 6AN8 before I took the measurements (as I noted in my original post), but I could pull a known working 6AN8 from one of my other Sunn amps and try it in the 100S to see if that makes a difference.

Sometimes new parts are defective. I re-capped my 1955 Fender Pro with Sprague Atom caps about 30 years ago and it sounded terrible.....hunted and hunted and couldn't find the problem. I finally took it to an amp tech and guess what? One of those new Sprague Atoms was defective! True story.

[Ryan Phelps]

I finally got back to my 100S. I did change the 1.5M resistor that feeds pins 7 & 9 at the 6AN8 to a 1.2M and it helped the voltages there. I also found that the .1 uf coupling cap at the 12AX7 is way too large and causing distortion issues. Most other Sunn amps use a .022 uf cap in this location, so that is what I used. I think this may be an error on the schematic.
Lastly, I swapped out the 5AR4 rectifier for a 5R4GB. It draws the same current at the 5AR4, but is much less efficient, dropping about 65 volts instead of 10 volts at the rectifier output. This has the positive effect of pulling down node B & C voltages to near-spec (425 volts and 320 volts respectively) so the 6AN8 and 12AX7 are operating at the correct voltages. My wall voltage is 125 VAC, so this rectifier swap saves using a variac, bucking transformer or zener diode at the secondary center-tap at the power transformer. And no tweaking the value of the power supply dropping resistors. The only issue is that the 5R4GB is designed to see only 20 uf at a capacitor-input filter (30 uf is spec for the amp) and I had re-capped with 50 uf. This may shorten the tube life. So in future, I'll probably use a smaller filter cap at the A node. The amp now sounds fantastic. Thanks for all your help guys!