A SSS Build

[GAStan]

102 amp is still on, Operate, idle-no input. Transformer is very warm, I can only keep my fingers on it for 2-3 seconds. Similar in temperature to SSS transformer. This is after nearly 4 hours operation.

[bepone]

I only have one fuse, 3 amp slow on input power.

In Operate

118vac wall voltage
320-0-322 vac secondary. 320vac leg is also feeding bias circuit.

B+ 414vdc

Bias -360vdc at 5mA

PT's biased to 27mA with -49 volts.

if this is in operation all looks ok.. temperature is ok also little bit hot, 45 deg C or 115 in F

[bepone]

I had the same problem in one amp, when inside the box.. chassis and power transformer were extremelly hot.. while outside on the table, chassis and PT temp was just moderate, nothing to worry about... Then i discover that there is no sufficient air circulation in the cabinet, and tubes were heating up the chasssis (chassis upside down, tubes on bottom)..

[martin manning]

102 amp is still on, Operate, idle-no input. Transformer is very warm, I can only keep my fingers on it for 2-3 seconds. Similar in temperature to SSS transformer. This is after nearly 4 hours operation.

Would your SSS run 4h and be at a similar temperature? It has four more preamp tubes, so 1.2A more heater current, probably 8-10 mA more HV current.

[GAStan]

I had the same problem in one amp, when inside the box.. chassis and power transformer were extremelly hot.. while outside on the table, chassis and PT temp was just moderate, nothing to worry about... Then i discover that there is no sufficient air circulation in the cabinet, and tubes were heating up the chasssis (chassis upside down, tubes on bottom)..

Thank you bepone for sharing your experience, it is greatly appreciated.

102 amp is still on, Operate, idle-no input. Transformer is very warm, I can only keep my fingers on it for 2-3 seconds. Similar in temperature to SSS transformer. This is after nearly 4 hours operation.

Would your SSS run 4h and be at a similar temperature? It has four more preamp tubes, so 1.2A more heater current, probably 8-10 mA more HV current.

Yes

I did figure out the ~20 volt difference. The transformers I used in my ODS builds were from Mojotone and rated 340-0-340 @ 400mA. The one I used in the SSS is from Amp Parts Direct and rated for 320-0-320 @ 450mA. This still doesn't explain the inconsistent voltages I've seen in the SSS, I'm beginning to suspect an intermittent shorted winding.

[timrobertson100]

Hi Glenn

Apologies if this is me being stupid.
Given that the #102 is providing 428V with a 340-0-340 @400mA PT isn't it reasonable you'd get the lower B+ reading of 414V from the 320-0-320 @450mA PT in the SSS with the same wall voltage?

Could it be the higher 440V reading you noted was taken under some different operating condition (higher wall voltage, fewer tubes, cold start, or so)?

[GAStan]

Hi Glenn

Apologies if this is me being stupid.
Given that the #102 is providing 428V with a 340-0-340 @400mA PT isn't it reasonable you'd get the lower B+ reading of 414V from the 320-0-320 @450mA PT in the SSS with the same wall voltage?

Could it be the higher 440V reading you noted was taken under some different operating condition (higher wall voltage, fewer tubes, cold start, or so)?

Not stupid at all. Yes, entirely possible.

[GAStan]

Spent the morning putting a PT from Mojotone in. Now the B+ is 458vdc in standby. Put it to Operate and saw a MAJOR arc in V8 and inside the amp at the same time. V8 heater opened and evidently shorted to B+. Cut the power immediately. Both 100 ohm heater to ground resistors smoked.

Replaced 100 ohm resistors and pulled outer two tubes. Bias to power tubes was around -70vdc so I wasn't too worried about possibly red-plating them. Put it to operate and checked bias on 1 ohm resistors, it was bouncing from 0 up to 70+mA. Checked power tube bias voltage, it was also bouncing crazily. Went back to Standby. Checked a few voltages, B+ still ~460vdc, bias voltage from rectifier board -390vdc, bias to power tubes -70. Decided to see what B+ was in Operate so took it out of standby. 100 ohm resistors arced again. Didn't see power tubes arc so not sure what happened this time. I checked the power tubes with my Fluke meter, heaters check good and nothing shorted. I don't have a tube tester to properly test them with.

I believe the fluctuating tube bias is the root problem. I think my first task will be set bias voltage to -360 vdc since that is what worked before with the other transformer. I haven't thought much beyond that yet.

[martin manning]

Once you have an arc occur there may be carbon tracking on the tube sockets which must be completely removed to prevent more arcing. If the socket material has been burned, replacing the socket may be necessary.

So what caused the initial arcing? I think it was most likely an oscillation in the power stage.

I’m not a fan of the high negative voltage on the driver tube, I’d rather go with the bipolar supply running off the 60VAC bias tap.

[GAStan]

Once you have an arc occur there may be carbon tracking on the tube sockets which must be completely removed to prevent more arcing. If the socket material has been burned, replacing the socket may be necessary.

So what caused the initial arcing? I think it was most likely an oscillation in the power stage.

I’m not a fan of the high negative voltage on the driver tube, I’d rather go with the bipolar supply running off the 60VAC bias tap.

I'll inspect the sockets with a magnifier I have and replace if necessary.

I agree about the oscillation being the probable cause. Tough to see what's going on with a DMM, times like this make me wish I had a Simpson 260, but it would get so little use it's hard to justify the cost.

I was trying to keep the power section as close to original as I could. However I actually have everything needed to convert to your bipolar supply, I requested the board from Erwin when I ordered the chassis and got the necessary components my last few orders. This option is looking like it might happen.

[bepone]

This is bad tube problem.. pls measure resistance directly on the tube pins, between 2-3.

[bepone]

Sockets are how i see new.. and Belton, so means high quality.. Never failed any in my life. So there is only 0.1% of chance that sockets are a problem.. Probably output tubes..

[GAStan]

This is bad tube problem.. pls measure resistance directly on the tube pins, between 2-3.

Using a Fluke 87 DMM, one tube has no continuity on any pin to any other pin (open heater). The other three only have continuity from pins 2 to 7 (heaters), no other pins have continuity to any other pins.

I'm putting in the bipolar power supply/rectifier board. I have another matched quad of 6L6 tubes but want to verify all else is good before I put them in.

[GAStan]

Sockets are how i see new.. and Belton, so means high quality.. Never failed any in my life. So there is only 0.1% of chance that sockets are a problem.. Probably output tubes..

Yes brand new Belton sockets. I found nothing wrong, no signs of tracking and no residual carbon from the resistors that arced. The tube that arced was internal arc, no damage to socket. All solder joints are good, no wire whiskers to cause shorts, plenty of room between solder tabs.

[GAStan]

Completed installation of bipolar power supply. Amp is now working again with better B+ and other voltages.

When plugging in 8 ohm dummy load had a déjà vu moment. I used Puretone jacks on the unswitched outputs because they have better contacts than Switchcraft. However they are a little harder to insert a plug into, having 2 distinct clicks. I nearly didn't fully insert the plug, feeling the first click similar to a Switchcraft jack. I'm pretty sure I didn't have the plug fully inserted before when the tubes were arcing. This could explain why they arced.

I never did fully troubleshoot the erratic output of V7 to the power tubes so I'm not sure if it was related.

Question: with the decreased voltage the bipolar supply provides V7 does this limit the clean headroom? Volume? Is there a noticeable tonal difference between the original and bipolar supplies?