B+ ramping up in a Superlead build

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Elad E
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B+ ramping up in a Superlead build

Post by Elad E » Tue Feb 20, 2018 6:21 pm

Lately I've been feeling a major degradation of my Superlead's tone - it seems that over the past couple of months the amp started sounding stiff, brittle, one dimensional, non-musical and muddy.

I decided to check bias and found out an outrageous 560VDC on pin 3 of the power tubes.
Bias current was 51mA, measured on pin 8 of the hottest of the four output tubes.
Once I set the bias to an optimal 31mA range I played for another 30 minutes, measured some minor fluctuations, rebiased and got a reading of 579VDC on pin 3 of each EL34.

Soundwise the amp pretty much regained (no pun intended) its former glory but I can't figure out what's up with the crazy voltages and I'm not sure the tubes would last this way (that is if they themselves are not the culprit).

The amp is pretty much a Metro 1969 kit which I built in 2015 with '68 filtering. It sports Classic Tone iron, power tubes are Mullard reissues which I've had since completing the amp some three years ago. I biased the amp some 7-8 months ago and measured a friendly 460-ish VDC B+ back then.

What could cause this upsurge of B+? bad diodes? filter caps? tubes? PT secondaries?

Any help shedding some light on what is the cause for the ramped up voltages will be highly appreciated!

Tazin
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Re: B+ ramping up in a Superlead build

Post by Tazin » Wed Feb 21, 2018 9:44 am

If your amp has an input voltage selector then check to make sure it set correctly to match your wall voltage. Double just any connections surrounding the primary wires for the mains transformers and make sure there is no shorts between the primary wires. Check the AC voltage right before the rectifier to see if the secondary of the mains transformer is putting out the correct AC voltage. Check the Heater voltage too. If everything is good up til that point then the problem probably lies either with the rectifier or maybe the mains filters. Also check the grounds for the rectifier and mains filter caps.

Elad E
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Re: B+ ramping up in a Superlead build

Post by Elad E » Fri Feb 23, 2018 7:34 am

Tazin, thanks for the thorough response!

I found the culprit to be the multimeter. after checking the grounding on the filter caps, prior to powering up I checked the wall voltage on the power switch and saw a whooping 280VAC which pretty much made me change course and replace the multimeter battery after which the readings were all in the right ranges.

Two strange things happened - first, the optimal bias setting (17.5 / B+ = xMa) seemed to dictate a position on the bias pot not so distant from where it was set when the sound was brittle and dull, the amp however sounded great now.

second thing that happened is while I was checking the bias again, after some noodling and before powering down, I accidentally bridged pins 8 and 7 of V6 with the bias probe and a spark and then a flashing flame appeared for a what seemed like a few seconds (probably lasted for a 1.5-2 seconds in reality) and then died down before I managed to turn down the amp.
upon inspection the HT fuse burned and some slight burn marks appeared on the tube socket and on pins 7 and 8 :bang:
I took the tube out and saw no noticeable damage to the tube or socket, put it back, replaced the fuse and turned the amp on - B+ and bias were right and the amp sounded as good as before.

Any idea if I should change or check anything after this unfortunate happening?

Tazin
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Re: B+ ramping up in a Superlead build

Post by Tazin » Fri Feb 23, 2018 9:29 am

Glad to hear it was just the multimeter and not a problem with the amp. Regarding the arcing accident with the meter probe, check and make sure there isn't any carbon trace residue on any of the pins for that octal socket that may lead to arcing between pins. If everything looks good (before or after checking/cleaning the octal socket) and amps voltages are good and stable you should be good.

Elad E
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Re: B+ ramping up in a Superlead build

Post by Elad E » Fri Feb 23, 2018 1:11 pm

I cleaned the carbon residue with isopropyl alcohol and reflowed the solder on pins 7 and 8.

Regarding the arcing I want to emphasise that the flashing and flame were external and not internal, it seemed as if the tube base was emitting burning gasses upward (the amp was upside down) like a burning match.
Would still be considered standard arcing or is that something worse?

Tazin
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Re: B+ ramping up in a Superlead build

Post by Tazin » Fri Feb 23, 2018 6:53 pm

Pin #7 is one half of the Heater voltage (3.15Vac) and pin #8 is ground, so you accidentally shorted out half of the heater. Surprised the Mains fuse didn't blow and not the H.T. fuse. I think you'll be fine if all the voltages are within specs.

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