Sovtek Tube Midget 50

[lowell]

This amp came in with a harsh distortion on the attack of notes played. The amp sounds loud and decent, just the attack has an odd "crossover distortion/loose solder connection" kind of a sound. I think the amp may have a bad OT but I want to see what yall think.

I checked the bias current and at 460v on the plates there's only 6ma of quiescent current. I adjusted the bias pot and nothing seemed to be happening on the meter. In fact as I lowered the negative grid bias voltage, the current on the meter was reducing. Until a certain point and then the power tubes began drawing lots of current and redplated.

From what I can recall of previous experiences where it seemed the power tubes COULD NOT be biased it ended up being a bad OT. Any thoughts?

Fyi I've checked screen voltages on both power tubes, plate voltage, and cathodes are grounded. I also tried new power tubes.

[MWJB]

If you checked the plate, screen & negative grid voltages, any chance that you could share them? What swing in negative dc do you get at the grids?

If plate voltage & current are similar from tube to tube it may not be the OT to blame, usually one side shorts more than the other, throwing off the plate currents. Check dc resistance from each power tube plate wire to the centre tap. I would expect some "crossover distortion" at 6mA per tube.

[Gtr_tech]

I've heard this kind of thing before. Amp was a late 60s Sunn Solarus IIRC. The "fartyness" didn't occur until you got it up around "3" on the volume. Metallic icky sounding noise like vibration induced intermittant connection. Turned out to be the output transformer that was at fault. It was non original...could have come from a 4 tube amp since it was larger than what you would expect for a 1 pr EL34 amp. Replaced it with the correct one and it sounded normal.

So....try a replacement transformer and see if that cures it. When they arc internally due to damage, it will cause that kind of noise.

[Bruce / Mission Amps]

I just replaced a blown 50 watt Sovtek OT last week with one from a JCM800... tricky in the can but was do-able after taking it all apart and unwinding the OT and power supply harness.

[lowell]

If you checked the plate, screen & negative grid voltages, any chance that you could share them? What swing in negative dc do you get at the grids?

If plate voltage & current are similar from tube to tube it may not be the OT to blame, usually one side shorts more than the other, throwing off the plate currents. Check dc resistance from each power tube plate wire to the centre tap. I would expect some "crossover distortion" at 6mA per tube.

Plate and Screens are around 460v. The negative grid bias swing was (if I remember correctly) -60 to -20'ish. I'll check DC resistance of the OT primaries on Monday.

[lowell]

The primaries are both 1k to center tap. I'm gonna sub in a deluxe OT and see if it helps.

[lowell]

Definitely the OT. I could dial the bias in now AND the sound was better.

[MWJB]

Yeah, there's no way that the primaries should measure 1K to the CT, 70ohms to 150ohms would be more typical for a 50W amp. Even a champ is only around 270ohms accross the primary.

[lowell]

Okay cool thanks.