Ampeg BA-115: What is the acceptable DC output voltage?

[cgoepel]

I have taken in 3 BA-115 amps in the last week. Complaint is: Loud hum after playing at rehearsal or gig. One of them had a shorted U4 (NE5532). Now I have 3 amps that all pass signal just fine. The bias is about 5 mV with no signal applied. However, the output voltage ranges between .2 and .35 volts. Usually if the amp comes in with a complaint of loud hum, it usually means the output FETs are shorted. I don't want to send these amps back to their owners till I am satisfied that they are stable. I got out the heat gun and tried to see if any of the amps were in Thermal runaway. I heated the bias transistor, Q6 separately and bias current drops with increasing temperature. I am asking for help/advice - What is the typical or recommended dc offset without input signal or load? Vd-s should be about .3 or .4 volts, which seems about right. What am I missing?

[J M Fahey]

That Offset value is acceptable and definitely wonīt damage anything.

I bet those amps were *very* hot, impossible to touch heatsinks, and *maybe* they were halfway towards thermal runout, only *maybe* idle current didnīt go beyond, say, 1A or somewhat more.

That would account for scorching heatsinks ,continuous high current draw and increased ripple but still not killing power Mosfets.

Which by the way are incredibly tough, I have seen just repaired amps *melting* the thermal grease because something else was still bad, and they kept running.

Flange must have well surpassed 100C at least.

Of course, lots of guessing here, those "maybe" will turn into certainty only after testing.

IF you wish, you may load them with something harder than normal, say 3 x 8 ohm loads in parallel for a nominally 4 ohm rated amp and drive it hard for 2 hours at least, so everything gets hot: transistors / chassis / transformer / even plugs inserted there and check what happens ... not worse than a typical rehearsal in a padded room.

Your choice of course, if it blows customers wonīt want to pay for that, "since you received it working".

[cgoepel]

Yes - Thanks so much.

I have thermally stressed them with a heat gun for a few minutes, but not really torture tested them as you suggest. I will do this soon and report back.

[cgoepel]

JM,
I torture tested two of these this weekend. Amp was placed on top of a crockpot set on high, Output at 30 volts peak, with a blanket covering the amp. Ran for over 2 hours. Output self-reduced to 26 volts, which I assume is a normal self-limiting feature. Chassis temperature reached 160 degrees F. This is a very stable amp - I can't blow it up.

[J M Fahey]

Ouch!!! .
Ok, now you have *proven* the amp is fine.

Will it thermally cut off again at customerīs house/studio/gig?

Maybe (Iīd even say "probably") .... but then itīs proof there is "something else" going on he didnīt tell you.

What could it be? ...... *anything* , such as him hitting it with some weird booster , poor cable shielding so amp oscillates at some inaudible frequency, amp sitting over a tubed amp or by some strong lamp, etc.

I once had a very important customer who was playing bass with then most important Rock star in Argentina, a high visibility job which brought me countless customers, but at the same time could nuke me if amp blowed up on stage, but amp mysteriously cut off only on *one* stage.

He should have introduced his new band in a couple sold out dates at Luna Park, our Madison square Garden, but he preferred 36 nights in a row at a small Club, think London Marquee or similar, for an intimate feeling.

The amp, a 500W beast driving 8 x 12" speakers was cutting off every night, although it performed flawlessly in Stadiums at full power!!!!!!

To make a long story short, the very small stage was surrounded by a wrap around heavy velvet curtain, which was only pulled back fully at night ... where it sat snugly against the amp back, killing ventilation.
I had been testing it at my shop , and at the Club, but during the afternoon, no audience so curtain not fully pulled back, so people standing at the sides of the small stage could see the Musicians.

Talk about chasing ghosts !!!!!!!
That did NOT happen at home, rehearsal studio or other places.

[olddawg]

I used to not waranty repairs like this unless I could check their speakers and cable.

[Enzo]

The BA115 is a small kickback combo, so the speakers and cables are inside it.