bad output transformer ?

[black68]

hello everyone.my laney gh100l has a problem in that their is very low volume.i have checked everything i can think of that could cause a problem like this. i have checked the effects loop send and return jacks, reflowed most of the preamp board and output board, substituted a different set of tubes and checked all interconnecting wiring for anything disconnected,and bias voltage. i noticed that when playing the amplifier the output tubes just start to red plate even though the volume is low with volume on 6. if the volume control is at 2 the tubes will not red plate.i dont know if the red plating means anything or is a possible clue as to what is wrong.i would like to know how to check the output transformer to find out if it is good or bad. i read many reviews on harmony central on this model where owners had the same problem all related to bad output transformers. any help is greatly appreciated.

[Satamax]

What you're describing makes me think of bad case of ultrasonic oscilation. Check this site, http://www.geofex.com/ampdbug/ampdebug.htm if you want, it's usefull. If you realy want to determin if your output transformer is shot check this one.
http://www.radioremembered.org/outimp.htm

Bye.

Max.

[unclejed613c]

also check your grid bias to make sure it isn't floating. red plates are from too much plate current, and the tube hasn't got much gain when it's in saturation.
the best way to find out what's going on is to put an oscope on the plate. also check your tube voltages, the grid should be more negative than the cathode (the cathode may be somewhere between ground and about +20v, and the grid should read at least 3v more negative than the cathode. you can also figure out your quiescent current by measuring across the cathode resistor, and dividing the voltage by the resistance.

if it does turn out to be oscillation, the most likely cause is a bad filter or bypass cap.

to check out the output transformer, disconnect it and connect a .01uf cap across the secondary. apply a 1khz square wave to the primary, and connect a scope to the secondary. you should get a square wave on the secondary that has a good amount of ringing on the transitions. if ringing is not present or the secondary waveform is not there, then you could have a shorted transformer. also acheck the resistance from each winding to the core, it should always read open. if the transformer is a push-pull, applying the above mentioned square wave across one half of the primary should yield almost the same voltage across the other half of the winding.

[Steve Conner]

If it was ultrasonic oscillations like Max said, then the tubes would probably redplate at high volume settings even if you weren't playing anything. I once had some trouble like that in a homebrew, when the master volume was turned past a certain setting, the plate current would suddenly shoot through the roof even with no guitar plugged in. A shield on the first preamp tube cured it.

If it's bad bias, the tubes will redplate as soon as you take the thing out of standby.

With a shorted OT, the tubes will only red plate when you start playing at high volume, the amp tries to make noise, but the power that should go to the speaker can't get through the OT, and gets burnt up in the tubes instead, overheating them. I think shorted OT is the most likely explanation, especially if they bought from the same trannie supplier as Trace Elliot

[Sanfton]

I I think shorted OT is the most likely explanation, especially if they bought from the same trannie supplier as Trace Elliot

Steve, what's the griff with Trace Elliot trannies??

I've got a V4 bass here (4 x KT88, B+ = 680V, Vg2=375V) that seems to 'break over' if you play hard. It was OK until a KT88 lost vacuum and blew the mains fuse. With an 80Hz signal, the speaker output just cuts off to nothing when the breakover happens.

At lower voltages the turns ratio is spot on for 2500 ohms a-a. Could the short-circuit event have burned up the trannie insulation?

thanks!

Rod

[pdf64]

I'vehad that happen with an OT. Ok at low volumes, but cuts out when pushed. Eventually after being pushed it stayed in the cut out mode, and an obvious imbalance in the primary could be measured. Caused by a tube short - when I did an autopsy, there was a charred region in the primary, where the excess current from the short must have overheated the insulation, which voltage peaks could then break through.
I'd guess you're in the early stages of that. Likely that your speaker cab bass resonance is 80Hz, causing a voltage peak at that frequency, creating the break through.
I now put quick blow fuses between each power tube cathode and ground, so that future tube shorts won't get chance to last long enough to overheat the OT primary.

[Sanfton]

I'vehad that happen with an OT. Ok at low volumes, but cuts out when pushed. Eventually after being pushed it stayed in the cut out mode, and an obvious imbalance in the primary could be measured. Caused by a tube short - when I did an autopsy, there was a charred region in the primary, where the excess current from the short must have overheated the insulation, which voltage peaks could then break through.
I'd guess you're in the early stages of that. Likely that your speaker cab bass resonance is 80Hz, causing a voltage peak at that frequency, creating the break through.
I now put quick blow fuses between each power tube cathode and ground, so that future tube shorts won't get chance to last long enough to overheat the OT primary.

Thanks, that's just the kind of thing I suspected. The stress event was caused by one KT88 retainer breaking and whipping into the valve. The overcurrent blew the mains fuse, eventually. The HT fuse did not blow - it is T2A, which seems a bad choice, and I would prefer your quick blow selection.

Anyone know of a quality 200W bass amp trafo, 2500 Ohms a-a?? Can handle 700V B+? Ouch. Sounds expensive!