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scutterflux 26th February 2012 03:59 PM

Help, Amp distortion after on/off/on???
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I'm having trouble diagnosing the problem with my AMP.

It starts up fine, runs good sounds good, but if I turn it off for less than half hour and turn it back on again it provides (sounds like tube i.e. lowered headroom) distortion, even if I flick the switch off then on again after a clean startup it will switch itself to broken, however It's a dual monoblock design and only shares the 12ax7 off the input. The distortion comes out of both sides so I believe it's limited to the very front end. I've tried tube swapping with poor results, and I am prepared to open it up to have a look.

What should I look for?

I'm suspecting a capacitor??? but how can I tell if it's a bad cap?

I think I've attached the schematics.


razorrick1293 26th February 2012 04:11 PM

I was about to say the best thing to do is to check all of the voltages but 1kv sounds scarey :P if its a headroom issue then the voltage would be unusually low. if the cap is bad, it may be a short, so the resistance accross it would be less that a few ohms maybe?

scutterflux 26th February 2012 04:39 PM

It is kinda scary, my multimeter only goes to 550V! So I'll first off check voltages below 500V on a clean startup and make sure the sound is there, and then switch it off then on and see what changes. Hopefully I can find a change and narrow it down from there.

In a physics class I took, if my memory is correct, we studied a LRC curcuit that had a remaining charge on a cap before being turned on, this altered the votage in that curcuit negatively, I have a feeling that's what's hapening, seeing as how it takes time, maybe 30minutes off before the amp can be turned on again and working properly, it's like the caps need to drain first but it takes a long time whereas the amp was fine a week ago.

razorrick1293 26th February 2012 04:59 PM

well the 900v is only for the power section so if you're getting plenty of volume you shouldnt have to worry about it. Ive never heard of a negative effect with an LRC circuit unless a standby switch is in series with a choke which can cause some big flyback voltages. but what you're saying makes sense. However i cannot see a choke or inductor in the provided schematics.

DF96 26th February 2012 05:06 PM


even if I flick the switch off then on again after a clean startup it will switch itself to broken
Two things could give this 'bistable' effect: DC positive feedback, RF instability. The 30 minute delay to correct itself could be a sign of a thermal effect, or maybe an open circuit cap bleeder reistor.

scutterflux 29th February 2012 12:31 PM

well I spent some good time poking around, and I can't find the problem,

I found bias was out by allot on the 300B drivers, The 12ax7, and 6922's aren't getting their 6.3, not even close, and that was after I brought the B+ to where it should be, as well as a few other odd spots it just seems like too much for me right now.

I'd like to note that the amp seems to be built well, but in shipping it was dropped, some of the parts inside appeared to have been knocked loose, so now I wonder if something shorted? But I couldn't find any evidence of that.

So thank you guys, but I in the end I took it to a tech it's just beyond me right now.

DF96 29th February 2012 12:37 PM

It might have helped if you told us it had been dropped. We are good, but we don't do mind reading.

riccoryder 29th February 2012 04:54 PM

Do let us know what it turned out to be. None of the tubes have the filaments properly fed, so what you see there may just be the bad design common to all of these amps. I am particularly curious if you might have experienced a failure seen in certain 300Bs prone to one side of the filament going intermittent or failing. As with many amplifiers of this type, the DHTs are unnecessarily exposed to multiple sub-optimal operating conditions, and some tubes survive the abuse better than others. No doubt without fully realizing what the issues are, some serious corners were cut in the design.

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