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Replaced caps, now amp is too hot

Started by tonescience, June 16, 2014, 12:13:09 PM

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tonescience

I just replaced all of the capacitors in an Earth Sound Research Original 2000 head.  Interestingly enough, once I replaced all of the capacitors the output section of the amplifier now runs super hot.   Any ideas of why this may be?

All capacitor values match design values.   The amp itself has a 4ohm output and I am running it through an 8ohm cab.

My guess is that this may be normal, but I don't remember the amp being this hot before I re-capped it. 

 

Enzo

Did you replace the caps in the bias supply?  Note the positive end of those two goes to ground.  Are they in right?


In any case, is the bias voltage up to snuff?  Check pin 5 of each power tube, is there something like -55v there?

tonescience

I replaced all of the caps in the amp.  This has the Model 340 solid state power amp section.  I didn't take a look at the bias on the output transistors (which seems like a great place to start).  I will be sure to check this out tonight.

phatt

You may have to wait for Enzo to reply,,, meantime check for DC volts at speaker terminals, this reading needs to be close to Zero VDC. About~ish 100 to 300 mV. If higher than 1 or 2 Volts you have problems so disconnect speaker until you solve the issue.

Sounds to me like you have created a problem while trying to fix something that may have been working fine.

Can I ask Why you replaced ALL caps? :o
As this is generally not how to fix amps,, You need to establish what the problem is First then replace only the failing components.
Phil.

Enzo

Oh, sorry, I was looking at the tube version.

Either link a schematic, or tell us which Peavey amp it is a clone of.  I don't remember.

tonescience

I've got about a little less than 100mV at the speaker terminals. 

I come from the world of analog synthesizer repair and often times replace all the capacitors in a synth as part of preventative maintenance.  If this isn't how it's done with amps, dually noted.   :) 

There were a whole series of issues that needed to be repaired in this amp.  One of the filter caps was toast.  There was a transistor that was bent / shorted in the pre-amp section.   All easy fixes.  Everything sounds great now, I just worry about the temperature of the output section. 

I believe this amp is a clone of the Peavey Standard 260H.  I do have schematics I can scan at a later point if this would be helpful.  I appreciate everybody's input! 

Enzo

Looking at 260H, I see a dual diode and another diode with parallel resistor as the bias string.


Offset is not what makes amps run hot, but underbias does.   If the outputs are running hot, find that dual diode.  It should be on the heatsink.  SHort across it.   I want to reduce the number of diode drops between the upper and lower drivers.


Does that cool it off?

tonescience

I just shorted that diode and it did cool it off.   :)