PPPECKKK in my amp...

[tubelectron]

Hi All,

What I know well is tubes - for solidstate, I am too often lost... That's why I call from help !

I was recently given a 1987 PEAVEY Special 130, all original and in excellent condition :






This amps works completely and is loud, but there is one issue :

When I power the amp, there is a big PPPECKKKK in the speaker, accross which I measure a peak voltage of 28 to 33V (250µS), so to speak : around the max power of the amp (28V=130Wrms), specially if the reverb pot is not at zero (but say at 5/10 - a normal setting).

I replaced all the lytic coupling caps by plastic ones, changed all lytics of higher value by new ones, suspecting leaks, the only remaining being the filter caps (4700µ and 1000µ) because these tested good, and I havn't axial convenient replacement caps for it.

This has no effect at all... I do not see where to search... Peste soit de ces transistors !

Any idea about where to look ? Any supposition ? Idea about that abnormal behavior ?

Thanks !

A+!

[blooze_man]

The people at ssguitar might be able to help you out.

[petemoore]

  Had that in an LM3886 build that caused what sounded like coil-form-er-backslam there are fixes for such, I put speaker switch in because I have other amps and I'm lazy.
  One fix involves switching the power on to let it stabilize, then allow current through the output a second or so later, I read of a chip intended for enabling this 2 stage startup automatically.
  I was going to say capacitors but you already did, that it works as such with old and new capacitors [sounds like you got all the old ones out].
  It's not exactly safe to assume it's in the output but that is likely given the timing sequence you described: pop at turn on.
  Switch on the speaker is ugly and ups user requirements, but does fascilitate amplification without speaker-poppage. I though I might have blown one of my Coral 8'' 2-way drivers, just one of those ''PAP'' of overXmax excursia into slamming coil into solid object needed fixed, unplugging/plugging in the speakers of course would work too .
  Kinda depends on how motivated I am and how many amps I really wanna buy new chips for instead of strings and put them in instead of enjoying an amplifier, I decided the switch is ok for this one...was a lot of work but very small investment, mostly the PS torroid.
  Could just be one of those things that develops, I was unaware of input transistor as cause of hiss until Mark read and changed one to a 2n5088 and then RG weighed in and changed my perception of 'solid state'.
  Perhaps an input junction breakdown causes rising startup currents and that throws the output to follow ? Seems unlikely but possible, about all I can offer on the second day of contemplating such matters...
  Does it hiss ?
     

[tubelectron]

Hi petemoore,

Perhaps an input junction breakdown causes rising startup currents and that throws the output to follow ? Seems unlikely but possible, about all I can offer on the second day of contemplating such matters...
  Does it hiss ?

Good suggestion to check, by the way. There is somewhere a rising startup current... Where does it borns is the question.

No, the amp doesn't hiss, it is very quiet. Nothing seems to be wrong except that "thunder pop at turn on"... The amp shouldn't have been factory-made with such a problem, IMHO.

I already know that the PPPECKKK comes from the preamp section, not the power amp stages : I checked it by disconnecting it via the power amp input jack, it ceased, and I could check that there was a starting voltage jump at the preamp out jack (+1,2Vpeak giving +33Vpeak at the speaker).

More than that, it comes from the reverb circuit (or this circuit is more sensitive) because it nearly cease if the reverb pot is at 0 when powering on). I removed the reverb RC4558 and replaced it, along with the caps, but no effect.

Same results if I disconnect the reverb can. Even the same with other caps and RC4558 replacements in the rest of the preamp... No improvement.

Here is the schematic of the PEAVEY Special 130 for reference (sorry, it's poor quality) :



A+!