Big Help Prosonic issues

Travisthedog

Arrrrrghhh 

Two days before arguably one of the biggest gigs of my life my beloved Prosonic starts to shit out on me

i thought I noticed it last gig but wasn't sure.

Basically youre going at it - the Amps doing what it does best - absolutely singing and all I've a sudden theres slow ish drop in volume and farting to the point of nothing . Leave it alone and it comes back after a while.

@icbm please say it's a valve

uncledick

Grandmother/eggs etc., but does it do this when you're plugged straight into the amp?  I had a similar thing at a couple of rehearsals and it was a dying battery on a pedal.  Can't remember the full details but it freaked me out initially.

Travisthedog

Nope does it when you straight into the amp too

ICBM

Could be - or a fault in the filament supply. If it fades out over about 5-10 seconds or so it's most likely because a valve (or all of them) is going cold. Try to make the fault repeat itself and have a look round the back to see what's glowing. If it's a preamp valve you need to check that *both* filaments are working, there should be two spots of orange light at the top of each valve. If it's just one valve it could just be a bad contact between the valve and the socket - either replace the valve to be on the safe side, or take it out, squirt some contact cleaner into the socket and wiggle the valve about as you put it back in.

If that fails, take the Bandit…

octatonic

I had it happen to one of my Prosonics.
A full retubing fixed the problem.

Travisthedog

Yeah lots of tubeys in th bugger too

FelineGuitars

Travisthedog said:

Yeah lots of tubeys in th bugger too

if it's power tubes surely just a pair of 6l6s

DJH83004

Does it make any difference if you change positions on the rectifier switch, e.g tube or SS, as it may be the rectifier Valve, also there is a separate power supply pcb, with a ceramic (I think 15watt) resistor, that does run v hot and may be unsoldering itself. 

Travisthedog

It is a very hot amp. I don't have time to get a tech to look at it so have go with the tubes and hope for the best at this stage. 

Travisthedog

So revalved the amp and it's ok but no I've seen small wisps of smoke coming out of the back here

https://imgur.com/gallery/eKKxv

Its hot hot as hell and smell it too but it's always smelt hot and you can't touch the rear panel after use, the speaker cable is super hot too. I changed a speaker out last week, that wouldn't have done anything ? It's 8ohm - arrrrgggh 

https://imgur.com/gallery/dXAZK

@icbm ??

ICBM

It looks like the amp is meant to have a 16-ohm load with only one jack in use, which means the speakers should each be 8-ohm. I doubt it's anything to do with the speaker anyway, but that hot and especially wisps of smoke coming out of the amp mean something is not right inside. Unless it's paint burning off the new valves, but even that is worrying - it would mean they're running much too hot. If it has been that hot inside then what DJH said about a component unsoldering itself is possible.

You are not going to like this, but if it was me I wouldn't trust it for the gig - it could fail in a major way at the worst possible time. I'd get it looked at immediately if you can find anyone who will do it today or tomorrow, or failing that if you can't, you absolutely must take a backup amp which you should have on stage, plugged in and ready to go. Personally I'd just use a different amp from the start, but I understand you may not want to...

octatonic

My Prosonics ran a bit hot but there was definitely no smoke and you could touch the back panel without getting burned.

I would get out looked at and find something else to use for your upcoming shows.

DJH83004

Which rectifier position do you run the amp in? TubeA/B, SS A/B, or Tube class A ? Speaker connections look fine, it may be worth putting a multimeter across the jack tip and sleeve and measuring the dc resistance, they are series connected, guess 2x 8 ohm? Are the output valves showing any signs or red-plating, just wondering if there is a bias issue, also worth noting in 'class A' position, the amp is cathode biased, hence the reason for the original question.

Travisthedog

Spoken to tech and hes of the opinion  that is been running so hot that something like a SG resistor is breaking down and it's running way to hot. 

Have do the gig gig with my Budda - good job it's a rock festival!!!!

Travisthedog

It's in A mode at present @DJH83004 ;

octatonic

Travisthedog said:

It's in A mode at present

Could could try SS A/B and see if that affects anything.

DJH83004

Travisthedog said:

It's in A mode at present @DJH83004 ;

In which the amp is running cathode biased, via a 220 ohm cathode bias resistor, this is without doubt the hottest mode the amp runs in, tube a/b being the coolest providing the amp is biased correctly. Would be surprised if it is a screen grid resistor as they normally fail open circuit, which has the effect of 'shutting down' that particular valve rather than running hot. In any event I would suggest your tech does take a look, as anytime you see smoke, it does need investigating, especially as you clearly do some serious gigging!

SunDevil

Greater luminaries than I already commenting, but my two penneth FWIW would be to ask if the valves red plating?

 The way you described the initial symptoms is what I've had before with a hot cathode bias and valves that couldn't handle the current

Travisthedog

Well, I used my spare amp at the gig and to be honest I didn't notice i wasn't using my beloved Prosonic. The onstage sound was made up of the lead guitarist and his BV's and nothing else. I've never played a big stage like that where the engineers were so clueless.

2 songs into our set, one of them re appeared on stage with a couple of mikes - they'd forgotten to put two rack tom mics in and somehow missed it until that point.........