Guitar Amp Capacitor Identification

[Cicero]

Hi all,

I have a Peavey 6505 112 guitar amp that I need to repair. Suspect some leaky caps.

I need to source caps with the following specs from the BOM (attached a schematic and BOM I pulled from the net):

• 0.022UF 400V 10% PE AXL (C9,10,11,,12,13,52)
• 1000PF 630V 10% PE AXL (C6,7,16,24)

Would you say they're electrolytic? C9 etc don't look like it from the overlay though - so its thrown me off.

I would just look, but the amp is currently 6000miles away. I will have the amp in my possession in a couple of weeks, but want to order the caps so I can start the repair straight away.

 

[audioguru]

Good luck trying to find capacitors that are 60 years old. They are not electrolytic, they are paper and oil or whatever they made for old high voltage vacuum tube amplifiers long ago.

 

[Cicero]

Nah, the Peavey 6505 is a recent amp. USA design, made in the east to reduce the cost.

 

[chemelec]

Not sure Why you picked Those Caps?
They don't usually Leak.

Before your Order any Parts, You should do some Checks on the amp to try to find the REAL Problem!

 

[Cicero]

Not sure Why you picked Those Caps?
They don't usually Leak.

Before your Order any Parts, You should do some Checks on the amp to try to find the REAL Problem!

Well thank you for your...erm help.

I suspect those caps because after a while, the lead channel cuts out to very low volume, I can switch to the clean channel and it's back. So something along the lead channel line is broken. I have changed the valves and it's not that. It takes a while for it to happen, 10s of minutes. So if one or two of them is leaky, and not blocking DC completely, then it'll have that effect as the voltage builds up.

Feel free to suggest what you think is the actual REAL problem.

 

[chemelec]

It could also be a Resistor on one of the Plates.
Or Very Possibly a "Cold Solder Joint" somewhere.

I would do some Voltage Checks When it is Working Properly and Mark them on the Schematic.
Than when it Goes Bad, Re-Test these Voltages.

 

[Nigel Goodwin]

I would say it's almost certainly NOT the capacitors, and they obviously aren't electrolytic as they are far too small a value.

 

[Nigel Goodwin]

then it'll have that effect as the voltage builds up.

No it won't, historically leaky coupling capacitors were a problem (but don't give your symptoms), but those were old crappy capacitors - and almost exclusively one of a couple of poor makes (the worst was Wima, I can't remember the other?).

First port of call on valve amps is the anode loads of the triodes, these ALWAYS go high resistance - you should check the anode voltages to see if they are low voltage, if they are then change the resistors. I would strongly suggest you replace them with higher wattage ones, and preferably metal film - this should stop them ever failing again.

 

[Cicero]

Hmm, I don't really agree that a coupling cap doesn't show the symptom I listed, but fair enough, points noted. It's gonna change the DC bias surely, which in turn can kill any signal let through to the next stage.

Regardless, I will update this thread once I get my hands on it again in a couple of weeks.

 

[AnalogKid]

You can replace the caps with 630 V polyethylene. Here is what Digi-Key in the US has in stock in 400 V and 630 V:

https://www.digikey.com/product-sea...1&stock=1&quantity=0&ptm=0&fid=0&pageSize=500

I see one input jack and one output transformer, so it's not clear what the lead and clean channels are.

10 minutes means that whatever the problem is, it is aggravated by heat. It doesn't take 10 minutes for voltages to build up, that happens in milliseconds. But it does take 10 minutes for heat to soak into the deepest parts of the components, make the chassis expand enough to move something one microinch, alter the conductivity of an iffy solder joint, etc. Get a couple of cans of freeze mist, the ones with the long narrow guide tube, and start poking around.

ak

 

[Nigel Goodwin]

Hmm, I don't really agree that a coupling cap doesn't show the symptom I listed, but fair enough, points noted. It's gonna change the DC bias surely, which in turn can kill any signal let through to the next stage.

Presumably you don't come from the valve era

A leaky cap will obviously upset the bias, but doesn't produce the symptoms you described.

You said in post #3 it's a 'recent' amp - assuming you don't mean pre-1960's?, then you won't have leaky caps

As I said, by FAR the most common fault is anode loads - I can't even remember last time I changed anything else in a valve amp?.

Regardless, I will update this thread once I get my hands on it again in a couple of weeks.

I look forward to it - as I said, check the voltages on the anodes, pins 1 and 6 (from memory )

 

[Nigel Goodwin]

It doesn't take 10 minutes for voltages to build up

It does in valve amps

Although not as long ten minutes.

 

[Nigel Goodwin]

Just to put my comments in context, I've been repairing electronics professionally for 44+ years, well back in the valve era.

Certain things fail, and certain things don't - any remotely modern capacitor won't leak - anode load resistors, including the ones still fitted today DO go faulty (in large numbers), simply because they still fit undersized resistors.

Historically, across all kinds of gear - the most common value to fail was 47K - values higher than that are more likely to fail as well, but 47K was always the most 'popular' failure.

 

[chemelec]

I have also Worked-on and Repaired Many Tube type Amps in my life and I am 70 Now.
Considering this is done on a PCB, I would be more inclined to Suspect a Cold Solder Joint.
As the PCB Heats up after 10 Minutes, the Solder/Wire can Separate in a Connection.

As I Said Previously: I would do some Voltage Checks When it is Working Properly and Mark them on the Schematic.
Than when it Goes Bad, Re-Test these Voltages.
When you find the Problem Area, Re-Solder some of the Connection and see if it solves the problem.

 

[audioguru]

Gary, I am also 70 years old.
My Yamaha stereo receiver is about 25 years old and its selector switch wore out causing intermittent loss of one channel.
I am now re-using my HH Scott stereo receiver that is 51 years old and its tape switch might have silver contacts and contact cleaner did not fix it permanently.

 

[chemelec]

Gary, I am also 70 years old.
My Yamaha stereo receiver is about 25 years old and its selector switch wore out causing intermittent loss of one channel.
I am now re-using my HH Scott stereo receiver that is 51 years old and its tape switch might have silver contacts and contact cleaner did not fix it permanently.

Yes I know you, We Talked on the phone a few years ago.
Contact Cleaner Only Removes Existing Oxidation, It Won't permanently fix any contacts and especially not ones that have lost silver plating.
More Oxidation is bound to come back over time.

I Know because even my Brain is getting Oxidized now.

 

[KeepItSimpleStupid]

This https://www.stabilant.com/sizes.html works nicely on switches and contacts.

I suspect those caps because after a while, the lead channel cuts out to very low volume, I can switch to the clean channel and it's back.

It's hard for me to interpret the meaning of the above sentence. I take it to mean:
1) Inject signal into faulty input/channel - dies in about 10 minutes.
2) Inject into other input/channel and the "other channel" works fine.
3) return to the "faulty input/channel" and it works for 10 minutes again.

If that's the case, then a capacitor is a possibility.

This thing has OP amps in it, so it's not of the "valve era".

 

[chemelec]

<I can switch to the clean channel and it's back.>

Is this using a SELECTOR SWITCH?

POSSIBLY BAD CONTACTS IN THE SWITCH?

 

[Mikebits]

Looks like a switch, I guess for lead guitar, or rhythms guitar. Usually the lead has more treble for those solo riffs. I think. He did post a schematic.

 

[Mikebits]

Seems like it would be easy enough to isolate the faulty stage with a scope. Could you bypass a tube stage by pulling it out and putting in a Cap between Grid and ANode? I would think this would allow you to audibly find the bad stage?
Last time I did any tube work was in the Navy, back in the 70's I need to brush up on gain calculations.

--------------------edit------------------------------------
I guess if the problem is between the anode of 1 stage, and grid of next stage, then the problem the cap in socket method won't work.