Crate Vintage Club 50 - advice please

[Bobcube]

Hi

This amp works fine on the bench, but when reassembled back in the cabinet runs for 5 minutes, then there's a crackle, loud hum, and no audio.
So, probably a heat related problem.
The EL84's were running very hot, two were red plating.
I swapped out the EL84's for a quartet of Mullards, went to brew the cup that nourishes, heard .. a loud crackle & hum, and saw one of the Mullards was not just red plating but glowing like a light bulb.

Not wishing to sacrifice any more vintage EL84's, where do I start looking? Any advice gratefully received.

Bob

[Jazz P Bass]

Sounds like something funky is going on with the bias.
The cathode bypass cap, C29, may be shorting out.
Remove it & monitor the cathode voltage for stability.
Schematic is here:http://music-electronics-forum.com/a..._vc3112pow.gif

[soundguruman]

mmmmmm, generally, these amps should be thrown in the trash, if we had any brains.
But hue-mons, as stubborn as they are, become determined to make them work.
OK the bias is crapping out, and the tube turns red...
Which either means that the bias voltage circuit path is failing, or the socket, internally, is intermittently burning through the insulation...
So (knowing Crate and Crate-Pegs), my educated guess is that the power tube sockets need to be replaced.
Also I would be suspicious of a carbon track forming through the fiberglass, between the control grid and the screen grid...
Shine a flashlight through the board, and see if there is any burned spots between the circuit tracks.
If that burn spot is in there, it has to be cut out of the fiberglass. (the carbon on the board is forming an extra resistor)

But my money is on the power tube sockets. I think they probably burned through. (until I see photos or get more info)
I would also change the screen grid resistors, to 1 Watt...or 5 or what ever the bigger size is.

But as far as the future goes, once you successfully fix it, later (probably sooner) it will fail again for a different reason.
Which is why (if we had any brains) we shouldn't fix Crate amps.
You see, when it fails again, the customer will blame YOU. Even though it's failing, due to defective design / cheap crap parts, etc....

The thing about Crate is defective thermal management. The heat has no place to go, and the amp self-immolates.
And so it goes, with pretty much any amp Crate has ever produced.
Crate has claimed that their amps are for "occasional use only" or "temporary use only."
And further they have claimed that: "no amp is built for continuous operation." BUT they are talking about their OWN amps.

We know that all professional amps ARE built for continuous operation. So, go figure.

[Bobcube]

Hi, thanks. But... there's no C29 fitted - possibly a later revision board. The layout is more like that on Blueguitar: http://www.blueguitar.org/new/schem/...ge_club_50.pdf

Resistors 44 & 45 have previously got hot and been replaced with 1 watters. Also R52 has got seriously hot & replaced with a 7 watter.
I don't have X-Ray vision but there's no apparent hot spots between OP tube tracks, and no shorts, when cold at any rate...
The board is quite clean; there's no evidence of cracked tracks and I've remade a few suspicious joints.
Bad news is that I've got two of these amps, but maybe not for long... just as soon as i find out what'e wrong with this one.
What's likely to be causing the bias voltage problem only when the amp has run for 5 minutes?

[Jazz P Bass]

Oh, it's fixed biased not cathode biased.
Whell then, in that case monitor TP17.
Without a signal.
If that point fails, for whatever reason, the output tubes have no choice but to red plate.

[Enzo]

Look in the cabinet, is there foil or other shielding material? And any chance a loose flap of it might hang into the works? Looking for reasons the cab matters. 5 minutes is not long enough to heat up the innards to the point of melt down.


Once you know for sure the amp is 100%, stuf whatever tubes you like into it, but really, for the shop tests, just use a set of plain old Sovteks.

SGM suggests replacing the tube sockets because he thinks everything in a Crate always fails. I am not sure why the common Belton sockets would fail in a Crate, but not when they are used in a Marshall or a Peavey or any one of a dozen other brands.