Cornford Roadhouse Issues

[CraigGa]

I've been having issues with my Cornford Roadhouse 30 recently whereby when I first turn it on it crackles for a bit then it's ok but yesterday it cracked then blew the HT fuse.
Previously I have had time to turn the master volume down and that had no effect on the crackles.

That leads me to think it's the EL34's that are bad, I've replaced the fuse and run it without the OP tubes in and it seems ok, I'm getting a signal out of the effects send so that proves the preamp is good,I'm just a bit wary about replacing the EL34's in case something bad happens.

I bought the amp 2nd hand from a shop and when I demoed it, it began to crackle and red plate and smell hot so I turned it off.
The shop tech changed the EL34's to some jjs that were boxed but I'm not sure if they were new or not and it seemed to be ok so I bought it with some extra discount.

What I don't understand is how can a cathode biased amp redplate? isn't that usually caused by a bad bias supply?

any ideas please?
I've attached the schematic but please note that it covers the 30w and 50w versions so the bias circuit is a bit confusing (the 50w is fixed bias)

Thanks
Craig

[CraigGa]

I just had a thought that if the coupling caps from the PI are leaky then that could upset the bias couldn't it?

Craig

[sluckey]

yes

[CraigGa]

I just measured the grids and they're both at 0v DC

Craig

[xtian]

I'd replace those power tubes!

[Stevem]

The crackling is telling you either a tube(s) are bad and arcing internally, or the socket is not making good contact with the tube pins, or the OT is shorted internally.

Older style JJ outputs had pins with no lead in taper that distroyed tube sockets, so hopefully the ones in the amp are not that old, but anyway the first thing I would do is to drain out the amps power supply and retention the tube sockets if you can, next before replacing the tubes I would derate the HT fuse by 50% do that if the tubes do arc over and or red plate you will not harm the OT.

[CraigGa]

Thanks for all the replies,
I'll try stevem's suggestions first, the tube pins do seem rounded at the end are you saying that they were flat ended before?

I did wonder whether the red plating in the shop could have damaged the ot but it has been working fine for years since then.

I could try the battery and neon test on it, it connects to the board on spades so it should be easy enough to do.

Craig

[JD0x0]

I just measured the grids and they're both at 0v DC

Craig

If the grids are at 0V on a fixed biased amp, you've lost bias voltage, and you're red plating the power tubes, I would think..

The schematic appears to show a mix of cathode biasing and fixed biasing, so you may have lost the fixed bias supply on the amp.

I'm not sure of the values of R9 and R6 in parallel with the 180R cathode resistor, but it's possible with a loss of the fixed bias voltage, the cathode bias values could set the tube too hot.

Edit: Nevermind, you seem to have the 30 watt version, which only uses the cathode biasing, my mistake.

[CraigGa]

So I decided to follow Stevem's advice,

I didn't have any 250mA fuses so while waiting for those I tried to re-tension the octal tube sockets, that didn't go well because I cracked the ceramic base on one of them so I ordered a pair of those too!

Anyway I've changed the sockets and all seems good so far, I'll play it for a couple of days before buttoning it up just in case.

Thanks for the advice.

Craig

[Stevem]

I am sorry, I should have noted to you that the retentioning process can't be done on ciramic type sockets.

[CraigGa]

I am sorry, I should have noted to you that the retentioning process can't be done on ciramic type sockets.

No worries Stevem, changing them out seems to have fixed the problem.
The actual pins were a fork style rather than a rolled tube, are they know to be problematic?

Craig