Cornford Carrera - Crackling/hissing

[Marky]

Hi chaps.

I know what you are thinking; bad output valve. However, it doesn't seem to be that to me as I tried replacing each tube in turn. The noise happens even with no guitar plugged in and sounds a bit like a detuned radio, but less consistent (more erratic sound).

The weird thing is this. The Carrera has switch-able output valves and the sound only occurs when the 9 pin is selected. It is perfect on 8 pin. I am suspecting a bad cap, but I wonder if there is an easy way to check these without replacing all 5 electrolytics (the usual culprits). The amp is only 3 years old, though I realise you can get a bad cap at any age.

I'd be grateful for any of your pearls of wisdom oh great oracles of the music world.


Thanks in advance, all the best

Mark

[pdf64]

I know nothing about these amps, but try the same 'subbing in a known good one' tube thing with the pre-amps, as a dying one could easily have those symptoms.

[MWJB]

"The Carrera has switch-able output valves and the sound only occurs when the 9 pin is selected." I wouldn't really be suspecting a cap (they are reasonable quality & usually last for decades), I would guess that there is an additional power supply resistor in the B+ chain when the 9 pin power tube is selected and it is dying...if it's anything like the Romany I'd be looking for a 1W/2W resistor that is burning up & going brown?

Or keep it in 9 pin mode, play hard and see what fails, to bring the culprit to light. I would guess that this resistor is bypassed when in 8 pin mode, so the amp should still work fine in that mode. Replace resistor with the same ohms but twice the W rating of the original.

[Marky]

Hi there

Thanks for responding to my post.

I already tried swapping out each tube in turn and it made no difference. That was my first thought too. I am suspecting a bad filter cap. They are quite tricky to get at without hurting anything else in there so I am reluctant to start the same subbing out process with those; it may come to that though.

Cheers

Mark

[MWJB]

"I am suspecting a bad filter cap. They are quite tricky to get at without hurting anything else in there so I am reluctant to start the same subbing out process with those" Don't, if it were a bad filter cap (unlikely) why would the 8 pin mode work fine? You are more than likely looking at something to do with the parts involved in switching modes.

[Marky]

Hi there

I had the same view about the caps too but couldn't think what else it could be. It just sounded like maybe some AC was leaking through one of them.

Great suggestion though, thanks. I will try that next time everyone is out of the house.

Would the resistor appear out of tolerance on my scope do you think? I couldn't see any off colour ones when I checked previously. It will take me a while to test them all as I have to look each one up (no electronics training besides a bit in A level physics centuries ago.....).



Cheers

Mark

[MWJB]

If the resistor is dropping the voltage to the plate of the EL84, with the amp off & unplugged from the wall, check with your meter for continuity (0hms) from pin 7 of the EL84 to one end of a resistor, this could be the suspect, should be physically larger than those say around the 12AX7.

The scope is only going to confirm what your ears are already telling you, that you have unwanted noise, in 9 pin mode, with no signal. If the problem is what I think it is, as the resistor burns up further, tone will degrade & the amp will eventually stop producing sound in 9 pin mode. You could just leave the amp on in 9 pin mode, not playing through it, the EL84 will still be pulling current through the resistor & may hasten its demise.

[Marky]

Good point. I guess I assumed that each mode must use its own filter cap, but that wouldn't be sensible design I guess.

Thanks again

Mark

[Marky]

Hi again

Hmm. No weird observations regards the resistors. However, when I fired the amp up on its side, the crackling wasn't there any more! This would suggest a bad connection wouldn't it? I tapped all the components with a chopstick and nothing crackled; ditto for each of the tubes. Maybe it is cured?


(I don't think so either)

Cheers
Mark

[Marky]

Another observation about this fault; it is intermittent. Sometimes no noise at all apart from background hiss. Then the crackling comes in with varying erratic intensity. It then often just fades out again. This is with no guitar or lead plugged in, not touching anything at all either (only happens on 9 pin mode). Still looking for a duff resistor.

Maybe a Poltergoost.....

Mark

[Glass Snuff]

Many years ago, I remember fixing this problem in a friend's Fender. I wish I could remember what I did, as right now my Marshall is doing the same thing.

Could it be a bad blocking cap?

[tedmich]

We most definitely need a detailed schematic of this $2k amp to help you in any meaningful way!





pleez oh pleeez....

[Marky]

I'd love one too....

Would a photo of the guts be of any use in the mean time?

Mark