[Sorted] Slightly crackly Marshall Origin 20w head noise - swapped valves but still there.

Philtre

You know that sound you get when you have a slightly noisy valve? Like a quiet soft crackly sound, or slight static. Not really a problem but slightly annoying and perhaps an indicator that a valve might need replacing at some point.

Marshall Origin 20w head. Swapped out all three 12AX7s, noise still there. Swapped out each EL34 one at a time (I only have one spare) and noise still there.

Either it's both EL34s or something else. Put De-oxit in the valve sockets and reseated.

Not a critical problem, just curious as to what else to check.

ICBM

Static/crackle sounds are often caused by carbon comp plate resistors. It looks from this pic as if Marshall might use them, specifically for these positions - they're thought to sound better, and at least one other company (Blackstar) does the same thing...



The plate resistors are the large beige-coloured ones, which look like they may be carbon-comp - unlike the blues ones which are modern carbon-film and the larger grey ones which are wirewound.

Philtre

Thanks for the reply, @ICBM

This is the classic valve-type noise (not really a harsh static sound more like a quiet windy rumble or soft crackle), that I've solved before now by replacing the valve. Given that things slightly improve when one of the power valves is swapped out, I'll check again when I have two spare EL34s.

Philtre

After more testing with my spare EL34 I think probably one of the EL34s is the main culprit. I have one spare EL34...do I really have to fit a matched pair with a cathode biased amp? I guess I do... ;-)

ICBM

Philtre said:

After more testing with my spare EL34 I think probably one of the EL34s is the main culprit. I have one spare EL34...do I really have to fit a matched pair with a cathode biased amp? I guess I do... ;-)

Yes ideally, if it has a shared resistor - which this amp does appear to have.

Ironically a cathode-biased amp with separate resistors is the least sensitive to mismatching since each valve self-biases itself independently... but that doesn't help you here .

Philtre

Cheers, @ICBM :-)

Modulus_Amps

ICBM said:

Static/crackle sounds are often caused by carbon comp plate resistors. It looks from this pic as if Marshall might use them, specifically for these positions - they're thought to sound better, and at least one other company (Blackstar) does the same thing...



The plate resistors are the large beige-coloured ones, which look like they may be carbon-comp - unlike the blues ones which are modern carbon-film and the larger grey ones which are wirewound.

Hmm, you generally know better than I do about valve amps but...

those beige ones are 1w carbon film, the blue are high precision metal film and the grey ones are metal oxide.

and really any tone difference between that lot is really minor, maybe I should record a blind test of them all one day.

That doesn't help Philtre though, hopefully the EL34 swap will sort it, is it still under warranty?

Philtre

Modulus_Amps said:

That doesn't help Philtre though, hopefully the EL34 swap will sort it, is it still under warranty?

No need to bother with warranty if it's a dodgy valve, especially for a £249 amp. I'll get some new EL34s soonish.

ICBM

Modulus_Amps said:

those beige ones are 1w carbon film, the blue are high precision metal film and the grey ones are metal oxide.

Think you're probably right, although I have some wirewounds that look like those grey ones and carbon films like the blue ones too. A quick search actually brings up quite a selection of case styles relative to types...

Modulus_Amps

ICBM said:

Modulus_Amps said:

those beige ones are 1w carbon film, the blue are high precision metal film and the grey ones are metal oxide.

Think you're probably right, although I have some wirewounds that look like those grey ones and carbon films like the blue ones too. A quick search actually brings up quite a selection of case styles relative to types...

Nothing is certain anymore, those resistors just look like the ones I stock.

Thing to watch out for it some wholesalers are selling off 1/2w resistors as 1 Watt ones, I ordered box of 1000 1w Carbon film resistors from RS and the box arrived from the manufacturer and was marked as 1/2w on the original label and 1w on the RS label

Modulus_Amps

Philtre said:

Modulus_Amps said:

That doesn't help Philtre though, hopefully the EL34 swap will sort it, is it still under warranty?

No need to bother with warranty if it's a dodgy valve, especially for a £249 amp. I'll get some new EL34s soonish.

Thats if it is a valve issue

Pull the phase invertor valve (normally nearest the power valves) and see if the noise stops, if it doesn't stop, then it is almost certainly the EL34's, output transformer or mains transformer

ICBM

Modulus_Amps said:

Thing to watch out for it some wholesalers are selling off 1/2w resistors as 1 Watt ones, I ordered box of 1000 1w Carbon film resistors from RS and the box arrived from the manufacturer and was marked as 1/2w on the original label and 1w on the RS label

That's not good... I would have expected better from RS.

Do you think they're compensating for good design practice being to run a power component at no more than half its rating?

Modulus_Amps

ICBM said:

Modulus_Amps said:

Thing to watch out for it some wholesalers are selling off 1/2w resistors as 1 Watt ones, I ordered box of 1000 1w Carbon film resistors from RS and the box arrived from the manufacturer and was marked as 1/2w on the original label and 1w on the RS label

That's not good... I would have expected better from RS.

Do you think they're compensating for good design practice being to run a power component at no more than half its rating?

I don't know, I complained and they sent me another box of the same resistors.

Somewhere along line the spec has been mixed up, the size of the resistors they sent were typical of 1/2w resistors too, so they looked like 1/2w resistors, were marked as 1/2w resistors by the manufacturer, I am pretty sure they were 1/2w resistors.

Maybe they were temperature de-rated by RS to meet the 1W mark. hard to know

ecc83

Just to set the record straight!

Blackstar used carbon comp Rs in the Artisan series only (and even then, dropper/decouplers were metal film) . The reason being the "cache" that has grown up around them* . The techdirect was never happy about it since you are technically using old, less reliable technology and charging a good wedge for it!

Have to say I never had one fail.

*Yes CCs ARE less linear than MF but you have to hit them with 100V + to measure even a tiny amount of distortion. Not likely to be heard in a guitar amp.

Dave.

Philtre

Update - I got two new EL34s from @karltone and the problem is solved. Sounds a lot better too (although that is subjective)