EL34 rattle?

thewiddler

I've been running an EL34 as the power tube in my Cornell Romany for a month or two now. It's a Winged C and I've really liked it versus the default 6L6 - smoother, more detailed mids. However, tonight noodling away the amp's started a dull, resonating rattle, particularly around certain low notes. 

As it sounds a bit like long springs crashing around I though it might be the 'verb… or my missus rattling her chains down in the basement. But after doing the whole 'tap each tube with a pencil rubber test' there's no obvious microphonics. Swapped it for a 6L6 and the problem's gone. Is this a fatal mechanical fault with the EL34 or anyway to stop the rattle?  

ICBM

It's probably had it. You could try putting some high temperature o-rings on it to see if it will stop, but the rattle is most likely coming from the internal elements and being amplified through the speaker, rather than the glass.

I have a whole set of four closely matched vintage Mullard XF2s which do it as well .

thewiddler

Oh well, I thought as much. Thanks ICBM. Are EL34s more susceptible to failure in the backwash hellhole of a combo like this, hence why it's recommended to use 6L6s?

ICBM

I don't think they're more susceptible really, 6L6s often do it as well. There was a period in the 90s where good new EL34s were hard to get though - Mesa used to recommend 6L6s in the Rectifiers (which can also take both types) for this reason. It was so bad for a few years that Marshall even temporarily switched their whole range over to 5881s, despite that not being a popular move for tone reasons.

jpfamps

I've had real trouble with =C= EL34s rattling over the last couple of years, to the point where I've stopped using them.

This is a shame as I really like the sound of them.

Regardless, I understand that they are now out of production.

Beam tetrodes (eg 6L6s) are inherently more reliable than pentodes (EL34 etc) due to lower screen grid currents due to the method of constuction.

thewiddler

I shall bear that in mind... Just much prefer the Romany with thicker, smoother mids. Sounds a bit anaemic now I've reverted to 6L6. If I did go EL34 - what's the best for price/reliability?

ICBM

jpfamps said:

Beam tetrodes (eg 6L6s) are inherently more reliable than pentodes (EL34 etc) due to lower screen grid currents due to the method of constuction.

Doesn't seem so in practice though. While EL84s definitely seem to have the highest failure rate - although they are the most commonly run well over their ratings, unlike the other types - the failure rate of EL34s, 6V6s and 6L6s seems to be about equal, at least in my fairly limited experience. Given the number of burned out screen and 'choke' resistors, screen failure is the most common cause of death for 6V6s and 6L6s too.

jpfamps

ICBM said:

jpfamps said:

Beam tetrodes (eg 6L6s) are inherently more reliable than pentodes (EL34 etc) due to lower screen grid currents due to the method of constuction.

Doesn't seem so in practice though. While EL84s definitely seem to have the highest failure rate - although they are the most commonly run well over their ratings, unlike the other types - the failure rate of EL34s, 6V6s and 6L6s seems to be about equal, at least in my fairly limited experience. Given the number of burned out screen and 'choke' resistors, screen failure is the most common cause of death for 6V6s and 6L6s too.

Beam tetrode are inherently more reliable by design. This is a statement of fact.

Of course you may see more failed 6L6s than other valves for plenty of other reasons, for example, there are more amps out there with 6L6s in, or perhaps many of these amps are designed badly, or the quality of 6L6 production is poor.

I see far more failures in EL34s than any other octal-based valve.

I very rarely see shorted 6V6s.

I regularly see shorted 6L6s in Fender Devilles and SF ultra-linear amps, along with toasted screen resistors. In my opinion this due to running the screens over voltage, typically over 500VDC in these amps. This is over spec.

ICBM

Exactly - I think there are other reasons which outweigh the theoretical better reliability of beam tetrodes (which I'm not disputing), in the conditions they're exposed to in guitar amps. The spec often goes totally out of the window. I have seen a lot of 6V6s with screen failures too - again that's likely to be because they're operated at 400V or more. As Dave says, in radio and TV circuits they can last decades with ease.

Similarly the notorious poor lifespan of EL84s is probably because designers from Dick Denny onwards insist on running them at over 15W at idle. If you find them in vintage equipment where they're run within limits they seem to last perfectly well - I'm pretty sure the one in my 1964 Centurion is the original .

The main cause of EL34 failure in my recent experience has been the simple mistake of building a Marshall JCM2000 and putting them in it .