Do dying tubes sometimes sound better with hotter bias?

mixolyd

The output EL84 (the last of my NOS Mullards from when I bought a bunch ten years ago) in my Rat Electronics Valve Special has been hard to dial in of late and I was thinking of replacing it.  While fiddling around for the nth time I switched the preamp to the louder (hotter?) bias option and the overdrive tightened up, really lost the horrible flubby mud which had taken over.

Is this a sign that the valve has seen better days?  It actually sounds pretty good in this setting which I was never a fan of before, but I seem to remember the amp sounding better in the past.  It's difficult to compare with the assortment of tubes that have been in there over the years.

thermionic

I'm sure I read from one of the forumites here about how amazingly glorious an AC30 sounded in the few minutes before it burst into flames.

ICBM

Sometimes, yes. But usually not for long.

What is the 'bias' switch doing? If it's in the preamp it has no bearing on the power valve anyway.

mixolyd

ICBM said:

Sometimes, yes. But usually not for long.

What is the 'bias' switch doing? If it's in the preamp it has no bearing on the power valve anyway.

Ah I see.  Here's what the Rat site says 

8. Three Position Preamp Valve ‘Bias’ Switch - Switches the Preamp ‘Bias’ between - ‘Negative Feedback’ -or- ‘Sharp+Clean’ -or- ‘Medium/High Gain’. Exclusive toRAT and fitted on All Amp

The amp sounds awful (on sharp&clean setting) when the power section is pushed hard, not so bad when the input gain is cranked instead.  It also sounds bad when the power scaling is applied to the output valve, that's why I'd assumed that the output was the problem.

DJH83004

I think the design is based on the Harley Benton GA5 / Epiphone valve junior, which was originally a single ended, cathode biased EL84 but I'm not sure where RAT have taken the design, maybe switchable cathode bias resistors?

mixolyd

I think I get it now.

Using the more aggressive preamp bias is producing more output in the preamp section so the output is having an easier job for a given volume level.  That fits in with the idea that the EL84 has had it.

mixolyd

Update:

Got an new JJ EL84 today and chucked it in: success.  Cleans sound better, overdrive sounds better in all preamp bias settings, power scaling working extremely well.

I had been thinking of replacing this amp, but now that it's sounding good again I may think about getting it a proper cab instead of throttling it with the little 8" combo speaker.