Biasing my Allen amp--settings?

[Phil M]

I finally got around to installing the new Tung Sol 5881s in my Allen Old Flame head yesterday. I took out the old 5881s and put the new ones in and set up my multimeter. The ampwares site said that 6L6/5881s should be set 30-35 (one of the old tubes was at 35, one at 39). I set the new ones at 34, but I'm wondering if I would hear any difference in setting them at 30 vs. 35?

Just thought I'd also mention that this Allen design with the test points and bias adjust on the outside is an incredible feature. I never envisioned that I'd do this work myself. It's the sort of thing that I used to end up paying somebody $50 to do. I'd get bottom-shelf tubes and be without my amp for a week. Now it just costs me the price of a new set up power tubes. I wonder how easy this would be on my Rivera head...?

[Blue Strat]

It takes less time to try different bias setting than it does to post:) Try it and let us know what you think.

[Phil M]

Understood...very true. How high past the 35 can you go before risking doing damage?

One more question: This amp has a Sovtek solid state rectifier plug in place of what would normally be a GZ34 tube. What differences might I expect by going to a tube? I don't have one handy...

[Blue Strat]

How high you can bias is also a function of the plate voltage which is higher with the solid state rectifier than with a tube rectifier. I wouldn't go higher without knowing your plate voltage.

A tube rectifier will give you more compression at higher playing volumes. SS rectifiers are known for tight bass, more highs and a stiff feel.

[Phil M]

Thanks, Mike. I'll probably stick with the SS rectifier for now.

[MikeP]

Just thought I'd also mention that this Allen design with the test points and bias adjust on the outside is an incredible feature.


Yes I loved that feature on Allen amps.
Also lets you check how closely matched your tubes are as you also found out ;)

[Phil M]

After setting the bias back in February I played the amp for awhile. I meant to re-check the bias but never got around to it until last night.

I had set both tubes at 34 and when I checked them again, one tube was at 31 and one was at 35. I put them both back to 34 and the sound became a little warmer.

Is this kind of drift normal with brand new tubes? I will check it again in a few weeks.