more Deville bias escapades

[slapbackdaddy]

Hello again all,

Have been doing some work on my friends Fender Blues Deviile and have given it an electrolytic re-cap job and fitted a bias trim pot as i have done on my own Deville. The bias current is quite lop-sided between the two 6L6's. They have plate voltages of 507 & 504v but if i bias the 504v tube to say 30ma the other sits at around 38ma. I've tried 3 different pairs of 6L6's including the pair from my own amp which i know are very closely matched and i get the same mis-match, one side is 7 or 8ma higher than the other.

1. Is this a problem other than the fact that it seems to hum a little more because of this?
2. What could be the cause of this and where should i go looking first?

I've also tried 3 different PI tubes too.

Thanks in advance for your help.

[Alex R]

Yes imbalance in current draw will make a little bit of hum, by slightly reducing the humbucking abilities of a push-pull amp.

It may be that your tubes aren't all that closely matched. Or that imbalances in the amp itself - which are quite normal in guitar amps - are causing the problem. Either way, it doesn't matter. Given the imbalance in plate volts some slight difference in the output transformer windings might be the cause. But if the currents are stable there's no major problem. The bias levels you report are giving safely under 70% of plate dissipation wattage, the usual recommended bias for push-pull amps.

Sometimes an unbalanced pair can interact with an unbalanced amp to produce level current draw. It might be worth experimenting.

Some people say a bit of imbalance sounds better.

You might be beginning to wonder why so much fuss is made about matched pairs of output valves. It's what marketing men call 'added value'. Like flame-grilling your whopper. Cryogenic treatment is another such wheeze. Whatever will they think of next.

I buy valves in small bulk quantities to repair amps. I do actually bother to match them fairly closely on my tester, because the amps hum a bit less. That's my only reason, others may think different. However I turn out plenty of amps with current differences like 30/38mA, I admit it.

[slapbackdaddy]

Thanks Alex, i was actually thinking this. Bottom line in that the amp sounds great, just wanted to be sure that it wasn't an easily correctable quirk. I can live with it as it sounds good.

One thing though, i was aiming for around 30ma per tube so how would you balance that out with what i have here?...one around 34ma and one around 26ma or 30 and 38....wouldn't the 38 be over the 70% dissipation mark...safely asuming 25w for these tubes....i'm not sure i trust em at 30w.

[Alex R]

507v x 38mA gives 19.3 watts; 17.5 watts is 70% of your conservative 25 watt dissipation for 6L6. I think that's ok - but tweak it down to 34 if you like, there won't be a difference in the sound I betcha and the other one will still be conducting well.

[Enzo]

Measure the resistances of the two sides of the OT primary - I bet that is where the difference lies. And don't worry about it.

[Mr. Droopy-Drawers]

Another thought is that you may have a small parasitic oscillation happening, and making the one tube draw more current. Connect a big coupling capacitor from grid to ground on the one 6L6 and see if the current comes back down.

[Twist]

Check your screen grid resistors out of circuit to see if they have drifted.

[drewl]

My thoughts exactly, the screens often blow on these so if one is starting to open that tube won't conduct as much.
It the screen resistors are good, try swapping the tubes around to try and even the bias.

[Enzo]

His amp is not smoking, he just has an imbalance of about 8ma betwen sides of his push pull.