Biasing and plate Voltage

[benabloom]

Hi all,

I have a weber Bias rite. I have a modified Bandmaster reverb with original Trannies and hoffman board in it. With the tube recto I am using (5u4gb), and a new pair of Winged C 6l6gc tubes, I got a reading of 415 on the Vp setting, when the Ik setting was reading 37 ma. I assume that to mean that the plate votage is 415 volts when the tube is drawing 37 ma of current. If I use the formula for dissapation assuming that 6l6gc dissapates 30 watts

30 divided by 415 v = .o7 or 70 ma max

70% of 70 ma = 49 ma , so, my 37 is on the cold side and sounds it.

I increased the bias so that the tubes read 49 ma, and now the Vp setting reads 405 volts, so it is decreasing in plate voltage as I am turning the bias up on the tubes. I don't want to go higher that the 49ma without checking with you all to see if I am aproaching this the right way, or if I am way off on something...

Does Vp = plate voltage?
Is it normal for plate voltage to decrease as current draw increases?
What should I bias these tubes at based on the info I have provided?

Thanks for all the help!

Ben

[phsyconoodler]

Yes it is normal for voltage to drop when increasing current.But if it drops that low you may be stressing the PT a little.There are two reasons for setting the bias on your amp.One is obviously to make the tubes live,and the other is to get a great tone.I personally would not bias those tubes that hot unless they sound so freakin' awesome you can't live without the tone you are hearing.Try bias settings from about 30ma in increments up to the upper limit(in your case 49ma) and listen to the amp.You might find that it sounds better at a lower current setting,but that's up to your ears.
and recheck that setting after the amp is cooking hot to see if it stays there.Check the PT and see how hot it's getting.
I don't like using 5U4 rectifiers because they draw quite a bit of current compared to a 5AR4 but again it's your ears that have to listen to it.

[Blue Strat]

Rule #1: Whoever said that 70% is gospel should be shot (with "all the lawyers" [and lobbyists]).

Rule #2: Forget about 70%. That's an absolute maximum setting. 50% to 60% is more realistic...especially for today's tubes which aren't particularly robust.

[Hacksaw]

Rule #1: Whoever said that 70% is gospel should be shot (with "all the lawyers" [and lobbyists]).


:AOK


:D

[benabloom]

Ok.. So if I used my earlier reading of 415v on the plates..what would the "upper limit be" for setting the bias for cuurent draw on these particular tubes. (I understand that JJs are pretty reliable cooking at 50ma but not sure about the winged Cs)

Thanks for the input guys..

Oh and I switched from a GZ34 recto to the 5U4GB to get more sag, and earlier breakup. It worked great.(also dropped the plate voltage reading about 30 volts).....as I understand my observations are to to the inefficiency of this type of recto.

Ben

[phsyconoodler]

You need to go by your ears and stop worrying so much about a magic 'number'.However,that said,you have to ask yourself if you want the tubes to live or can you replace them every couple of months?
And don't just listen for one aspect of the tone and that's it.It needs to be balanced to get a good lead and rythym tone.
Sometimes conservative does not mean poor tone.Listen to the amp.Listen to your ears.I think those tubes will cook themselves biased at 49ma.

[Old Tele man]

"70% of Ppd is a LIMIT, not a GOAL"

[jetlag]

I'd pop a GZ34 in there and run your svets between 60% and 65%. See where it sounds better. I rarely run 6L6s up over 40ma or 42ma in classic fender (fixed bias, push-pull Class AB) amps, regardlezzzzzzss of the percent of max dissapation - and they've always sounded great. Plus, in those amps, I like the sound of the 6L6GCs running at a higher plate voltage than what you are seeing right now in your amp.