Tube biasing

Jon Light (deceased) · Post by **Jon Light (deceased)** » 16 Dec 2003 3:26 pm

disclaimer----I may use terms I don't understand at all

On my new '74 Dual Showman Reverb. I have a Weber BiasRite with four heads. I am reading plate voltage of 462, pretty even across all 4. I am getting a current reading of 12.5, 10.9, 10.9, 10.9, mA respectively after tweaking the balance pot. Isn't this WAY low? Don't I want to be in the 35mA range? And isn't tube # 1 out of whack with the rest? Does this mean that I need to change out resistors and stuff? If so, even though I intended to learn how to do this stuff, I don't want to make the new amp my classroom so I reckon I'll be off to a tech.
The amp sounds good though. Should I just leave it be for now?

Ken Fox · Post by **Ken Fox** » 16 Dec 2003 3:40 pm

Really low there! 30 to 35 ma is good for us country pickers! Some blues and rockers crank them up to 45 ma or more! Tubes do not last long at that and you are losing headroom at those levels.
Within a millampere or two is about as good as it gets, even with "matched tubes". Sounds like you have old tubes! They will get low when older. You can change the bias resistor or just Blackface the bias control area to allow for adjustable control.
What you get with low bias is crossover, or notch distortion. It is very audible on the low notes. You can see this readily with an oscilloscope and a sine wave generator. That distortion is caused when the note first starts and the tube is not quite on (idling at proper bias). A bit of the signal is lost as the tube starts to conduct. I like to think of it like setting the curb idle on a carburator. If it is too low, the car lugs when you give it gas, then takes off and goes!

Jon Light (deceased) · Post by **Jon Light (deceased)** » 16 Dec 2003 4:06 pm

Thought so. Well, these were JJ's with only a couple of dozen hours on them. But I just put in a different quad (no name) and they range from 34-39. I can't figure what happened to the JJ's but it would seem that they are spent.
Actually, they spent their short life in a BF Showman with no biasing. Maybe it is way out of whack. Could explain why I was never happy with the amp. One of the reasons I want to learn how to do this stuff. Thanks Ken.

Brad Sarno · Post by **Brad Sarno** » 16 Dec 2003 4:39 pm

Jon, dont forget the golden rule for working inside a live tube amp. Keep one hand behind your back, for real. No need to let your hands and heart complete a 400V+ circuit. It's lethal and we'd like to see your name continue to show up here on the forum.

Brad Sarno

Jon Light (deceased) · Post by **Jon Light (deceased)** » 16 Dec 2003 5:09 pm

In some place other than "Our Extended Family", I presume. Thanks Brad.

Ken Fox · Post by **Ken Fox** » 16 Dec 2003 5:20 pm

I would not toss them. New ones can vary quite a bit on bias range! That's why Groove Tubes sells different "hardness" values. You need to test them with a tube tester that measures mutual transconductance. Good ones will be in the range of 5000 micro-mhos.
I use an Hickock tester, a real life saver for my hobby! I actually have a Jackson model tester as well. If you do not have one, keep your eyes open for one that is a true trnasconductance tester. It will tell the tubes ampflication abilty, not just a bad/good indication.
Be careful to always have a load on a tube amp. If the output sees an open, it can destroy output transformer when a signal is amplified. Most all Fender tube amps had a shorting jack on the main speaker jack, to protect against an accidental open. Much unlike a transistor amp, they can tolerate an open very well.

Jerry Hedge · Post by **Jerry Hedge** » 16 Dec 2003 8:44 pm

John you might check the resistor on the bias pot. It's probably a 27K. Change that to a 15K and see if that helps get the bias current up.

Mike Simpson · Post by **Mike Simpson** » 16 Dec 2003 8:45 pm

Jon, how did the change in power tubes and bias affect the sound of the amp?

Murnel Babineaux · Post by **Murnel Babineaux** » 16 Dec 2003 9:47 pm

Jon,

Check your grid resistors going from pin 6 to 4 on all four output tubes on your Fender Showman. Bias resistors between 1 and 5. With 462 volts of plate, you should have at least -49 volts of negative bias.

What is your negative bias voltage?
Read it on Pin 5 of tubes 8 and 9.<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Murnel Babineaux on 16 December 2003 at 09:48 PM.]</p></FONT>

Jon Light (deceased) · Post by **Jon Light (deceased)** » 16 Dec 2003 11:25 pm

Jerry---this is a SF Fender with the bias balance pot. So all I can do is balance the readings between 1&2 and 3&4.

Mike---had to go out before I had a chance to spend any time with it and it's too late to play at meaningful volume now. But oddly, there wasn't all that much difference to my ears other than what I would expect from different tubes. But since I live in an apartment, I don't think that I'd hear much of anything we're talking about at the living room levels I'm restricted to. I won't get to crank it until my weekly gig Sunday (and that will be the first time I've taken this amp out so I've got nothing to compare it with).

Murnel---thanks for the suggestions. But what you don't know is that I am an absolute beginner. My bias probe is a set of adapters that go in the tube sockets--the tubes go in the adapters and the meter comes off this assembly with a switch that chooses between the plate voltage and the cathode current. In other words I haven't dropped the chassis and I haven't been messing around inside a live amp. Although I'm working my way towards such stuff--I have a project amp and I have books and a video etc., I haven't gone there yet. I'm not ready to check what you suggest. I'm smart enough to know what is over my head. I feel frustrated enough by stuff like this to want to learn more. And I will.
Thanks.