Tube biasing

[treguiers]

Hi everyone,

I'v searched here for instruction on how to bias a tube amp. but to no avail.
Mine is a fender blues deville hot rod.

Firstly do I need one of those tube testers with the specific connection that goes between the tube and the tube socket? or can i use a normal multimetre.

Secondly I believe that Fenders amps have only one overall biasing adjuster, which seems to defeat the purpose as I thought it would be necessary to be able to adjust the tubes individually( my amp was made mid 90s)

I am very aware of the hazzards involved and will tread very carefully in any endevour regarding this.

I'm sure this subject was covered on this site I just can't find it......so any help will be warmly welcomed
thanks

[thetragichero]

looks like your amp is fixed bias, so the only way you could adjust the bias is if you bias pots installed (well, you could swap out resistors)
basically, i would take it to a tech

[sumgai]

Oct 19, 2010 9:13:11 GMT -5 thetragichero said:

looks like your amp is fixed bias, so the only way you could adjust the bias is if you bias pots installed....

Errr, not quite. The term "fixed bias" means that means that the bias does not vary in relation to the incoming signal. Contrast that with a cathode biased circuit, where the bias varies as the incoming signal varies. More importantly, the cathode bias method is indeed "cast in concrete" and cannot be adjusted by us, the user/techician, except to physically swap out parts.

Now normally a fixed grid bias circuit has an adjustment pot, and in this case, the Blues DeVille, that pot is missing. Sad, but not a disaster. Simply insert a 25K Linear taper pot between the two points marked "C-" on the circuit, and all should be well. I'd also recommend that you (treguiers) check to make sure that when the pot is rotated fully one way (making it appear, electrically, as if it weren't in the circuit), that the expected voltage on the tubes is a bit more than necessary. That way you can back off a bit with the pot. Strictly speaking this isn't necessary, but if the tubes want more bias voltage, putting in the pot won't help - it'll be set to the point of least resistance (imitating a straight wire), and something else will have to be done.

Now, that something else. If indeed you require more negative voltage at the tube's grid(s), then work your way back up the supply line.... see that 12KΩ resistor? Replace that with a 4.7KΩ or a 6.8KΩ unit, and you should see the expected/desired voltage on the tube's grid(s). But don't bother to change this unless required, or in other words, if the pot wouldn't do you any good in the first place.

As for your "defeated purpose" by having one one adjustment for all (power) tubes, you're looking at a different thing. A fixed bias pot adjusts for the overall amount of bias voltage available to any/all of the power tubes. A "bias balance" pot would adjust for each side of the signal (the two halves of the power section, one on each side of the transformer) getting just the right voltage so as to give the best possible signal results with the least amount of distortion.

Meanwhile, back here in the real world, we aren't dealing with laboratory precision - we're dealing with things that have a wide latitude as to how they are interpreted by those hearing them. I'm talking about the final output... where, even if we were to take the most exquisite care in adjusting every little nuance we can find, it still wouldn't amount to a hill of beans, when the chips are down. All that means is that Fender knew it back in 1946 - you don't need to sweat the small stuff, the end results are still gonna satisfy the customer. (Which kinda goes to the economics of the thing, or so I believe anyways.) And I don't think I'd like to argue their sucess in building amplifiers for the past 60-some years, would you?

As to how to actually bias your amp, I'm gonna leave that for a bit. First though, I wanna know how it is that you've determined your amp needs to be biased.

HTH




sumgai

[treguiers]

Hi Sumgai,

As always your knowledge on all things guitar is astounding.

Why do want to bias the tubes on my amp? The amp has a serious amount of milage on it (gigged 2 to 3 times a week and the tubes haven't been changed for 2 to 3 years).

This is my first attempt to change the tubes myself, and reading up on the subject seems to indicate that when you change tubes you have to re-bias them.

I know its not as simple as whipping out the old ones and sticking in some new ones.

One more thing, my pilot light dims to nothing as the amp warms up. i don't know if that has anything to do with anything.

I'll go now and try make sense of what sumgai has just written.
thanks

[treguiers]

So getting back to my original question: If I change the tubes on my mid 90s fender blue deville, Do I just take out the old ones and put in the new ones or do i need to re-bias the new tubes.

Any nuggets of wisdom would be greatly appreciated. ;D

[sumgai]

tre,

In your case, with the Blues DeVille, the answer is yes, just stick in the new power tubes and go. Unless you want to play with your soldering iron and insert that pot I mentioned, there's no way to adjust the bias voltage.

As for age/use, etc,..... There are several reasons to re-bias when replacing tubes, but in the end, it's not an absolute necessity - at least, not most of the time.

When would it be necessary? Good question. I never check the bias on an amp's tubes when I replace them. Instead, I drag out the 'scope and check the output waveform. If that is too distorted, in either direction, then I start twiddling the bias adjustment. Where there's no user-adjustable pot, I either insert one, like what you'll need to do for your amp, or I start messing with the cathode bias resistors, when that's appropriate.

Interpreting how much distortion is too much - that's subject to the technician's experience. Some might look at what I call acceptable, and consider me to be crazy (i.e. it's way outta whack). Others might call me anal for fiddling around with the adjustments when the waveform looks perfectly fine to them. It's all a matter of subjective interpretation, there's no saying "this waveform is correct" or otherwise.

I counsel you, don't be overly concerned here. The bottom line is, what does your amp sound like to you, not to someone else? But of course, if you really dislike the sound of your amp with the new tubes, at that point you can take steps to restore your tone, adjusting the bias being one of those steps.

As for your pilot lamp dimming during warm up, I wonder.... does it come back up to normal brightness when your amp is warmed up and ready to play?

HTH




sumgai

[treguiers]

Hello sumgai, and thanks for the reply. Always informative.

I have trouble getting my head around solid state electronics, but tubes(or valves as we call them this side of the pond) is off the scale.

I hear that you can get spec of tubes that will give you more head room(i.e. you can go louder before it distorts) Is this true, and if so what specs do I look for.

Is there something about fine tuning tubes so as to improve the tone and their life expectancy?

With regard to the pilot light: I switch on the amp the pilot light is bright. But as it warms up it dims, and after about 10 mins there's no light at all. The amp is still functioning.

Am I happy with the sound of my amp....yes. But I hear other ones that seem to sound better....but some players are masters at pulling tone out of an amp.... I suppose I just want to make sure it's firing on all cylinders!

[morbe]

So what happens if you change out tube with other than original parts and don't re-bias?
How do you know if it was re-biased with differ tubes?

[yug]

I guess I'll throw in my $00.02 worth. When I re-tube my amp, I don't worry about the"bias". More on that in a minute.
When I re-tubed my Roland Bolt 60, I just installed the new tubes, turned the amp on in "standby" and left them to sit in the basement/studio overnite. Before I went to work next A.M. I turned off the amp and let it cool down. I haven't had a problem with the rig at all. I was told( by a well respected tech) that the critical part was the "burn in" process.

Now, to "bias". When I was a kid my dad was an electronic tech trying to learn the new and exciting world of solid state. PNP, NPN, FET, MosFET, etc, etc. To pay for this, my dad would pick up television sets off trash heaps and replace tubes to fix them up for re-sale. Did pretty good at it for a while. Back then, 7-Eleven had an RCA kiosk with vacuum tubes and a tube tester. You could test your TV tubes and replace the bad or marginal ones. This is where "bias" comes into play. Old TV sets had many tubes that performed a variety of fuctions. Audio reception and amplification, video reception and signal recognition, as well as power supply and rectification. All those tubes had to work together as well as find the audio and video signals. Tube bias was critical in those days for receivingand amplifying those signals clearly. That's why tube bias was so important then.All the tubes had to play their own little part in putting up an image on that 8in screen in a cabinet the size of a refrigerator. Not so critical in a guitar amp that just has to amplify a dedicated signal to a dedicated output.

I know this was a bit wordy but the subject is very complicated, as I am finding out.

Hope this helps!

yug...