Peavey Classic 50 watt amp setting bias

[DrGonz78]

This is the Peavey Classic w/ two 6L6's on the output of the amp: hybrid style solid state preamp section. The year on the amp is 1976. I am currently fixing almost everything up on this amp in an attempt to keep myself busy and learn. My question is really something that will pay off in a few weeks when I finally get the amp all back in order...

I am wondering if anyone here can give instructions/advice for setting the bias on the amp? I have yet set bias on any tube type amp and would like this amp to be my starting point. I am asking this way in advance as to get all my info in place before I attempt this feat. This may seem like simple stuff for most but to me it is my first time. I have two multimeters that can be connected to the amp for measurements. I have what I think is the right schematic...





I included the Preamp schematic just in case someone here finds this thread and needs that since it was tough to find without calling Peavey.

Any help on this will be much appreciated as always.

P.S. - The amp is named the Classic 100 series amp but it is 50 watts output... Just to clarify my title better.

[soundguruman]

Well you turn up the heat until the tubes start to turn a little red...
Then you back it off, until the red "just" disappears. That is the basis for all bias adjustment.

The tubes run as hot as possible, without turning RED.

Ha ha ha, but it's true.

Now get out your O scope, your load resistor, and your sine wave generator.
Set the bias so that the crossover distortion is absent
A. until "just" before clipping (groove tubes method)
B. until "just" after clipping (hot bias method)
The hotter the bias is set, the better the amp sounds, and the shorter the tubes last.
The colder the bias is set, the worse the amp sounds, and the longer the tubes last.

And so, you can see why groove tubes wants the bias set on the cold side. So that crossover distortion appears just before clipping.
This runs cooler, and makes the tubes last longer.

And, you can see why guitar player wants the bias set hotter, so that amp sounds better. And so crossover distortion appears just after clipping.
Tubes do not last as long, but sound much better.

So its a compromise. Hotter sounds better, colder lasts longer.
This can be decided by the tech who is setting the bias, according to the application.

[DrGonz78]

Thanks for the advice there SGM that helps with my thinking of what I need to accomplish setting the bias. Also, I was wonder about where to place probes for the O-Scope...? Right now still waiting for the right buy on a scope and signal generator, but definitely going to get one soon. I always read about recommended bias settings on amps where they connect two DMM to measure some form of "mv" voltage to match the paired tube outputs. Lets say I had no scope or signal generator... Would there be another way to set the bias for this amp?

[frus]

Well you turn up the heat until the tubes start to turn a little red...
And so, you can see why groove tubes wants the bias set on the cold side. So that crossover distortion appears just before clipping.
This runs cooler, and makes the tubes last longer.

why would they want to make tubes last longer? they want to sell more tubes right?

[mac dillard]

Might help if you read this:

Biasing

Quick way would be to just set the bias voltage at the -55 vdc called for on the schematic.

[soundguruman]

Thanks for the advice there SGM that helps with my thinking of what I need to accomplish setting the bias. Also, I was wonder about where to place probes for the O-Scope...? Right now still waiting for the right buy on a scope and signal generator, but definitely going to get one soon. I always read about recommended bias settings on amps where they connect two DMM to measure some form of "mv" voltage to match the paired tube outputs. Lets say I had no scope or signal generator... Would there be another way to set the bias for this amp?

I already told you...
You want it hot but not so hot the tubes turn red...
But not so cold that it sounds like mud.
Somewhere in between.
If you have no test equipment, that's one way to set it...

Why would groove tubes want the bias set cold???
Because the tubes are guaranteed, and they don't want them to blow too soon.
But the groove method will leave crossover distortion, before clipping, and sound muddy.

[frus]

c'mon they won't blow. they'll just wear out faster, but not THAT fast. what is the guarantee period for tubes anyway? a month?

[soundguruman]

On groove tubes, it's 180 days...but I can't see one failing that fast, even if the crossover distortion is slightly "after" clipping.
Sure they will take some abuse, I guess how much abuse they take (without blowing) is how "good" they are...

Buy a $425 quad of Bugle Boy EL34
set the bias so they red plate just a hair, and blast them really loud....
now THAT does sound good.
About 20 hours life span?

(yes this has actually been done)
After highly trained technicians, tube scientists, laboriously burned in and tested four matching output tubes, carefully documenting (in writing, signed and dated) and measuring each perimeter with a gigantic Hewitt Packard spectrum analyzer...Boxed them in their own individual form fitted foam lined coffins, with individual QA stamped specification sheets...stating that all grids and plates were carefully tested and matched within 1%...meticulously tested for harmonic distortion, hum, noise, microphonics...
Then , having the Bugle Boy Logo carefully stenciled upon each one...
We deliberately red plated them, and over drove them, until they screamed and begged for holy mercy, and burned up in an unfogotten blaze of glory.

Man, did that sound killer or what?

[Enzo]

why would they want to make tubes last longer? they want to sell more tubes right?

Who? No, they don;t. Peavey certainly didn't want their amps needing tube replacements every couple months. Groove tube sure doesn't want to get a reputation for their tubes lasting two months while everyone else's tubes last 6 months. Peavey designs tube amps with power tubes purposely biased cool. They rely on the preamp for tone, and they want the power tubes to live long reliable lives. They will tell you as much.

Crossover distortion is a concern for hifi amps. It is not the basis of guitar amp adjustments. Red hot tubes and backing off is not real precise, but is better than crossover. But no need to set it that hot. Yes, there are guys who like the hotter the better, and stories about about how the best some amp ever sounded was when its tubes were all cherry red, until the power tubes failed soon thereafter. Screen grids can melt, you know. But not everyone agrees that hotter is better.

The classic rule of thumb is 70%. I don;t buy it, but it is the most common method among hobby techs. That means you set the idle dissipation at 70% of the tube's rated max. The modern 6L6 has a 30 watt rating, and 70% of that is 21 watts. Dissipation in a tube is the product of the current through the tube and the voltage across it. Your tube cathodes are gounded, so the voltage on the plate is the voltage across the tube. Using a bias probe, you read the current through the tube and multiply times the voltage - that gives watts, which you want at around 20-21 by the 7-% thing.

You don't have to use 70%, 60% is fine, as is any other setting that sounds good to you. But there is also the factory spec. Look at your drawing. The factory spec is -55v. Same spec PV uses on the 6505/5150 series and numerous others. It is not about dissipation, it is about the design that Peavey knows will work just fine and to their liking at -55v.

So what is going on? 70%? 60%? Crossover? Red hot plates? -55v? Guess what, regardless of the breathless worship of bias adjustment on the internet, it simply IS NOT A CRITICAL ADJUSTMENT. If the tubes are not getting red hot and it sounds good to you, then it is OK. If you like it to grind hot, then set it that way. If you never play real loud, then power tube overdrive is not going to enter into it.

A little about screen current. All the current through a tube flows through its cathode, most to the plate, but a small percentage flows to the screen. SO if you only measure cathdoe current - which is what bias probes do - then the reaqding includes the screen current added in. What to do? You could use other more dangerous methodes to measure plate current. Or you could just subtract a couple milliamps from your reading. It will be close enough, remember - not critical. You could even measure the voltage drop across the screen resistort and use Ohm's Law to get the exact screen current. Or... just ignore it. That extra couple milliamps will result in a dissipation figure slightly higher than it really is - a sort of built-in safety margin. So your 43ma will in reality be only 41ma or 40ma.


Me? When I get a Classic in needing adjustment, I set it to -55 as the factory specifies, and darn it all, it sounds like teh factory designed it to.

Gonz, your amp is a Classic - that defines what it is. The Classic was only a 50 watt amp, never 100, regardless of any 100 series in the series literature.

[Jazz P Bass]

I just could not figure out how to reply to the previous post.
(in a non abusive way)
Enzo, you said it all.
Thankyou.

[mac dillard]

+1 on the -55 vdc....Don't forget about the Classic 50's (Son plays a Classic 50/410) little brother the Classic 30.

[Enzo]

Hi mac. remember, Gonz has a Classic, not a Classic 50. That was why I mentioned the name earlier. Classic 30 is a very cool amp though, yes.

[frus]

On groove tubes, it's 180 days...but I can't see one failing that fast, even if the crossover distortion is slightly "after" clipping.
Buy a $425 quad of Bugle Boy EL34
set the bias so they red plate just a hair, and blast them really loud....
now THAT does sound good.
About 20 hours life span?

yes, you're right, that would be too hot
but that brings me to the question, what if you buy groove tubes and do the same, and 20 hours later return to the shop and claim the tubes are shot and you want replacement? how can they check what bias you ran them at?


Enzo, if they recommend cold bias, their tubes could gain (undeserved) reputation of sounding bad, that's maybe as bad as "short lived"?

[52 Bill]

yes, you're right, that would be too hot
but that brings me to the question, what if you buy groove tubes and do the same, and 20 hours later return to the shop and claim the tubes are shot and you want replacement? how can they check what bias you ran them at?

One way would be by the color of the silkscreened GT logo on the glass. It is designed to darken with heat. If the tube comes in with almost black silkscreening you know that the tube has over heated.

In fact that is one of the visual clues when an amp comes in blowing fuses, if one of the outputs has a blackened logo, I check it first thing for loss of bias.

[DrGonz78]

Yeah when I posted the title for this I meant Peavey Classic 100 series amp... Then it read like Classic 50... I was merely meaning to state it was a 50 watt amp... LOL. My P.S. at the end of the original post was to point out that my title totally sucked! Sorry guys.

Thanks so much for all the tips here! When we are talking about "Bias Probe" is that a DMM or should I have some test equipment more suited just for this test. I think though we are really looking the voltage of 55vdc and screen current is something as another avenue of exploration? So I guess I need to know if I use the DMM... which I am pretty sure that is what I use... Or I need a Bias Probe...?

I will no doubt be asking where to put my probes. So we are measuring a probe that has a 1ohm resistor between the cathode to ground. That will basically use ohm's law to help determine what the anode/plate current is, if I understand this correctly. Just not sure exactly how to measure for this 55vdc, but I have a few books that show some about biasing. So I might figure something out and then ask to make sure later. Example, like which pin is what and yadi yadi yadi. Thanks for all the patience to help a guy learn this stuff.

Sorry if I ask a lot of questions, but just going to do all the research now. Later I will return to this page when I go to perform the bias procedure when the rest of the amp is ready. Well I leave it here for now but will definitely be back to ask some questions as I go. Thanks again to all here helping out!