Tube matching and biasing

[Randall]

I have a set of JJ 6V6S in my current 5E3 build that are dissipating 14.3 watts with a cathode voltage of 22v and plate to grid voltage of 349v. 268 ohm cathode resistor. The JJs went microphonic after about an hour of dubbing around with them, so I sent them back to AES for another set. I had planned on trying the zener diode trick to drop the B+ down 10 volts or so to get things within maximum dissipation of 14 watts for a JJ. Before doing this, I tried a couple of other used sets and got 12.8 watts and 13.5 watts respectively. So this got me to thinking, is it possible to request a set that will draw less idle current vs more, from the matching numbers on the box? Something like, "this amp runs hot, so please give me some cooler tubes"? Does it work this way, and if so, how?

Thanks.

[soundguruman]

Yes there are low and high current...
you are going to make a special request to New Sensor, or Antique Electronics, etc...
The output tubes should not "go microphonic." Something else, probably.
A lot of amps have a "rating" for the tube so the bias will be right. The bias is non adjustable.
But that's kind of goofy, it's better to adjust it rather than trying to "match" it.

But then, there's always gotta be a joker, like a Mesa 50 caliber.
The bias supply is also the cathode bias for the phase inverter.
And you can only really change it a small amount, without goofing up the PI.
And so in that case, you really are stuck for a certain output tube, and a certain PI tube...that matches the preset circuit.

[Randall]

I know they shouldn't go microphonic guru, but they did, and one worse than the other. A ringing on certain notes that is stopped just by touching the glass. Noise when tapping lightly with a fingernail or gently wiggling the bad one. Swapped sides, problem followed. No noise at all from the used EHs in there now. I've had JJs do that before.

[kleuck]

The JJ can stand a lot of power, more than actual 6V6 (they have a 6L6 plate) so don't bother, you just had a bad tube

[Jazz P Bass]

And did not Groove Tubes capitalize on the fact that all tubes have a slightly different bias point.
That is what there numbering system is all about.

[soundguruman]

Unfortunately, it's not a standard number system.
Everybody has a different rating number for the same tube.
A Groove tube #2 is not the same as somebody Else's #2...they are tested in different non standardized circuits.
So, until you measure it yourself, there is no cross-compatibility.
And like, for example, if a Mesa Boogie needs a "certain" rated tube, they NEVER tell you or label the amp "which" rating is required.
It's utterly scatterbrained.
What you need is a "standard" rating system, that everybody uses.
But this would allow you to buy tubes from "anywhere." Which is exactly what the manufacturer DOES NOT want.
The whole idea is to FORCE you to buy a certain tube, from a certain source. The standard rating would end this extortion.

[R.G.]

There is a standard system - tube current. But that's too technical for many people.

The idea of numbering systems to bin up tubes that have similar current flow at a given bias voltage is probably an attempt to simplify biasing for the masses - and, as you note, to make a buck doing it. Like fast food simplifies cooking.

It's funny. The sane thing to do, now that we have over half a century of electronics advancement under our belt is to design the amps to make biasing output tubes easily, and independent per output tube.

I may see if the boss wants to aftermarket the Workhorse biasing system as an add on for amps. I'm sure that would get the tube sellers to condemn it.

[soundguruman]

Yes, current, under what circuit?
Everybody is testing with a different circuit / bias / voltage. That's why all the different numbers on the same tube, depending on WHAT circumstances it was tested in.
Groove is rating tubes as: #1 #2 #3 etc...
New sensor is rating tubes: 34/5.5 or 24/2.8 etc...
Wheres the cross compatibility? There is none folks. It's deliberate.

And furthermore, an amp that has "locked in" bias, has no label to indicate "which" tube belongs in it. (thanks, Mesa)

NO WONDER so many techs modify amps for adjustable bias. It's a no-brainer.
And how unfortunate you are, to have an amp with "locked in" bias.
When your amp blows a tube one hour before a show, relief is just 10 days away!

[R.G.]

Well, maybe.

There's a quotation attributed to Napoleon Bonaparte that says something like "never ascribe to malice that which can be explained by incompetence". It's been said by many.

The tube sellers do indeed want to make it easy and quick for people to buy their tubes and holding out the hope of not then paying for someone to bias them is good salesmanship, if tainted with all the other issues of good salesmanship. They would say they're doing it for their customers to be better served.

What got me was Groove Tubes blathering about the distortion characteristics being tied up in their binning system too.

[bob p]

Independent bias? RG if you were marketing a 6-way setup I'd probably buy one right now.

I just finished breathing life back into a DOA Super Twin Reverb. It came to me with 5 different brands of 6L6 in it's 6-tube output section! Yikes! Matching up what it's got just isn't going to happen. Short of buying a new "matched" sextet, the best that can be done is to run through a pile of tubes and find 6 that would compliment one another. The time and effort involved in doing this has made me think of building a 6-way independent bias setup. The biggest problem? Lack of panel space.

[soundguruman]

Sextet of matched JJ 6L6 is
$100 + $5 drop ship fee + shipping (whatever that is)
+ CA sales tax if you live in CA...
and I will get them for you if you want

[R.G.]

Independent bias? RG if you were marketing a 6-way setup I'd probably buy one right now.

As my IP negotiating team partner used to say, there's always a number.

I just finished breathing life back into a DOA Super Twin Reverb. ... The time and effort involved in doing this has made me think of building a 6-way independent bias setup. The biggest problem? Lack of panel space.

Hmmm. Interesting problem. For just the independent biases, doesn't this reduce down to finding a way to mount six pots, each of which can withstand about 75V minimum and vanishing small current?

How about a small add-on panel on the underside of the chassis at the back that mounts six mini-pots and runs wires back to the tubes? I'm not hearing that you need independent red-light/green-light indicators.

But thanks - your post clarified some things in my mind and made me come up with a better way to do the independent quick-bias with indicators. That kick in the pants may get the boss to go along.

And @SGM: Thanks for that bit of market research. That's pretty much going to set the retail price, I think.

[soundguruman]

As my IP negotiating team partner used to say, there's always a number.


Hmmm. Interesting problem. For just the independent biases, doesn't this reduce down to finding a way to mount six pots, each of which can withstand about 75V minimum and vanishing small current?

How about a small add-on panel on the underside of the chassis at the back that mounts six mini-pots and runs wires back to the tubes? I'm not hearing that you need independent red-light/green-light indicators.

But thanks - your post clarified some things in my mind and made me come up with a better way to do the independent quick-bias with indicators. That kick in the pants may get the boss to go along.

And @SGM: Thanks for that bit of market research. That's pretty much going to set the retail price, I think.

That price is from CED wholesale supplier. Can't get too much of a break on 6.

[g1]

Amplified parts has that set on their ebay store for $103.50, free shipping
6L6GC Vacuum Tubes JJ Brand Matched Sextet Six | eBay

[bob p]

As my IP negotiating team partner used to say, there's always a number.

Yes, there is. But I was making somewhat of a rhetorical comment for the sake of giving you feedback, because I'll probably work out my own solution to the problem by the time you get around to bringing a kit to the market. I do think that a kit like the one you proposed would have some value. If one were available today, at the right price, that would fit into this project, then I'd be all over it. I think that other people might be interested in an off-the-shelf solution. For them to sell well, it would have to be compact enough to fit unobtrusively into the "average" amp.

One potential problem in marketing such a kit is that your target market is comprised of people who understand the biasing problem well enough to be willing to seek-out and pay for a canned solution. There's the risk that people who understand the problem well enough to seek out a solution might also be inclined to roll their own solution. There's a risk that you'll be aiming at sort of a self-cannibalizing market, so to sell the solution it would have to be cheap. Unfortunately, cheap tends to put a limit on your ROI.

I'm not hearing that you need independent red-light/green-light indicators.

I don't NEED to have idiot lights. I'm more than willing to do it the old-fashioned way. But that isn't to say that if I had the quick-adjust indicators that I wouldn't appreciate having them, or that I wouldn't utilize them. I do like the idea of using smart circuits and solid-state devices to make our lives easier.

I think that the LED-indicator is an excellent idea. But sometimes it difficult selling excellent ideas to people who don't appreciate what you're bringing to the table. There are a lot of players out there who don't worry about bias the way people here do. Maybe they won't appreciate what you're doing because it goes over their heads. It's hard to sell excellent ideas to a market that is trained to settle for good enough.

It sounds like the target market for this kind of device would be pretty narrow: people who know enough to understand the problem and want a solution, but would rather buy off the shelf rather than DIY.

Hmmm. Interesting problem. For just the independent biases, doesn't this reduce down to finding a way to mount six pots, each of which can withstand about 75V minimum and vanishing small current?

How about a small add-on panel on the underside of the chassis at the back that mounts six mini-pots and runs wires back to the tubes?

In this application real-estate is a real problem. No space on the back panel, no space inside of the chassis, no space on the underside of the chassis at the back:



I don't really like the fixed bias circuit as it exists in this amp. It has an adjustment for push-pull balance, but no adjustment for magnitude. That will have to change. Since there's no real space on the back panel or the rear underside, I'm thinking that the best solution in this case is to just build a replacement board for the bias adjustment and mount the mini-pots there. I think this is just going to be a case where I'll have to pull out the chassis when I want to do maintenance.

As far as market research goes, the cost of around $80 for a new sextet of "matched" JJ's probably will set the upper limit on what you could charge for an independent control solution that allows you to avoid buying a new set of "matched" tubes. But as we know, the problem with buying "matched" tubes is that unless you buy tubes that are matched after the burn-in drift has already occurred, then you're really not getting a set of tubes that are going to stay matched.

I think that in an application like this one, where someone is trying to match 6 different tubes so that they can use a single bias control, they'd have better luck if they were trying to herd cats. Independent bias definitely has appeal in this application if you're willing to jump over the hurdles to get there. I don't know if I represent the demographic for your target market or not.