Range of bias voltages for power tubes in fixed bias?

[R.G.]

I'm trying to fill in a bit few of the areas of my ignorance, for my own nefarious purposes.

What is the range of grid bias voltages you see for 6L6, EL34, 6V6, etc. in Class AB1 fixed bias amps?

The data sheets talk in terms of -25 to -40V for various tubes, but I suspect that the working techs here know better.

I know that measuring grid bias voltage is not a good way to bias, and regularly advise people not to do it that way. This info is for other nefarious purposes.

[bob p]

for my own nefarious purposes.

It's no coincidence that I have been having some nefarious thoughts of my own.

As you know, there's going to be a range of desired bias currents, depending upon factors such as B+ on the tubes. If I were to rely on an automated circuit to perform bias sensing and adjustment duties, I'd want to be able to program it with the desired target bias current for the application. I'm thinking about DIP switches.

I think we had discussed that I have a 6L6 amp on the bench now that has a B+ of 530V. That exceeds the limits you'll find on the data sheets and the bias suggestions that you'll find in popular tables like Jim Jones' table:

http://www.webervst.com/tubes1/calcbias.htm

So I'm going to tag along on this thread to see what kind of feedback you get.

[soundmasterg]

I'm trying to fill in a bit few of the areas of my ignorance, for my own nefarious purposes.

What is the range of grid bias voltages you see for 6L6, EL34, 6V6, etc. in Class AB1 fixed bias amps?

The data sheets talk in terms of -25 to -40V for various tubes, but I suspect that the working techs here know better.

I know that measuring grid bias voltage is not a good way to bias, and regularly advise people not to do it that way. This info is for other nefarious purposes.

I honestly don't even go by the negative voltage. I measure the cathode current and whatever negative voltage is required to get the tube to idle in a safe range that sounds good, that is what I choose. More important to me is the percent of dissipation. I may have written it down the last time I worked on my amp that I made, but that uses 7868's so you may not care about what that is....haha. My amp is also using power scaling which changes things somewhat. Most of the commercial amps that I have fixed in the past get negative bias voltage numbers that are way different than what the schematic shows and in fact, I recently fixed a Sunn for a friend that was cooking the tubes with the bias set to the schematic negative voltage level. It had to be set for more negative voltage, but the actual voltage amount wasn't as important to me as the dissipation so I didn't bother to take note of it.

Greg

[Steve Conner]

-40 isn't enough for 6L6 type tubes. Something like -60 would be better.

I came across one tube type for which even -60 wasn't enough. The S11E12, advertised as a "6550" by some optimistic EBay sellers.

[Steve Conner]

-40 isn't enough for 6L6 type tubes. Something like -60 would be better.

I came across one tube type for which even -60 wasn't enough. The S11E12, advertised as a "6550" by some optimistic EBay sellers.

[MWJB]

6V6 fixed bias amps tend to be around 8-10% of the plate voltage, measured at the junction of the 220K grid loads. 400-440vdc range on the plates.

6L6/EL34 amps might be 11-12%, same procedure. Maybe 13% for upper end of B+ range (520-530vdc) with hot tubes, or at coldest bias. Maybe 10% for low plate voltages (~400vdc). Certain, cooler running, "big" tubes like Sovtek 6550WE & EH KT90 will fit the general 6L6/EL34 profile also (KT88/KT120 & some NOS 6550 will need more negative voltage).

EL84 amps 3-5%. Lower end for plate voltages ~300vdc, higher for 400vdc+.

These are ball-park targets, subject to fine tuning by mA.

[R.G.]

Everyone, thanks for responding. As I mentioned, I know that biasing by the grid voltage is not the thing to do, and that biasing to a current is the correct process, even if in the end you're trying to get a certain tube dissipation.

My "nefarious purposes" involve designing an adjustable bias circuit, and I need to know both how much is enough, and what a rough "typical" value is. My experience is limited to small(ish) numbers of 6L6s and EL84s and very limited numbers of EL34s. Even my limited experience with these has taught me that the nominal G1 voltage in the datasheets is not a good guide for the tubes we can actually get today.

I very much respect the real-world experience of people who make a living by (among other things) re-biasing amps, so I would like to have any shared knowledge you will share with me about the real-world needs of power tubes that you see.

So if you can, help me with the grid voltages. Otherwise, I'll guess, and that always gets me into a heap o' trouble.

[bob p]

On a related nefarious note:

I've got a 6x6L6 amp here that was spec'd 30+ years ago for B+ = 500V with -61V of non-adjustable bias (balance only). On today's voltages B+ = 530, the non-adjustable bias comes in at -61V and the amp sounds quite cold. Unfortunately it's got 3 pairs of 6L6 that are driven in parallel on a grid bus arrangement, which doesn't simplify the bias process. Before doing anything else I'm going to have to un-bus the output section so that I can get a handle on directly biasing individual tubes, using current sensing resistors on the cathodes. Part of the logistical problem with this project is that at these B+ levels there isn't good data on current production tubes. And the typical bias current tables like the ones I mentioned above don't even list B+ that high for any 6L6.

I'm thinking that any sort of bias-assist circuit that would provide high/low indications for making adjustments would need to allow the user to specify the desired range of acceptable currents for the specific tube application, because there are so many different operating points that you'd need to be able to specify to cover all the bases. That's why I mentioned DIP switch configuration earlier.

[Gingertube]

Generally accepted as 10% of the SCREEN voltage but obviously influenced by tube gm. When designing your adjustment range allow for up to a a maximum 15% of screen volts and you will be fine.

Bob P - how are you at solid state - here is a bias servo, use one per tube with a common reference and a common -ve supply. Just set the reference voltage to 10mV per mA desired idle current, e.g. set the reference to 350mV and all the tubes will idle at 35mA.
This is not my design, its one I borrowed from the HIFI crowd. It is actually a nice scheme, for Class AB bias servo you generally have to clamp the current sense at 2x idle to take account of the natural clamp as the tube goes into cut off (to get a consistent measure of average current). This scheme does a similar thing but clamps in a much narrower window around the desired idle current point.

The LED and the bottom transistor form a current source load for the actual bias amplifier section which adds great noise immunity from any residual "crap" on the bias supply. It has a +/- 1.5 to 2 mA sink/source capability so is tolerant of tubes with grid current (not that you are likely to be designing say a 300B git. amp but this servo would be fine for that too).

Cheers,
Ian

[Chuck H]

Not knowing exactly what R.G.s end circuit is supposed to do (I'm guessing something like a circuit you can plug ANY octal tube into and it automatically adjusts itself?) I thought perhaps a current sense controled variable voltage type bias circuit (like a power scaling circuit) might work. But there's just too many other variables in the tube types that should be accounted for. If you stay with just big bottle types it should be possible. Though you couldn't get top perfomance from 6550's and kt88's without biasing too hot for el34's. Nominal is the key. But nominal feels too much like mediocrity.

For octals used in guitar amps (other than the 7591) and a moderate blanket voltage something like 400Vp I'd think a bias range of -30V to -50V should do it. With lower voltages you may need a low end of -25V and higher voltages might need a high end of -70V (sorry to any who want to nitpick how I used the terms high and low WRT negative voltage). But I'm guessing there will be a nominal plate voltage if adjusting the bias for different tubes is the criteria. If just big bottles were employed then a B+ of 470Vp and a bias range of -40 to -55 would probably be enough. If it needs to be a concise range. Otherwise, where's the harm in having more range than you would need? The bias supply itself shouldn't care a tic if it's adjustable from -20V to -70V.

It would help to know a little more about the project. But I understand if that's not possible yet.

[Chuck H]

Yeah, something like that.
We were posting at the same time...
Funny, when you wrote "servo" I expected to see a motor driven potentiometer. Thanks for the neat circuit.

[Gingertube]

Chuck,
Glad you caught up with my edit to add the bias "servo".
I always value your insights, opinion, experience. A necessary balance to my to purely technical approach and fairly average musician skills,more times than not.
Cheers,
Ian

[R.G.]

Not knowing exactly what R.G.s end circuit is supposed to do (I'm guessing something like a circuit you can plug ANY octal tube into and it automatically adjusts itself?)

Nothing nearly so fancy. It's a simple individual bias-to-a-current setup with one pot per tube. I'm just trying to make it easily adjustable and customizable. Optimized for users, not for fancy stuff.

While customizable ought to mean that it can be set to whatever, you have to know the limits on 'whatever' to be able to make sense of things.

I've done the auto-bias thing. It's more sophisticated than is really needed, and adds unnecessary complexity. This is different, and simpler. It may also wind up being a pile of... er, well, unpleasant stuff ... so the unmentioned nature is more me not wanting to look quite so silly if I have to say in my best Emily Litella voice "Never mind."

[Gingertube]

R.G.
Making it easy for customers is as much about physical layout as what you actually wire.
This is one I did fro a friend Neale. Shots during construction
2mm Banana Plug Sockets which take you multimeter probes directly. Black 0V, Push and Pull side Cathodes, Current Sense 10 Ohm resistors wire right on sockets , Put the multimeter black in black hole red in red hole and adjust adjacent pot, when ready for final tweak put probes red to red and adjust one control for a reading of zero. 20 turn Bourns pots.

Another hint, Those mains switches are the cathode/fixed bias and triode/pentode switches. Don't throw those keyed to the slot on the bush ON/Off plates they come with away, stick them on the inside, at assembly time, wipe some 5 minute epoxy on the wings and they become an anti-rotation device for the switches.

Cheers,
Ian

[R.G.]

That's nice, tidy work, G.T.

I hope he appreciated the effort.