Anybody ever tried zero bias with the screen and control grid tied together?

[Austin]

I was reading on wiki about beam tetrodes and at the bottom of the page it talks about "Class B zero Bias" briefly. I looked it up and I guess it is something that would work with a 6v6 or the like, making it equivalent to a high mu triode. It says class B but I don't see why it couldn't be done with a se amp like a champ. Anybody ever experimented with it?

Beam tetrode - Wikipedia, the free encyclopedia

Here it talks about it a little: 811A's, Dual Grid and Class B triodes

Happy new year!

[loudthud]

I've seen screen grid drive of SE amps. Usually the grid is grounded along with the cathode and the screen grid is driven with some form of cathode follower. KOC has a project for push pull screen drive amp in one of his books. Meant to be a power booster driven by a small amp. It also grounds the cathode and grid(s). Uses a small output transformer in reverse to drive the screens. More of a class B design, IIRC the screens have zero volts with no signal so both tubes are cutoff.

[Jazz P Bass]

Class B, zero bias.
(Man will that piss off the amp bias jockeys. After all zero is zero)
That is a radio thing isn't it?
Check out this link:
RCA Ham Tips

[loudthud]

I don't know about tying the control grid to the screen. Seems like the control grid would burn up long before significant plate current would flow. I'd like to look at the reference sited to see if the author of the Wiki article understood what the authors of the reference were talking about.

[jazbo8]

Search for right-handed triode and you will see some design examples (sometimes G3 is tied together with G1 & G2 as well). Here is an example for the LS-50 (GU-50): Right-handed Triode

Jaz

[Steve Conner]

If you tie all the grids together and drive them, it'll take quite a lot of current because the control grid has to be driven positive. You'll need a follower to drive it.

Large tetrodes like the 4-400 were sometimes used in RF amplifiers with both grids grounded and the cathode driven. Cathode drive needs a lot of power, but most of it gets fed through and contributes to the output, so it was a good match for a 100 watt transmitter that you wanted to boost to 400W, 1kW or whatever.

However, the 4-400 has sturdy grids, somewhat like cages made of coat hanger wire.

[Jazz P Bass]

Won't that (Class B) sound distorted as all get out (in a bad way)?

[Steve Conner]

"Zero bias" means that no grid bias voltage is applied, it doesn't necessarily mean that no plate current flows. With all its grids grounded and 2kV on the plate, the 4-400 passes a few tens of milliamps. Most triodes designed for zero bias use behave the same, so if you made a push-pull audio amp with them, it would be Class AB.

[Jazz P Bass]

Thanks for the clarification.

[jazbo8]

Won't that (Class B) sound distorted as all get out (in a bad way)?

It does not have to be Class B, here is a design that runs in A2, the designer posted some measurements but those picture seem to be lost (or could be just my pc/connection).

https://dl.dropbox.com/u/1326040/GU-...rtosalo%29.jpg

More information at Russian GU-50 Amplifier

Jaz

[soundguruman]

I was reading on wiki about beam tetrodes and at the bottom of the page it talks about "Class B zero Bias" briefly. I looked it up and I guess it is something that would work with a 6v6 or the like, making it equivalent to a high mu triode. It says class B but I don't see why it couldn't be done with a se amp like a champ. Anybody ever experimented with it?

Beam tetrode - Wikipedia, the free encyclopedia

Here it talks about it a little: 811A's, Dual Grid and Class B triodes

Happy new year!

Collins used to do it all the time, in AM radio transmitters, and other products...
You will probably find some cool stuff in old Collins schematics.
I used to work on a 5000 watt one, that had 0 volt bias on the output tubes.
That transmitter had 5500 VDC plate voltage...one night, a rat climbed up inside the transmitter and sat down on the filter capacitor terminals....but that is another messy story. hahahahahahahah!

[Austin]

There might be a couple reasons you would want to do this, one is if it made the pentode into a high mu triode then it would take much less drive signal than normal triode mode to get the same power output, but would still sound like a triode and have the low output impedance.

Another reason might be to have less distortion. Say you use a el34 and tie all the grids together, at zero volts relative to the cathode the tube will be in cutoff I assume. So if you actually had a positive bias then it would be conducting and also drawing grid current all the time which would remove the abrupt transition from no grid current to grid current as with a negative bias.. This guy explains it better than I here: SE outputs

"When the grid is negative it is a high impedance, when the grid goes positive it becomes a low impedance. There is a large abrupt change in grid impedance when the grid transitions into the positive region. If your driver stage is not designed for this it will distort badly."

Funny about that rat....

[loudthud]

Here's a link to a project I saw a few years ago. It uses an EL509/6KG6 with a little cathode bias in an SE circuit but drives the screen. Curves are posted.

Screen Driven, DC Coupled, Single-ended EL509 Amp

[Austin]

Here's a link to a project I saw a few years ago. It uses an EL509/6KG6 with a little cathode bias in an SE circuit but drives the screen. Curves are posted.

Screen Driven, DC Coupled, Single-ended EL509 Amp

I tried dc coupling a 12ax7 to a el84 screen and it worked ok but I don't think the 12ax7 had enough current to really sound good. Or maybe a el84 is a bad choice with the screen not sensitive enough. btw, Did you guys catch the 12ax7 class b zero bias claim in that link I posted in the first post?

"The old ARRL handbooks (50s?, 60s?) had a listing in their tube tables for
using a 12AX7 as a class B zero bias output tube. They said it could
produce 7.5W. I never saw a circuit that actually used one this way,
however."

That would be cool for a one tube amp, you wouldn't even need a driver tube if the 12ax7 were used as the power section. 7 watts seems like a stretch though to me.. Still if it would work at all and drive a speaker, the amp could be made as small as a pedal on a board and that would be cool.

[Steve Conner]

There was a hi-fi amp that used about a dozen 12AX7s as output devices. I think it was one of the E.A.R series designed by Tim de Paravicini. In an interview he said that the 12AX7 grids had to be driven quite far positive to get enough output power.

So, I wouldn't be surprised if the grids were positive even at idle, which would make it "zero bias", and imply that he got the idea from the old ARRL handbook.

The amp got some pretty good reviews at the time.

All of these zero bias schemes have the disadvantage that the grid drive circuit needs to source considerable current. It is more like driving the base of a BJT than the grid of a tube. The signal from a guitar would be nowhere near strong enough, so I don't see how a "one tube amp" could be possible. If you want a one tube amp, use something like an ECL86, or 6BM8 or whatever.

In zero bias push-pull, driven by an interstage transformer, the driver circuit is always loaded by one grid or the other, so the distortion issue is not so bad.