Chuck H's Zener bias mod

[eschertron]

Looking at the 6V6 data sheet I see that max grid-to-ground (grid-to-bias-reference?) resistance is 500k for cathode bias, but only 100k for fixed. If I'm limiting bias droop with a zener in parallel with the cathode RC network to effect "faux" fixed bias, should I observe the fixed-bias value? I guess the question I should be asking is what's the engineering reason for the different values?

[Chuck H]

Because cathode bias is self adjusting the grid to bias reference CAN become less critical as the bias cools with stress on the tube analogous to current. Fixed bias cannot do that so the safe zone drops to the lowest common denominator (to put it in laymans terms, which is all I am capable of ) Since the relative fixed bias level on "my" designs is at a moderate, say 75 to 80%ish level, and the cathode bias is near class A, I would say that the fixed bias grid load is the right call because THIS is the range where those denominators begin to matter. The lower grid load will almost certainly change the shift in bias level so there may be some experimenting necessary to find the cathode voltage rise/zener value/ideal fixed bias current circuit. It probably won't actually be that hard, but you will need to have components +/- on hand to make adjustments. Base your bias calculations for the fixed bias voltage (the zener voltage) on the plate to cathode voltage rather than plate to 0V.

EDIT: And... It's not really MY circuit. I saw it first on the nyquist plot site and it's actually been used since early in Zener diode history sometime in the '50's

EDIT2: Screw 100k. That's pathetically low for the dynamics needed in a guitar amp. Go with 220k. It'll be fine! Just consider all those Deluxe Reverbs and their clones running at 420V+ with JJ 6v6 tubes

[Enzo]

Off the top of my head, I might think in fixed bias they are expecting the grid resistor to be connected to the bias supply instead of ground. In cathode bias, the grid resistor goes to ground.

I didn't read the thread you cite about a zener. It caps the cathode voltage, but cannot hold the voltage up, is that correct?

[eschertron]

Off the top of my head, I might think in fixed bias they are expecting the grid resistor to be connected to the bias supply instead of ground.

And that's one reason I wonder if the grid-to-bias-reference is treated differently than ground. Is it possible that the low current capacity of the fixed bias source is taken into account with the leak resistance choice?

I didn't read the thread you cite about a zener. It caps the cathode voltage, but cannot hold the voltage up, is that correct?

At idle, the zener does nothing. As dissipation increases, the voltage dropped across the cathode R increases to where the zener breaks over. This is the 'cap', preventing the cathode R from doing its power degeneration 'thing'. So does the tube need a smaller grid leak value to maintain tube safety at this point?

[Chuck H]

220k

[eschertron]

I understand the basis for the grid-leak value is to prevent bias runaway if electrons intercepted by the grid accumulate enough to change its potential. The electrons must drain (or 'leak') off the grid to whatever the design reference potential is. I'm still confused as to why there would be two different values. So in my befuddled brain:
1. in fixed bias a smaller value is required because when grid current starts to flow, there's a voltage gradient across the grid leaks that must be managed, or
2. the bias circuit itself may start to drift, or
3. alternately in cathode bias the larger value is allowed because when the voltage across the cathode R increases due to increased tube dissipation the voltage across the leaks due to grid current helps 'keep up' with the voltage drift of the cathode.
4. OR my poor brain is really over-thinking this, and the folks who wrote RDH were playing it safe.

[Malcolm Irving]

It’s useful to think about what might happen if the grid was left floating (i.e. infinitely large grid-leak resistance). In a tube with a good vacuum, some electrons leaving the cathode would ‘land on’ the grid making it more negative and tending to bias the tube off – no problem.
However, for a more ‘gassy’ tube, the electron stream will hit gas molecules creating some positively charged ions. Some of these landing on a floating grid would make it more positive, biasing the tube ‘on’ which creates more electron flow – more ionised gas molecules – more positive grid --- thermal runaway and tube destruction.
The maximum allowed values for the grid-leak resistor are to prevent the grid from floating, to drain away grid current and prevent an unwanted bias voltage building up.
In a cathode-biased circuit, any unwanted increase in cathode current is self-correcting as higher voltage is developed across the cathode resistor tending to bias the tube off. This means that a larger value grid resistor can be tolerated.

[eschertron]

Electron build-up on the grid makes the bias colder. Thanks for correcting my misconception. I was sleepy last night, should have been able to think that straight.
But that means minimum leak resistance is not to protect the tube, but to keep the gain from degenerating?

[Malcolm Irving]

There's a good explanation, in the section on Grid Current, here:

Tubes 201 - How Vacuum Tubes Really Work

[frus]

Also more in depth here, pages 58-61

http://www.tubebooks.org/Books/RDH4.pdf

[pdf64]

Electron build-up on the grid makes the bias colder. Thanks for correcting my misconception. I was sleepy last night, should have been able to think that straight.
But that means minimum leak resistance is not to protect the tube, but to keep the gain from degenerating?

I think that a reasonable grid leak resistance mainly should be seen to help stabilise the operating point in order to accommodate grid current of either polarity.
But especially with power tubes, the worst case scenario is that more positive ions get emitted and land on the grid the hotter the tube gets.
Hence plate current can spiral into self destruction if the grid leak resistance is excessive, especially if the applied bias voltage is fixed.

[Chuck H]

220k

[eschertron]

I think that a reasonable grid leak resistance mainly should be seen to help stabilise the operating point in order to accommodate grid current of either polarity.
But especially with power tubes, the worst case scenario is that more positive ions get emitted and land on the grid the hotter the tube gets.
Hence plate current can spiral into self destruction if the grid leak resistance is excessive, especially if the applied bias voltage is fixed.

From my brief look at the references provided, it appears thermal runaway is an issue with gassy tubes (by positively-charged gas ions hitting the grid), while electron collection is more likely in hard vacuum. Good info, all of it, even though RDH4 requires the depth of engineering background I don't possess.

220k

Yes, Yes, I think I agree. I had 220k leaks plus 100k stoppers, I've lowered the stoppers to 10k each, and have yet to test it for blocking artifacts. So 230k grid-to-ground is probably as high (and maybe as low?) as I want to go. Maybe.

[Gingertube]

THis may be helpful:
http://music-electronics-forum.com/t32978/
Cheers,
Ian

[Chuck H]

Read the essay Most of it applies "best" for something like a table radio or RF receiver/transmitter where you're expecting to get five years of consistent performance out of the tubes. Of course understanding the information is relevant to guitar amps too! But I'll be the first to mention that WRT guitar amps it can be cumbersome to adhere to the minimum grid bias resistor value and still achieve the feel and tonal characteristics that we're after. That's why I vote for 220k.

1) Used reliably in a great many amps that operate with JJ6V6s tubes at relatively high voltage without issue. .

2) These amps simply wouldn't have the same dynamics in feel and gain with a lower value. I've tried reducing this value and the detriment was obvious Beating the tar out of the tubes, such as guitar amps do, the tubes don't often stick around long enough to become gassy such that they will run away and die due to the grid bias resistor value.

3) The tonal imperative is calling. Wander from the path at your own risk. I think the track record for this value in both tone (for guitar amps) and "acceptable" reliability is proven.?. Idealizing is great. But try to reinvent the wheel and your car might roll bumpy.