Bias feed resistors

[hearforever]

Can someone give me a quick explanation of bias feed resistors or point me to a relevant article online? I'm specifically interested in their application to a simple single ended amplifier

I don't really understand how they work and what effect changing the value of the bias feed resistor has on the output tube.

I'm including some pictures to help understand....
In the Masco and Fender circuits below, doesn't the pot preceding the output tube (6L6 and 6V6 respectively) act as the bias feed resistor because part of the voltage divider is the bias feed resistor? In that case, doesn't the 1M volume pot of the champ circuit exceed the Max value listed on the chart I've attached for the 6V6?

On the resistance coupled amplifier chart I've included, would Rs correspond to the bias feed resistor in a single stage pentode amplifier?

http://www.hereforever.org/ma8n.jpg
http://www.ampwares.com/ffg/schem/champ_5c1_schem.gif
http://www.mif.pg.gda.pl/homepages/f.../001/6/6V6.gif

[MWJB]

Smaller values attenuate signal to the power tube grid, larger values give more drive. both schematics you reference are very old examples, long before designers were achieving any relative headroom. With the pots fully up the 6V6 would have got quite a hammering, no cleans.

In fixed bias amps 220K is the most common value, in cathode biased amps 220K-470K are the common range.

You won't find many (any?) production amps that meet the criteria on that 6V6 data sheet, many sucessful designs buck the trends with regard to data sheets.

[Steve Conner]

Larger values of bias feed resistor also make the power tubes more vulnerable to thermal runaway as they get old. The fundamental purpose of the bias feed resistor is to drain away any leakage current to the grid and make it sit at the DC voltage you want. The leakage current is caused by contaminants on the grid and gas in the tube, and increases with age and tube temperature.

The fact that it absorbs signal by loading down the previous stage is just an unfortunate side effect, which amp designers tried to mitigate by making the resistor bigger than they really should have.

There is another way that bias feed resistors affect the tone, can anyone guess what it is?

[martin manning]

There is another way that bias feed resistors affect the tone, can anyone guess what it is?

Umm... together with the driver stage coupling cap it forms a high-pass filter?

MPM

[jrfrond]

Good points. One thing I'd like to add is that I routinely replace them in aging amps. It was a lesson I learned the hard way. Several years ago, I was working on a 6G6-B that looked good during static and playing tests, but when pushed into hard distortion, one of the power tubes literally went into meltdown. I fried two more tubes (that's why I use cheapies for test now too, those were Svetlana 6L6's), before I figured out the problem: one of the 220K bias feed resistors was opening under load. The LAST thing you want is for bias to disappear when the tubes need it the most, hence the meltdown. First time I'd seen that in my 30+ years repairing amps! Anyway, now that I've seen it once, all vintage amps get the swap with new carbon comps, period.

Just an FYI.....

[MattT]

'Bias Feed' resistor might not be the best name IMO. In a cathode-biased amp they aren't functioning as 'bias feed' resistors or anything 'bias'...they're just grid load resistors. And they react similar to grid load resistors for your preamp tubes...lower values attenuate gain and bass (because of the HPF formed with the blocking cap mentioned above).

[R.G.]

Good points. One thing I'd like to add is that I routinely replace them in aging amps. ...one of the 220K bias feed resistors was opening under load. The LAST thing you want is for bias to disappear when the tubes need it the most, hence the meltdown. First time I'd seen that in my 30+ years repairing amps! Anyway, now that I've seen it once, all vintage amps get the swap with new carbon comps, period.

Of course, if it's the only time you've seen it in 30+ years repairing amps, that puts it down in the "once in a lifetime" category. I once saw a Fender amp that had no solder on the output tube sockets. It worked that way for 20 years, then became intermittent. Not many of those around either

It's likely that the resistor was internally cracked at the junction of the metal end cap and the internal element and as the resistor got hot from the hot air around it in the amp, the thermal expansion opened it. I've seen a few caps and solder joints with that problem, and hear of a few resistors that did the same. A cracked component has that as one of its failure modes.

But you're right, it's cheap insurance.

Having seen many loss of bias problems, I would replace the bias filter cap(s) in every amp first. Then there's those series resistors in the bias supply; intermittent opens do the same to the amp there, as does an intermittent open in the bias rectifier diode.

But I guess replacing all of those is cheap too.

[R.G.]

You're right Matt, the guys in the Golden Age called them grid leak resistors, as Steve points out.

[hasserl]

'Bias Feed' resistor might not be the best name IMO. In a cathode-biased amp they aren't functioning as 'bias feed' resistors or anything 'bias'...they're just grid load resistors. And they react similar to grid load resistors for your preamp tubes...lower values attenuate gain and bass (because of the HPF formed with the blocking cap mentioned above).

You're right Matt, the guys in the Golden Age called them grid leak resistors, as Steve points out.

In a cathode biased amp isn't the path to ground via these resistors how the grid remains at ground potential, providing the bias voltage? If the resistor opens under load as JR explained in his post, won't the amp loose it's bias voltage?

[Steve Conner]

Yes. It doesn't matter whether the amp is fixed or cathode biased, the resistors are still setting the DC level of the grid, and the tube will still burn if they go open.

The tube only cares about the voltage difference between grid and cathode, it doesn't care if the grid is at -40v and the cathode at 0v in a fixed-bias amp, or the grid at 0v and the cathode at +40 in a cathode bias amp. It's still 40 volts of bias either way.

The high-pass filter thing is cool, but there's still one more way.

[jrfrond]

Of course, if it's the only time you've seen it in 30+ years repairing amps, that puts it down in the "once in a lifetime" category. I once saw a Fender amp that had no solder on the output tube sockets. It worked that way for 20 years, then became intermittent. Not many of those around either

It's likely that the resistor was internally cracked at the junction of the metal end cap and the internal element and as the resistor got hot from the hot air around it in the amp, the thermal expansion opened it. I've seen a few caps and solder joints with that problem, and hear of a few resistors that did the same. A cracked component has that as one of its failure modes.

But you're right, it's cheap insurance.

Having seen many loss of bias problems, I would replace the bias filter cap(s) in every amp first. Then there's those series resistors in the bias supply; intermittent opens do the same to the amp there, as does an intermittent open in the bias rectifier diode.

But I guess replacing all of those is cheap too.

Just for the record, it wasn't a thermal issue. It happened ONLY when you hit the amp hard. If the amp was played at low volume only, it would have never happened. We always do a balls-to-the-wall test, and this is just one more reason why.

You are SO right about the bias diode and cap. If it's a vintage Fender, the six rectifier diodes get yanked as well, often replaced with just two 6A10's.

[BackwardsBoB]

The high-pass filter thing is cool, but there's still one more way.

If the tube is driven to saturation, grid current increases substantially, charging the coupling cap. The value of the grid leak resistor working with the value of the coupling cap determines the recovery time. Right?

The other thing I note is that if you cascade two inverting gain stages and drive them to clipping, with a large coupling cap and grid leak resistor, your output approaches a constant amplitude square wave during clipping. But since the clipping of the first stage is one-sided, a low frequency signal has been added which matches the ADSR dynamics (i.e. the envelope) of the input. A smaller cap and grid-leak resistor will block some of this, helping to preserve input dynamics.

[hearforever]

You won't find many (any?) production amps that meet the criteria on that 6V6 data sheet, many sucessful designs buck the trends with regard to data sheets.

I'm working on rewiring a single-ended 6F6 amp with a single stage 6J7 pentode...right now it has a 470k resistor in this position, going from the grid to ground on the 6F6. If I switched this to a 1M resistor or 1M volume pot would I then get more gain? And I shouldn't worry that 1M is twice the max value on the datasheet?

[R.G.]

If I switched this to a 1M resistor or 1M volume pot would I then get more gain? And I shouldn't worry that 1M is twice the max value on the datasheet?

This is like that song "The Gambler"; you gotta know when to hold em', know when to fold 'em.

If the grid is being driven by a follower, cathode or otherwise, the impedance of the follower can drive the grid no matter what the grid leak is, within practical limits. 10 ohms won't work, nor will 1K. But probably 10K on up will. If you drive it from the plate of a 12AX7, the rp of that connection is about 60K in parallel with whatever plate resistor is in use, so you're looking at a 30K-40K driving impedance for typical 100-200K plate resistors. A 100K grid resistor eats a significant amount of the signal by pure voltage divider in that case.
So --> The driving impedance matters. <-- and that is what makes the change in gain. It's actually not a change in gain, but a change in the signal that the tube gets to see.

And yes, you might want to worry about it being more than twice the value of the recommended grid leak. The reason they specify a grid leak resistor maximum in the data sheet is that each tube has a certain amount of... well, grid leakage... that being electrons that residual positive ions in the tube grab off the grid by bumping into it. This makes the grid go more positive, increasing plate current.

There is a certain level of gas left in all tubes, as it's not economically possible to make them perfect. EL34s, for instance, were lower vacuum tubes than 6L6s, at least in the Golden Age, so they needed lower value grid leak resistors to keep the grid leakage pulled down. This is important because the hotter a tube gets, the more gas is released from where it may be stuck temporarily to the tube's inner surfaces.

And here's the important chain of events: hot tube -> gas release -> more grid leakage -> less "off" bias -> hotter tube. This is the thermal runaway Steve mentioned. Low values of grid leak resistors protect against thermal runaway. Manufacturers recommend grid leak resistors to protect their average tubes against runaway.

With todays tubes, who knows what residual gas you're getting? MAYBE the maker is following a recipe that will be OK with bigger grid leak resistors. Maybe it is marginal with the recommended maximum.

It's a game called "You bet your tube". The only bad effect of a runaway is that it puts your power transformer and output transformers at risk. If you have protected somewhat along the lines of my recommendations for the Immortal Amplifier, you'll only lose the tube and you get to try again.

(For long time readers, I'm publishing the Immortal Amp stuff in Premier Guitar in a series of columns, starting with July; see "Tech Views"; it's on line and free)

[tubeswell]

(For long time readers, I'm publishing the Immortal Amp stuff in Premier Guitar in a series of columns, starting with July; see "Tech Views"; it's on line and free)

What's the website for that R.G.?