Grid Leak Bias

While working on my new amp I played around with gri leak bias abit, and got to wondering about what is going on with the tube in this situation, the effects on the signal and so on. Also is there any reason i have never seen grid leak biasing done on anything other then 6SL7s, or is 6SL7 just the most common tube for this because the 6SL7 data sheet has values for this in the resistance coupled amplifer table?

Thanks,
adam

You could almost explain it with that cool avatar!

As electrons leave the cathode and sail towards the positive plate, some of them "leak" onto the grid wires which are in between the cathode and plate. If you use a big enough resistor from grid to ground, you can keep these electrons "trapped" on the grid. That is why typical grid to ground resistors in a grid leak circuit are usually 5 to 10 megohms. If you can collect enough of these electrons on the grid, they will form a negative charge large enough to bias the tube, which will keep the plate current where you want it.

Measuring bias volts on a grid leaker can be tough without a vacuum tube voltmeter, as the voltmeter's internal resistance can bleed off some of the electrons, giving you a lower than normal reading.

Grid leak bias is sensitive to the surfaces of the tube elements. This is why only a handful of tubes are usually good candidates for grid leak usage. So I guess you could say that it is kind of a touchy way to bias a tube. I believe that as a tube ages, it's grid leak bias may change more than a cathode biased tube. But, grid leak bias tends to produce lower distortion if the plate current is kept down to 1 ma or below.

The bias voltage of a tube used in grid leak mode will generally be between 0.1 volts and 1.1 volts, depending on the tube, and this bias may change as much as 0.4 volts during the life of the tube.

In a typical cathode bias circuit, the grid resistor is usally around 100 K to 1 Meg, which lets any electrons picked up by the grid flow back to ground.

I believe fets can display this same grid leak phenmenom, but I forgot how. I guess electrons getting to the gate somehow and not having a way to get back into the Drain Source channel.

I believe grid leak got it's start in the IF sections of radios.

It is used with other tubes, but mostly in the early days of guitar amplifiers. Noise and hum are sometimes a problem with grid leak bias. You can get away with a grid leak guitar amp at home, and maybe at a house party, but if you take it down to Jam Night at the local club, you will be in for a rude awakening, due to stage lights making your amp sound like a Kenworth hitting the jake brake coming down the grapevine. In order to keep the complaints down, Fender and others moved away from grid leak and over to self or cathode bias.

Some people complain about a lack of headroom with grid leak bias. This might be because typical bias voltages with grid leak are usually lower than a typical cathode bias voltage. I have two amps that use grid leak, one is a Vox AC-15 circuit that grid leaks an EF-86, and the other is a Champ circuit that uses a 6SJ7 metal tube in grid leak. I have no headroom problems, even while using stomp boxes. I like the mellow tone of this bias technique as opposed as to you garden variaty 12AX7a/1.5K input stage.

Grid leak bias tends to give you a warm, compressed sound. My guess is that this is due to the fact that as the signal to the grid increases, more electrons flow to the plate, which means more electrons hit the grid, which means more negative bias, which means less plate current. So there is kind of a self limiting thing going on.

cj

"I believe fets can display this same grid leak phenmenom, but I forgot how."

You can operate JFETs in what is known as floating gate mode. The normal gate leakage current is from the drain end of the channel to the gate. If there is nothing to bleed it off, the gate for an N channel device gets biased positive with respect to the source until the forward gate bias causes the gate-channel diode to conduct. At equilibrium the two currents are equal in magnitude and opposite in polarity. Actually the transconductance of the FET is higher than for Vgs = 0. There is more current noise, since the two currents that cancel add as noise powers.

This works when the resistance of the source is super-high, like a cooled reverse-biased photodiode or a pyroelectric cell. Or, even just a cap-coupled circuit where someone forgot to put a gate bias resistor after the cap ;-). The voltage swing will tend to be pretty limited since the gate-channel diode is already forward-biased and will conduct hard with another few tenths of a volt going positive. With feedback to the source you could increase the dynamic range a lot.

One could make a Whitlock/THAT style buffer that worked to d.c. and didn't infringe on Bill's patent, if one had half a mind to.

[quote author="CJ"]You could almost explain it with that cool avatar![/quote]
Here is the source of it
www.sdiy.org/oid/pat879532.pdf

As electrons leave the cathode and sail towards the positive plate, some of them "leak" onto the grid wires which are in between the cathode and plate. If you use a big enough resistor from grid to ground, you can keep these electrons "trapped" on the grid. That is why typical grid to ground resistors in a grid leak circuit are usually 5 to 10 megohms. If you can collect enough of these electrons on the grid, they will form a negative charge large enough to bias the tube, which will keep the plate current where you want it.

Click to expand...

This is about what i expected, but I had nothing but a hunch. and I did not relise that the large grid resistor was just allowing the electrons that normaly leak to the grid to collect and bias the tube. I figured the resistor was pulling them down, increasing the leak.

It is used with other tubes, but mostly in the early days of guitar amplifiers. Noise and hum are sometimes a problem with grid leak bias. You can get away with a grid leak guitar amp at home, and maybe at a house party, but if you take it down to Jam Night at the local club, you will be in for a rude awakening, due to stage lights making your amp sound like a Kenworth hitting the jake brake coming down the grapevine. In order to keep the complaints down, Fender and others moved away from grid leak and over to self or cathode bias.

Click to expand...

Are metal tubes with the metal grounded less prone to this pickup of stray noise, or tubes useing an external sheild?

Some people complain about a lack of headroom with grid leak bias. This might be because typical bias voltages with grid leak are usually lower than a typical cathode bias voltage. I have two amps that use grid leak, one is a Vox AC-15 circuit that grid leaks an EF-86, and the other is a Champ circuit that uses a 6SJ7 metal tube in grid leak. I have no headroom problems, even while using stomp boxes. I like the mellow tone of this bias technique as opposed as to you garden variaty 12AX7a/1.5K input stage.

Click to expand...

I had no problems with lack of headroom, and I found the sound to be a very welcome change from the beaten to death 12AX7 preamp. I actualy built that champ quite a few years ago, it was the only amp I had all the parts for so it was the first amp I built, but I never relised that it was running grid leak bias. I may have to rebuild that one again so I can appreciate its grid leak bias properly.

Grid leak bias tends to give you a warm, compressed sound. My guess is that this is due to the fact that as the signal to the grid increases, more electrons flow to the plate, which means more electrons hit the grid, which means more negative bias, which means less plate current. So there is kind of a self limiting thing going on.

Click to expand...

I tried to figure out a way to use the scope and meter to figure out exactly what is going on here, but my scope is an old tube one and not that good, i do not really trust it. But I totally agree on the sound, I am really enjoying it.

Thanks
adam

> wondering about what is going on with the tube in this situation

It is actually quite complicated. Radiotron has several pages just about grid current effects.

But it isn't just the grid. Grid-leak is "dangerous" because the tube conducts almost wide-open. Do this with a fat tube and it will melt. You only do it with a large plate resistor. Remember the plate resistor has a lot of control of current, just like the grid except Mu times higher. If current goes high, plate voltage falls, current can't run-away. This makes most sense with high-Mu tubes that like having large plate resistors. 6SL7, but also a LOT of 12AX7.

It is roughly equivalent to using a fixed grid bias battery, except no battery to go bad.

Effect on the signal? Because bias voltage is very low and bias current is high for the plate voltage, gain is high and maximum input level is low. And because input impedance at zero-signal is high (about half the grid resistor) you can use smaller cheaper coupling caps. You do need a cap: a DC-coupled input would zero-bias the grid, so any positive swing would forward-bias the grid-cathode diode and short-out the signal. Anyway the input level is so low and so close to clipping, you want a cap so positive peaks will charge-up the cap, shift the bias, and stay clean (after the initial charge-up transient).

Disadvantages are that different tubes will bias different due to small differences in grid dimensions and materials. It was common in HiFi phono preamps, but only inside a feedback loop which semi-stabilized overall gain.

In radio, they use the term differently. A radio grid-leak detector uses more like 47K, is roughly zero-biased for no-signal. On strong signal, the coupling cap charges to keep the grid out of conduction, driving the average grid bias negative, to lower gain. This is crude compression, necessary in radio because strong stations give 1,000 times more signal than weak stations. Grid-leak fell out of the mainstream in the 1930s, except the local oscillator always used grid-leak to give large gain for starting and then lower gain to the edge of oscillation to give a clean beat signal.

It did not even phase me to look in Radiotron for this, there is alot in there dealing with grid leak bias, and the grid current effects. I have got plenty of reading for the night.

Thanks
adam