Max value of grid leak resistors in small signal tubes.

[Lee_M]

I've seen max grid leak resistor (Rg1) values listed on some datasheets, the Mullard ECC83 datasheet specifies it as 2M maximum under the "limiting values" section on page 5, but I often see that value exceeded in real life circuits, such as the Ampeg SVT (which uses a 5M6 grid leak resistor on the first stage), or the Hammond AO-28 amplifier (which uses 3.3M at the 2nd section of V4).

What are the implications and risks of exceeding the rated Rg1 values for small signal tubes like the ECC83, 6SL7, 6SN7, etc?

http://www.r-type.org/pdfs/ecc83-1.pdf

 

[Heikki]

Using higher value grid leak resistor than maximum value will shift the operating point. If you have significant resistance in series with the tube and sensible B+ voltage, then there will usually be negative voltage on the grid.

Edit: I took a look at the Ampeg SVT schematic. It doesn't have a cap at input so when you connect your guitar to it the grid leak is 5.6M || guitar volume pot || guitar mic.

 

[CJ]

resistors are so cheap, so buy a bunch and have a shootout,

 

[merlin]

The max grid leak resistance is not really a 'hard limit', and is more important when the tube is running at high current with fixed bias (unless specified). In most designs like the SVT the tube will be running at very low current which means you can get away with a higher grid leak resistance (because there is less ionisation in the tube at low currents). Cathode bias also allows you to safely multiply the grid leak by 1+gmRk, due to the self-stabilisation of the operating point. The SVT uses a 3.3k cathode bias so is running within limits.

 

[trobbins]

If the grid leak resistor is not aiming for grid leak biasing (ie. a typical cathode bias circuit) then you can check that the grid leak resistor value is not too high by measuring the idle anode voltage and confirming that voltage doesn't change when the grid leak resistor is shorted. That type of test is also valid to identify if a valve has a noticeably leaky input grid.

 

[moamps]

That vacuum tube ECC83 was also used in some microphones as an impedance converter with grid-leak resistors of very high value (100Mohm or similar), so it is always good to experiment to determine how far you can go. Someone will notice the grid voltage of -2V in this schematic.

 

[Lee_M]

resistors are so cheap, so buy a bunch and have a shootout,

It's not the resistors that I care about damaging, it's the fancy glassware that I'm worried about

The max grid leak resistance is not really a 'hard limit', and is more important when the tube is running at high current with fixed bias (unless specified). In most designs like the SVT the tube will be running at very low current which means you can get away with a higher grid leak resistance (because there is less ionisation in the tube at low currents). Cathode bias also allows you to safely multiply the grid leak by 1+gmRk, due to the self-stabilisation of the operating point. The SVT uses a 3.3k cathode bias so is running within limits.

That's good to know. It's possibly worth noting that the Mullard datasheet specifies 2Meg max for Rg1 when using automatic (cathode) bias, so I'm guessing they already factored in the 1+gmRk equation.

Btw, is your last name Blencowe, by any chance?

 

[scott2000]

Btw, is your last name Blencowe, by any chance?

Aye..That be he....
https://groupdiy.com/threads/merlin-blencowe-designing-power-supplies-for-valve-amplifiers.78166/

 

[Lee_M]

Aye..That be he....
https://groupdiy.com/threads/merlin-blencowe-designing-power-supplies-for-valve-amplifiers.78166/

That's a very funny coincidence, I was reading that exact book earlier this afternoon, just before I checked back on this thread.

It's quite a surreal experience to put down a book and log on to the computer, only to find you've received a message from the author of the book you were just reading

 

[OneyedK]

My 2 cents:

The ECC83 in a typical cathode follower topology:
Up to 2M Rg : bias is determined by the value of the cathode resistor
From 2-10M Rg : gray zone, hard to predict how a tube will be biased, will vary strongly with the tube used.
10M and above : grid leak bias, Rk can be 0 Ohm, bias is fully determined by Rg. (sample schematic included as illustration)

 

[Lee_M]

My 2 cents:

The ECC83 in a typical cathode follower topology:
Up to 2M Rg : bias is determined by the value of the cathode resistor
From 2-10M Rg : gray zone, hard to predict how a tube will be biased, will vary strongly with the tube used.
10M and above : grid leak bias, Rk can be 0 Ohm, bias is fully determined by Rg. (sample schematic included as illustration)

I think I understand now.

So the grid resistance isn't a "hard limit" which will damage the tube (exactly as Merlin said), but by exceeding the rated value (for fixed bias) you run the risk of your amplifier stage shifting from cathode bias to grid leak bias. Is that correct?

 

[abbey road d enfer]

"During operation the grid becomes hot since it is close to the cathode, so it does, in fact emit a few electrons of its own. The electrons leaving the grid constitute a tiny reverse grid current and, if the grid leak is very large, a positive voltage will develop at the grid". (Designing Tube Preamps … page 35).
The flow of electrons from the grid could excess the flow to the grid and result in thermal runaway.
It is more likely to happen in power tubes because of the higher dissipation.