Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Craftyjam on June 04, 2021, 02:21:05 pm

Title: Question about g1 circuit resistance
Post by: Craftyjam on June 04, 2021, 02:21:05 pm: Hello, first post :icon_biggrin:

I would like to know if an attenuating resistor adds to the circuit resistance of g1, and if so, how far the maximum circuit resistance can be safely exceeded.

Here is a picture of what I'm working with. Building a simple SE guitar amp that due to design constraints is more prone to oscilation. It sounds pleasing to me when using this attenuating circuit, but I do not wish to damage my tubes.

Thanks!
Title: Re: Question about g1 circuit resistance
Post by: pdf64 on June 04, 2021, 04:31:54 pm: Quote from: Craftyjam on June 04, 2021, 02:21:05 pm
... I would like to know if an attenuating resistor adds to the circuit resistance of g1, and if so, how far the maximum circuit resistance can be safely exceeded.

Here is a picture of what I'm working with. ...
The grid circuit resistance is pretty much what can be measured with your meter, the resistance between the control grid and its cathode.
As the max possible anode dissipation in the circuit decreases, then it’s safe to increase the grid circuit resistance.
Note that almost no fixed bias valve guitar amps abide by the 100k grid circuit resistance limit of many valve types.
Title: Re: Question about g1 circuit resistance
Post by: brewdude on June 04, 2021, 04:53:09 pm: I had an amp build that was not pleasing me many years ago and in an attempt to improve it, I tacked some resistors across the existing grid leaks to give a total of ~100k instead of 250k. I thought it helped this amp. In fact I’ve used the recommended limits at the grid leaks in 3 or 4 amps since. I’m not saying it’s necessary or better or worse, I just thought I’d share.
Title: Re: Question about g1 circuit resistance
Post by: PRR on June 04, 2021, 05:18:12 pm: Welcome.

That plan shows 221.5k to the tube. Without knowing the tube type or bias system we can't say if that is OK.
Title: Re: Question about g1 circuit resistance
Post by: Craftyjam on June 04, 2021, 05:51:30 pm: Quote from: PRR on June 04, 2021, 05:18:12 pm
That plan shows 221.5k to the tube. Without knowing the tube type or bias system we can't say if that is OK.

Ah ok, so the total grid resistance is comprised of the grid stopper and the the grid leak, and not the attenuating resistor as well.
The power tube is a 6AQ5 and the datasheet states that the max g1 circuit resistance is 500kohm when using cathopde bias, which I am.

Thank you for clearing that up.