Grid leak as reverb recovery?

[Chuck H]

I'm slow to get to a project that needs doing. I have a build with a noisy tube reverb (aren't they all to some degree?) and I need to try and quiet it down as much as possible. Not sure when I'll actually get to it but I'm penciling notes for when I do. And I was thinking...

The grid leak bias system is typically shunned for modern guitar amplifier inputs. I assume this is because it can barely handle a signal even as small as the average humbucker puts out. So, good, we just used the cathode bias arrangement and forsook the grid leak circuit completely. This change was done by most amp builders in the 50's, I think, before the advent of built in reverb. But the grid leak circuit should have the lowest noise for the most gain. That seems like a good thing then you must amplify an input signal no bigger than 10mV. So I guess my question is:

Is there any disadvantage to using the grid leak circuit as a reverb recovery stage? Also, is there any misconception on my part of the potential advantages? Other reverb noise issues not withstanding, using the grid leak circuit for recovery seems like a sensible start.

[uneumann]

I have a THD Flexi 50 - it's a recent design and uses a grid leak input stage. I'm quite happy with it. However a bias is developed, it's all the same to the tube. Vgk of 1v is 1v regardless of how it's generated. That said, there are issues with how stable and tube-sensitive a grid leak bias voltage will be. I note quite a range of behaviors as I change the Flexi tube. That's neither good nor bad - it depends on how you look at it and whether stability and tube invariance is important - IMO it's not that important since I rather like to play with the variations that different tubes offer in that amp. For reverb recovery, I think such variations are probably less desirable. It is hard to beat a bypassed cathode-bias stage for stability and low noise and gain if that's what you're after. I don't see how a grid leak bias stage offers any advantage.

[Chuck H]

I have a THD Flexi 50 - it's a recent design and uses a grid leak input stage. I'm quite happy with it. However a bias is developed, it's all the same to the tube. Vgk of 1v is 1v regardless of how it's generated. That said, there are issues with how stable and tube-sensitive a grid leak bias voltage will be. I note quite a range of behaviors as I change the Flexi tube. That's neither good nor bad - it depends on how you look at it and whether stability and tube invariance is important - IMO it's not that important since I rather like to play with the variations that different tubes offer in that amp. For reverb recovery, I think such variations are probably less desirable. It is hard to beat a bypassed cathode-bias stage for stability and low noise and gain if that's what you're after. I don't see how a grid leak bias stage offers any advantage.

I'm not familiar with the THD Flexi 50. Grid leak seems like an odd choice for the input of any modern amp since the bias voltage will almost certainly be less than a volt. And many EQ, boost and distortion pedals can easily overdrive that. Even some hot humbuckers can peak over a volt at times. Because a reverb pan puts out less than 10mV the low input signal tolerance of the grid leak circuit isn't an issue. The reason grid leak is quieter than cathode biasing is that the cathode is grounded. This virtually eliminates any hiss, buzz or hum on the cathode circuit and prevents it being amplified by the valve. Since reverb recovery stages must amplify A LOT any noise reduction is a plus.

[Alan0354]

I'm not familiar with the THD Flexi 50. Grid leak seems like an odd choice for the input of any modern amp since the bias voltage will almost certainly be less than a volt. And many EQ, boost and distortion pedals can easily overdrive that. Even some hot humbuckers can peak over a volt at times. Because a reverb pan puts out less than 10mV the low input signal tolerance of the grid leak circuit isn't an issue. The reason grid leak is quieter than cathode biasing is that the cathode is grounded. This virtually eliminates any hiss, buzz or hum on the cathode circuit and prevents it being amplified by the valve. Since reverb recovery stages must amplify A LOT any noise reduction is a plus.

But if the cathode is bypassed by a big cap, it is literally grounded. So what is the advantage of a grid leak? I don't see noise advantage of noise performance.

[Alan0354]

I am going to hijack one post here. I have been studying cascaded frontend schematics lately, Badcat uses a grid leak as the first stage. What is the advantage? From my understanding, the input impedance is quite low on top of input drawing current when signal goes positive at the grid. That will make the guitar sound very muddy. It is known that the output impedance of the pickup rises with frequency, a low impedance load will cause voltage divider effect on the highs.

It is proven if you use `100K pot for volume and tone, you'll reduce a lot of highs of the guitar signal, this is even more obvious for humbuckers that optimized for loading of 250K(500K volume and 500K tone in parallel).

Is the designer purposely use this to lower the highs for the cascade amp?

[Tom Phillips]

I am going to hijack one post here....

If you already know that you are hijacking then why not start a new thread? That would be to your advantage since you are asking questions and therefore expect replies. Your dedicated thread will probably receive more attention too.

[Alan0354]

If you already know that you are hijacking then why not start a new thread? That would be to your advantage since you are asking questions and therefore expect replies. Your dedicated thread will probably receive more attention too.

OK!!!

[Merlinb]

But if the cathode is bypassed by a big cap, it is literally grounded. So what is the advantage of a grid leak? I don't see noise advantage of noise performance.

Grid leak is typically noisier, because grid current is itself noisy. However, we're talking about nanovolt differences that would never be noticed in a reverb recovery scenario. I would think heater / PSU hum was the greatest offender in any typical reverb circuit.