Help to get rid of hum please.

[Mike K]

I mean I guess it makes sense - the grid (at least in the model) has infinite impedance. As I increase grid leak resistance toward infinity, I approach that final curve.

I know at some point, if you were to make your grid leak too big, you would build up a charge and develop a DC voltage. I honestly don't know why the very old guitar amps used grid leaks at all with cathode bias - I can perhaps see Helmholtz's point that with active pedals, there may be a real need for it for function.

And I suppose noise isn't really a concern for the first stage - although I was thinking more generally about all other grid leaks contributing to noise. I've typically always used very low noise resistors for the first grid leak but perhaps that's not really a necessity.

[Helmholtz]

Output taken at junction of C5 and R8....

Miller capacitance C6 should go after the grid stopper R7 as it acts directly at the grid, constituting a LP filter together with R7.

[Mike K]

Miller capacitance C6 should go after the grid stopper R7 as it acts directly at the grid, constituting a LP filter together with R7.

That's cable capacitance. Miller capacitance is included in the tube model.

Actually I think the 470p was supposed to be the cable. I pulled up this old model and didn't realize that - I thought it was the pickup only and added in a capacitance for the cable when I hooked the input tube up. At any rate, they are in parallel, so it's just a really lossy cable

I'll fix that in my model so I don't forget - but I think the general idea here is still valid....

[Helmholtz]

And I suppose noise isn't really a concern for the first stage - although I was thinking more generally about all other grid leaks contributing to noise. I've typically always used very low noise resistors for the first grid leak but perhaps that's not really a necessity.

The first stage is the main source of thermal noise in an amp - simply because its noise gets amplified most by the following stages.

As long as there's no DC current, resistors only generate Johnson noise.
The Johnson (or thermal) noise of all resistor types is the same, as it only depends on R value and temperature.

DC current causes excess noise and here's where resistor type/technology comes into play.
The only place where a low noise type might make a difference is the plate resistor.

[Mike K]

Here's a more clear circuit with that "extra" cable capacitance removed. I also moved the cable capacitance after the pots, so hopefully it won't trip me in the future.

[Mike K]

The first stage is the main source of thermal noise in an amp - simply because its noise gets amplified most by the following stages.

As long as there's no DC current, resistors only generate Johnson noise.
The Johnson (or thermal) noise of all resistor types is the same, as it only depends on R value and temperature.

DC current causes excess noise and here's where resistor type/technology comes into play.
The only place where a low noise type might make a difference is the plate resistor.

Right - I knew Johnson noise was responsible and amplified the most from the first stage, but I wasn't actually aware that low noise resistors were not affected by this. On forums I used to frequent, I guess it was always assumed that low noise resistors lower all noise effects. I also knew from experience that the first grid stopper value was important in this because I've played around with different values and noted the changes - I have never used anything but a 1M grid leak for a cathode biased first stage, so empirically I had no experience there. But I have (mostly) always used low noise resistors there because it has been purported to do so in many discussions. But what you say makes sense and I plan to give it a test at some point. I'm not mad that I used a 50 cent resistor vs. a 10 cent, but I like to know who serves their purpose best where.

[Helmholtz]

So much nonsense told on internet forums .
I prefer to trust standard literature, my meters - and last not least my ears.

[catalin gramada]

So much nonsense told on internet forums .
I prefer to trust standard literature, my meters - and last not least my ears.

...Well standard literature I may have acces did not approach how to g3t rid of grid leak counting on dc resistance of the source, or how to get rid of noise when you circuit is floating without an earthing like in my case...

[catalin gramada]

Yes, but was the DC really clean?
Maybe try a 6V battery.

OK, clean DC for heaters is pretty tricky but think I get it. I had to make few turns over PT core to get a separate winding still but the result is quite right I suppose. I ended with 1 mV ac ripple for 300 mA dc draw.

[Helmholtz]

I ended with 1 mV ac ripple for 300 mV draw.

300mA?

[catalin gramada]

300mA?

Yes 300 mA / 6.4Vdc. Is only the first tube I supplied. The ac ripple is 1 mV.