John G wrote:First photo shows, at the extreme right hand end a 10Meg ohm to a ground.
The top end has the input coax connected, and most probably a 33K ohm resistor going up to the first grid.
I'm agree with your point about the presence of this resistor connecting the upper right tag to pin 2 of the socket tube (same as the vertical 220K resistor to the other grid).
The remaining connections to that socket (including the speculation about the heater supply) are hidden in all the pictures, so were guessed.
The last components to solve are the two cathode bypass caps and confirm how the spdt switch related to the tone pot are implemented.
I build one in my test box and it sounds really good. There is little bit of oscillation when the tube is close to smps supply (extreme settings gain and tone).
Also wiring is spagetti type, so that could introduce oscillation too, but I used shielded cable for input/output and fx in/out..
Ben N wrote:I don't understand how the tube is biased without grid leak resistors.
They are cathode biased, ahem...
Right, but so are all common cathode preamp stages, and AFAIK, they typically have a resistor from grid to ground. Go look at pretty much any preamp.
Edit: I guess the main function is just to provide a DC path to ground so the tube doesn't self-destruct, but it also has a biasing function in conjunction with the cathode R.
Most have grid stoppers, too, but they are, AFAIK, not essential, just good practice.
Ben N wrote:I don't understand how the tube is biased without grid leak resistors.
They are cathode biased, ahem...
Right, but so are all common cathode preamp stages, and AFAIK, they typically have a resistor from grid to ground. Go look at pretty much any preamp.
Edit: I guess the main function is just to provide a DC path to ground so the tube doesn't self-destruct, but it also has a biasing function in conjunction with the cathode R.
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Most have grid stoppers, too, but they are, AFAIK, not essential, just good practice.
In this configuration, gread leak resistors are present to drain DC potential from control grid.
Here we have a gain pot @ 2nd stage, acting as a grid leak.
But the first stage is definitely missing something like 1Meg from grid to ground.
Your initial post was
Ben N wrote:I don't understand how the tube is biased without grid leak resistors.
I beleive, draining DC potential and biasing are a bit different things.
brejna wrote:There is 10M grid resistor to the ground. Also I tried 68k grid resistor and I didn't here big change in the sound.
It's not about the change in the sound, but limiting RF frequencies from destabilizing the tube (grid block resistor together with Miller capacitance form low pass filter, effectively cancelling everyting above audible range).
I think it's better to leave 68k grid stopper at the input, together with pulldown resistor.
Right, my question was really about the first stage, as I understand the gain pot serves that function for the second. 10M -- ok, not on the schematic -- isn't that a tad on the large side? I think the biggest I've seen there is 3.3M.
plush wrote:
I beleive, draining DC potential and biasing are a bit different things.
I readily confess that what little I know about it (other than general familiarity with amp schematics) comes mostly from an But he clearly says that both functions are part of the role of the grid leak in a cathode biased preamp stage, and I found some confirmation of that poking about the internetz.
plush wrote:
I beleive, draining DC potential and biasing are a bit different things.
I readily confess that what little I know about it (other than general familiarity with amp schematics) comes mostly from an But he clearly says that both functions are part of the role of the grid leak in a cathode biased preamp stage, and I found some confirmation of that poking about the internetz.
Well, he's right. In an ideal circuit, these are must-have things. In reality, there's a lot of amps that do not have grid leaks and stoppers and still somehow operate and do not self-destruct.
I do not advocate for such design practices though. Sorry if misguided you.