[sdiy] table radio preamp vs. guitar amp preamp (saf-t amp revisited)

[anthony]

>> The cool thing is that it looks like I won't need very many parts at all.
>
> Except for the power supply. :-)

some 1N4007's, a couple caps, a resistor...
I have this part made right now.


> The biassing as shown (which I suppose is grid leak biassing) is a 
> consequence of the rectification diodes which also work against the common 
> cathode, in such tubes you usually ground the cathode. But since you don't 
> need them (I would simply ground the plates of the diodes) you can 
> experiment with higher cathode voltages.

Yeah I noticed that th 12SQ7's properties are a lot like a 12AX7A, amp 
factor, plate resistance etc. So I just decided that I'd make a typical 
preamp stage. I'm going to use a 220 KOhm plate resistor and about a 7.5 
KOhm resistor bypassed with a 22 uF cap and also bypassed with a 0.68 uF 
polypropylene cap. (The output bias resistor is bypassed by a 100 uF Sprague 
Atom and a 2.2 uF polypropylene cap.)

>> The plate voltage is about half of what the preceding step is would that 
>> effectively bias the grid? Also given that I only have one triode to work 
>> with I'm not sure where the best place to put the volume control.
>
> If you would use the grid leak, with its consequently small grid bias, you 
> need that high valued pot wich easily catches hum and what not. A 470k pot 
> right before the power tube seems more managable to me.

So putting the volume control - assuming I'n only using one because that's 
all I /am/ using of course - between the pre and the output tube would be 
best for noise.

> Don't forget to put a higher valued resistor (2.2M or so) directly from 
> the grid to GND, in case the wiper looses contact this saves the final 
> tubes life.

I've decided not to make a small SE amp out of my EL84's. I think I'm just 
going to buy a Valve Junior and swap tubes. This amp will be my amp project 
for a while. My AC15 (with tremolo and reverb) will be a long-term project. 
I need to learn how to do tweed and all that...

>
> Cheers,
>  René

thanx,
aa