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

[René Schmitz]

Hi Anthony and all,

anthony wrote:
> OK I have some good schematics to get me started on the amp that I 
> mentioned a while ago - you know the one using a 12SQ7GT and a 50L6GT. 
> The cool thing is that it looks like I won't need very many parts at all.

Except for the power supply. :-)

> But I can't seem to figure out the best way to set the 12SQ7GT to have 
> the kind of gain you'd want in a guitar amp. The way the tube is biased 
> doesn't make sense to me. The cathode just goes straight to ground and 
> it doesn't look like the grid is biased negative. 

Well, an easy method for biassing is to use a couple of 1N4148 in series 
instead of a cathode resistor. One or two might give you some extra bias 
voltage.

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.

> 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.
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.


Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159