Restoring old 30's Lafayette Model 408

[hearforever]

I am in the process of restoring an old Lafayette tube amp from the 1930's. I'd like to use it for guitar.

It has a 6SC7 preamp tube and a 6N6 power tube. I am getting very distorted output not pleasant distortion but very harsh past about 5 on the volume knob. I would think the 6SC7 would have enough gain to get decent output but maybe not? Are there any improvements anyone would suggest on this circuit?

I was thinking of getting rid of the grid leak bias. The power supply is pretty funny looking too. Should I replace the .1 and .25 capacitors in the power supply to electrolytic caps? And what about the .5 cap across the choke?

Thanks!

[Tom Phillips]

First I don't think that the schematic is correct. There are multiple things that don't look right. Someone either drew it wrong or there have been some goofy parts changes made before the circuit was traced out to make the schematic. For starters, the .5 uF and .25uF caps in the power supply are way too low capacitance and the interstage resistors are way too high. Plus none of the tube stages are properly biased.
Can you post photos of the condition of the amp?

[Tom Phillips]

The Fender Champ Model 5C1 may be a good candidate circuit to build into your chassis. Here is a link to the schematic.
http://webphix.com/schematic%20heave..._5c1_schem.pdf

[hearforever]

The schematic is correct to what is there but as you say some components may very well have been added in. I'm thinking that just putting in a proven Fender circuit will probably be the best way to go. I am leaning towards the Princeton 5C2 since it uses a 6SC7

[Tom Phillips]

...I'm thinking that just putting in a proven Fender circuit will probably be the best way to go. I am leaning towards the Princeton 5C2 since it uses a 6SC7

That sounds like a good idea. If you strip out the small components and build in the 5C2 you should have a nice end result. The existing circuit appears to be a mess of mistakes and I'm sure that is why it sounds so bad. That amp has had an interesting 60-70 year life so far.
Regards,
Tom

[hearforever]

One quick question: why no grid leak resistor on the 5C2 circuit? http://www.freeinfosociety.com/media/images/1265.gif
I see that one of the 75K resistors acts as a grid leak resistor when only one input is plugged in, but what about when there are 2 inputs?

[Tom Phillips]

I don't know why the 5C2 was designed as shown on the schematic. However, if two inputs are being used at the same time then the grid leak function is provided by the series path through the guitar cable, the pickup coil and back to chassis ground.
Tom