Historic Harmonica Amplifier

[gingertube]

For your possible interest.

I have this amp on my bench for repair. The lady who brought it to me says it was her grandpa's Harmonica Amp and would like restored. Did a circuit trace. Very interesting tone control, treble boost/cut by feedback around the output tube. I looked up the tone control in RDH to find that the entire design is a 1942 W.N. Williamson design, Yep same Williamson who did the famous (well certainly the most copied) HiFi Amp design. Could have saved the circuit trace effort if I had know that up front.
6J7 triode mode to 6J7 pentode mode to 6V6 with an 80 rectifier. Lamp in standby switch line as a HT fuse.

Cheers,
Ian

[kevinkr]

Hi Ian,
Cool, any pictures and further story to relate?

[rok-n-rol]

Gotta love those Harp amps....
Thanks for the post...

[gingertube]

Finally got the old digital camera working.
Here it is:
1st pic of the thing out of its case. The metal can along side the amp is the original home brew mains filter which just had to go. You can see in the 3rd pic that I just replaced it with an IEC Socket with an integral filter. The original mains input hole in the case was covered over with a brass plate. So where do you get a convenient brass plate for this? Any old hardware store - if you took the plate off you would see that the other side has engraved "NO ADVERTISING MATERIAL ACCEPTED -THANK YOU". That is it was intended for a mail box.
2nd picrture is of the amp back in its case, you can see that as well as replacing the mains input, I fitted an in-line mains fuse.
4th pic shows the "groovy" throtle knob standby switch. Pull out for standby.
The input 6J7 is in that shield at the rear left. The first thing I did for this amp was turf the 6V6GT and substitute the 6V6G ST shape tube (in the cetre).
All tubes tested good but have sent it back with a full set of spare tubes (2 x 6J7, 1 x 6V6G, 1 x 80).
Cheers,
Ian