I got this nice Kelly amp. 50W, 3x12AX7, 2xEL34. To be checked and somewhat restored.
John Kelly was a tech working for Selmer (end of ’60s, beginning of ’70s).
Great links found on the web :
I tried to draw a schematic from the guts of the amp as I saw them.
(Schematic upgraded October 2011)
Voltage measurement, with tubes :
- A0 = A1 = 400V
- A2 : 394V
- A3 : 330V
- A4 : 270V
- EL34 plate : 400V
- EL34 grid2 : 395V
- Bias current : 40mA
A pretty ECC83 Mullard …
Power filter capacitor C22 was obviously out of order … so I replaced all filter capacitors
The problem is : very low power output … (less than 1W). So I inject a 1000Hz 70mV rms input signal and follow up and measure at each stage the signal / gain. Before distorsion, I get 20V rms on the grid (Vgrid1 / cathode) of each EL34 (that seems ok) but only 14V rms plate to plate EL34, and 0.9 V on loudspeaker side (0.05 W under 16 ohms !)
Output transformer seems to be dead, that’s why there is this very low power output. To be fixed or replaced.
And then, I finally replaced the OT with a IGPW (ex-Shinrock) transformer for JMP50. It works (and sounds !) very well !
Many thanks to forum ProjetG5 members for the help !
2 pictures of a Kelly 50W with a choke in the power supply, thanks to Greg Cunningham :
Power supply with choke schematic drawn by Greg Cunningham, thanks Greg !
Pictures of a Kelly slave amp : (Credit : Olivier Foxen ampstack.wordpress.com)