Selmer All Purpose 30

Serial #51120

Schematic (GIF 30k)

This amp is very similar to the Selmer Compact 30 I found at Obsolete Electronics. The main differences are the absence of negative feedback, two way rectifier instead of bridge and the use of a ECC82 for the input stage (although I don't know if this is the original tube). Unhappily, the EL506 (Brimar) has long been discontinued and there is no replacement (anybody out there got a pair?). So I have to take good care of them...

 

 

Repair

This was my first bass amp back in '76. When I took it from the attic some time ago (after more then 20 years), it still worked to some degree but had some bad hum. So I drew the schematic and bought a book on tube amp repair. After some thorough study of the book and the schematic (and some measurements with the scope) I was able to tell that one of the grid capacitors had blown. To be sure, I replaced both of them with 1000V types (the blue ones on the middle pic) and now this little amp is working fine again. I also replaced the 100n cap in the phase inverter, although this would not have been necessary. The small electrolytics in the preamp look like they had been replaced already. I tested the big power supply filter caps, they were ok.

 

 

Modifications

The amp had been modified already before I owned it. A separate power switch was built in next to the volume pot to replace the defective switch on the rear of the pot. This was a two pole on/off switch with one pole still working, so I used it as a standby switch between ground and the cathodes of the power tubes (no high voltages there).
Having gone this far, and without the need to keep any "original condition", I didn't feel too good with the 220V power connection running across the input jacks. Placing a new switch on the rear side of the amp kept the primary circuit limited to the far left (right photo, right side). As you can see, I had to move the round plate with the serial number to the other side. The speaker connector is isolated to avoid ground loops and has a shorting switch. This can help to protect the output stage, at least for some limited amount of time.
Last thing I did was to to use the now obsolete switch in front to simulate a tube rectifier. I put a 150 ohm resistor (middle photo, front left) in series with the 440V output of the rectifier diodes that can be shorted with the switch. The effect is a slight compression at higher output levels (maybe I'll use a 220 ohm for more of it).