DD101 - The Low Gain Tube Amp

I got hold of an old PA/central radio amp few years ago. I wrote something about it to the old KP here. The amp was not working at the time but had some nice parts: Four EL34's, EF86 and ECC83 in preamp and huge transformers. Also a big and "good looking" (but a little bent) chassis.

Apparently the amp was made in Finland in 50's or 60's. Now the amp got a new life as a low gain (bass)guitar amp. It features two EL34 output tubes, 12AX7 DC-coupled PI and 12AX7 Preamp with Tilt. Mid and Volume controls.



Dis-assembly

I traced out the schematic of the amp and decided that I would not use it as it is. I decided to dis-assemble everything. Most electronic parts went to thrash. Old can electrolytic caps and scary looking mains connectors had to go. Actually I only kept the tube sockets and two red lamps on the front panel. After removing all parts I removed most of the old paint from the chassis.



Output transformer

The OT was huge. It had UL-taps in the primary (actually 57% instead of 43%). The output transformer secondary was for 100V line. I disassembled the transformer core and wound 10 turns of wire over the existing windings. Then I fed a 1kHz signal to this 10 turn coil and measured voltages from all coils. I found that primary had approx. 2000 turns of wire. I calculated secondaries so that I got approx. 6kOhms primary impedance. 76turns for the 8ohm output. I was supposed to measure the tranny but did not have the patience for that. I assembled it to the chassis and wired to the circuit.




Chassis

The chassis is a true classic. Huge and ugly. Bent and scratched. Originally the chassis was painted with blue hammer-tone but someone had painted it over with some white paint. Latex or oil based paint used to paint interior walls in houses or something. I wanted to keep something original in the amp so I painted the chassis with something that claimed to be "Black Hammertone" in a spray can. Well, not quite. It is actually gray and does not look like hammertone. You cant win every time... I did not have to drill any new holes except for the new IEC socket.

Schematic





UPDATE 13.6.2010: Schematic rev. 1.5. With new PI, PPIMV and subsonic filter before poweramp.

UPDATE 21.3.2010: New schematic uploaded.

UPDATE 15.3.2010: Now the amp is pentode connected with NFB to PI. Preamp tubes changed to 12ax7 for more gain. Schematic soon. New Mid controll also coming up.

Sound

Big and clean

Old info: The amp has really low gain and huge headroom. I tried to push the input stage to distort with a booster but 10Vpp signal was not enough for that... The tone control works good at least with a guitar. Nice "Tilt" control with some mid scoop adjustment. Tone control is a version of the Big Muff PI tone. It works nice but I would like a better control for the mid frequencies and may change this in the future. Seems to work great with distortion pedals.

Problems

Due to the 57% UL taps I only got approx 20W of clean output. I was hoping to get over 30W clean and near 40W full distortion. This is a setback. Now I need to add second pair of tubes or change to pentode circuit. That sucks because with pentode I have to use feedback and post PI master volume is not optimal with it. UPDATE 13.6.2010: I have now changed the amp to fixed bias with NFB and PPIMV and it seems to work better than expected. About 60W at full clipping. Maybe more. Definitely more if a 4 ohm cab is plugged to the 8 ohm output.


Pekka