Mark's Valve Guitar Amp Page

First of all I would like to thank the helpful guys at  AX84 - The Co-operative Guitar Amp Project   for their advise, wealth of knowledge and information in the discussion forum.   Also thanks to Fallsy for the NOS valves he gave me, which gave me the inspi ration to get the project underway. 

The amp has two, foot-switchable channels, clean and overdrive, with independent bright switches & gain controls, shared TMB tone stack and active effects loop. The power amp section is a cathode biased push-pull design with no negative feedback. The power amp has the choice of  6BQ5/EL84,  6V6GT  or  6CA7/EL34  power output valves. 
The 6BQ5's and 6V6GT's are biased class AB and produces a loud 18 watts RMS into a 7600 ohm output transformer primary impedance.  The 6CA7's were intended to be biased class A and should have produced 22 watts RMS into a 3800 ohm primary impedance.  I measured closer to 30 watts so the bias is probably just in the class AB region due to a lower bias current and slightly higher than expected supply voltage. The 6CA7's measured ~18 watts into a 7600 ohm primary impedance.

The power transformer is a Hammond 370HX and the output transformer is a Hammond 1650F built onto a Hammond 1441-28BK3 406x203x76 steel chassis. The output valves used are NOS Mullard 6BQ5, Mullard 6CA7 and AWV "Super Radiotron" 6V6GT. The preamp valves are Electro-Harmonix 12AX7EH, the rectifier is a Sovtek 5AR4 and the speaker is a Celestion G12H  30W - 8 ohm in a homemade 1x12" speaker cabinet.


Images                                     Sound Samples

Chassis Wiring                                             6V6GT clean finger-picked by Keith Wilson
Chassis - front view with 6CA7's              6V6GT Crunch
Chassis - rear view with 6BQ5's                6V6GT Overdrive
Speaker                                                         6CA7 clean
NOS valves                                                   6CA7 Overdrive
Amp and Guitars                                         6CA7 Layered track sample (midi drums)
                                                                          
Schematics

MJL-18 Preamp
MJL-18 Effects Loop (optional)
MJL-18 Power Amp & Power Supply



Links

AX84 - the co-operative guitar amp project
Duncan's Amp Pages - tube data, schematics, software etc.
Aiken Amps - good source of technical info
Electronic Valve & Tube Company - suppliers of transformers, chassis, valves etc. in Australia
Guitar Amplifiers - more info