Ok. This one in English. I've been building this amp for four years now. Last two years I was just happy actually playing guitar through it but then I decided to change the power tubes. From EL84 to 6V6GTA. Last month or so has gone in rebuilding the amp. I was only supposed to do quick rebuild on the power amp. Eventually I ended rebuilding the entire thing and actually went through several different versions before settling with this one. Also built a new cabinet.
The topology of the amp is quite traditional. 3 gain stages, cathode follower, PI and pentode Push-Pull power amp. Three 12AX7's and two 6V6GTA's (NOS RCA). The amp is now basically AX84 SEL preamp with 20W core power amp. The preamp has been modified to my taste. The power amp and PI was built from components I had available. This is the first time I have implemented Presence control. The schematic shows the current state of the amp.
Features and specs:
- 2 x 12AX7 preamp
- 6V6GTA (RCA NOS) PP power amp with 12AX7 PI
- 5AR4 rectifier tube
- UralTone power transformer
- Hammond Mfg 125E output transformer
- Approx. 20W output power at full tilt
- Gain, Bass, Treble, Middle, Presence and Volume controls
- Two switches for fine tuning the bass in the first gain stage
- 3 stage preamp with cathode follower
- Traditional tonestack
I think I'm happy with the amp now. Had a lo of issues in the rebuild. The old preamp just did not work with the first version of the new power amp. The power amp also oscillated at 30kHz is volume was set too high. I got the oscillation under control but the sound still was not right. Then I ripped out the entire old amp and rebuild it. First the PA, then preamp and then PI. Some fine tuning in the preamp and now things seem ok. Enough gain, cleans up ok from guitar volume. Not too bright but also not "muffled". No strange noises, crackles, hisses or buzzes. Not even at full tilt. The only thing that bothers me is that the Presence control is effective only in the last third of the rotation. Maybe I'll look into it some day...
Sound samples may not become available sometime in distant future.
EDIT 6.2.2010 - Added an adjustable feedback. Turned down the NFB and to my great surprise the sound opened up and really started to growl. I guess it is more Marshall now. The presence does not do much without NFB...
EDIT 9.2.2010 - Bias adjustments, 560ohm G" resistor and JCM800 style 2nd stage with switch. 270nF Presence cap.
Pekka