What I built this Christmas !

[hamfist]

Well not exactly "this Christmas". It's taken me a good 3 or 4 weeks. And thanks to a few gems of advice from this board, it's all working beautifully !

I started off with a 1970's Selmer, 6 channel, 100W PA.
Opened it up to be greeted by a quad of good Brimar EL34's, 7 x Mazda grey plate 12AX7's, a Brimar 12AT7, 2 x Brimar 12AU7's and an EF86. Also huge Partridge (of Hiwatt fame) transformers. Not forgetting 18 x Mustard 0.022 caps !!

I've now converted it into a two channel guitar amp. Channel one is a bog standard Marshall 2203. I've been through a number of higher gain pre-amp configurations for channel 2 and ended up with a clear winner - which is closely based on a B-52 AT100 (schematic shown below).
The B52 pre-amp is an absolute winner, from low-mid gain, right up to really quite high gain. Although at the upper end of it's gain range, it gets pretty hissy ....this is a big old hand-wired amp though, so not that surprising. I'd never use that much gain anyway (and I do like quite a lot !). In general the noise floor is very low ...certainly low enough for me (I did use quite a lot of shielded cable throughout).

FX loop is passive, and is situated straight after the MV's for each channel, and pre-PI. I've had a lot of luck with this loop recently with another amp. My DD-20, set on line level seems to get on very well with it.

Channel switching is by footswitch, which activates a DPDT relay - a big "first" for me, and probably the most challenging part of the build, as I found there was actually not too much free info available on the www about circuits etc.
Both the pre-amps are completely independant of each other, and the relay switches inputs (directly before the input resistor of the first gain stage of each pre-amp), and outputs (straight after the traditional Marshall Tone stack and MV, used in both pre-amps).
I ended up powering the relay tapping off the 6.3V AC heater winding - using the circuit shown below - the voltage tripler powering a 5V voltage regulator, powering a 5V relay.

The front panel is much as the amp came like (but with some stuck-on labels for the new controls). I like to think of it very much as a wolf in sheeps clothing.
Also below are several gutshops. And I'm quite embarrassed at the poor standard of my soldering. I'm very much in the "effective but not beautiful" camp of soldering at the moment !

I would welcome any comments about how I might improve my layout, or in fact anything else about the amp.
Here's the pics and schems.

[hamfist]

Just in case anyone's interested, I did a schematic drawing for the whole amp.

Page one at http://www.box.net/shared/static/9643sxsk1f.jpg

Page two at http://www.box.net/shared/static/c87z7pxzls.jpg

[Bruce / Mission Amps]

Be careful with your bias pot setting... if you set it too far towards zero -voltage... it could go to zero.
Those EL34s will run severely under biased (zero bias voltage) and could blow up.

[Enzo]

I'm with Bruce here, why not add a resistor in series with the bias pot so if you turn it to zero, there is still bias. 33k, 50k, whatever.

[tubeswell]

FWIW as well as what Bruce and Enzo said, (if you haven't already done so) wire your pot wiper to one end of the pot, so if the wiper contact fails, you've still got some bias. (apologies for the miniscule schematic)