An inconspicuous amp, the sound quite rocking on the guitar .... and how about the harp? A look at the circuit diagram shows: a mini bassman with additional gain. There's something to be made, without drilling a hole.
Good labeled board, good components, good workmanship, all circuits protected ... well done. Unfortunately, a two-sided board which means hard to solder because there's always something somewhere. With much gain it is not low at all, too small transformers maybe ... but as long as it works? The reverb / FX loop is realized by OPs and sits below the output jacks - also works fine.
The other side .... we are getting closer. The jack for "LO" got removed, It's not really needed, but I need it - as input for harp. The contacts on the board are closed. On the top right you can see the tone stack, behind the potentiometers. A stack with Marshall values - which has simply too many mids for Harp. Before the second tube, there is space for a signal relay that disconnects the first two triodes and changes the wiring of the third triode.
For switching we need appropriate voltage .... you could take it from the reverb, but I take it from the heater, which has as opposed to the specifications in the plan no 3.15V - 0 - 3.15V but a 0 to 6.3 V winding.
The stuff around the relay. You see the two separated conducting paths on the tube. The socket can be reused (also because of the optics) - just rotate it. Feel free with the wiring. The relay can also be glued straight.
Oh yeah, the tone stack. In case of changes, we don't want to disassemble the whole box, so I placed it on the back. This way, everything is reachable. That was it, a few things can still be changed to adjust the sound ... more crunch, less treble, or vice versa .... Quite noisy this little one.
If you plug in "Hi", it's a guitar amp as before that just has a different tone control, which makes it possible to turn down the mids. All controls work normally. In the picture, you can also see the jumper at the input LO.
If you put a cable in "Lo", the ground contact of the relay coil is interrupted - the relay opens, the second tube no longer receives its signal from the first tube but from our input. Because this way we would have too little gain and the operating point would be suboptimal, the cathode wiring gets changed. Gain and Bright have no function anymore - it is a harp amp!
You can use the old tubes, possibly a 5751 in V2. The standard speaker, Celestone G10, is also OK.