Reel to Reel amp conversion opinions

[nickfl]

I just bought an old Columbia (sony) C-690 TR reel to reel cheap on craigslist and I am looking for some advice on converting it to use as a guitar amp. This tape deck uses one 12ax7 and one 12BH7 with a grid leak biased input and one half of the 12BH7 in a single ended power stage. Here's a schematic of the original amp:

Columbia C690-TR schematic-001.jpg

I only had to move one wire to be able to plug a guitar into the inputs and play through it. It has tons of gain, a pretty decent amount of volume, but a bit of a muffled tone without much treble. It is usable as is but it really seems like it would benefit from some tweaking to make it more guitar friendly...

Here is a first draft of a modified schematic with some proposed changes in red:

Columbia RTR mod schematic.png

I've changed the input jack wiring and a few cap and resistor values as well as adding a tone control and a grid stopper to the power tube. I don't really want to rewire the whole amp, just make it a bit more guitar friendly and usable. Thoughts and suggestions would be much appreciated!

[Andy Le Blanc]

Adding a bypass cap to the second 12ax7 stage, or the first 12bh7 stage should brighten it up, experiment, doesn't have to be a large value... .5 to 5 uF. You might change the first gain stage from zero bias to cathode bias as well. Nice to see 12bh7 as a SE amp.