Converting tube PA into guitar amp

[EETStudent]

I recently have taken on the challenge of converting an old Rauland 2135 into use for a guitar. It actually sounds pretty good without any modifications, but clearly there is more potential. Im fairly new to this whole DIY world, so bear with me. Ive had a few, actually several, troubles with this. and not having the schematic doesnt help.

1. I simply dont know where to put a 1/4" jack. Its been almost a week and ive been getting by using the XLR connections with a 1/4" adapter, but I speculate the connection isnt all that great because almost always I dont get a signal until i jiggle it around, which I know probably makes the issue worse. I read an article suggesting that you place it between the grid of whatever stage your using(stage 4, 1 and 2 dont work) and the ground to the circuit ground connection of the cathode. Couldnt get that to work, probably meaning I went wrong somewhere

2. An extremely loud hum/buzz. The tube lineup is six 12AX7's, 2 EL34's, 1 5AR4, along with master volume, phone stage, R.C. connections and four inputs. I suppose all this activity could be causing alot of unnecessary noise? I wouldnt use the phono or the r.c. so I would like to take them out, but I dont have enough wire tracing skills to remove them and bridge the connection back together. Also, this thing is dated 1966, so I know a cardinal rule is to replace the capacitors. Any thoughts on how I might could troubleshoot the excessive noise source? I can live with the bad connection, but the noise has got to lessen.

3. Master Volume. The master volume comes directly after the input volume controls, which I have read to be pretty lame for guitar use. Any thoughts on where else this can go?

Any help or additional advice would be greatly appreciated!!

[drewl]

You can wire up a 1/4" input where one of the XLR inputs is.
Buzz and hum could be a bad ground or filter cap problem.
An easy master volume can go after the tonestack, before the phase splitter.
Or a post phase inverter master.
Of course the tonestack values should probably be changed for guitar use.
Dig up some simple Fender or Marshall schematics....they'll point you in the right direction.

[Marc]

www.geofex.com

The guidance articles there are an invaluable resource!

Marc

[EETStudent]

hey thanks guys! i had previously read through a geofex article on how to convert a PA into guitar amp, and the way he suggested the 1/4" jack should be installed just didnt work for me. More than likely I was doing something wrong, but drewl I did what you said and it worked! so thanks for that.

I have tried some different tone circuits, but Ive had some problems. 1) I couldnt find a consistent Fender schematic. Looked at a handful and just about all had some variation to it. 2) When reading the schematic, I had trouble identifying which terminal was which on the pots. I wired both a standard Fender and a Brownface, but I think I did some things backwards and the interaction was all wrong. Can anyone help me indentify which terminal(other than the obvious wiper) is which when reading potentiometer symbols?

[Regis]

I have tried some different tone circuits, but Ive had some problems. 1) I couldnt find a consistent Fender schematic. Looked at a handful and just about all had some variation to it. 2) When reading the schematic, I had trouble identifying which terminal was which on the pots. I wired both a standard Fender and a Brownface, but I think I did some things backwards and the interaction was all wrong. Can anyone help me indentify which terminal(other than the obvious wiper) is which when reading potentiometer symbols?

See Here about pot wiring.

[rf7]

If I were you, I'd get an ohmmeter and try to draw out a schematic of what you've got there. It'll really help you out in the long run. I've done it for a lot of amps that I did not have schematics for, and it has always been worth the pain of drawing it out.

If you are worried about the excess inputs, you could just jump (or solder) the unneeded ones to ground.

[EETStudent]

Thanks everyone for all of your input. Has really helped tremendously. I did note one more problem

- Inconsistent tone performance. Seems like sometimes Ill plug in and the highs seem fairly muddled out, even with the treble control maxed and the bass completely cut. Then again sometimes Ill plug in and the treble will be very clear and present with the same settings as before. Anyone know what types of things cause this?