amp mod question

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antichef

antichef

pornk rock
Thanks to an ebay spree about a year ago, I have several small tube practice amps from the '50s and '60s. These are fairly simple point to point wired amps, usually with a 12AX7 preamp tube and either a single EL84 or 6V6 power tube. (I've also got an Epi Valve Jr. head that sort of falls into this category for me). I pretty much like them, and they all sound a little different, which is nice. Recently, for convenience, I've also been recording into my audio interface and through microphone preamps that have instrument inputs, and I'm finding that I like the sound of these preamps. I've been re-amping, too, with moderate success, but when I do it, I can't help but dwell on the fact that my guitar signal ultimately travels through two different preamps (being converted, stored, re-converted, and attenuated in between).

My question: is it sane to think about modding one of these small tube amps to provide a "line in" that will route an incoming line level signal straight to the power stage, circumventing the built in preamp? That way, I could, for instance, run the guitar into a Maudio DMP3, and then run the output of the DMP3 straight into the EL84 stage of a little amp, skipping the amp's 12AX7 stage.

I mean this question in a general way, although I'm happy to post a schematic for one of the amps if need be. If I'm talking like someone you used to know who was electrocuted, please let me know that, too.
 
Don't think it's an unreasonable idea - what you're talking about doing is essentially adding the "in" side of an effects loop. On amps with effects loops, the "out" comes out of the preamp to the effects pedal and then the return from the effects goes to the power amp via the "in" (Sometimes called send & return). If you plug direct into the effects loop return rather than the main input you would be bypassing the pre-amp. Some folks do this so they can use digital modelling processors like a POD into the output stage of a tube amp. I'm not the one to be answering the actual details like how to run the circuit, but you may be able to find schematics of effects loop mod's for the amps you have posted on the web. I hope I don't offend you with this question, but I have to ask - you do know the dangers of poking around inside a tube amp, and how to discharge capacitors?
 
encouraging - thanks! Not offended in the least. I know about discharging capacitors in the abstract sense, but I need to practice :) - preferably doing it the correct and non-painful way
 
Adding line in mods like that is one of the first things I ever did with an amp! It's super easy. You just use one of those shorting jacks, so that it breaks the connection between the preamp and power amp when you plug into it. Depending on which wire you connect to which lug, you can have it be either a power amp in or a preamp out.
The only tricky part is finding the correct spot in the circuit to put the jack. In this case, maybe it's best to work backwards from the output tubes. First you have the output tubes, then the phase inverter/driver stage... you will probably want to put the jack right before that.
 
Never mind. I was multi-tasking and misunderstood the question.

Nothing to see here.
 
Last edited:
antichef said:
encouraging - thanks! Not offended in the least. I know about discharging capacitors in the abstract sense, but I need to practice :) - preferably doing it the correct and non-lethal way
Click to expand...

there, corrected. ;)
 
I know this might not be exactly what you're wanting but have you considered using low gain preamp tubes? I'm thinking like a 12AU7 or even a 12AT7. It would still have some preamp sound but would significantly clean up the signal. Might be worth a shot before getting into the modding of an amp which, by the way can be a lot of fun if you're up to it.

As others have mentioned, bypassing the input stages can be done, get a schematic (obviously) which will be invaluable. Finding the right spot to open the signal path and install an impedance matching insert should be fairly simple.

Just a thought.
 
I have been putting 12AT7s into a couple of them -- it does work great - particularly when a weaker power tube is getting pushed to hard (causing fartiness) by the 12AX7.

So, just hypothetically, if there were a schematic that looked like this:

http://recursor.net/epiphone101schematic.png

(the image is too big to be viewed comfortably inline)

we're talking about in between C7 and pin 2 of V2?
 
Yep,

That is indeed the input to the final. The biggest challange will be ensuring a good impedance match and proper coupling with your drive/signal source. It would be a good idea to first put a volt meter and o-scope to various points throughout the circuit and document them. It will be helpful when troubleshooting and for comparisons.
 
It'll be cool as a project, and useful, but really I wouldn't seriously consider it from the angle of making your signal path purer or whatever. It's already been through 8 kinds of crap already, and about 100 kinds if you go digital. Skipping a few caps and resistors at the end doesn't really matter.

That's the same logic a fat guy uses when he washes down a whole pizza and a quart of ice cream with a diet soda. He's fucked in the first place, the diet soda just makes him feel better about it.
 
you may need to add at a minimum coupling caps... the signal as it is going into the final stages often is sitting on a sustantial dc voltage and is cause for concern...
 
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