Weird question. Is there anyway to mod a tube amp to act just as a power amp? Like to where none of the tone knobs work just the volume, and then run a pre-amp through it?

Yes. I would think that it is too much work though. Just buy a preamp. Isn't that the right answer.

Possibly, dependent on the design.

You could possibly add a break-jack prior to the power amp section with some consideration.

Roland JC 120s have "power amp in" jacks. I think there are other commercially-available amps that have this feature as well, but I can't think of them now. Maybe Ampeg V-4s?

the old ampegs have line out jacks (B25, V4, SVT). i've plugged a guitar directly into the line out jack of an SVT to inject sound *into* it, and got sound out, but that was just for troubleshooting. I got sound out. I dunno how good it would work, only because of impedance issues. somebody else who understands it well and can look at a schematic could probably answer that one.

b-vapor wrote:Weird question. Is there anyway to mod a tube amp to act just as a power amp? Like to where none of the tone knobs work just the volume, and then run a pre-amp through it?

Yes. Most tube amps can be modified in this way.

A buffer of some sort may be needed before the power amp stage, depending on bias currents/voltages at the power amp's input.

The old Ampeg heads, many of them actually have 'power amp in' jacks in addition to line outs. But if there's no 'power amp in', the line out will usually work as an input on a tube amp.

The reason is that the impedance into the preamp stage is the output impedance of the preamp, which is very high. And the impedance of the power amp stage's input will be quite low. The signal you inject will follow the path of least resistance, into the power amp. It might not be optimized for this use, but it will often work.

I just ran a modern SVT preamp into an old V4 power amp to see if it was better than the modern power amp we were using. It was. A lot better.

interestingly I actually have an ampeg v-4b, haven't looked at the inputs/outputs though. The head i was thinking about doing this mod to is an old sunn solarus. Just something I was curious about because I just got a traynor ts-25b for $80 and was looking to use it as a pre amp.