Turn amp in preamp

[Bernardduur]

Hey all!

I had a small question; how can I turn an full tube amp into a preamp, the common way? I have a Boogie that a friend wants to use as a standalone preamp (and switch the preamps in his playing). He wants to use it without a load........ How can I do this without destroying components???

Thx!

[Albert Kreuzer]

Just pull the power tubes.
Preamp voltages will be slightly higher after that, but not dangerously higher.

Cheers,
Albert

[Bernardduur]

Thank you very much!

I was thinking way more difficult then this. It is simply removing the power tubes; then you can run an amp without a load as a preamp?

Sweet!

[Albert Kreuzer]

The only thing you might want to check: Is there any NFB that goes from the speaker output to a point BEFORE the line out?
If so, you should use a dummy load and keep the power tubes in.

If not: Yes, it's just that easy

Cheers,
Albert

[chipaudette]

Like most amps, Boogie's do have negative feedback. Your plan might have some difficulties (of course, it might also work just fine).

But, if you just want the preamp, the easiest thing to do is to use the signal that is output by the amp's effects loop. That's what it's there for. Just plug a cable into the "send" and use it for whatever evil purposes you want.

Then, because the "return" is buffered, you can pull the power tubes despite the negative feedback. The buffer automatically isolates any changes on the power amp side from the pre-amp side. Pull the tubes, and you get silence. Now it's safe to unplug the speaker. You're done.

Chip

[Bernardduur]

Thank you!

The amp here is a Mesa Boogie Caliber 50+.

schematic on the preamp
Schematics on the poweramp

[JoeM]

I use one of my amps as a preamp. It has a pre-out - pwr amp in jacks (effects loop). I dont even unplug the speaker, I just use a shorting jack in the power amp in.