Trainwreck For Vocals & Harmonica

[Dave Lotek]

Has anyone ever experimented with your amp build for small club vocals or bluesharp? I know there's alot more dynamic range using guitar, but I wonder what the vocal texture would be like using a Trainwreck.

[rfgordon]

If I were gonna use a TW-style circuit for harp, I'd use one based on the Blues express. You have to overcome the TW principle of building up a huge clean signal, though, by dropping the plate voltage on the first gain stage to around 80 to 90 volts. This will make it growl at almost any input, plus it will brown/darken the tone quite a bit, which is necessary for blues harp. If you do this, though, you need to put a cap on the input to that gain stage, say a .022mF on the grid, because that low a plate voltage will cause grid emission, and put dc onto the input jack. If your harp player uses a mic with a volume pot, as many new ones do, it'll sound scratchy if there's dc across it.

You can also tinker with the value of the load resistor between stages 2 and 3 to get the right amount of signal onto the PI.

Lastly, wire the speaker out of phase with the input, and you'll be able to give more volume with less feedback.

[Dave Lotek]

Thanks for the reply Rich. This looks like it might be useful for guitar also in lower volume situations.

[rfgordon]

Super-low plate voltages can be useful for guitar IF you want an amp that has basically no clean headroom! I recently built one like that. With a 220k plate resistor and 88 vdc on the first gain stage. Extremely growly beast. Compression and sustain for days. Rather addicting to play thru, actually. No clean tone to speak of, but the customer wanted it dark and agressive, so that's what he got. I'm fixin' to start a harp amp with reverb, and I'll use the same trick.

Gotta remember, tho, to isolate the input jack from the grid with a cap, or you'll get "scratchy pot syndrome."