Building/converting a power amp

[tlainhart]

I've got a discontinued Allen amp kit that's basically the normal channel of a Fender BF, about 35-40watts w/ 6L6s. I'd like to try and build this such that I can take a variable pre-amp out of a modeler and put it right into the power section of this amp, sort of like Atomic Amps Reactor.

Can someone point me in a direction on how I could learn to proceed on this path?

[Bob-I]

Sure. Can you post a schematic?

[tlainhart]

Sure. Can you post a schematic?

Here's a link to the schematic: http://lainhart.net/Amps/5F2V-Schematic.jpg

Trading mail with David Allen, he's assuming that the PI can take line-level inputs but he's not proven it or worked it out. He also recommended that I check out Rivera-era Fender amps and their effects loops for some clues/insights. Being a noob, trying to interpret those circuits is probably above my current paygrade.

What I'd like to do is to have one input use the tonal/power section of the amp as designed, as use the other input as the line-out of my modeller/fx.

[tlainhart]

(Didn't realize that the comment edit wouldn't bump the thread)

[Jana]

I think a place to start would be to have an input jack, run the jack to a 1 meg pot and then the wiper of the pot goes to the input of the phase inverter (through the cap of course).

That way you can attenuate the input signal a bit if it is too hot, if not, just turn the pot up all the way.

[tlainhart]

I think a place to start would be to have an input jack, run the jack to a 1 meg pot and then the wiper of the pot goes to the input of the phase inverter (through the cap of course).

The input device in this case will have an attenuator on its output.

Which cap(s) does the shielded cable go to?

[Jana]

okay, maybe just wire up the input jacks like it shows on the schematic and then instead of connecting them to the grid of V1a, connect them to the .0047uf cap going into the phase inverter.

[Bob-I]

I'd do this a little different. I'd take the input, go through a 68K resistor into pin 7 of V1. Remove all the components connected to V1a, including the tone stack, it's not necessary.

Out of V1B plate, pin 6, replace the 220k resistor with a .022uF / 600V cap then into a 1M pot pin 1. Take pin 2 (wiper) of the pot into the .0047uF cap and pin 3 to ground.

The signal path would be:

Your preamp -> gain stage -> coupling cap .022uF -> attenuation -> phase invertor.

If that's not clear I could draw it up. Let me know.

[tlainhart]

Hey Bob -

Thanks for the reply. I didn't get notification to the thread update, and just now thought to check it.

Your mods description - I'm taking this to mean that your design is dedicating the amp to just the power amp support (I'm a beginner, obviously). Is there a design where I can still use one input for the tone stack, and the other for the preamp->poweramp?

-- Todd

[Bob-I]

Hey Bob -

Thanks for the reply. I didn't get notification to the thread update, and just now thought to check it.

Your mods description - I'm taking this to mean that your design is dedicating the amp to just the power amp support (I'm a beginner, obviously). Is there a design where I can still use one input for the tone stack, and the other for the preamp->poweramp?

-- Todd

Yes, that's not difficult at all, in fact maybe easier than redesigning. You'd simply use a switching jack to input to the power amp, and break the signal path from the preamp. Look at the typical loop circuit and use just the return side.

[tlainhart]

Yes, that's not difficult at all, in fact maybe easier than redesigning. You'd simply use a switching jack to input to the power amp, and break the signal path from the preamp. Look at the typical loop circuit and use just the return side.

Thanks. I'll look around for an FX loop circuit.

-- Todd