Adding an external preamp to a Bassman

Tube or solid-state, this section goes to eleven!
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dave_b
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Post by dave_b »

Hi all,
I've been reading Martin Blencowe's preamp book and I'd like to use the power amp of my 1976 Bassman 50 to test out some ideas. I've had the chassis in a cradle several times since '91 so I'm comfortable modifying it. If it matters to anyone, this wouldn't be a permanent thing. I just want to mess around first before I commit the cash needed to build a dedicated power amp.

The idea is to build a high-gain preamp with it's own power supply, then tie the output into the Bassman's phase inverter. I'm assuming I'll need to disconnect the existing preamps since there isn't any other isolation there, but that's just a guess on my part.

Does anyone see an issue with this idea or have any suggestions?

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phatt
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Post by phatt »

Hey Dave,,
I'd do a bit more reading first as you have drawn the common of your preamp add on directly to the 400VDC rail of the main Amplifier. [smilie=a_reading.gif]
You will likely kill the amp or worse,, kill yourself. [smilie=pope.gif]
Frankly I avoid modding amps as it's often counter productive,, a few tweaks maybe but pedals now are very good at altering the sonic signature.
Phil.

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Post by dave_b »

phatt wrote:Hey Dave,,
I'd do a bit more reading first as you have drawn the common of your preamp add on directly to the 400VDC rail of the main Amplifier. [smilie=a_reading.gif]
You will likely kill the amp or worse,, kill yourself. [smilie=pope.gif]
Frankly I avoid modding amps as it's often counter productive,, a few tweaks maybe but pedals now are very good at altering the sonic signature.
Phil.
Eeks, thanks. For what it's worth, that was careless drawing. I wouldn't have hooked it up that way. :) It looks like I can't edit that original post so here's the revised schematic.

I see your point on pedals and I don't have an issue using them. My goal here, though, is to apply what I've learned about tube preamps without building a power amp (for the time being). If I wait until I have the funds to build the whole thing, I'll probably never get it done.
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Post by phatt »

Yo all good ,, yes I thought it might be just a mistake but better to be sure. :thumbsup

Just remember it's not just One part that makes magic,, it's the combination of the Whole circuit. Just changing the value of resistors along the supply can make substantial changes to how the whole amp
circuit responds to the signal. Your circuit is not the more common Bassman circuit so if you are wanting more crunch you may need to alter more than just the preamp.

Your Amp has an AT7 PI which will likely make the output stiffer (clean).. An AX7 there will likely make more dirt but you may have to adjust the voltage as well to make it sound
convincing.
I recently rebuilt a Marshall TSL100,, (long story) but the 5k resistor that feed power to the PI (AX7) was replaced with 15k and that removed the brittle hard edge clipping,, a much more realistic and more playable Amp. Smoother transition into OD
Also Feedback adjustment may help change things, on some circuits you can remove it,, but I'd research that first. :hmmm:

Others here will know far more and hopefully they may be able to give more indepth options. :thumbsup

Oh yeah,, I just read you other posting *John Mcintyre Fender Bassman Mods* Your Amp is AA371 so it's going to respond in a different manner,, nothing like the other bassman
Phil.

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