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Author Topic: Preamp Based On Modified Fender 6G6-B  (Read 4502 times)

jaylow

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Preamp Based On Modified Fender 6G6-B
« on: April 25, 2013, 08:48:06 PM »
Hello!

So I took a modified Fender Bassman 6G6-B schematic and borrowed some ideas from the Professor Tweed (using FETs and a low pass filter at the end) and came up with this schematic, which I want to ultimately use as a preamp for a DIY combo amp. I've kinda gotten stuck on it and can't seem to get it to work. I was able to biased the FETs to half the voltage (biased to about 7.5V from a 15V power supply). But when I try it out on my amp, I don't get any sound at all. Can anyone suggest why I'm not getting any sound? Will this hypothetically work at all right now?

Professor Tweed Schematic: http://www.runoffgroove.com/professor.html
Modified 6G6-B Schematic: http://web.archive.org/web/20020317060405/http://home.cfl.rr.com/dbhammond/baby_bassman.gif
My Schematic: http://www.freestompboxes.org/download/file.php?id=21284&mode=view

Here's the stripboard layout I came up with. I've built it but I don't get any sound out of it.
Stripboard Layout: http://oi49.tinypic.com/1zdlkl1.jpg
« Last Edit: April 25, 2013, 10:29:24 PM by jaylow »

Roly

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Re: Preamp Based On Modified Fender 6G6-B
« Reply #1 on: April 26, 2013, 04:09:01 AM »
Welcome @jaylow!

Well, nothing at all coming out is actully better than "slightly muzzy" or whatever.

{Looks at Prof Tweed circuit}  AAARRRGGHHhhhhh... OH NOEZ, not THIS  :duh  thing AGAIN.  I do wish runoffgrove.com would withdraw this abomination.}

We have already had several threads about this so-called "design".  You may notice that in the "Baby Bassman" NOT ONE of the anode loads is a trimpot.  The reason for this is that the person who originally designed the Bassman, Leo Fender, actually knew something about electronics.

The long and short of it is, if you were going to make anything adjustable to compensate for FET spread it would be the Source resistor, not the Drain, because the Source resistor only effects the DC conditions, while trimming the Drain changes both AC and DC conditions at the same time (and incidentally allows the Drain load to be "adjusted" to zero which is an absurd situation in a preamp).

So, what can be done?

Set all trimpots to the maximum resistance (lowest Drain voltages).

In each stage reduce the trimpot value until the Drain is at half the supply.

Check that the Source voltages on each FET are a fair bit lower than the Drain voltage (i.e. the FET isn't saturated).

Connected to an amplifier you should be able to hear crackles and pops when you probe around the output with your meter.  Follow the circuit back towards the input probing at each Drain and Gate point to confirm you are getting pops and crackles.  Where this stops happening you have something wrong, a wiring mistake, a missing strip cut, a FET in backwards, a solder dag shorting tracks, etc etc.

There is no way to predict the overall gain because the "design" is totally at the mercy of the FET spread, when a good design is independent of component spread.

This is my take on a FET preamp - no trimpots at all.



http://www.ssguitar.com/index.php?topic=2912.15
If you say theory and practice don't agree you haven't applied enough theory.

jaylow

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Re: Preamp Based On Modified Fender 6G6-B
« Reply #2 on: April 28, 2013, 04:00:26 PM »
Thanks for the debugging tips! Hopefully I can find the problem and fix it up. It's my first foray into DIY circuits.

jaylow

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Re: Preamp Based On Modified Fender 6G6-B
« Reply #3 on: May 01, 2013, 08:05:29 PM »
I've had to put my project on hold. Turns out I had a bunch of bad transistors.

Roly

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Re: Preamp Based On Modified Fender 6G6-B
« Reply #4 on: May 02, 2013, 12:30:50 PM »
I've had to put my project on hold. Turns out I had a bunch of bad transistors.

That's quite unusual unless you got them second hand.
If you say theory and practice don't agree you haven't applied enough theory.