Fender BF Bassman mods
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Fender BF Bassman mods
From ic–(at)–bbot.Colorado.EDU Thu Mar 2 11:57:42 CST 1995
Article: 42213 of rec.music.makers.guitar
From: ic–(at)–bbot.Colorado.EDU (John Mastrangelo)
Subject: Re: BF Bassman mods?
Date: 2 Mar 1995 09:59:12 -0700
Organization: University of Colorado, Boulder
Xref: geraldo.cc.utexas.edu rec.music.makers.guitar:42213 alt.guitar:37395
In article <3isums$lt--(at)--cd1.fm.intel.com> dcovel–(at)–rx702.intel.com (David Covell – MPG DA) writes:
I own a ’68 Bassman (AB165) which I like to use as a project amp
so I am speaking directly from experience.
>I picked up a mid-’66 Bassman last weekend; it’s an AB165 circuit and sounds
>really great as it is (stock). However, I’m still interested in mods for it to
>get a bit more gain at lower volumes without sacrificing the clean tone.
More gain? Mine breaks up at 3 on both channel with vintage
Strat pickups. How much gain do you want?
>I got the Bassman because the extra tube and larger output transformer make it
>a promising mod platform for me to experiment and learn on. I see two simple
>mods and a more complex one as good candidates:
>1. Put a pot (1M or larger) in series with the 470K resistor in the feedback
>loop across the extra gain stage (preceding the splitter) to allow variable
>reduction in negative feedback and pick up some more gain into the splitter.
>Question here: will that overdrive the splitter, the output, or both? And is
>that a good idea?
WARNING!!!!!!!! THIS IS A TERRIBLE IDEA!!!!!!!!!
It is a very unsafe practice connecting plate
voltages to panel mounted controls, especially 99 cent pots.
I know of _no_ amp designs that implement any of these
Increasing the value of that resistor will have no audible
effect. The resistor is there to stablize the gain stage, not
to set the gain. In fact, the gain of that stage looks something
Gain = mu/( 2 + x/Rf)
where mu is the gain of the tube , and “x” is much smaller
than Rf (the feedback resistor).
(I did this analysis a few months ago and have it documented,
although it is not handy, I’ll post it tommorrow…..)
So you see, the feedback resistor has vitually no effect
on gain untill it is reduced to a value less than “x”, which
is much smaller than Rf ( again, I’ll post a value tommorrow).
The previous Bassman AA864 circuit uses a 220K/220K voltage
>divider into this stage rather than negative feedback. That would indicate a
>%50 reduction in signal into that stage, right?
Yes, and the .001 cap creates a low-pass filter.
>Can I assume then that the 470K R in the AB165 is performing roughly
>the same reduction?
Absolutely not. One is a negative feedback circuit. The other
is a voltage divider/lowpass filter.
>2. Put a double pot of at least 250K in series with the 220K feedback resistors
>from the output plates to before the coupling caps to reduce negative feedback.
Yikes! Please don’t connect the plates of your power tubes to
any panel mounted pots. Don’t take this personally, but this is
a perfect example of the saying “A little bit of knowledge is
a dangerous thing”
>Or, should I remove these and configure the feedback loop like the standard 820
>used in most Fenders?
It is safe to remove the resistors .
These resistors limit the gain of the power stage. Removing them
will give you roughly 3dB more gain (depending on what tubes
Converting to the “standard feedback circuit” as seen in most
Fenders will give you an 18dB boost at 60 Hz. It’s like turning
your bass control up 10 more numbers. It also _decreases_
gain by 3dB from roughly 1kHz up.
I know this for a fact because I performed this mod on _my_
AB165 and documented it with a pink noise generator and
>3. Add a cathode resistor and bypass cap and insert the unused half of the
>extra tube either before or after the tone stack.
With your current level of understanding, adding a gain stage is
way beyond you (not a flame).
Can extra stages be added
>without the pronounced high-pass effect they seem to have in Boogies?
>complaint with Boogies and other cascaded-stage overdrive amps I’ve heard is
>that as soon as the extra stage is switched in the lows are just gone. Is that
>intentional to avoid flubbiness at high gains?
I’m looking for subtle gain
>enhancement and fatness rather than massive distortion so I want to preserve
>the frequency response on the clean tone whan I add more gain.
Now you’re talkin’.
Remove those 2 pesky 220k feedback resistors
off the power tube plates. You’ll get about 3dB
more gain and the bass will “tighten up”. Jack Zucker
and I both did this mod and we both got similar
results and liked it. This may be all you need.
If this is not enough, e-mail me and we’ll take
the next step.
>As is likely obvious from the above naive speculations I’m still on the steep
>part of the tube amp learning curve. I’d thus appreciate any advice about how
>to optimize the Bassman (or my Showman head) for fat clean tones and subtle
That’s o.k. We all have to start at the bottom. Tube amps
are both very delicate and very dangerous things. Remember,
you are playing with fire.
On a side issue, several weeks ago you wanted to “warm up”
your HiWatt. I gave you a tone stack cap change that will
give a you a warmer low end. Did you do it?