From: META000 <META@UNB.CA>
Subject: RE: misunderstanding about Bassman
Organization: The University of New Brunswick
Date: Wed, 23 Dec 1992 16:39:30 GMT

It seems that, once again, I've been talking off the top of my head
(busy little place, that) with respect to the Bassman, and there
have been some misunderstandings, primarily based in terminology.

1) The two CHANNELS on the Bassman appear to have the same gain
structure.

2) The two INPUTS to each channel do not present the same signal
level to the pre-amp tubes.

Glancing at the schematics for not only the Bassman, but a whole
array of Fender, Gibson, and Ampeg amps, whenever there are two
input jacks for the same channel, one generally tends to find the
following arangement:

  Input 2    to-preamp    Input 1       Ground
    |----68k----|----68K----|------1meg----|>

To the untechnical among us, from the Input 2 jack's perspective, it
looks like it is going into a 1.136meg volume control (1.0M + .068M
+ .068M = 1.136M) that is set to about 6% down from the top of its'
range (the 6db and 6% are mere coincidence, I think, or are they?
Don, Dave, RG, help me out here).

That is, from the point where the Input 2 signal hits the first
tube, there is a 68K resistanceto the signal source, and a 1.068M
resistance to ground.  In the case of Input 1, there is no such
"voltage divider" present.

Sowhat does this mean with respect to the "stupid trick"?  Let's see
how I screw this one up....

Consider that if we take the signal appearing at Input 1 in one
channel and tap it out of Input 2, and THEN run it to Input 1 on the
OTHER channel, it has now run through TWO 68k resistors before
encountering the 1meg resistor going to ground.

      Input 2A    to pre-amp    Input 1A      Ground
         |-----68K-----|----68K----|-----1 meg----|>
      To input                     ^.....................From guitar
        1B .........................
                                   V
         |-----68K-----|----68K----|-----1 meg----|>

This is tantamount to a 1.136M potentiometer set to 12% down from
the top of its range, which means that the signal hitting Input 1B
is a lower level than that at Input 1A.

Now, if this wasn't complicated enough, consider that the jacks at
Inputs 1A/2A and 1B/2B are closed circuit jacks! What this means is
thatwhen nothingis plugged into Input 2A, it is effectively tied to
Input 1A so that the two 68K resistors are now in parallel (making a
34K path tothe tube).  When you plug into BOTH 1A and 2A, that
connection is brokenand the 68k resistors now act like 68K
resistors.

Okay, Dr. Stupid, put on the patented stuponitron helmet, and tell
me what it means.

What it means kids is that the "slaved" second channel, while having
the same gain as the first, is seeing a somewhat lower level signal
than the first channel, so whileit won't result in nearly as much
distortion in its OWN pre-amp stage, when the net effects of Channel
A and the slaved Channel B are added together at the mixer stage,
the total will be sufficient to more easily overdrive the mixer,
albeit without as much high end as Channel A alone.  I suppose
thatif you want a smoother sound, you simply turn down the brighter of
the two channels and turn up the muter one.  If you want an even
mellower overdive sound than that (Moody Blues or Cream-era Clapton
anyone?), you can defeat the bright capacitor on the bright channel.
Fortunately, the Bassman creates most of its distortion (at least
the non-speaker, non-transformer type) BEFORE the tone controls,
rather than after them, so cutting out the harsh high end can be
easily done.

In retrospect, not such a "stupid" trick after all.

Hope you all get that sunburst, rosewood, whatever that you're all
hoping for.  Me, I got my Bassman to finally work, just in time for
Chanukah. (Have you ever considered what "I had a little dreidl"
would sound like if Billy Gibbons played it?  Geez I need a holiday
badly!  This is one sick puppy)

Best of the holidays to ya!  Mark Hammer