59 bassman
Kevin O'Connor tips to backdate your new Fender Bassman RI.
"A customer of mine had the same complaints (i.e. too trebly and twangy) about his "re-issue" Bassman. The amp is not an exact copy of the 59, so there are a couple of things you can have a tech do:
1) There are likely to be rattling problems if you try the tube rectifier route. I installed a couple of power resistors in series with the solid-state rectifiers to achieve tube-like sag with better reliability and no tube rattles!
2) Replace the 'presence' pot with a 1-meg audio taper
3) Add a pair of 1k5 grid-stopper resistors to the power tubes (one per tube)
4) Rewire the 'presence' control to be a 'cut' by: slashing a trace on the control PCB; move the end of the 4k7 to ground; with a shielded 2-wire cable, tie the new pot across the grid-leak side of the 1k5 grid-stoppers, with a 5nF-400V plastic cap in series with one of the pot leads (NOTE: You only have to use the wiper and one end of the pot)
5) For best versatility of tube selection; Add a second bias circuit and connect the existing one to one power tube and the new one to the other. This allows hum-free performance to be obtained from any set of tubes.
6) Install some 1-ohm current sense resistors in series withpin-8 of each power tube to ground. This makes reading bias currents a lot easier and safer (than the transformer-shunt method)."
The grid-stoppers roll off the high end a little in the power amp, and makes the circuit truer to original form. The re-wired 'presence' gives at '10' what tone you used to have at '1'-- now you can smooth out the highs at will, while a non-zero setting of your 'treble' pot allows some high-end sparkle. These mods allow a mellow vintage tone to be obtained from the new amp, with greater reliability.

My personal bassman RI tweaking.

Tube set-up:

  1. 1st - 12AY7 (Philips NOS)
  2. 2nd - 12AX7 (Philips NOS)
  3. 3rd - 12AU7 (Mullard NOS)
  4. 4-5 - 6L6GB (Sylvania NOS)
  5. 6th - 5U4GB (Sylvania NOS)

Speakers:
4 Jensen P10R

Circuit changes:
1. Change R38 (4.7k) with 10 k/1W: this mod reduces voltage on V1 and V2.
2. Change R17 a.k.a. slope resistor (100k) with 56 k/1/2W: this mod makes the bass clearer.

Control settings:

  1. Plug in normal 1
  2. Vol normal 6
  3. Vol bright 6 *
  4. Treble 4
  5. Bass 4
  6. Middle 2 (according to your taste and grip it may range from 0 to 4)
  7. Presence 0

* Strange enough but it influences the overall tone by grounding out some highs !!

Please note that these values should be viewed as guidelines only, as several factors come into play here, most notably the Bassman's sensitivity to it's environment. Remember an empty room sounds different from a croudy one. When the room fills, you may be forced to increase vol a notch or two. Other factors include: the sensitivity of the element in a particular mic, its output strength, e.g., crystal versus ceramic vs. controlled magnetic or reluctance vs. dynamic. The presence of the volume pot also influences the results. Last but not the least, your gripping technique and basic acoustic tone.

Jeff Harpdog Hale great experiments with pre-amp tube swapping.
"I thought these recent "tests" might generate some interesting discussion, approaches, and comparisons. Bassman RIs feedback quickly with an all AX pre-amp configuration. So, I loaded a series of alternative combinations for a test. I Placed a fully-open (volume pot at 100%) Green Bullet mic 10 feet in front of the amp and 6 feet high with mic faceplate facing away from the amp. The amp settings for the test were Pres 5, Mid 6, Bass 10, Treb 2.5, Bright 5. The amp has a tube rectifier.

TEST RESULTS

Tubes Tested: AX, AT, AY, (actually used NOS 6072) AU.
(first tube in chart below is on the right looking into the back of the amp)

TUBES VOLUME SETTING FOR INITIAL FEEDBACK
AX,AX,AX too hot to even care about
AY,AX,AX below 4
AY,AX,AT 4.2
AY,AY,AY 4.7 (Harpster Hal used this set up. I liked it)
AU,AX,AT 4.8
AU,AX,AY ??? (a Ray Beltran set-up not included in this test but makes sense)
AY,AX,AX below 4
AY,AX,AU 4.9 (Steven Rousseau set-up)
AU,AX,AU 6.6 (my current set-up, works well in medium size club settings)
AU,AT,AU 7.3 (one of Dr. Abe's suggestions)
AU,AU,AU 12 (it didn't feed at all)

In each case I found the feedback point, went to the mic, picked it up and feedback kicked-in, I backed of the pot to approx. 3/4, went to full cup and did a play test. The testers ears were still full from a cold and no accurate data could be obtained but subjectively I liked AU,AX,AU the best. Good headroom, real loud, easiest to control feedback in a 1.5 to 2 notch range. Some other settings went from slight feed to monster feed in less than 1/2 a click.
By way of information I am including some additional information to help explain the workings of Bassman tubes from Steven Rousseau (off-list, so don't take him to task for any of the data below since it was not initially intended for everyone although I believe it to be very useful and accurate information, but then again, what do I know).

First position is the right most tube, when standing behind the amp. Out of the box Bassman RI configuration.

  1. 1st - 12AX7
  2. 2nd - 12AX7
  3. 3rd - 12AX7
  4. 4/5 - 6L6GC
  5. 6th - 5AR4 rectifier
As you probably know, 12A?7 pre amp tubes are double triodes, which means there are really 2 triodes in one tube. So, lets use the following naming system to specify exactly which tube, and which triode in it we are talking about: V1A, V1B = Tube 1 (rightmost) , side A or side B. So, a bassman's Normal channel signal path is as follows: input -> V1A -> volume pot -> V2A -> V2B -> tone controls -> V3AB -> power tubes. Bright input signal path is as follows: input -> V1B -> volume pot -> V2A -> V2B -> tone controls -> V3AB -> power tubes A Bassman with the regular, all 12AX7 setup: V1A + volume pot = 29 times signal amplification V2A + V2b + tone controls = 38 times V3 power tube driver = 7.6 times power tubes = 8 times So, all together that gives 29 x 38 x 7.6 x 8 = 67000 times signal amplification."
Jeff Harpdog Hale with special thanks to Steven, Dr. Abe, Harpster Hal, and Ray Beltran.