Fender SF Bassman 100 missing 220k mix resistors in preamp

[nevetslab]

A client brought me his replacement Fender SF Bassman 100, after loosing his previous Bassman 135 amp that I had serviced for him years ago in an apartment building fire. This replacement amp appears to have been dropped on one end, perhaps in shpg from where he bought it (ebay?). It had apparently been serviced recently, recapping the unit, which I did find. The Output xfmr's mounting ears got bend in the fall, so it was cattywampus from normal, and he was reluctant to even power it up without having it looked at.

After I removed, straightened the mtg flanges, tightened up the core bolts and put it back into place, along with the choke which likewise took to leaning from the fall, I then went thru the top side of the tag board inspecting the build, and replaced the aged White Mallory 5uF cathode bypass cap of the Input stage tube. That's when I noticed the two 220k mix resistors off of the common output stage tubes, which share a common 820 ohm cathode resistor. The lead wires simple had a jumper tack-soldered to them where the resistor bodies had been. (My GRN probe points to the locations).



Fender-Bassman-100-Schematic.pdf

So, I have to ask.....why would these mix resistors be removed, placing the two preamp stages' plates in parallel? Any reason to restore the 220k mix resistors, or just leave it as is, assuming there would be additional overall gain as a result?

I found the Death Cap was still in place on the rear panel, which I clipped out.

The Bias circuit had been modified. The 470 ohm resistor from the bias tap had been replaced with a 2.2k resistor, 100uF/100V following the rectifier, then that fed the top of the bias pot, the tap had been removed, with a 15k resistor off the end of the bias pot to GND, so that bias balance circuit was changed to traditional bias adjust pot. I haven't yet checked the power tubes.

[pdf64]

The mod turns it into shared anode mixing. I don’t think there will be much if any gain advantage.

[g1]

The newish 'custom' series of SF re-issues has similar modification. In the attached circuit, V1B and V2B plates are tied together.
It makes reverb available to both channels, and as Rob Robinette has pointed out, puts the 2 channels in phase so you can put a jumper cable between the inputs like you can with the tweed amps for thicker tone.

With no reverb in the bassman, it doesn't seem there is much advantage for that amp.

[Helmholtz]

The mod turns it into shared anode mixing. I don’t think there will be much if any gain advantage.

Yes, total gain is essentially the same with both wirings.

Without the mixing resistors each tube loads the other and gain is halved.

With mixing resistors gain is halved by voltage divider effect.

[nevetslab]

I had thought about the Custom Deluxe Reverb, it too having the two 2nd stage tubes' plates connected in parallel, along with their cathodes. Which did allow the Normal Ch to have the Reverb. I'll leave the mod as is.