Help with modded Bassman

[exluthier]

I was thinking of having my Bassman modded so I opened her up to see what the interior was like since it looked like it had been modded before (two unused extra holes in the chassis) anyway it looks like the guitar side is still the ab165 circuit but I don't know what's going on with the bass side. Any help? Thanks.

http://i887.photobucket.com/albums/ac77/ozimandius00/j1067.jpg
http://i887.photobucket.com/albums/ac77/ozimandius00/j1068.jpg
http://i887.photobucket.com/albums/ac77/ozimandius00/j1072.jpg
http://i887.photobucket.com/albums/ac77/ozimandius00/j1076.jpg

[smolder]

I would not worry about unused holes in the chassis so much. Pull down the layout diagram from (ampwares.com) and compare. It looks like a 68 or 69 given the cloth wiring and the blue molded caps (good stuff)... the brown blob caps are the next phase of part fender used after blue blobs (which followed the blue molded that you have). I have never seen the brown blob and blue molded in the same amp, but that's not saying it didn't happen.

The black spragues are not uncommon replacements for electrolytics. I have seen orange caps in that position a lot.

The two abnormal things are the cap across V1 and V2... and the cap/resistor pair from the board to V2.

If you want it back to spec... a good tech could do it without much trouble. There are many mods for making the bass channel usable. How does yours sound?

[exluthier]

It's actually a blackface. I've always thought it sounded great on the guitar side and I never really messed with the bass side much though it looks like it's been modded so I was playing around with it yesterday. There's an extra wire running from jack2 on the bass side so I'm going to play with that today and see what it sounds like. I was planning on having the bass side modded but it looks like someone already got there so I just wanted to understand what was going on before I start talking to an amp tech.

[davemccarthy707]

Someone has added in the extra triode that is usually unused in the bassman. That's what it looks like anyways

[exluthier]

Would that only affect the bass side? I really appreciate the input.

[Ronsonic]

I don't think it's "the bass side" anymore. Play it, what's it do?

[exluthier]

Okay on both "sides" input one is considerably hotter than two. Input one on the normal side is the punchiest of all of them and I needed to roll the treble off a bit to keep my ears from bleeding. On the bass side I needed to roll the bass almost down to 1 to keep it from being flubby but other wise it has a nice tight crunch to it. I had both volumes set to the same level to compare everything. Damn I forgot how loud I could crank this thing.

[donnievaz]

I have never seen the brown blob and blue molded in the same amp, but that's not saying it didn't happen.

I have a '66 that has both.

[WaltC]

so here's what it looks to me (from the pics) like has been done to your bassman:
- the orig. model has an available 1/2 12ax7 unused and someone has modded your amp to use that triode as an additional gain stage by :
-- moved the initial input wire from V1, pin 2 to V2 pin 2
-- connected a 100K resistor to the B+ rail to get power to the plate at V2, pin 1
-- connected the plate of V2 (pin 1) to the grid of V1 (pin 2) to get the guitar signal back into its usual channel, but w/ 1 triode more gain
-- added a paired cathode resistor/bypass cap to the cathode of the first half of V2 at pin 3

- some more-or-less normal looking maintenance seems to have been done, like:
-- filter cap job, including cathode bypass and probably the bias filter cap
-- changed the PI/power tube coupling caps to orange drops, looks like maybe .1uF

that's all I can see from the pictures

[exluthier]

That's a great summary. Thank you very much.