COMPLETE Fender Bassman AB165 to AA864 Conversion (w/ Schematic)

[lightbath]

I am in the process of converting an AB165 Fender Bassman to AA864 specs. In the schematic below, I have outlined every component difference in red.

So far I have removed the 500pf and .01uf cap in parallel with the 100k resistor in the preamp and removed the two 220k resistors in the 6L6 feedback loop. This has opened up the amp ALOT. I am already blown away by the improvement. Removing the caps added a lot of high frequency back into both channels but removing the 220k resistors was the biggest change. The amp had an odd compression and squash before the mod. Now the amp has a sparkle and feel more like (but not quite as awesome as) my stock 66' Showman.

UPDATE: This morning I removed the 470k resistor in the feedback loop of V3. On first listen, this is not an improvement but I imagine the amp will sound better with some of the modifications ahead. I anticipate increasing the value of the resistor in the negative feedback loop from 47k to 820 ohms will go a long way in reducing the gain and volume, as well as giving it a sound more similar to my beloved Showman. The amp currently has way too much gain and the cleans have suffered. The amp is louder (obviously because I removed some of the negative feedback), some of the high end is gone, and the amp does not seem to have nearly as much punch and clarity as before.

SO MY QUESTIONS IS: Does anyone have any personal experience with any of these modifications and how they will effect the tone and response of the amp? (i.e. changing the value of the coupling capacitors, altering where the feedback loop feeds into the circuit, changing the bias circuit back to aa864, etc). Are any of these changes not advisable.

I have attached an annotated AB165 and AA864 schematic for reference.

[Paul G.]

I have done this to my AB165:

Mods
1. Remove output stage negative feedback circuit -- highly recommend.
2. Convert bias balance to true bias -- the stock bias is very cold, true bias lets you use a variety of tubes.
3. All brown turds replaced with Sprague Orange Drops -- not necessary.
4. Remove plate bypass caps on Normal and Bass Channel -- as you've noted, opens up the trebles.
5. Convert to AA864 negative feedback scheme, 820R feedback resistor,
reverse OT primaries, reconfigure Phase Inverter (100K/82K plate resistors) -- not sure how much this did, I do like the response of the amp, so...
6. Put both plates of V3 on the same B+ node -- I was chasing down a bit of noise, I think this helped but you don't need to do it if you're ok.
7. Reconfigure Bass channel to Trainwreck spec's
250pf silver mica treble cap/.022 mid cap/.022 bass cap/100k slope resistor -- this configuration gives a rich tone with less mid-scoop but still punchy. Nice to have different sounding channels.
8. Remove Deep circuit, replace with Bright switch (120pf cap) -- useful.
9. Shielded cable input to grid, volume to grid V1, V3.
10. Add 1R cathode resistors to assist checking bias.
11. Switchable NFB using ground switch.
12 Hum balance control -- not necessary.

I wouldn't take the third gain stage of of the normal channel, first -- why? second -- if you do modify one of the channels, you can jump them to get more variety.
I didn't touch the local feedback on V2.

Amp as is is killer, responsive and lively. Once the volume is above 2-1/2 it sounds great, and it can be used at low volumes, but is ridiculously loud if you want it.

[BobSimpson]

[lightbath]

Thanks for the helpful info Paul. I have done most of the mods in the schematic and some you have listed and I can confirm the amp sounds better than ever. BUT it is also louder and breaks up WAY early. I know the Bassman is known for early breakup but the
volume is just way too much and the drive becomes out of control at anything above 4-5. I can play at higher volumes with the volume rolled back but the amps drive / distortion just doesn't sound right at super high volumes. And the usable range on the volume knob is limited (from about 0 to 3)

I have 2 other vintage Fender amps. An original 66 Showman and a 71 Bassman with essentially all the same mods.The volume knob on both amps has a much more usable sweep than my 65 Bassman. My Showman is fairly clean all the way up to 10 and the 71' Bassman has a perfect gradient of volume and gain increase all the way up the knob. Never too harsh or overly distorted even at the most extreme settings. My 65' Bassman is probably the same volume (and tone) on 3 as my 71' Bassman on 10. It's almost like the volume and drive just keep going.

[brewdude]

Are you sure you have a log taper volume pot?

[lightbath]

I checked the pots and they are indeed log taper. I am at a loss here. The amp sound GREAT. It is just LOUD. Louder than my DC30..

Biased around 60%. Tubes fairly matched. All voltages check out. Not sure what the problems is. Again, any suggestions appreciated!

[Davidg]

Check your gain staging .the AB165 stick used 3 gain stages in the bass channel plus the mixer with 470K local negative feedback loop around that triode. The AA864 used only 2 gain stages in each channel plus a mixer. You need to lose some gain with either a voltage divider or byoass a gain stage.

[lightbath]

Thank you Davidg. You are right on. I ended up rewiring the normal channel to have 1 less gain stage and the amp is near perfect (could maybe use a slight adjustment on the bass channel but I am already very pleased with the results). It sounds good all the way up the dial on the normal channel, and the volume is much more what I was expecting.

[G-SPACE]

I am in the process of converting an AB165 Fender Bassman to AA864 specs. In the schematic below, I have outlined every component difference in red.

So far I have removed the 500pf and .01uf cap in parallel with the 100k resistor in the preamp and removed the two 220k resistors in the 6L6 feedback loop. This has opened up the amp ALOT. I am already blown away by the improvement. Removing the caps added a lot of high frequency back into both channels but removing the 220k resistors was the biggest change. The amp had an odd compression and squash before the mod. Now the amp has a sparkle and feel more like (but not quite as awesome as) my stock 66' Showman.

UPDATE: This morning I removed the 470k resistor in the feedback loop of V3. On first listen, this is not an improvement but I imagine the amp will sound better with some of the modifications ahead. I anticipate increasing the value of the resistor in the negative feedback loop from 47k to 820 ohms will go a long way in reducing the gain and volume, as well as giving it a sound more similar to my beloved Showman. The amp currently has way too much gain and the cleans have suffered. The amp is louder (obviously because I removed some of the negative feedback), some of the high end is gone, and the amp does not seem to have nearly as much punch and clarity as before.

SO MY QUESTIONS IS: Does anyone have any personal experience with any of these modifications and how they will effect the tone and response of the amp? (i.e. changing the value of the coupling capacitors, altering where the feedback loop feeds into the circuit, changing the bias circuit back to aa864, etc). Are any of these changes not advisable.

I have attached an annotated AB165 and AA864 schematic for reference.

First post in this forum - hello all!

It's funny that I just found this thread. I began a similar conversion of a new (to me) '66 Bassman that I started right around the time you did. I, similarly, didn't like the massive gain present in the Normal channel, which I attributed to the 3rd triode, and bypassed it in a nearly same way as your schematic shows. I did, however, keep all the filtering that it looks like you removed, and directed all my 1st stage stuff to that last node. The amp right now is extremely quiet, and I'm very happy with how it's progressing.

With that bypassed, I decided to use both the unused triode on V2 and the now freed-up triode for a Plexi/JCM preamp on the Bass side, which will be configured much like a Marshall JMP 1987 with a cold clipper stage (a-la JCM800) that I will have on a switch.

As it stands, both preamps are currently working. I'm extremely happy with the Normal/Fender side, which has a lot of the tone that we get from the '73 Pro Reverb, which we've also rebuilt and sounds gorgeous. It stays clean all the way into the upper 4s, and then breaks up gradually. Goosing it (little drive, level set a little past unity) with a Tube Screamer is absolutely killer with my LP (humbucker-sized P90) and Strats.

The Bass/Plexi side is still a work in progress. With the tone stack and controls reconfigured to a 3rd-stage cathode follower, the amp is a bit dark and the tone controls don't seem to influence the tone as much as shaping it slightly. I'm planning to try different slope resistor values and see where that takes me, along with some different bright caps (current 500pf is super ice-picky).

The other problem I'm going to be working through is that I'm getting ghost notes at certain frequencies.

Here's a copy of my schematic I've drawn up with the changes I've listed and more. Still to do is adding a master volume (will probably just be on the Plexi side, right after the Treble control) and a bias switch that will let me switch between standard negative bias and cathode bias on the power tubes.



I've done a fairly extensive build thread on another forum I'm involved with. Please check it out: https://www.guitargearforum.com/forum/v ... =5&t=48867

And, if you haven't seen this, it gives a really great representation of a stock AB165 tone: