Fender Bassman AA864 variant - mods

[sliberty]

I've built a single channel AA864 Bassman head that is essentially combination of the bass and normal channels. It uses the topology of the Bass channel, but the tone stack of the Normal channel plus a mid pot. I found the Bass channel in its stock form to be much too dark and a little dull (no sparkle). Note - I will be using the amp for guitar. That's why I switched to the Normal channel's tone stack, although it was still not where I wanted it to be. So, I've made a few additional small changes.

1) Changed the bright cap to 500pF (stock is 100pF - helped a lot with the desired sparkle)
2) Removed the .003uF cap from the plate of V2b (this also helped brightened things up - better clarity overall - much more open sounding)
3) Changed the coupling cap after the plate of V1b to .022uF from .1uF (not sure I hear much difference)

The amp sounds really good, but for my needs, it might have a little too much gain. I am thinking of changing the voltage divider after V1b to throw away a little more gain than it already does. I am also considering removing the 001uF cap on that voltage divider to brighten things up a bit more.

I also plan to add a pre-PI master volume. I will add a cap after the mixing resistors, and then run that cap over to the pot, with the wiper of the pot connected to the 500pF coupling cap on the input to the PI.

Any thoughts on the following would be appreciated:

1) Is tweaking the voltage divider the best way to reduce the gain a bit, or do you have another suggestion?
2) Would reducing the 250pF tone stack treble cap to 100pF brighten things up some more? Do I need to make any complimentary changes?
3) What are your thoughts on the MV?
4) Is there anything that you recommend I do? What will the effect be?

For those of you who have modded an AA864 for use with guitar, please also let me know which mods you've liked.
Thanks,
Steve

[Enzo]

Since the two channels are the same but for the tone stack, I must assume the topology you refer to is adding the phase inversion stage. Since you want it for guitar, something the Bassman has been used for for decades, one wonders why you built the Bass channel. But that's just my view.

Look at a stock Bassman, other than the tone, the two channels work about the same, one is not twice as loud as the other or anything. That means that the third triode is not there to increase gain or up the level. It is just there for tone shaping. You removed the .003, yes, that would brighten it some, shrinking the 0.1 to 0.022 might reduce the bottom some, not really increasing the top, other than relative. But look at the phase inverter input, 500pf. I'd think that would limit the low end quite a lot right there. And you want to change the input to the stage. How about getting rid of the stage altogether? And that brings me back to building the Normal channel. Add the mids control or whatever tone stack stages, but seems to me a lot of effort to turn that third triode stage up above into something that acts like it isn;t there.

You want a MV, in my world I might replace that 220k/220k voltage divider with a 500k pot, then run the wiper over to the 500pf PI input. Eliminate the tube. I am not excited to have a MV there, but there are limited opportunities to place one. You stuck in keeping the stage, then replace the 220k stuff with my 500k pot right where it sits. Then you can adjust the signal level all you want whenever you want.

Mixing resistors? What is there to mix?

[sliberty]

Enzo - you seem a little miffed/impatient with my question. Not sure why.
I am looking for suggestions, not a slap on the back of the head.

>>other than the tone, the two channels work about the same

Well, actually, the Bass channel has a third triode that the normal channel does not. That is why I built the Bass channel topology. And, the Bass channel is louder and much more aggressive than the Normal channel as a result.

Like I said, I am just looking for suggestions.

[J M Fahey]

Won't offer any, given the way you receive them .

[Enzo]

Not in the slightest. I tried to answer directly, and I included my opinion, for what it is worth. I'd rather just tell you what I think, than beat around the bush and equivocate. As long as you are still experimenting, try some of the things I mentioned, like replacing the 220k pair with a 500k pot. It not only is the MV you wanted, but also will demonstrate that changing that voltage divider either will or will not get you where you want to be. And try patching its wiper direct to the PI cap, just to see. Those are two separate steps, you need not do both. Well, or either for that matter.



I know the bass channel has the third triode, that was what I was lobbying to eliminate. I called it a phase inversion stage. perhaps a poor choice of words since it might confuse with the phase inverter for the power tubes. My intent was that the two input channels hit the mixer at opposite polarities, due to that stage. I have never thought of the bass channel as stronger than the normal myself, but if you prefer it, that's fine.

They went out of their way to keep that stage tame. They put in top end removal, which you are deleting, that results in a stronger signal coming out, which you want to diminish with your voltage divider. That may work, try it.

I;ll sum this , then I won;t belabor it further. It seems, if I read right, that you liked the tone of the normal channel better, so you used its tone circuits, then you changed the third stage in various ways resulting in a stronger signal than you want, so you want to diminish it. But you report you thought the bass channel was stronger to begin with. So it just seemed practical to build the normal channel which sounds right and does not have the excess gain, rather than build the other channel; and try to mold it into sounding like the normal.

My comment on the mixing resistors merely meant with one channel there is no mixing to do. Not intended to be flippant.

[Justin Thomas]

Sliberty,

My personal opinion of MVs in these amps is that they are particularly ineffective... I own or have owned an AB165, factory hybrid Bassman 50, two Bassman 100s. I've dimed the channel volumes on all of the preamps of the silverface amps and kept the Master low, but Fender's idea of "any desired amount of distortion" is um, hmmmm... The main thing is, these are not high gain amps by their nature, and the preamp crunching is not exactly what we expect. All of my amps sound best when the master is on 10 and they're played at a ahem, "respectable" volume. Not a bedroom amp. They're quite pedal friendly.

Incidentally, when I bought my first Bassman 100 (which has only 2 stages in each channel) the BASS channel sounded better with a guitar, and the GUITAR sounded better with a bass. At least to us. Go figure.

As far as having too much gain, these things were never known for their ability to stay clean at volume... it's why the guitarist & bassist would swap the Bassman for the Showman. If it starts to seriously crunch around 3.5-4, it's probably completely normal! And I think a lot of the magic with these is power amp crunch.

Re: the mixing resistors, Enzo is right - there' nothing there to mix, as you only have one channel. Do you mean the 220k after the 3rd stage feeding the 500pF into the PI? And as far as the comment about "modding the AA864 for guitar," I agree - there's no modding needed. There's a perfectly usable channel already in the amp. Or if you like the sound of the Bass channel, plug into it... about the only mod I know of is to make that 3rd "gain" stage shared by both channels so you can bridge them without phasing issues, or tweak the tone stack. No need to hotrod or Marshallize these designs - they is what they is!

I think Enzo was saying you're doing a lot of work for a result that can be achieved much simpler! Stick around, you'll get used to us...

Justin

[sliberty]

The reason I tried the Normal tone stack was that the amp was much darker than I expected when I initially built it with the stock Bass channel tone stack. That was my own fault - my expectations were not in line with reality. But, in retrospect, it seems that the tone stack may not have been the problem - it was more likely the two filter networks that Fender included that were killing the highs. Now that I have removed those caps (.001uF on the voltage divider, and 003uF on the plate of V2b), the amp is plenty bright and sparkly, and I could probably go back to the Bass channel tone stack and be happy. In fact, I might try it again and see.

Here is a specific question - a friend has suggested making the 250pF treble cap in the tone stack switchable with a 100pF (or even 50pF) as an alternate way to implement a bright switch. The idea is that this way, the bright boost would still be effective even with the volume pot cranked up. Any thoughts on this? Is it a bad idea to switch what is effectively a coupling cap like this?

Regarding the mixing resistors: yes, the 220K after V2b. I've kept the both 220K resistor in the circuit, and grounded the one that would have been connected to the Normal channel. The way it was explained to me is that this creates a voltage divider that would provide gain levels closest to that of an amp that had both channels. How do you feel about that?

I'll leave the MV discussion for another day.

Thanks,
Steve

[Enzo]

Using the mixers as a voltage divider will indeed cut the signal in half there, but that did not happen in the two channel circuit, because neither one was grounded. Un ground it and see what you think. Then again you may not want the increased signal level, after all you are trying to squash it at the 220k pair as it is. But it is a simple experiment, easily done and easily undone. And either way, it will not harm the circuit in any way regardless.

The treble cap in a tone stack determines the frequency range the control controls. If you change the 250pf to 100pf, that will then make the treble control only work on higher tones. The treble control would then not reach as far down into the upper mids as it does now. Likewise increasing it to 500pf or something does the opposite, it increased how far down into the upper mids the teble control reaches. If you wanted to switch in a parallel extra 100pf or 200pf or whatever, tack a cap in and try it. It might do what you like.

If we imagine a 31 band graphic or something similar, we could see the 250pf treble cap means the control would cover like the top 10 bands on the graphic. Change it to 100pf and it now is like controling only the top 5 bands, and if to 50pf, the top 2 bands. That is hardly scientific, I made up th numbers to make the point. So please, no one calculate and tell me the "correct" numbers.

The volume control bright cap is there to boost highs at lower volumes to compensate for your ears that hear less highs at lower volumes.

Go over to Duncan amps and find the Tone Stack Calculator. it is a bit of free software you can download. It shows freq response of a number of common tone circuits. You can adjust the controls and watch the change in response in real time. And in fact, it has the ability to change the component values to see what that does as well. That should provide tons of fun and answer many questions about the tone stacks.