Tweaking a Fender AA964

[sliberty]

As mentioned in my other thread, I've completed a Fender Princeton AA964. I had an oscillation at first, but I've solved that, and now the amp does what I suspect it is expected to do. It is pretty lifeless on volume settings below 7 (I hear that is normal for this model - is that your experience?). But from 7-10, this thing is really sweet sounding. So I have begun considering some tweaks. At this point, I don't think I want to do the "sacrifice the trem" mod, but who knows - that could change.

I have:

1. moved the PI to the unused ps node to provide a little more voltage. I am also going to try switching to a SS rectifier slug in place of the GZ34 to see if that adds some additional punch.

2. installed a switchable tone stack bypass. I did this by switching the volume pot upper leg to a .022uF cap connected to plate of the first stage.

The tone stack bypass is a bit too effective - the amp gets aggressive and a bit fuzzy - I just want more guts, not necessarily an overdrive monster. Some drive would be fine, but not into the fuzziness.

Would the results be much different if I lifted the ground side of the 6.8k mid resistor instead of connecting around the tone stack?

I plan to experiment with reducing some of that "re-captured" gain when the tone stack is bypassed. First, I'll try installing a parallel resistor / small capacitor network in series with the cap and volume pot to throw away some of that gain, while boosting the highs a bit. I'll report back on this tomorrow.

What else should I try overall? My goal is to have more punch / gain than the stock tone, but not so much that the amp loses its sweet sound. Punchier clean to clean-ish tones would be ideal.

Note - I built this amp into a Blues Junior cabinet that I grabbed from a guy on CL. Due to speaker baffle and back panel orientation of the BJ, I decided to reverse the layout (left to right) so that the transformers wouldn't collide with the speaker, and power tubes would be exposed by the back panel cut out. I used the standard AA964 board from TurretBoards, but had them put the eyelets in from the back so that I could flip the board over. All this worked out very well, and the bottom line is that the layout is standard, but reversed.

[robrob]

Another way to bypass the tone stack is to disconnect the bass pot from the 6.8k resistor. This removes the tone stack's ground so the stack "disappears" from the preamp circuit.

One way to kill some preamp gain would be to put the second half 7025's cathode bypass cap on a switch. This would add some headroom and could be used with or without the tone stack.

[dvnator]

If you have a spot for it I would get rid of the fixed 6.8k mid resistor and use a 25k or even a 50k mid pot. Then you can dial it in like stock or crank up the mids for fatter, punchier sound.

[sliberty]

Another way to bypass the tone stack is to disconnect the bass pot from the 6.8k resistor. This removes the tone stack's ground so the stack "disappears" from the preamp circuit.

One way to kill some preamp gain would be to put the second half 7025's cathode bypass cap on a switch. This would add some headroom and could be used with or without the tone stack.

Should I expect that this approach to the tone stack by pass will sound pretty much the same as the approach I took, or will there be different tonal characteristics?

[andyfromdenver]

*from someone who has built that circuit*

you are experiencing the "meh" of (imo) Fender's misstep amps, the no verb Princeton and no verb Deluxe.

They make ok kybd amps, but are so lacking for guitar.

I rewired the preamp and tonestack to the brwnface Princeton specs, but kept the bface pwr section with gz34 and roughly 405VDC B+ biased @ 25mA (w voltage dropping zeners). The amp is great. I call it the Cocoa Loco

You may not be able to do that depending on control panel aesthetics, but it is a great sounding amp now.

[sliberty]

*from someone who has built that circuit*

you are experiencing the "meh" of (imo) Fender's misstep amps, the no verb Princeton and no verb Deluxe.

They make ok kybd amps, but are so lacking for guitar.

I rewired the preamp and tonestack to the brwnface Princeton specs, but kept the bface pwr section with gz34 and roughly 405VDC B+ biased @ 25mA (w voltage dropping zeners). The amp is great. I call it the Cocoa Loco

You may not be able to do that depending on control panel aesthetics, but it is a great sounding amp now.

Does it sound much different from a Brown Princeton? I already have one of those. I was kind-of looking for a cleaner tone from this one.

[andyfromdenver]

Does it sound much different from a Brown Princeton? I already have one of those. I was kind-of looking for a cleaner tone from this one.

It does! the gz34, extra filtering, extra voltage if you use jj6v6 and no zeners makes for a tight clean pwr section.
I call these amps the "in-betweeners" and like to imagine some EEs and techs at Fender did this in the lab

You have a useable clean up until half, and it doesn't sound anemic at low volume. not quite as lively as low volume tweed deluxe on the bright input, but not as dead sounding as the noverb bface princeton without the volume maxed.

if I get around to making a demo vid, i can post it (but it will be iphone quality). I actually just put the finishing touches on mine and put it in a mergili cab for a 10" speaker, it's a boutique-y little combo with the celestion gold alnico.

did you add a variable bias pot?

[robrob]

Should I expect that this approach to the tone stack by pass will sound pretty much the same as the approach I took, or will there be different tonal characteristics?

If the new cap and tone stack are paralleled during the tone cut then there might be a tonal difference. If the tone stack is actually bypassed and not paralleled then there shouldn't be a tonal difference.

[sliberty]

Thanks for yours suggestions and responses. So far you are the first person who has honestly described the AA964 as anemic - exactly the way I would describe it. This at least confirms that I didn't mess up the build

did you add a variable bias pot?

Not yet, but that is in my plans.

I have been thinking about how to handle this amp's shortcomings, and with your suggestion, now have 4 main options:

1. Just tweak the circuit (tone stack bypass, NFB, etc.) - not too happy with the results so far
2. Sacrifice the trem so that I can add the extra gain makeup stage of the PR (but I don't want to lose the trem if I can avoid it)
3. Shoehorn in another tube so that I can add the extra gain stage without losing the trem (given my current tube layout, this is not ideal).
4. Replace the Treble pot / Bass pot / fixed mid resistor style tone stack with a single tone pot ala 6G2. Might need additional tweaks to keep it fairly clean and a bit more scooped than the Brown (which is my goal for this amp).

I'll probably keep playing with the 1st option a bit longer, if for no other reason to satisfy myself before I start to consider the more drastic options. But I think #4 is high on the list now. If it gets me where I want to be, and all it costs me is new faceplate, I can live with that.

Thanks,
Steve

[andyfromdenver]

Thanks for yours suggestions and responses. So far you are the first person who has honestly described the AA964 as anemic - exactly the way I would describe it. This at least confirms that I didn't mess up the build

did you add a variable bias pot?

Not yet, but that is in my plans.

I have been thinking about how to handle this amp's shortcomings, and with your suggestion, now have 4 main options:

1. Just tweak the circuit (tone stack bypass, NFB, etc.) - not too happy with the results so far
2. Sacrifice the trem so that I can add the extra gain makeup stage of the PR (but I don't want to lose the trem if I can avoid it)
3. Shoehorn in another tube so that I can add the extra gain stage without losing the trem (given my current tube layout, this is not ideal).
4. Replace the Treble pot / Bass pot / fixed mid resistor style tone stack with a single tone pot ala 6G2. Might need additional tweaks to keep it fairly clean and a bit more scooped than the Brown (which is my goal for this amp).

I'll probably keep playing with the 1st option a bit longer, if for no other reason to satisfy myself before I start to consider the more drastic options. But I think #4 is high on the list now. If it gets me where I want to be, and all it costs me is new faceplate, I can live with that.

Thanks,
Steve

no problem Steve!!

option 3, option 3!! lol
that way you can keep the famous scooped mids plate fed stack and all the fender bface glory. you just need 1 triode unfortunately, bypassed or un- to taste. I am a tremolo junky, a little, constantly on, is very pleasing.
best wishes to ya!