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Tone stack and other mods for blackface Bassman AB165

PostPosted:Mon May 25, 2020 5:12 pm
by phogg
Hi all,

I have blackface Fender Bassman AB165 amp that I want to modify to get 1975 JGB type tones. I know this is quite a different circuit than the TR AB763, but it's what I have right now. This is a 50 watt amp with nice big transformers so that is good. I have a 1x12 open cab with a JBL D131 that sounds pretty good.

I am not doing these mods myself and have no experience working on amps. My preferred local amp tech is not a specialist with Jerry mods, but he is very skilled. I'm looking for help with communicating to him how to modify this amp properly. I have tried searching this forum and other resources and it seems that much that was available is gone.

Any help, comments, or notes are appreciated. I'm especially looking for how to modify the stock AB763 tone stack to match Jerry's 1972-1974 preamp sound. feel free to DM me.

Here are the modifications notes I have right now:
- Modify the circuit back to an AA865 to correct the weird issues with the bias supply and power amp.
- Modify 'normal' preamp tone stack to sound like an AB763.
○ swap ceramic capacitors for silver mica?
- add a 'mid' control and negative feedback control. Use the second input jacks for the knobs to avoid drilling.


Thanks!

Re: Tone stack and other mods for blackface Bassman AB165

PostPosted:Tue May 26, 2020 11:48 am
by Jon S.
Here are some potentially helpful resources.

(1) Digest this article, The Tone Stack Explained in English for Humans: http://pickroar.com/1003/the-tone-stack ... or-humans/

(2) Here is a helpful RUKIND post, from 2018, that I cut, pasted, and saved before the big bruhaha. EVERYTHING BELOW WAS AND REMAINS IN THE PUBLIC DOMAIN.

Re: SF twin gut shots/ Help with some minor mods pleeeeze
#162385 by Pointclear
Tue Aug 28, 2018 10:04 pm

Maybe you’ve moved on from your Silverface twin project, but I still want to chime in. I’ve got the same amp (late 73 twin reverb with all rca tubes. Mine doesn’t seem to have the power tube suppression caps either) and I’ve done a lot of the preamp mods mentioned in this forum and elsewhere about how to achieve Jerry tone. I’ll give you a little rundown of what I’ve done, and encourage you to dive in:

- changed the tone stack .022,.022, and 250pf - Sprague 225p and silver mica

- dale metal film plate resistors, RN series

- Sprague tantalum 150 series cathode bypass caps, 22uf

- black-faced the phase inverter and bias pot with dale metal film resistors, xicon metalized polypropylene caps - I like these in the phase inverter over the orange drops: responsive, 3-dimensional and textured

- changed the phase inverter input cap to .oo27 Sprague 225p - fattened up the tone just enough

- disassembled the master volume circuit, and disengaged the tremolo

- after lifting the wire to the Tremolo pot, I changed the cap on the right of the phase inverter from .1 to .047 (Sprague 715p) this tightened up the bottom end in a necessary way - I don’t see too many people talking about this but it seems important to my ears

- I changed out a few capacitors here and there to 225p orange. Drops

- AND just today I swapped out the reverb driver resistor on v4 from 850 ohm to 2200k metal film and found the sonic edge I’d been looking for. It added a bit of gain and breakup overall and a quality to the reverb that is just killer. I feel I’m almost there - think early spring ‘78

I run an e120 and k120 in the combo and an ext cab with the same speaker set up.
waldo041 liked this

And also posted in the same thread:

The main reason I disconnect the vibrato is to achieve more gain. The vibrato circuit bogs down the signal a bit, and lifting the lead on the intensity pot gives each note more life. Apparently there is an impedance mismatch, or something, and the .1 capacitor on the right side of the phase inverter section needs to be dropped down some ( mine to .047 ) and the bass tightens up quite a bit.

Re: Tone stack and other mods for blackface Bassman AB165

PostPosted:Tue May 26, 2020 11:52 am
by Jon S.
And (3): More tips I cut, pasted, and saved before the bruhaha. EVERYTHING BELOW WAS AND REMAINS IN THE PUBLIC DOMAIN.

As to what others do, best to ask them. I can tell you, though, that if you do some simple internet word searches, you'll quickly find many comments like these in the public domain that may assist you:

* I just saw on a respected sources website that Jerry's Twin had .022's in the tonestack instead of the normal .1 and .047. (Imagined Days)

* .022s in the mids will keep more mids around than some of the other higher value capacitors commonly used in fender amps, and .022 in the bass will cause the bass frequencies to drop off at a higher value and lose a lot more overall bass than the .1. So, theoretically, .022s seem to make sense for jerry (more bass cut and less mid scoop) (Stevo123)

* it is by no means a secret mod, others have done it. using the duncan tonestack calculator, and the known jerry twin settings, show a pretty identical curve between the stock values and the .022uFs. the difference being the sharper bass rolloff with the .022uF's. (Mike Wald)

* Just got my twin back from the tech with all the healy mods done and played a farm gig over the weekend. YOW! Talk about clean power and clear highs and lows. (JB Arnold)

* Let me suggest a place to start: First, in addition to changing all of the caps, you will want to change all of the plate resistors for non-inductive metal film versions. Also change all of the cathode resistors and use tantalum capacitors for the bypass caps (I use military grade axial lead versions). This, especially for steel, will greatly improve the high frequencies yielding the pure and clearer bell like tones. (JB Arnold claiming to be quoting Healy)

* I've done most of the mod that JB quoted except for the non-inductive metal film plate resistors. That's next. The other mods were amazing. I've never heard a Twin so open and sweet and clear and hi-fi before. The steel just sings thru it. (Brad Sarno)

Re: Tone stack and other mods for blackface Bassman AB165

PostPosted:Tue May 26, 2020 1:32 pm
by phogg
Jon s.,

Thank you so very much for sharing these notes. I should now have enough info to have the conversation with my amp tech. I found JB Arnold's notes from Healy to be very interesting and I think I was able to find the source in his 2003 post replies on steelguitarforum.

I'm reading through the 'tone stack for humans' article and it's very helpful.

Re: Tone stack and other mods for blackface Bassman AB165

PostPosted:Thu May 28, 2020 5:06 am
by Searing75
Good stuff from Jon S.

I wonder though......did Jerry have the .022’s in 72-74? I know that most believe he did at some point, but I wonder when exactly the tonestack mods took place? I prefer the .022’s in my amps non the less, and I can certainly get tones similar to 72-74.

Re: Tone stack and other mods for blackface Bassman AB165

PostPosted:Thu May 28, 2020 6:01 am
by Jon S.
Searing75 wrote:
Thu May 28, 2020 5:06 am
Good stuff from Jon S.
Thank you, though, of course, I merely shared others' ideas which I've done my best to attribute correctly to the individuals first posting them. :-)

Re: Tone stack and other mods for blackface Bassman AB165

PostPosted:Thu May 28, 2020 5:27 pm
by PurpleTrails
Jon S. wrote:
Searing75 wrote:
Thu May 28, 2020 5:06 am
Good stuff from Jon S.
Thank you, though, of course, I merely shared others' ideas which I've done my best to attribute correctly to the individuals first posting them. :-)
Yes, but you had the presence of mind to save some very useful and hard to replicate knowledge when things turned upside down here for a bit. I did the same with guitar wiring diagrams and some of the notes waldo posted on construction and components thereof, but didn't think to do so with the amp stuff.