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FDP Forum / Amp Mods, Repairs, and Projects / Installed a PPIMV on my 69 Bassman AB169 head

pcalu

usa Thumb area Mi

Feb 16th, 2019 12:59 PM Edit Profile
Used the standard 1Meg pot.

The mod is staying. Been wanting to do this for a while. The cool thing with the 1 Meg pot, when I open it up, can't tell it's there. That was important.. not a fan of MV that squish the tone.

Anyone put a PPIMV on a Fender?

pdf64

UK

Feb 18th, 2019 05:23 AM Edit Profile
The terminology used is rather non-specific*. As Ken Fisher went to the trouble of publishing a nomenclature for the various master vol types in the Trainwreck Pages, it seems a good idea to use that, see p32

* ie I don't know whether you mean a type 3 or type 4.
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Leftee
Contributing Member
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VA

Toxic Humility
Feb 18th, 2019 09:05 AM Edit Profile
In general I've used it a couple times and I have an amp, now, that came that way (a VHT Standard 18 head).

It is "the way" to do MV.

To Pete's point, I don't know which one. It's been a long while since I did one and I haven't looked that closely at this area of the VHT schematic.

pcalu

usa Thumb area Mi

Mar 2nd, 2019 09:07 PM Edit Profile
NO#3 (if this helps)

1 meg pot tied into the two phase inverter output streams.

IMO Pretty simple to do & works really well. Really transparent wide open (Dimed) I hear no difference vs MV and no MV when wide open.

Used braided/shielded wire and dress/wound them to cancel hum/oscillation.

I haven't tapped into the unused half of the second triode that goes unused in an AB165 Bassman. I'm using JJ-5751s which IMO is a solid substitute for a 7025s and even using a 12AT7 in the phase inverter (and for me) there is plenty of gain/overdrive using the MV.

Nice American voiced Classic Rock overdrive. I don't want to work in a Marshall tone stack, I want to keep it with a Fender voice as much as possible. Yet, I can see how people would want to do it. I also installed 3-way negative feedback switch 820/none/47 using the leftover ground switch.

Makes for a very versatile amp. (Good lord is this a loud 50-watt amp... which is why I wanted to install the MV)

Leftee
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VA

Toxic Humility
Mar 3rd, 2019 05:32 AM Edit Profile
Those old Bassmans are such great amps.

Roly
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Whitehorse Canada

I don't get out much
Mar 3rd, 2019 05:46 AM Edit Profile
Did you use a dual 1 meg pot?

pcalu

usa Thumb area Mi

Mar 3rd, 2019 07:51 PM Edit Profile
No.

I believe that's what people are referring to as a Ken Fisher "Type #2" with a duel 250k gang pot.

I used a regular 1 meg pot. ( a Ken Fisher No#3 master volume circuit) Internet says when doing the No# 3 modification, the 1 meg pot is the most transparent in this type of modification.

Like a Trainwreck type tone stack, I dial the amp to overdrive, because of the simplicity in the master volume circuit, I just roll back the guitar's volume and the amp really cleans up.

We've all had pedals and amps do this "obviously right :) but I've never had an amp do this to this degree. I'm mean it really cleans up without getting muddy or dull.

pdf64

UK

Mar 4th, 2019 02:57 AM Edit Profile
I find that type 2 or variant thereof (see link, scroll down to p31) work well with most all LTP designs.
The type 3 seems fine with a 12AX7 LTP, but it seemed harsh when I've tried it with SF Fender 12AT7 LTPs (47k plate load resistors).
I've not actually tried it the BF 12AT7 LTP, ie with 82k & 100k plate loads.
All of them are pretty much 'out of circuit' when turned up full.
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FDP Forum / Amp Mods, Repairs, and Projects / Installed a PPIMV on my 69 Bassman AB169 head

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