AB165 bass channel=>plexi mod

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Postby Hazy_2004 on Sun Jun 22, 2008 6:24 pm

Hazy_2004 wrote:
jamme61 wrote:I posted (easy baseman mod) a mod that I got from the weber book to get back some of the gain you lose in the tone stack. It works great IMHO and its one resistor and cap.


Did you hook the 0.01 uf cap to the Normal or Bass channel volume? Must try!


I don't even have a preamp tube in V1...I don't use the Bass channel. :? It looks like V3 is the Normal channel 1st stage. That's where I'm going to solder the 820 ohm resistor. :shock:
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Postby jamme61 on Tue Jun 24, 2008 3:39 am

[quote="Hazy_2004"][quote="Hazy_2004"][quote="jamme61"]I posted (easy baseman mod) a mod that I got from the weber book to get back some of the gain you lose in the tone stack. It works great IMHO and its one resistor and cap.[/quote]

Did you hook the 0.01 uf cap to the Normal or Bass channel volume? Must try![/quote]

I don't even have a preamp tube in V1...I don't use the Bass channel. :? It looks like V3 is the Normal channel 1st stage. That's where I'm going to solder the 820 ohm resistor. :shock:[/quote]

That's the one, your right. I know you don't use the bass channel and I don't blame you if it's the stk setup but, that channe can make a real killer marshall tone IMHO. once you get the weber book you'll be able to tweak it to a marshall and have two great channels for guitar.
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Postby Hazy_2004 on Tue Jun 24, 2008 11:46 am

jamme61 wrote:That's the one, your right. I know you don't use the bass channel and I don't blame you if it's the stk setup but, that channe can make a real killer marshall tone IMHO. once you get the weber book you'll be able to tweak it to a marshall and have two great channels for guitar.


Thanks. Can't wait to get the book. I do a lot with my amps but I don't have any in-depth knowledge of how the circuits are set up electronically.

BTW, I have two Bassman Amps and I rewired this one for EL34s. It sounds great and it another reversible change. My other is stock and I don't touch it.

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Postby jamme61 on Tue Jun 24, 2008 8:29 pm

[quote="Hazy_2004"][quote="jamme61"]
That's the one, your right. I know you don't use the bass channel and I don't blame you if it's the stk setup but, that channe can make a real killer marshall tone IMHO. once you get the weber book you'll be able to tweak it to a marshall and have two great channels for guitar.[/quote]

Thanks. Can't wait to get the book. I do a lot with my amps but I don't have any in-depth knowledge of how the circuits are set up electronically.

BTW, I have two Bassman Amps and I rewired this one for EL34s. It sounds great and it another reversible change. My other is stock and I don't touch it.

Hazy[/quote]

I only have the one and it's a great amp IMHO. when I got it (aa165 used GC $350) it didn't sound that great. With just a few changes it's a killer amp. I love my Plexi but my bassman can't be touched. It has fender clean on the one side and balls out Marshall on the other side.You'll love the book it's 95% fender amp stuff.
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Re: AB165 bass channel=>plexi mod

Postby bereanbill on Thu Jun 24, 2010 10:16 pm

Hi...my first post. I have done this with a few AB165. Clip out the nested feedback loop on the output tubes and disconnect the loop that goes to the output jack. It's the only wire that goes to the board from the output jack. You can replace the cathode resistors on the input tubes with 820 ohm and experiment with their bypass caps. I prefer leaving them at 25uf myself. Also reworking the bias circut to BF specs is a plus, but not entirely necessary. Don't forget that you can bridge the channels a la plexi as well. I did these mods and recapped the board with Orange Drops with one, then sold it to a friend in Canada. His response was " This thing thinks it's a f***ing Plexi !!" Don't forget to replace all the tired old electrolytics with Sprague Atoms...including the ones on the board. Bump the Bias cap up to 100/100 and double the values of the caps for the plates and screens of the outputs. This will tighten up the bottom end a lot !!
Then ( in the words of Beetlejuice ) " Turn up the juice and see what shakes loose !! " :mrgreen: :mrgreen: :mrgreen:
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Re: AB165 bass channel=>plexi mod

Postby dean on Thu Jun 24, 2010 11:39 pm

IF you are looking to 'tighten up' the low end, you can try bumping up the capacitance in the first stage of filtering. Two 100mfd's in series increases the cpacitance almost 50% over the stock 70's. Two 220's in series really yields some serious punch.
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Re: AB165 bass channel=>plexi mod

Postby worldoftone on Fri Jun 25, 2010 2:08 pm

The Fender/Marshall mods have been around for years. I recently did one and tweaked a bunch of the circuit in a late 60's Bassman. I had to voice it to the guy's guitar. Eventually worked out great.

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Re: AB165 bass channel=>plexi mod

Postby sultan_of_sound on Fri Jun 25, 2010 5:49 pm

Couldn't help but notice that you guys resurrected a thread that was last posted on exactly the same date two years ago (June 24, 2008)!

An interesting thread regardless. I have an AB165 that I picked up cheap late last year and had to go through it and back out a number of screwy mods that had turned the tone to crap (and added a lot of noise). Back to stock it now has a great tone... fortunately had all original iron and C's & R's in the tone stack.

So now I have to decide what to do to take it to the next level. Sounds like most (not all) think that getting rid of the balance bias and going to an AA864 feedback scheme for the power section are a given. I'll start there..... But where do I get a schematic to look over for turning the bass channel into a plexi?
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Re: AB165 bass channel=>plexi mod

Postby thirdman333 on Sun Jun 27, 2010 8:30 pm

+1 to sultan if any of you guys could draw out a quick schematic of how yours looks with the bass channel re wired as a plexi that would be awesome i too dont even put a tube in there and would like to expiriment with that channel i have a ab165 btw thanks
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Re: AB165 bass channel=>plexi mod

Postby orrong65 on Sat Jul 03, 2010 12:03 am

I have done this plexi mod on two-channel blackface Fenders, usually on the under-used normal channel. I also do an ultra-clean mod for tele players, but thats another story.

The key problem in the blackface tone circuit is the 100k slope resistor, its the gain robber. We all know the 5F6A Bassman and the early marshall circuits have 56k or even 33k in this position.

To make a really useful plexi-voiced channel without major surgery do the following: change the slope resistor to 56k or 33k (see below); change the 0.1uf and 0.047uf caps to 0.022uf; increase the treble cap to 330pf or 500pf to lower the treble pot shoulder and add a bit more gain (see below).

There seem to be two main tone combinations in the plexi circuits - a 33k slope resistor and 500pf treble cap, or a 56k slope resistor and a 300pf treble cap. And then there are other combinations! The lower the slope resistor, the higher the gain. You need to experiment here and decide which tone you like best.

While this tone stack mod is probably not as good as having the tone stage as a cathode follower later on in the pre-amp, it still makes a big difference and gives you two very different channels.
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Re: AB165 bass channel=>plexi mod

Postby orrong65 on Sat Jul 03, 2010 1:12 am

I should have added this to my previous comments......

To specifically mod the AB165 bass channel for a lead plexi tone, do the following:

change the 100k slope resistor to 33k
change the 'top' two 0.1uf caps to 0.022uf/400v caps
change the ceramic 250pf treble cap to 500pf
leave everything else the same

If you do this mod, its then usual to change the Deep switch to a "Fat" switch. In its current design its not much use for lead guitar. Replace the 0.1uf cap attached to the Deep switch with a 1500pf/400V cap. That circuit may not quite work - if it doesnt, you need to remove the earth side of the switch and reattach to the treble pot wiper - V1b grid. The fat switch path should fully bypass the tone circuit.

Remember all new caps must be 400v or more rating, and also remember to keep all parts you remove to put back in if you want to one day sell it in original condition.
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