Fender PA 100 Phase inverter done; Preamp help please

[TerryD]

OK  I looked up Gerald Weber's Silverface twin to Blackface phase inverter section instructions on youtube in order to finish the section.  Followed that to the tee.  This sounds so good now I can hardly take it.  Completed power cap upgrade with the F&Ts that Doug has and now that they re burned in sound very good to me.

Thanks to everyone for all the help getting it in good order and direction in upgrades.

The preamp section is all screwed up in relation to what I'm used to seeing.  All the tone caps and whatever else is right on the pots and as mentioned that extender thing on the volume pot.  I'm reading and rereading Dan Torres book on amps to better understand what is actually going on in the preamp section, so I don't screw this up.

Thinking of 4 different sections or one old fender and a three stage marshall tonestack.  I have the beginnings of a regular guitar tone stack on one channel, probably leave the other channel on this tube stock (this is one 12 ax7)  thinking of changing/experimenting what other tube to use on the remaining two channels.  Kind of leaning towards the 4 different fender sounds than the one fender and then marshall preamp.

The questions are two:

Ideas, thoughts or advice??

How do I go from this mess on the pots to a regular guitar amp?

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[MFowler]

Here is something for one channel.

[drgonzonm]

I am not all that familiar with Gene Weber's work, although some consider him a guru.  Since the amp is being rewired, My question is:

"Is the grounding scheme for this amp being upgraded to more current safety standards?" 

After looking at the Bassman100PA, schematic, there appears a safety issue. 

[DummyLoad]

something else to consider - minimally invasive. where indicated "add cap" is optional - add 100pf-.001uF as so desired.


bond the channels - blend for bright/bottom.


--pete

[Willabe]

The preamp section is all screwed up in relation to what I'm used to seeing.  All the tone caps and whatever else is right on the pots and as mentioned that extender thing on the volume pot.

Putting the tone caps on the tone stack (TS) pots can be a good thing. I do that when I can, and I put 1 end of the coupling cap right on the volume pot. G. Weber has been doing that for years, that's where I got the idea from.

On a preamp tube gain stage the wire from the tubes plate that goes to the coupling cap is a plate wire. But after the coupling cap it's now a grid wire. Grid wires act like an antenna and will/can pick up any stray noise/hash in the air and then feed it into the tubes grid where it will get amplified.

So it's better to leave the plate wire long and the grid wire short. To do that you move the coupling cap and TS caps.     

[jjasilli]

The mess on the pots is standard vintage layout.


Not clear what you want from this amp.  The stock preamp is 4 "true" channels each with vol & tone controls.  IMHO there's nothing to be gained from different Fender voicings of the channels.  Even Marshall & Vox tones can be had by switching cap values within one channel.  I.e., it is not necessary to "waste" channels in order to have different voicings.  Torres & Ted Weber ( maybe Gerald Weber too) post schematics for this. 


The main other mod to a PA amp is to convert tube stages in the separate channels to cascading gain stages, like Mesa Boogie, later Marshals, Soldano, etc. 
 

[birt]

i would probably go for a standard fender channel. maybe a second one with a bit of a different voicing (but that can be done with the tone pots as well) with a switch to parallel them.


i would then use the other 2 channels together to make one with a higher gain.

[super&plexi]

Great topic, my Father & I went out looking and came back with one of these new. My 1st Fender.   Wish I still had it. We built big Cab for it, w/15's. At one time, it had '52 goldtop, es125, my Pops Mustang, and room for my buddy Anthony's Duo Sonic, or anyone else that happened to come by, (even if they had humbuckers), and come to think of it, those 15's handled a bass on more than one occasion. 


I've been impressed lately with single tone, no reverb, no tremolo amps. MusicMaster Bass (guitar) amp, & just last wknd stumbled on something called Bantam Bass. Was blown away by the round, fat, undeniable great tone. By the time I got to the second note played I knew it was going to sound  great, but I also love Fender 'Verb, & Trem,  so my thoughts go to T/S lift, maybe on board Re-Vibe, or 'Verb w/all the trimmings, or tube driven F/X, all switchable, out of circuit, for clarity.  When Ed Van Halen changed from ''you really got me''  tone, to ''come on baby, finish what you started'' (or something like that) tone, a big part of the change in sound  was parallel F/X, maybe something you'd like?  You could do it on-board.   What about true stereo? Just an OT away. Best regards,  however you choose, sNp

[TerryD]

Got it, Willabe.  So......that's why this is the quietist amp I've ever seen.  I'll be sure not to screw that up.

Birt, how would I do that with just two halves of a 12 ax7.  Seems the things Torres does is with three stages.

[birt]

sorry i thought you had a full tube for each channel. i should have paid more attention

[Willabe]

So......that's why this is the quietist amp I've ever seen. 

It helps but there's more to building a quiet amp than just that.

[MFowler]

If you don't like wiring up a new tone/treble caps on the pots you can use a turret strip in front of the controls to wire PTP.


This amp has J type pots which I would keep.  You can change out the coupling caps for each channel to come up with different voicing.  I would change V1 and V2 to have individual cathode bypass resistor and cap too, rather then shared.


The last one of these I did I keep one chaneel stock, another Ampeg B15N, another old Bassman like 5b6, and hotter tweed in the last.