PA 100 Preamp Section is goofing me. New! Schematics included

[TerryD]

OK I got the phase inverter from silverface to blackface with Gerald Weber's YouTube instructional.  Lovin that.

As I've said before the preamp section is different.  I've been going over Torres' preamp section of his book to actually try and figure out what is actually going on in a preamp.  I'm starting to.

Here is my trouble:

All his examples are standard Bassman type circuits.  I'm not having success in applying them to what is I'm seeing in the PA 100.

My values seem different than what is on the PA 100 schematic.  Not that I can read schematics perfectly.

My main problems besides figuring out what is going on in this thing:

It seems there is only 1/2 of the 12AX7 for each input and tone control.  then that goes right to V5 and on to phase inverter.  That's majorly different from the bassman type stuff I've worked on.  Aren't most One stage for input and one for tone controls.

The tone controls have wires directly to the input??  That's way different for me.

Here is where I really show my cards.  I want to eliminate one of the input jacks to put a midrange in.  I'm using a 68 and 68 with 1meg as instructed here.  When I move to one input, what do I do with that 1 meg?  What is that for any ways?

How do I set up a midrange pot?

How do I go from this to the other Fender preamps I want to mod this to?

Here is my tonestack as I actually see it (from the back of the pots) on the board

input to V1 pin 2

Volume pot:

left peg >.0022> left treble peg "plus"  a wire to input ground
center peg > 270k> extended post on back of vol pot>270k (to v5) and also from that post is a ceramic 100? to ground (back of pot) "plus" there is a a wire from the center peg of the volume to input I don't know what its for tab.
right peg to center treble

Treble pot:

left peg > .0022> left volume pot peg
Center peg > right peg of volume pot "plus" wire to 820k > center bass pot
right peg to 1kv 2.5 P (is that 250p?) ceramic disc to extended pole on volume pot to 82k to right peg of bass pot
"plus" wire from extended pole (same place) to ..01 cap and 100k resister on the board

Bass pot:

Cap right across from left peg to right peg looks like .0012 or c012

left peg wire to input ground "plus" center .0012 cap to right peg
center peg to 820k resister to center treble
right peg to center .0012 cap to left bass peg  "plus" wire to 820k resister to extended pole on volume pot which connects to 1kv 2.5p to right peg of treble pot and .01 cap and 100k resister on fibreboard

Any help would be appreciated.  I am trying to learn whats going on as well this time.

http://www.prowessamplifiers.com/schematics/fender/pa100.html

Thanks,
Terry

[uki]

About the Fender input circuit here a very good explanation about it.

See also:
CommonHookups

[TerryD]

I've watched a couple of other of Uncle Doug's videos.

[PRR]

> The tone controls have wires directly to the input??  That's way different for me.

Look at what it does. It shorts-out the preamp and tone control if there is no plug in the jack. It is how a four-input mixer can be fairly quiet when only 1 or 2 inputs are in use.

> majorly different from the bassman type stuff I've worked on.  Aren't most One stage for input and one for tone controls.

Well, there's 5F6a Bassman and then there is all the rest. Most Blackface and Siverface amps omit much of the Bassman's complication and just fed tone/volume from the first stage.

However.... the PA100 schematic study-material is not right at hand, most folks don't memorize odd models, so many helpful readers will move-on to some other topic.

[TerryD]

Thanks!

[jjasilli]

In one of your other threads I made the point that you need to know what you want the amp to do, before making random or arbitrary mods.  E.g., do you intend to use it for el or acoustic guitar.  Clean tone, overdrive?


Doug has the PA 100 schematic posted.


You are conflating layout with schematic.  It's a learning curve, but you must come to understand the schematic.  The actual physical layout, especially with point-to-point wiring like on Fender tone controls, can be very confusing.  A schematic is like a road map -- at first, a blur of lines, squiggles and random shapes.  But you can learn to distinguish between big cities, small tows, interstates, local roads, etc.  Eventually you can quickly pick-out a travel route.

The amp schematic has sections & subsections too:  power supply, power amp & preamp.  The preamp can be broken down into input; gains stages, and tonestack.  (There may be other subsections like reverb, tremolo, effects loop, etc.)  These section & subsections are not clearly delineated.  Your eye will learn to pick them out.  Books by amp guru's you mention break down the schematic into these bite-size segments to discuss them.  Once you "get" how a gain stage, or tonestack,  is drawn on schematic with its components and capacitors, you will become able to focus-in and pick it out of a complete schematic.  (You did pick a complex place to start.  It would be easier to build or mod a Fender Champ, than a 4 channel behemoth.) 


The best way to reconcile layout with schematic  is to post a copy of the schematic above the chassis.  As you identify ea leg of the circuit in the amp, trace it over with a highlighter pen on the schematic.  Do the same thing if you build or rebuild a circuit.  As you add a lead or component in the amp, trace it over on the schematic with a highlighter. 


***************
This tonestack is not typical Fender.  It strikes me as Traynor-like.  I wouldn't know how to place a mid pot in it.  My suggestion would be to convert the tonestack to typical FMV.   But why?  What purpose are you trying to achieve?

[TerryD]

Thanks for the thoughts.  What am I trying to achieve?  It sounds pretty good now. 

But, The idea of having a blackface twin preamp section with a midrange control sounds intriguing.  The second channel on that tube is stock.  That I'd like to keep as is. 

Then I'd throw another fancy tube in for the other two channels and probably chain those two together with a short guitar cord or internally.

 Or have one channel the fender blackface twin and use all the others for a Marshall cathode follower type tonestack.

The schematics.  I keep returning to them.  I'll keep doing that.  When lines cross they are just crossing, right.  They don't connect unless there is a dot on their intersection, right?

Thanks for noticing that the tonestack is NOT typical Fender.  What is FMV?

[shooter]

When lines cross they are just crossing

yes, mostly, ideally the drafter puts a *hump* over the line being crossed.  Having a good working knowledge of "where the line *should* go" comes in handy.

[jjasilli]

FMV is the lingo for Fender-Marshall-Vox.  They all use the same tonestack topology -- coming to prominence in the Fender Blackface era; though I believe it originated in the '59 Bassman 5F6.  The term tonestack is unique to guitar amps.  Elsewhere, like hi-fi or professional audio, it's called tone controls, equalization or EQ.  I think the origin of the term tonestack is in dispute.  I agree with the thought that it comes from the usual visual depiction in a schematic showing the tone pots "stacked" on top of another.  (there is often more than one way to draw a schematic so it's good to know conventions.)


Go to Doug's Library, Schematics Page, Fender:  #129   Fender_Champ-AA764-Schematic.pdf - 335 Kb  (above that is the Layout diagram)


The tonestack is between the 2 preamp tube stages.  That configuration has come to be known as the FMV tonestack.  It is the same topology but with different component values.  The fixed resistor in the Champ schematic, "under" the bass pot, could be replaced with a pot which would be the mid-control.

[TerryD]

Thanks.