74 Fender Quad Reverb

[johnnieboy2]

Can anybody tell me if this looks like a familiar mod? The amp does not seem to have the 'clean' I'm used to in Fender Twins. I'm OK with non push pull control. Thinking someone wanted a lot more overdrive from the amp. Thanks for any input.

[johnnieboy2]

...clarification...

This circuit is (was) a standard Twin Reverb type with the addition of two more 12" speakers, nothing exotic. I assumed the removal and replacement of the push-pull pot with a std pot was following a standard format (mod) and wanted to get an idea before I consider a change back to original p/p design. Aside from the added big .1 caps the circuit seems to be plain jane Twin.

[TD_Madden]

ummm..maybe folks haven't seen the inside of a quad-reverb...

Of course, mods made to twin-reverbs and super-six might be similar.....

If you want clean, rewire it back to original.

[pdf64]

It's really hard trying to work out a schematic from 1 photo like that. Could you draw it out? Is it a dual track pot on the MV?
Are you saying that with the MV on max / channel vol low, you can't get the big, clean, full twin sound? Peter.

[johnnieboy2]

I should update this...I had at least one badly harmonic tube and replaced all 4 6L6's, recapped the filter section (had physical leakage) and the distortion is gone now. I'm just dealing with the mod right now. It doesn't sound bad at all, but I'm wondering why it was done in the first place and what it might sound like removed and restored. If it was a mod it was not a complicated one and I have read lots talking about tone caps.

[Tom Phillips]

IMHO the pull boost never did sound good. In fact, it sounded really bad. Therefore, you would not want to "restore" that circuit. I believe what you have there is someone's mod to another type of master volume circuit. If you like the sound at all then it is apparently better than the original.

A popular mod for the Fender Silverface amps is to revert the circuit to "Blackface" You can Google that and find lots of information. I don't know your experience with tube circuits.
Don't be in a hurry. Just use the amp for a while and read the info that's out there.

Note: It would help a little if you moved the text box out of the way.

Regards,
Tom

[oc disorder]

Hi Johnnieboy2
This looks like the "post phase inverter master volume mod"
which removes the pull switch distortion and pre phase inverter master volume
and inserts the master volume just before the output tubes to enable the whole pre amp circuit including the phase inverter to be driven hard while
being able to turn down the output.
Most (but not all !) people don't like the distortion circuit introduced by CBS
that utilises part of the reverb circuit for more gain so the single gang pot with a pull switch is discarded in favour of a dual gang pot .
So the master volume is moved from before the PI to after it.
This is a popular mod as with the volume on full the amp is more or less stock.The value of the coupling caps determines the bass roll off.
I would leave it in but check the pot value is at least 500k.
Here is the schematic on the Blue Guitar site drawn by Bruce Collins in '98.
http://www.blueguitar.org/new/schem/...d/pi-m-vol.pdf
I'll attach it as well

[johnnieboy2]

Maybe???

That certainly looks like it could be the mod, except the (4) .1 caps are directly tied to the pot. The schematic shows two that way, but the other two caps are preceded by the 33k-100k resistors. I have a lot of experience in circuit repair but not design, wish I did! Thoughts?

I am going to remove the chassis tomorrow and do some point to point notes to check the rest of the diagram you provided. Thanks again.

[oc disorder]

Well it will work without the resistors.
The centre lugs of the pot go to the input (grid) of the output tubes.
I suppose if one cap failed (shorted) in series with the wiper and the pot
was turned down (wiper to ground) the bias would be shorted on that side.
Then again if that cap failed in a non MV circuit the HT from the PI would go straight into the grid with equally disastrous results.
Not sure why these resistors were inserted ..perhaps someone else would like to comment.
Its easy enough to temporarily remove it to see what it sounds like without it.
The two caps that come from the PI to pot- lift the pot ends and likewise the two caps from the pot lift the two ends from the 220k /1k5 junction and join up the two ends previously lifted from the PI to pot to the 220k/1k5 junction. Well a picture is worth a thousand words

[johnnieboy2]

Duh...

Well, I'm pleading temp insanity...been a long day. I should have asked do you have a diagram showing the circuit with 4 output tubes! My bad. My imagination is fading fast.

[oc disorder]

You'll wonder why you asked that in the morning.
The grid resistors all join at the respective junction to the 220k's.
See attached diagram.
Also see one of the original articles on a MV by Craig Anderton from Guitar Player magazine Dec'82 attached.
There are other discussions on this forum relating to PPIMV .
The search function works well you may have to input "variations on the theme" because of various acronyms used.

[johnnieboy2]

YES!!!

Wow, insight is a wonderful thing! It's also fabulous what a good nights rest will do for the mind. Positive ID has been made, it is the post inverter master volume mod. Thanks for all the help oc disorder!

I think I'll just leave the mod alone...maybe kill the feedback loop to increase the gain...OR, do you suggest a switch so I can select feedback if desired? I installed one on a Music Man HD130 that worked well.

[oc disorder]

Well you could alter the negative feedback but in ten years time you will have to explain to a poster with a query about "Can anybody tell me if this looks like a familiar mod?" with a picture of a switch in the back of a Fender quad....!
Seriously have seen a pot installed that can vary the amount of neg feedback
and when turned down returns the amp to stock.
If the amp has a 3 wire power cable and properly grounded you could use the hole that originally contained the ground switch.
The pot sits between the output and the 820 ohm neg f/b resistor.
100k seems to be a common value.

[johnnieboy2]

grin...

Humor well spoken and taken

Just wonder if it's worth the effort, would you fool with it on your twin?

Linear or audio taper on that 100k pot?

[oc disorder]

Just wonder if it's worth the effort, would you fool with it on your twin?

Linear or audio taper on that 100k pot?

I would probably try it if I could without any drilling or alteration of the chassis and then decide to leave it in or not.
As it can be returned to stock with a simple turn of the pot I would probably
leave it in.

Mmm.. probably linear as all the action would be in the first minute incrument turn with a log pot. Mind you with a lin pot probably not much difference after the first eighth of a turn.

lin/log choice not all that critical in that application.