New guy and first question.

[Bleachedfan]

Hello. I'm excited to be here as I realize this is a treasure trove. My question is this. There are a few old threads on the fender pa 100. I was wondering if anyone actually did any mods to this amp. I just acquired one in a trade. Not sure exactly what I want to do, but I would like more control of the tone. My first step is to replace the 47k resistor with a 1m on the input. Perhaps replace the filter caps. They look good, but who knows. I've heard this amp is like a twin reverb, but I've also heard it's like the dual showman. Any help will be appreciated. Thanks again.

[eleventeen]

"I've heard this amp is like a twin reverb, but I've also heard it's like the dual showman."

Those are pretty similar amps, all in all, of course depending upon what vintage you are referring to.

It's certainly a very ripe platform for trying out any number of preamp configurations....at the same time.

I would probably do the following:

1: Get an assortment of schematics of Twin reverbs schematics of various vintages and compare, put them in a 3-ring binder and start taking notes. Probably the main difference is the master volume control, present in some SF TRs, of course. Decide whether you like having that. Also get the parts layout dwgs, orient yourself as to which cluster of parts relates to which 12AX7, and figure which of the input channels might be easiest to get rid of or otherwise modify.

2: Decide whether or not tremolo is important to you. If so, you'll probably have to lose an input channel (no worries, with 4 of them) and build up the trem circuit. 

3: Decide whether or not Twin reverb power is what you want. For some, that is pretty loud these days. It's very easy to either pull out 2 qty 6L6 output tubes (make sure they are from opposite sides of the OT!) or install a switch that lifts the cathodes of same.

4: As a matter of general practice, one would probably want to replace the electrolytics and I really like to replace all the 100K ohm plate resistors with the 3 watt metal film ones Doug sells. This will produce a profound, non-subtle reduction in overall hiss and rat-noises. Very cheap. Check bias volts and get that right. We like to install small-value resistors (1 ohm or 10 ohm) under the 6L6 tubes and run the cathodes and a ground out to tip jacks on the exterior of the chassis to make adjusting/checking bias easy. Suggest you plan to do that when the soldering iron/drill come out.

5: I would get a 12AY7 tube or a 5751, throw it into one of the input channels in place of the 12AX7 for that channel, and see if you like the reduced gain of the first stage on that channel.

At this point, you haven't performed any mods, you've just refurbed the existing circuit. Poke around here and get some ideas to what types of mods would serve you best. I have never owned nor modded one of those, but it's obviously a very, very ripe platform to build almost anything you want.

[Bleachedfan]

Awesome advice. Current I only have two tubes in. (my buddy told me the correct way to do it, from Hayes amplification).
I was thinking about the master volume. I'm not sure the pros/cons of having one. From what I can tell, it lets me drive the channel volume, without getting the cops called. This link has a cheap mod for the master volume http://vintagefenderamprepair.com/mods-2/replace-your-master-volume-w/
As far as the twin reverb goes, I am choosing that amp based on the similarities in the amp's. However, if there are other amps that are similar or closer I would love to know. I would like tremolo, if is different from effect pedal tremolo. I would like the ability to install a switch to remove two tubes, my thinking it it would bias easier with the complete setup than half.

You mentioned parts layout dwgs, is that a typo? I'm not familiar with that term. I've been lookinf for the pa 100 layout.. No luck
That's it for now, thanks for the help so far.. Keep it coming!