I think I might understand the PA100 tone stack...?

[potatofarmer]

...but I probably don't. There, I admitted it!

Here's a schematic: http://www.prowessamplifiers.com/sch...nder/pa100.pdf

I'm fixing one of these up, and I'm going to take out one or two of the tone stacks and replace them with the standard Fender tone stacks. Definitely going to leave at least one of these alone though.

So, here's my thinking on this, and if someone would be so kind as to show me where I'm going wrong, I would appreciate it.

1. The input coupling cap forms a high-pass filter with the 39k resistor and 250k bass pot = high pass filter, -3db @ 55 Hz.
* In series with the 500pF capacitor, we get an equivalent 476pF capacitor.
* I'm ignoring the coupling cap from here on out - it's blocking DC and big enough to not really matter for anything else.

2. The bass control:
* We get wide-band attenuation from the 39k resistor in series with the 250k bass pot. There is additional wide-band attenuation from this resulting series resitance, plus the 120k resistor, going into the volume pot.
* That 120k resistor's primary purpose is to prevent the bass control from shunting the whole signal at low settings.
* The combination of the 39k resistor and the 3300p cap form a low-pass filter with the knee at 1.2 kHz.
* The bass control then attenuates the passed frequencies, and sends them to the volume pot.
* The overall result: we can slightly or drastically attenuate everything below 1.2 kHz.

3. The treble control:
* This bears a strong resemblance to the Tweed Princeton one-knob tone control, except it's around the bass control this time, instead of a volume control.
* I'm not really sure how to analyze this, but I see potential knees at 1.2 kHz and 135 Hz. Subjectively I understand how this control works in poets' terms (variable bright cap, becomes low pass).


Am I in the right ballpark here? I don't know how to use SPICE (or how to plot this without SPICE, for that matter) but it seems safe to guess that this stack can be set for basically flat response, wherever the midpoint might be on J-taper pots.

Thanks

[Chuck H]

Nailed it on the Bass control. The way I see it the treble control is just attenuating what passes through the 500p cap in series with the 4700p cap by loading. No variable bright cap activity since the bass function operates from a similar knee frequency.

[potatofarmer]

Awesome, thank you!

Yeah, I started posting the thread as "I don't understand at all..." but in the process of posting I managed to suss out chunks of it.

Seems strange to me that they went to all this trouble instead of just building a James. I flipped through RDH4 looking for a similar stack to this one but nada. Maybe it's a more common hi-fi stack than the James, or maybe they had a whole crate of 250k-J pots they wanted to use up?

[J M Fahey]

Itīs a hybrid.
The treble side is pure James, and allows almost normal treble boost/cut.
The bass side is a very simplified, boost only James, which becomes very close to a Fender Bass control.
Donīt mod it, downgrading it even more to simpler Fender wonīt help in any way and probably worsen it.

And yes, using 250K pots *everywhere* probablu solves some warehouse and supplier problems.

FWIW in the old days I made the opposite: my first Bass Preamps were an Op Amp version of Ampegs , with 100K log pots which I bought by the case, I also used those tone stacks for Guitar preamps , customers loved the fat sound, but for twangers I used classic Fender tone controls, with impedances lowered by 2.5X to match 100K Log pots.
Worked like a charm.

[potatofarmer]

Donīt mod it, downgrading it even more to simpler Fender wonīt help in any way and probably worsen it.

Well, I was thinking of modding it with an eye towards selling it. Make the first two channels into bassman tone stacks, leave the other two stock. Probably could get a little more from it that way, and if I offered it to "mod to suit" the seller I guarantee everyone's going to ask for a Marshall channel.

I've been buying too much gear lately, and I have too many ~100W amps, but it might come in handy some day so for now I'll probably just hang onto it and leave it as-is. Maybe recap that old Kasino, invite seven friends over and have a ridiculous jam.

[J M Fahey]

For Bass, I bet it will be better as-is (which is somewhat closer to an Ampeg tone stack) than downgrading it to a Bassman one, which is *really* a guitar tone stack.

If anything, turn it into a full James "Ampeg" stack, you are almost there.

[teemuk]

Well, I was thinking of modding it with an eye towards selling it.

If you are planning to sell it then probably best not to modifdy at all but sell in original condition. Modified amps are always a pig in a poke. Modified vintage Fenders might be something that lost huge quantities of their value.

[Chuck H]

If you are planning to sell it then probably best not to modifdy at all but sell in original condition. Modified amps are always a pig in a poke. Modified vintage Fenders might be something that lost huge quantities of their value.

I'll second that. Don't mod. Get it working and sell it if that's your plan. Anyone buying it to mod won't want your mods in there anyway. And anyone buying for a guitar amp doesn't want to shell out the $$$ for your work or they'd as soon buy a proper guitar amp and not a modified PA.