I have a PAB switch question.

[sjturbo]

I have a Music Man RD50 and I have a question regarding adding a PAB. I believe a PAB switch takes the tone stack out of the circuit increasing the gain, is that correct? If so my question is can I use a variable resistor (pot), instead of a switch? Would I need to remove the "mid" resistor in the tone stack? And would it act like a "blend" knob until it reached the extreme and took the stack completely out of the circuit? Second question is could someone recommend a R value for the pot and tell me where in the circuit I would place it. Lastly would this be somewhat like the Raw control on the Allen amps?

[sluckey]

Replace R8 with a 25K pot. Does that do what you want?

[sjturbo]

Thanks for the reply sluckey. The R8 location would get me a "mid" pot control but it's not quite like the location joostin posted in another forum. He added the PAB switch in a different location and left R8 alone. Could the "PAB pot" be placed where he placed the switch? Again would this location act as a blend pot? There were some additional mods by BOBW regarding D style changes to the limit channel. I have added a link to the schematic in hopes that others may have some thoughts on the PAB pot as well as the D changes also.
Thanks again sluckey!

[sluckey]

The R8 location would get me a "mid" pot control

The value I suggested would act more like a raw control to lift the tone stack.

[sjturbo]

Thanks sluckey, I guess I misunderstood. I'm not the sharpest tool in the shed when it comes to circuits. So correct me if I'm wrong, at 25k it is acting as a "mid" resistor and at 0 ohms it is lifting the tone stack?

[sluckey]

The higher the resistance, the more the tone stack is lifted.

25K is a typical value for a 'mid' pot in a tone stack made for tubes where all pot have higher values. But your TS interfaces with SS devices and resistance values will be much lower. Just look at the pot values in your MM. In this case, 25K should lift the tone stack like a raw control. If it doesn't, just increase the value of the pot. The raw control will gradually lift the TS. It doesn't act like a switch.

[sjturbo]

Thanks again sluckey! I was hoping someone would address the location of the PAB switch between the bass and treble pots. But I guess I forgot to post the schematic link, http://ampgarage.com/forum/download.php?id=2201

[shooter]

Sorry but you are not authorized to view or download this Attachment

[sjturbo]

Sorry shooter, I'll see what I can do about that.

[sjturbo]

Here is the schematic with the PAB mod.

[sluckey]

I see it just fine. You have to be logged in to TAG.

[sjturbo]

I guess some may not be members of AG. Thanks sluckey.

[PRR]

> the schematic with the PAB mod.

The "mid shift" red mod must be a typo; it does not do anything as drawn. Need to cut the black line under it. There should be a cap there stock, we's just changing the cap value.

The "PAB relay" is a complicated way to do essentially the same thing as breaking (or greatly increasing) the ground leg of the mid resistor, here R8 1.5K. Both ways end up feeding the next tube stage through the Treble pot. But with nothing else loading the treb pot, this will be essentially full-range full-strength audio, +/-0.5dB 40Hz-20KHz, -3dB at 20Hz.

R8 (here 1.5K) is Fender's location for a "Mid" pot. What the mid pot really does is cut *everything* down, an adjustable amount, then the Bass Treb pots add-back their ends. It's a cheap knob and somewhat useful.

A known mod is to replace the MID resistor/pot with something that can go MUCH higher resistance. That is what Sluckey was suggesting. A 25K variable resistor will have the stock-like 1.5K resistance down at the low end of the dial, but turn it to the high resistance end and the "cut-down everything" function is foiled, it all comes out full strength. An "audio taper" pot is best, because you can easily dial-up values like 0.5K or 1.5K or 3K (near stock tone) in the first 7/10th of a turn, then go all the way for no cut-back full-tone.

A simple switch under R8 gives you stock/LOUD, and is a popular choice.

> not quite like the location joostin posted in another forum. He added the PAB switch in a different location and left R8 alone.

Shop around. Try all possible variations.

Personally, I would not mess around in a MusicMan. They are fairly unique, and if not collectable now, probably will be in some years.

[sjturbo]

Thank you PRR for the explanation! It was very helpful. I did think the mid shift in red was interesting but I assumed that he just left the original connection line in there. As a tinker-er I am always interested in what changes I might make to my amps, pedals, and guitars but. AS I am the original owner of the amp and it is still all original I have been reluctant to do anything to it. Oh I did change the 12AX7 to a 12AY7.