Any New Ideas on the LC PAB Circuit?

[ayan]

I still haven't thought of/come up with anything that makes sense so far. Notes:

1. The middle pot does have a tail resistor, soldered to ground (most likely).

2. Whatever the PAB does, there are two wires on the PAB board (yellow and clear) that either connect to a common wire (black) in normal mode, or disconnect from it (via 22 Meg resistors) in PAB mode.

Any ideas?

Gil

[dogears]

It sure would be nice if someone could get a peek at all the other pictures.....

[dogears]

What if the midpot connect to ground with that resistor (what size do you make out?) and also with a direct connection through the relay. PAB is stack lift by resistive amount. Only if the resistor has an orange band and not red for decimal....

Or, what would happen if you broke the connection between bass pot and mid pot?

Just thinkin'

[ayan]

What if the midpot connect to ground with that resistor (what size do you make out?) and also with a direct connection through the relay. PAB is stack lift by resistive amount. Only if the resistor has an orange band and not red for decimal....

Or, what would happen if you broke the connection between bass pot and mid pot?

Just thinkin'

1. I read the mid tail resistor to be 1.8K, or X.8K, so not enough to lift the stack at that point.

2. I have thought that if the middle cap connected to the top of the bass pot, as opposed to the mid cap directly, then you could lift the whole stack by disconnecting the bass from the middle pot.

Either way, I can't make any sense out of the picture. Now, if it helps, you guys can refer to the pictures of #124 since both amps were made around the same time. That will allow you to see the original lead dress of the TS wires, and what each color is, and it goes like this:

1. Blue = from middle cap. Cannot see where it goes in the LC picture.

2. Yellow = from bass cap, and it goes to the J/R switch on both amps.

3. Purple = from treble cap on the board, and it goes to the MID switch on both amps.

4. Note the cap at the mid switch on the LC amp (and 234, for that matter) doesn't look to be 390pF but bigger. In the case of 234, it seems it's even a SM cap.

Gil

[dogears]

I wasn't sure if that resistor was a 1.8K or something much larger.

Anyway, I have a bunch of 3K rated orange ceramic discs that look like the one on the midswitch. Even a 330pf is that size so all bets are off me thinks...

On the other hand, I heard some BM amps had a 500pf there.

What if the midpot connect to ground with that resistor (what size do you make out?) and also with a direct connection through the relay. PAB is stack lift by resistive amount. Only if the resistor has an orange band and not red for decimal....

Or, what would happen if you broke the connection between bass pot and mid pot?

Just thinkin'

1. I read the mid tail resistor to be 1.8K, or X.8K, so not enough to lift the stack at that point.

2. I have thought that if the middle cap connected to the top of the bass pot, as opposed to the mid cap directly, then you could lift the whole stack by disconnecting the bass from the middle pot.

Either way, I can't make any sense out of the picture. Now, if it helps, you guys can refer to the pictures of #124 since both amps were made around the same time. That will allow you to see the original lead dress of the TS wires, and what each color is, and it goes like this:

1. Blue = from middle cap. Cannot see where it goes in the LC picture.

2. Yellow = from bass cap, and it goes to the J/R switch on both amps.

3. Purple = from treble cap on the board, and it goes to the MID switch on both amps.

4. Note the cap at the mid switch on the LC amp (and 234, for that matter) doesn't look to be 390pF but bigger. In the case of 234, it seems it's even a SM cap.

Gil

[ayan]

After a few hours worth of emails back and forth with Chris, this is a thought:

1. As suggested by Chris (and this would apply to 234 and LC's amp) the left side (looking at the picture and imagining the switch is rightside up) of the J/R would have, from top to bottom: .005 uF jazz cap, connectionto the tail of the treble pot, and the bottom lug (bridged) connects to both the wire from the bass cap at the board and to the input of the bass pot.

2. The other end off the .005 jazz cap would go to the PAB board and be conceivably grounded in normal operation, and lifted for PAB mode.

3. The right side of the J/R switch is business as usual. Top connection would be 100K resistor to wiper of treble pot, middle connection would be to the wiper of the bass pot -- as already stated, the bottom lug is bridged to bass cap.

4. And what about the other side of the boost? Shoot, lot of ideas and speculation to arrive at this thought. First, from the LC amp, we know that two connections at the PAB relay go someplace in common; from 234 (which uses two separate black wires) we can see (black wire) one connection goes to ground. Extrapolating from there and thirdly, form the LC amp (where the .005uF cap is hidden under the switch... but we can see he uses the clear insulation sleeve as in 234), we can assume the common connection is indeed ground.

The only connections not accounted for are the bass to the middle cap. What if the output of the bass is connected to the top and middle lugs of the middle cap? That would make sense, but then what about the tail resistor? I have a wicked theory... You have to blow up the picture of 234, like in the attachment, and then you can see that there are two funny things about the resistor: it does have a clear insulator on the right side (to prevent it from touching the middle pot's case) and I propose it has the same on the left side. So the left side, rather than being soldered to the ground buss is merely wrapped around it with one loop, for mechanical stability. Note that, from the picture comparing that to the other solder joints made to the ground buss, that would be a very wimpy solder joint otherwise.

And the rest would be history: the tail end of that resistor either goes to ground via the relay for normal mode, or it would be lifted for PAB an thus incorporate the oldest "boost" that we know. Any thoughts?

Gil

[ayan]

Anyway, I have a bunch of 3K rated orange ceramic discs that look like the one on the midswitch. Even a 330pf is that size so all bets are off me thinks...

Looking at LC's amp only, I would agree. Looking at 234, there is something weird going on... that cap most definitely does not look like a ceramic disc to me (whether it is coated with epoxy, and in LC"s amp, or not).

Gil

[dogears]

Damn.... Too bad that we can't see a few mm to the right in the LC picture.

So you suggest the 1.8K tail is connected to a wire going to the PAB? Hmmmmm. Kind of a sloppy way for the big guy to do it. Especially since that "other" amp we have really good pictures of has no tail and it is older than 234.

[dogears]

No doubt. 234 has a Silver Mica. Could be a 390pf. Or it could be a 470pf. Or anything really.....

Anyway, I have a bunch of 3K rated orange ceramic discs that look like the one on the midswitch. Even a 330pf is that size so all bets are off me thinks...

Looking at LC's amp only, I would agree. Looking at 234, there is something weird going on... that cap most definitely does not look like a ceramic disc to me (whether it is coated with epoxy, and in LC"s amp, or not).

Gil

[ayan]

Damn.... Too bad that we can't see a few mm to the right in the LC picture.

So you suggest the 1.8K tail is connected to a wire going to the PAB? Hmmmmm. Kind of a sloppy way for the big guy to do it. Especially since that "other" amp we have really good pictures of has no tail and it is older than 234.

That's one possible conclusion after looking at 234. I would agree that it is a sloppy way of doing it, but considering the clear sleeve used elsewhere on the same resistor and the .005uF cap, it looks like a possibility to me. Also note that the .005uF cap in 234 is already kind of sloppy, with the span from end of lead to end of lead being much longer than I have ever seen. Suspect, for that reason, it may be gooped to the side (234) or bottom (LC) of the J/R switch.

Gil

[ayan]

Especially since that "other" amp we have really good pictures of has no tail and it is older than 234.

Well, there is something else to consider: it is reaonable to suspect that 234 (which lives in L.A.) and LC's amps would have the latest tweaks, correct? So maybe the other amp is "dated."

Gil

[dogears]

Whoops.... I just realized LC has the dpdt where the midboost is. Does that look like a standard deep switch?

[ayan]

Whoops.... I just realized LC has the dpdt where the midboost is. Does that look like a standard deep switch?

Nope, look at 124 -- the amps are VERY similar -- Dumble just reused the old switch.

Gil

[dogears]

Where is the 4.7M ground on the midboost in 234? Just looking....

[ayan]

Where is the 4.7M ground on the midboost in 234? Just looking....

In the LC amp, it's very visible on the switch, on opposite side from where the orange cap is. In 234, it must be behind the SM cap, I can't see it on the other side... but it must be there cause I played that amp and it didn't pop when flipping the MID switch up.

Gil