What is this?

[tubenit]

Guys,

You have any thoughts about what this could be?  I'm thinking possibly some type of VVR device using a rotary switch instead of a pot?

With respect, Tubenit

[sluckey]

No idea what the rotary switch is doing.

The components I've circled in yellow is a low voltage dc supply. The positive side of the cap has a green wire connected. That green wire meanders down and to the left and connects to the cathode of a diode on a small perf board that probably has a small red relay on it.

I don't think those three black Dale power resistors are part of this circuit. One of them appears to be connected to the screen grid of an EL84?

[ACDCG400]

this looks like the inside of a soho amp. maybe =] im usually mistaken. that might be part of the tone/bump circuit? =/ not sure.

[tubenit]

The components I've circled in yellow is a low voltage dc supply. The positive side of the cap has a green wire connected. That green wire meanders down and to the left and connects to the cathode of a diode on a small perf board that probably has a small red relay on it.

I don't think those three black Dale power resistors are part of this circuit. One of them appears to be connected to the screen grid of an EL84?

The amp has a relay so that answers that question and I think you're right about screen resistors also.
Sluckey, you seem to always have a knack of figuring this stuff out.

Thanks for the reply, 
Tubenit

[tubeswell]

As to the switch itself (and I know that's not what you were asking about tubenit), it appears to be a 4T3P switch (I have a bunch of 'em) wired in a variable RC network, possibly for rk/ck combinations?

[tubenit]

Tubeswell,

Thanks for the information!  What is a rk/ck combination? Any chance that has to do with the cathode and tail resistor on a LTPI?

With respect, Tubenit

[tubesornothing]

What is a rk/ck combination? Any chance that has to do with the cathode and tail resistor on a LTPI?

rk - cathode resistor
ck - cathode bypass capacitor

Its generic designation, for any gain stage, PI, V1, V2, even cathode biased power stage.

I sure would be interested seeing the front panel.  I searched the net, but could only find a lo-res picture, it did not show the details.  Have any more gut shots? 

If you are going to duplicate the circuit, make sure to measure B+ voltages, tube voltages, and the OT impedance.

[tubenit]

Have any more gut shots?

http://ampgarage.com/forum/viewtopic.php?t=9948

[tubeswell]

Hmmm... , if the 'bump switch' is to switch to another tone stack, then maybe its like a Dumble style switchable tone stack? (So its a 6-position switch then?)

[topbrent]

Here is the front panel of the amp guts...

Click on the link, when the pic opens you can enlarge it well enough to see the lettering on the knobs and switches.
http://www.tonecentral.net/gearpics/65AmpsSoHo.jpg

[Heinz]

I have spent some time trying to reverse engineer the SoHo circuit from the gut shots back in the old giant SoHo thread. Here's what I came up with.
The MV is probably a passive control with some sort of dynamic filtering, not a VVR. However, a VVR might be worth trying. Since Geezer's HoSo is not a straight clone of the original circuit, another modification might not be a bad idea.
The original SoHo MV adds extra gain by engaging a cathode bypass cap on the second stage and inserts the MV pot after the tone stack. I'm also pretty sure that the bump control lifts the tone stack and the bump level is a switchable "mid" resistor. However, one thing puzzles me. Why did they use switchable resistors instead of a pot? Was it just a design choice or is there some necessity (e.g. dc through the wiper, power rating, etc)?

[tubenit]

Heinz,  I think your version is probably closer. Here is another one to consider with a VVR.

With respect, Tubenit

[Fresh_Start]

Why did they use switchable resistors instead of a pot? Was it just a design choice or is there some necessity (e.g. dc through the wiper, power rating, etc)?

I'm pretty sure that the "Bump Level" control is a 2-pole (if not 3-pole) rotary switch from the photos.  My guess is that it does double duty, switching the slope resistor and the "Mid" resistor.  Note that the amp doesn't have a Mid control despite all the knobs.  Don't understand how the "Bump" switch interacts with the "Bump Level" though.  Maybe the "Bump" switch is like a variable Raw control, with the Bump level selecting the resistor between the TMB tonestack & ground.  Anyway, a 2-pole/6-throw switch gives the designer a lot more control over the possible combinations.

Chip

[neon333]

That rotary switch is for the bump level.

[neon333]

I'm also pretty sure that the bump control lifts the tone stack and the bump level is a switchable "mid" resistor. However, one thing puzzles me. Why did they use switchable resistors instead of a pot? Was it just a design choice or is there some necessity (e.g. dc through the wiper, power rating, etc)?

That is a puzzler?  Could there be a clue here: 4:45 to 6:00 in

[Heinz]

I've watched the video and it does explain what the switch does, but it doesn't give me a clue how it is done. Still the question remains: why did they use a switch instead of a pot?
Is there more to it than switching resistors (which is obvious from the gut shots)?

If you bought a car that has a 6-position steering wheel, wouldn't you ask why it was done that way...?

[arjepsen]

Did the MV and switch ever get worked out? Been trying to go over the pictures, and think I'm hitting my head on the same place as everyone else :)
Did anyone do get some testing and results?

[HotBluePlates]

Did the MV and switch ever get worked out? Been trying to go over the pictures, and think I'm hitting my head on the same place as everyone else :)
Did anyone do get some testing and results?

Are you asking about how the "bump" switch is wired, and what it does?