Assistance please with the Gibson GA-1RT

[mresistor]

Hi guys - I still have a problem with getting this reverb echo to work.  To summarize previous findings, the schematic appears to not be correct for this one that I have on the bench. The original foot switch in this amp has a Single pole Single throw switch. The schematic shows Two Poles on the switch and when turned off it connects to the junction of  1M and 3.3K resistors which also tie to the positive side of the tank input coil and to ground. There is also a 33 ohm resistor in series with the input coil of the tank.
I have it wired up with a 1M resistor tied just past the 33 ohm resistor on the input coil and then to the pan chassis,  It works and passes a weak signal from an external amplifiers speaker, which the input to the reverb tank is connected two via two alligator clips. But there is a nasty distortion and  a larger amount of hum present.
I'm now thinking I have a ground issue.  The pan resistance is  190 ohm output  and 1.3 ohm input
Your thoughts? Any ideas?


https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga1rt.pdf

[shooter]

schematic appears to not be correct

  welcome to Gibson, I've done 1/2 doz repairs, I've spent more time finding the correct schematic than on repairs!

[mresistor]

Well I thought I had the foot switch in the on position..engaging the pan-   But it wasn't  - I switched it on and WOW   the echo reverb is very nice sounding and very cool..   I have it connected to a '68 Bronco amp's speaker which has the equivalent speaker to the Gibson .. both little eight inch alnico..

So .. the hum is when I have the foot switch in the off position and I am picking up hum from the atmosphere - probably because this is a SPST foot switch and then the wire that inputs from the tank  is left open ended.  so the schematic might be correct and this unit has had a wrong switch installed in the foot switch housing and two conductor wire instead of three..   
I wonder it there is some sort of workaround..  ?

[PRR]

If there was a simpler (1Pole) way, wouldn't Gibson had done that?

The possible compromise is to NOT have main amp feeding through the Gibson when reverb is off. Just let Gibbie sit silent. That just needs a shielded 1-pole to short the tank output. (No connection to the 1Meg:3K divider.) Though if Gibbie is now reverb-ONLY, you might as well at 6BM6 pin 3.

Otherwise you are looking at relays, which need small power voltage at not-small current, and general complication.

[mresistor]

Thanks PRR - I found some three conductor flexible cable and a DPDT footswitch so I'm going to wire it up like the schematic..  see what happens. Thing is - I have no idea where on the amp they physically had placed the 1M  and 3.3K - only clue is  the dashed lines of the reverb pan on the schematic - it appears they weren't located on the reverb pan... however they could be I suppose.

[Willabe]

They might have been mounted in the foot switch chassis/box/cover.

[mresistor]

Thanks Willabe   - that thought crossed my mind..  it would be very convenient to mount inside the fs if there is room.  Also my concern would be where to pick up ground  - using a three wire cord..   with four wires would be doable I think..

[mresistor]

Just thought I'd finish this thread off - SUCCESS !!    Got the lil bugger working exactly like it was designed and built to do. Installed a DPDT latching foot switch and three conductor wire. I wired the 3.3K resistor on the back of the foot switch from one pole to the grounded pole that is grounded at the output side of the pan. Then in the reverb tank I took  the other pole from the foot switch in the off position from the output side of the pan and with a 22ga wire tied it to a 1 meg resistor that connects to the input coil on the input side. I secured the wire along the inside of the reverb pan vertical wall. It needed that load for when the foot switch is off.  When the foot switch is on it takes the output of the reverb pan and sends it to the input tube of the amp.

Hooked it up to one of my Champs and it sounds pretty cool  -   and when switched off is very silent. 

Thanks for everyone who helped..