Gibson GA-1RT

[mresistor]

Does anyone here on the forum own an unmolested example of a Gibson GA-1RT amplifier?  I could use your assistance.

[sluckey]

Here's one. Old thread...

     https://el34world.com/Forum/index.php?topic=9593.0

What do you want to know?

[mresistor]

I understand what is going on with the reverb pan, it uses an external input and then when the footswitch is engaged the output of the reverb pan is mixed with the input to V1A. I guess when the fs is switched again there is a voltage divider the loads the pan.
But this one was modified by someone and the original wiring in the output side of the reverb tank has been changed. Also the pount of entry of the rca cable to the chassis has been changed.


I'm hoping to find a pic of the inside of the reverb pan, and also of the area around the input jacks on an unmolested amp to see how it was physically wired. Where the components were. The 1M and 3.3K that goes from input side of pan to a terminal in the fs and 3.3K to ground. I suppose the 1M and 3.3K were in the chassis near the input jacks. 


I'll probably get it figured out but seeing an original would be great.

[sluckey]

The 1M and 3.3K are not in the chassis. More likely inside the footswitch box. There are plenty of pics on the net use google. Pete (Dummyload) can tell you if Richard still has his GA1RT. Here's a pretty good pic...

[mresistor]

Before I posted I searched for quite a while and have found some very good photos,, but none inside the pan.   


I'll open up the fs for a look.

[mresistor]

Forget opening the fs, there is a rubber pad on the bottom and I don't want to distrub it.  I'll measure it with the meter..

[mresistor]

Thanks Sluckey,  I already have that photo..

[mresistor]

Footswitch is open one way and shorted the other.


The output transformer has two secondary windings.  Anyone ever see this before?   one winding going to speaker the other had been wired to go to the input of the tremolo pan. The OT has this written on it;
GA5-0
174A7
549-6129


I'm thinking it's the same trasformer set Gibson used in the GA-5 amp. I looked at that schematic and it also only shows one set of secondary leads. 

[mresistor]

I found this picture that clearly shows more than two wires coming out of the secondary side of the output transformer.  So  it appears on set of leads was used for the feedback resistor, which is mounted on the terminals next to the OT. 
I replaced the cathode bias resistor and capacitor on the 6BM8 and wired up only the original speaker and wow!  this little bugger sounds cool.. very cool distortion when turned up and the tremolo is very nice too.

[mresistor]

The ot has two green and two black wires on the secondary..  I'm hoping its like any other multiple tap ot.. each one has a different impedance.. just strange they are the same colors..

[sluckey]

The ot has two green and two black wires on the secondary..  I'm hoping its like any other multiple tap ot.. each one has a different impedance.. just strange they are the same colors..

May not be taps at all. Could be totally separate windings. Use your ohm meter to know for sure.

[mresistor]

Getting back to the footswitch.  I don't see how it's possible that the 1M and 3.3K resistors are contained within the footswitch.  The cable to the footswitch is coax and has only two wires. The Schematic shows 3 wires coming to the footswitch.  And the footswitch measures dead short and dead open between the wires.


1- from the fs output coil
2- from the center lead of the switch
3- from the fs input coil


As well the .001 capacitor is present inside the amp chassis.


So I am still in the dark as to where the 1M and 3.3K were located physically. 

[sluckey]

That cable from the amp connects directly to the output jack on the tank. The footswitch connects directly to the tank. Those components should be inside the tank. If your tank doesn't look like this pic it's probably not the original.

[DummyLoad]

The 1M and 3.3K are not in the chassis. More likely inside the footswitch box. There are plenty of pics on the net use google. Pete (Dummyload) can tell you if Richard still has his GA1RT. Here's a pretty good pic...

richard fried the OT in his shortly after acquiring it. he mostly rebuilt it - new PT, new OT. richard modified the preamp and reverb ckt. and added an EF86 front end. his amp ate 3 OTs before he got it sorted out. stock they don't seem to be reliable running the 6L6 so hot. 

richard did not like the stock tone. he's a weird one that one. richard did all the preamp mods, i helped revamp the output section and PS. he used the amp in early toxic water (his band) work. don't know what he's using now...

attached is the schema of what the amp evolved to. i's sure he won't mind me doing so...  sorry i can't be of any help. i do hope that you get yours sorted out.

--pete

[mresistor]

Thanks DL and Sluckey


The reverb pan is  "Reverberation Unit Type IV, made by Hammond Chicago Ill. "   I believe it is original. It still has the 33 ohm resistor present on the input coil side.  There are no signs of a wire or the other two resistors inside but they could have been there at one time. The pan looks just like one in the photo posted by Sluckey.

[DummyLoad]

Thanks DL and Sluckey


The reverb pan is  "Reverberation Unit Type IV, made by Hammond Chicago Ill. "   I believe it is original. It still has the 33 ohm resistor present on the input coil side.  There are no signs of a wire or the other two resistors inside but they could have been there at one time. The pan looks just like one in the photo posted by Sluckey.

is there a part number on the tank? measure the DC resistance of the input and output.


--pete

[mresistor]

DL - stamped into the metal on top of the pan the number 1122  and adjacent to that is an inked stamp number 6121

[mresistor]

190 ohm output
1.3 ohm input

[DummyLoad]

seems like that tank would work well with any of the fender reverb ckt. plans. check the jack grounding config. i have a couple that i salvaged from hammond organs that sound exceptional.


https://www.amplifiedparts.com/tech-corner/accutronics-products-and-specifications

input/output DCR measurement correlation to input/output impedances. grounding scheme of the input/output jacks is also relevant to how driver/recovery circuits are laid out.


https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared


--pete