Help with Reverb/Tremolo wiring Gibson GA15-RVT

[Firestorm]

my observations lead me to conclude that the braided wire at pin 2 is simply to prevent stray RF interference...the braided wire in the substitute GA15RVT footswitch appears to be bridge soldered to pins connected to the exterior metal housing of the pole switches and NOT to either side of the poles on the tremolo and reverb switches...

That still leaves me with the problem of why I don't have any tremolo on the first amp. I used the known good pedals from both amps and can't activate the tremolo circuit.

I took all the tubes from the known good amp and substituted in the problem amp and there was no change. I took all the tubes from the problem amp and placed them in the good amp and reverb and trem operated properly.

The shield IS only to prevent interference. The reverb cable is connected directly to a tube grid, so stray signals, will get amplified. The extra connections to the switch bodies are just to get the footswitch housing connected to ground, too.

So now you know that the footswitch works and your trem problem is not tubes. Next step is to check the resistors in the oscillator circuit and likely replace the three caps in the phase loop. When these leak current it can shut down the oscillator. Ceramics are supposedly better than films for this.

[Phil_S]

Sorry for stating the obviously simple....it won't oscillate if the tube is weak. Easy enough to make a swap if you haven't already done so.

[devilsindetails]

Next step is to check the resistors in the oscillator circuit and likely replace the three caps in the phase loop. When these leak current it can shut down the oscillator. Ceramics are supposedly better than films for this.[/quote]

Ok thanks...as stated before the Gibson Tag board with its numeric markings is confusing and seems to bear no real-world 3-D realtion to the schematic.....

Would the three caps in the Phase loop be C19 C20 C21?

Any idea as to which resistor/s are most integral to the signal modulation circuit: I'm guessing R34 R35 R36?

I am assuming that once I can get the tremolo activated and adjust frequency then I can move on to the depth part of things?

Thanks for your time and input Firestorm.

I've attached the schematic once again, if you're so inclined, to offer your eyes, so you don't have to scroll back.

[devilsindetails]

Sorry for stating the obviously simple....it won't oscillate if the tube is weak. Easy enough to make a swap if you haven't already done so.

Thanks Phil...I swapped the tubes from known good amp to troublesome one and visa versa...definitely not the tubes....

[tubeswell]

Would the three caps in the Phase loop be C19 C20 C21?

Yep

Any idea as to which resistor/s are most integral to the signal modulation circuit: I'm guessing R34 R35 R36?

yep

I am assuming that once I can get the tremolo activated and adjust frequency then I can move on to the depth part of things?

Yep - provided that the oscillator works then the depth pot will be able to increase/decrease the 'strength' of that

[devilsindetails]

Thks Tubeswell...will post results shortly.....

[devilsindetails]

Ok...replaced the three caps C19, 20, 21 no change...still no tremolo..

I also checked every resistor and only found two out of spec R4 and R52 (neither one in the trem circuit, of course, but while I was inside anyways....)

No effect on tremolo, of course, but definitely a lot more reverb gain and master volume. I have yet to fire it up after replacing R52 but as that resistor is not part of the trem circuit all I'm expecting is a little more gain off the instrument jacks.

So fellows.where do I go now? Thanks again.....

[tubeswell]

Time to take some (idle - DC) voltages at the relevant HT supply point(s), plate(s), grid(s), cathode(s) etc.

[Pierre]

I'm attaching a schematic for the GA15-RVT that I drew after tracing the circuit of one of these that I was repairing. It was drawn with express schematic. The schematic that is commonly available on the internet was not even close to the actual circuit. Hope this helps.

[devilsindetails]

What format is this in? The attachment doesn't show up in the post and my Windows 7 doesn't seem to be able to open it. Can you open it and save image and then attach to your post? Thks. Leon

[Colossal]

What format is this in? The attachment doesn't show up in the post and my Windows 7 doesn't seem to be able to open it. Can you open it and save image and then attach to your post? Thks. Leon

Try right-clicking and downloading the file directly to your desktop or download folder. It will then open with JSchem.

[tubeswell]

To reiterate.

Time to take some (idle - DC) voltages at the relevant HT supply point(s), plate(s), grid(s), cathode(s) etc.

[devilsindetails]

Nope...won't open..unknown program....

[martin manning]

You need to download and install ExpressPCB: http://www.expresspcb.com/ExpressPCBHtm/Download.htm

[devilsindetails]

To reiterate.

Time to take some (idle - DC) voltages at the relevant HT supply point(s), plate(s), grid(s), cathode(s) etc.

Hi Tubeswell....well I took the measurements...but kind of thinking I may not have done this correctly....

I used my DIMM connected black probe lead via insulated alligator clip to chassis ground and then with my left hand securely in my back pocket used the red probe to test DC voltage at the various pins on all five tubes.....

I know I muffed up the heater readings because I forgot to set my DIMM to AC which probably explains why I didn't get any readings there at all despite the fact the tubes were all operating As I said the amp operates all tone controls and reverb with lots of output so I believe it is safe to assume the heaters are working. I will take proper heater measurements tomorrow as I know it would be helpful to ensure the heaters are operating at the desired voltage across the board.

Anyways here are my other readings

(6EU5)
V1:
Pin 1--0 vDC
Pin 2--0 vDC
Pin 3--BLANK
Pin 4--1.3vDC
Pin 5--0 vDC
Pin 6--186 vDC
Pin 7--164 vDC
Pin 8--0 vDC
Pin 9--1.3vDC

V2:
Pin 1--0 vDC
Pin 2--0 vDC
Pin 3--BLANK
Pin 4--1.9 vDC
Pin 5--0 vDC
Pin 6--287 vDC
Pin 7--111 vDC
Pin 8--0 vDC
Pin 9--0 vDC

12AU7
V3
Pin 1--73 vDC
Pin 2--0 vDC
Pin 3--2.8 vDC
Pin 4--0 vDC
Pin 5--0 vDC
Pin 6--287 vDC
Pin 7--36 vDC
Pin 8--49 vDC
Pin 9--0 vDC

EL84
V4
Pin 1--BLANK
Pin 2--0 vDC
Pin 3--10.5 vDC
Pin 4--0 vDC
Pin 5--0 vDC
Pin 6--BLANK
Pin 7--294 vDC
Pin 8--BLANK
Pin 9--296 vDC

V5
Pin 1--BLANK
Pin 2--0 vDC
Pin 3--11 vDC
Pin 4--0 vDC
Pin 5--0 vDC
Pin 6--BLANK
Pin 7--294 vDC
Pin 8--BLANK
Pin 9--295 vDC