Gibson skylark ga5 with an external tube tremolo questions

[delray]

I bought a gibson ga5 from a local guy.  I cleaned it up a bit and brought it up on a variac. It worked to my surprise. I am planning to order the mojotone ga5 small parts kit to recap and replace the two prong cord with a grounded cord.

At some point someone added an interesting 6snwgt tube based tremelo circuit to the amp. This has two knobs which I assume are depth and speed. The speed seems to work although the depth doesnt seem to do anything other than make the amp hum if turned to 100%. It appears that this unit draws power from the 6v6 heater and then returns the signal back to the rectifier tube.

I will probably leave this unit in the amp and wanted to recap it and maybe replace the pots. I can read most of the values but there is one cap i cant read.

Does anyone have a schematic for this modification? Any explanation for how it is supposed to work?

[sluckey]

Whoever built that had access to high quality caps, pots, standoff turrets, etc., and was familiar with military point to point wiring techniques. It is unlikely that those silver caps are bad. They are higher quality than any replacements you might use. I suggest to leave them alone. That yellow filter cap is the only cap in that unit that may need replacing.

The circuit is likely a typical phase shift oscillator. With a big hi-res pic I could probably draw the schematic for the small chassis. But, I still need to see exactly how it is connected to the GA-5. The unit needs filament and B+ voltages from the GA-5 just to make the oscillator work. Then the output of the oscillator has to connect back to the amp. So, I would expect at least 5 wires between the two chassis.

Tell us more and post some big hi-rez pics, not those small 480x640 pics.

[delray]

Thank you for the reply. I can read the yellow one but there is one that is moldy or something that i cant really read.

It looks like power leads come in off the filament on 6v6 and then the output goes back in to the 5y3.

[delray]

Pics

[sluckey]

Thank you for the reply. I can read the yellow one but there is one that is moldy or something that i cant really read.

See if you can roll the cap over enough to see more writing.

It looks like power leads come in off the filament on 6v6 and then the output goes back in to the 5y3.

The wire connected to the 5Y3 is the B+ being sent to the tremolo unit. It is not the tremolo output. There should be at least one more wire between the two chassis.

Still waiting for hi-rez pics of the tremolo chassis.

[HotBluePlates]

If needed, upload the pictures to Imgur.com, then copy/paste the links here.  No size restrictions on doing that.

[delray]

Yeah i was hitting the limits on uploads















There are more pics here
https://mobolr.com/Ga5/

[sluckey]

I'm pretty sure the moldy crud is just on the plastic cover for that one cap. Use a utility razor knife to cut the plastic cover off that moldy looking cap. Then you should be able to read the value on the metal body of the cap. Removing that plastic will not harm the cap.

[kagliostro]

If you dismant that cap from the circuit it seems (from the photo) that you can read something, just try to read and make a note before to procede to skin it


That to be sure you save the more data you can, if happened a transfer from the metal to the plastic of the label this will help you to prevent data loss


Franco

[bmccowan]

If you have not ordered that small parts kit yet...Mojo made some changes. The most significant is the filter caps. You may want to compare the schematics. this one is confirmed and nicely drawn. And the filter caps you definitely should replace - some of the other parts you may not need.
BTW - those little "special" Jensen speakers do not hold up to high volume very well. You may want to replace the speaker and save the Jensen.

[delray]

If you have not ordered that small parts kit yet...Mojo made some changes. The most significant is the filter caps. You may want to compare the schematics. this one is confirmed and nicely drawn. And the filter caps you definitely should replace - some of the other parts you may not need.
BTW - those little "special" Jensen speakers do not hold up to high volume very well. You may want to replace the speaker and save the Jensen.

Yeah I noticed that the factory caps are 20/10/10 and mojo's are 16/8/8. I reached out to their tech support  prior to ordering and they said

Ga5's were originally designed around 110/117vac, so the voltages were considerably lower inside the amp than they are operating the same amp at 122~125vac. Going with the original filtering specs, it throws the b+ of the amp about 10v over where the voltages reside with the lower filtering in the kit.  This gives a slightly more true-to tone of the original, but maintaining the smooth, tweed-ish overdrive at higher volume levels.  This was one of the more popular mods that were incorporated into the GA-5 "Les Paul" reissues we built for Gison about 15 years ago.  They varied slightly from the original to bring the amp up to date to hang with Fender and Vox's smaller combos, and really propelled the GA5 to a cult status, bringing attention back to the 50's originals, driving up the value.


Hopefully these will work for me. There are a couple of other small changes as well. The kit is pretty reasonably priced and i get a few other pieces i may need (jacks, etc.).

[delray]

It looks like power leads come in off the filament on 6v6 and then the output goes back in to the 5y3.

The wire connected to the 5Y3 is the B+ being sent to the tremolo unit. It is not the tremolo output. There should be at least one more wire between the two chassis.

Still waiting for hi-rez pics of the tremolo chassis.
[/quote]

You were correct. There was an additional wire that goes to the 6v6 and it is shielded.

It looks like power/ground off of 5Y3, filament pins 2 and 7 of 6v6 and the shielded cable to pin 5 (control grid?). It seems like there should be an input signal and a return for this to work?

[sluckey]

It looks like power/ground off of 5Y3, filament pins 2 and 7 of 6v6 and the shielded cable to pin 5 (control grid?). It seems like there should be an input signal and a return for this to work?

The tremolo unit is an oscillator, ie, a signal generator. It does not require any input.

[bmccowan]

If you have not ordered that small parts kit yet...Mojo made some changes. The most significant is the filter caps. You may want to compare the schematics. this one is confirmed and nicely drawn. And the filter caps you definitely should replace - some of the other parts you may not need.
BTW - those little "special" Jensen speakers do not hold up to high volume very well. You may want to replace the speaker and save the Jensen.

Yeah I noticed that the factory caps are 20/10/10 and mojo's are 16/8/8. I reached out to their tech support  prior to ordering and they said

Ga5's were originally designed around 110/117vac, so the voltages were considerably lower inside the amp than they are operating the same amp at 122~125vac. Going with the original filtering specs, it throws the b+ of the amp about 10v over where the voltages reside with the lower filtering in the kit.  This gives a slightly more true-to tone of the original, but maintaining the smooth, tweed-ish overdrive at higher volume levels.  This was one of the more popular mods that were incorporated into the GA-5 "Les Paul" reissues we built for Gison about 15 years ago.  They varied slightly from the original to bring the amp up to date to hang with Fender and Vox's smaller combos, and really propelled the GA5 to a cult status, bringing attention back to the 50's originals, driving up the value.


Hopefully these will work for me. There are a couple of other small changes as well. The kit is pretty reasonably priced and i get a few other pieces i may need (jacks, etc.).

Oh yeah - those cap values will work just fine - no worries. But whoever wrote that reply to you certainly had his Mojo working.  They did a nice job with those reissues they built for Gibson, but they were EL-84 amps with a solid state rectifier - different beast. For their own GA5 kits, it appears they are getting a bit lower voltage with the PT selection, not by slightly reducing cap size. But again, those filter caps will work fine.

[sluckey]

Here's the schematic for that tremolo chassis. I guessed .1µF for the value of the dirty cap that's connected between pins 2 and 4. It should be very easy to move that cap up to see the value printed on the underside of the cap.

I also guessed 50µF for the big yellow Aerovox cap. Just roll the cardboard cover slightly to see it's printed value.

edit... updated schematic

[delray]

Here's the schematic for that tremolo chassis. I guessed .1µF for the value of the dirty cap that's connected between pins 2 and 4. It should be very easy to move that cap up to see the value printed on the underside of the cap.

I also guessed 50µF for the big yellow Aerovox cap. Just roll the cardboard cover slightly to see it's printed value.

Thie is awesome. The aerovox is a 20 MFD/50VDC. The other two a .1MFD 600V. The pots are .5 meg.

[Dave]

I don't think that 20uF versus 50uF would make a lot of difference. The idea for that bypass cap is for the cathode to be "fully" bypassed so that the oscillator will oscillate at its maximum. 20uF should do it, 50uF probably wouldn't be noticeably different. Some might argue that removing it and the resistor and replacing with a single LED would be optimum.


Dave

[sluckey]

Is this black resistor 1 Meg or .1 Meg?

[shooter]

looks point 1 from here

[delray]

Yes .1 is correct.

[sluckey]

Yes .1 is correct.

OK. I updated the schematic in reply #14.