Weird tremolo problem with a GA-5T Skylark

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Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Sun May 14, 2017 2:53 am

Hi guys,

Picked up an early 60's GA-5T Skylark. The Crestline model.
Volume, Tremolo On/Frequency, Power Off/On.
This one here:

Image

Amp sounds amazing, but the tremolo has a couple weird issues.
First, the tremolo has just a speed control with an off switch, but the tremolo will not work unless I turn the amp off and back on with the tremolo switch on. Not a tragic problem, but weird.
The other problem is that there's a weird distorted splat on the attack of notes or chords with the tremolo on. You can hear it in the clip I recorded of this amp here:

https://youtu.be/G4Y6Gwsiz5I?t=75

I'm not really sure what's happening there. The splat only happens when the tremolo is on. I don't know if it's related to the issue where the tremolo won't work unless you cycle the power, or completely unrelated. The power thing I can live with, but I really would like to fix the splat. Any ideas what to look at first there?

Thanks!
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Mon May 15, 2017 2:11 pm

It has been suggested to me that the pot/switch for Frequency of the tremolo might be bad, so I will attempt to just clip around that and see what happens. Other possibility mentioned is that some of those .01 and .02 caps might be failing, so I can try replacing those one at a time too, I guess, unless there's an easier way to identify which if any are problematic.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Mon May 15, 2017 7:57 pm

Not to make this all one-sided, but I cleaned the pot and with, and the tremolo now comes on reliably with the switch, but still crackles. I did notice that it crackles less with milder pickups, so I'm assuming I'm overloading the input of the tremolo circuit in a non-musical sounding way. Amp sounds awesome except for the tremolo, so I'll just leave it as is for now. It may well be that someone has heated up the input gain for other reasons, and it just doesn't play well with the tremolo circuit. It's one of those rat's nest, truly point to point amps, so it's a drag to replace things. Here's a shot of the internals, all four of them.

Image

I know it doesn't really count, but I measured the capacitance of those disc caps around V2, which is the tremolo tube, and they were all well within the 20% (!!) stated on the schematic.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Mon May 15, 2017 7:59 pm

For those playing at home, in the pic above, the volume knob is closest to the jacks, and the tremolo "frequency" and on/off is that stacked thing just to this side of the volume knob.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby JJman on Tue May 16, 2017 12:38 am

I would check the 300k resistor that is grounded on one side and on the trem and 220k grid leaks on the other side. I would then try a 220k, then 150k since the trem sounds "too" strong, if there is such a thing. Maybe its strength is pushing the outputs into cutoff causing the ticking. Check the biasing of the output tubes too.
-If it says "Vintage" on it, it isn't.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby stratele52 on Tue May 16, 2017 9:30 am

Tremolo issue ; replace all the 3 caps.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby Dean Jr. on Tue May 16, 2017 8:59 pm

Those electrolytics don't look like Gibson factory applications. Have they been replaced? Is all of that correct? The schematic shows 3 x 20MFDs, and Gibson usually used multisections mounted in retainers inside the chassis. What are the date codes on those two yellow e-caps....and where is the third one? Have all of the tubes been tested? I would ask if you have subbed in new tubes, but a 6C4 is not the most common of tubes. I have one or two NOS here, but ???? Have you done a voltage chart?
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Fri May 19, 2017 1:40 am

Thanks, guys. Dean, no, I haven't had time to do much of anything yet except play with it. I was told the amp was "gone through", and it doesn't have any hum, so it would not surprise me to find that the elctrolytics have been replaced, and/or that other mods might have been done.
I'll probably start, as suggested above, with replacing those caps in the tremolo circuit. They're cheapy ceramic discs, anyway, and I know those have a tendency to crap out in hot amps, so I'll start there. I also do need to try and see if I can follow the schematic and check values and voltages on things. Just wanted to see first if the problem shouted out anything obvious to anyone.

Thanks again!
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Fri May 19, 2017 1:42 am

And, while I have a really basic tube tester, I can at least see if the tubes all measure "good". I'm not sure if there are modern versions of any of those made. The current ones are all older US tubes.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Fri May 19, 2017 1:44 am

Also didn't realize my images were getting cut off so you can only see part of the schematic. Here's a link to it:

http://archive.gibson.com/Files/schematics/GA-5T.jpg
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Re: Weird tremolo problem with a GA-5T Skylark

Postby stratele52 on Fri May 19, 2017 8:35 am

bloodspoint wrote:And, while I have a really basic tube tester, I can at least see if the tubes all measure "good". I'm not sure if there are modern versions of any of those made. The current ones are all older US tubes.


Don't trust tube tester, put know good tubes.
On most tube tester if not al,l tube show good unless they are blow ( no heater ) or if they are short. Between that you can't know.

You'll waiste your time to try fixing a amp with bad tube....
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Fri May 19, 2017 12:03 pm

Well, I'd have to come up with new tubes. I don't have any extra 6AQ5 or 6EU7 in my parts stash.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby vez on Fri Jun 30, 2017 4:02 pm

bloodspoint wrote:Well, I'd have to come up with new tubes. I don't have any extra 6AQ5 or 6EU7 in my parts stash.

Have you fixed your tremolo yet? I'lll bet it's that oscillator tube 6EU7. Seeing you have two of those tubes, swap them and see what happens. Let us know.
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Re: Weird tremolo problem with a GA-5T Skylark

Postby bloodspoint on Fri Jun 30, 2017 7:32 pm

I tried swapping the two 6EU7's around, no difference. I have had zero time to dig into it any further, though. What's weird is it's not in the initial attack, just the "repeats":

https://youtu.be/1hlgWc3xAV8
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Re: Weird tremolo problem with a GA-5T Skylark

Postby stratele52 on Fri Jun 30, 2017 8:01 pm

Since May 16, did you replace the 3 tremolo caps ?
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