Gibson GA-18 Temolo Issues

[TxTeleman]

The tremolo works... sort of. The speed control works only up to about 50%, and if you turn it up past that the temolo goes away. The same with the depth control , if you turn it up past 50%, the trem goes away. All I can get is a medium slow trem.

Once it goes away, I have to turn them both back to 0, and then, even if I turn them both back to 4 or 5, there is a delay before the tremolo comes back on.

The tube (12AX7) and caps are all new.

Any suggestions would be appreciated.

[Markr1]

I have a 1959 GA-18T all original I bought a couple of months ago for 500 bucks which was a great deal considering they go on eBay from 900 to 1100 bucks on eBay. My tremelo works great.I don't know why yours doesn't work but surely an amp tech could figure it out.

[Prairie Dawg]

Unless you're working on this yourself do not assume that a 'recap' got these three caps in the trem circuit. If in fact they are new and of the correct value I'd start looking of a resistor in there that has drifted in value-fairly common in amps of this age. It's a relatively simple circuit and there are only at most a dozen components involved, they're easy enough to check.

[TxTeleman]

OK. Prairie Dawg, I'll take a look at those.

Markr1, how does you amp sound? $500 sounds like a good price.

[Markr1]

The amp sounded really good when I got it. It had been sitting in a closet for 30yrs. I had a 3 prong put on it and changed the speaker to a weber alnico and it just blew it away. Sounds great now and of course the original speaker is in the closet.

[Chuck H]

The amp sounded really good when I got it. It had been sitting in a closet for 30yrs. I had a 3 prong put on it and changed the speaker to a weber alnico and it just blew it away. Sounds great now and of course the original speaker is in the closet.

After 30 years powered down I'm very surprised the power supply caps didn't blow up within five minutes. They must be dry as toast. Did you have them replaced?

[Markr1]

I was told it had been taken to an amp tech and had some of the capacitors and whatever else they typically replace when they go over it. It works great and no problems whatsoever or bad sounds. I can post a pic of the guts to see if anyone can tell if they were replaced. I'm not good enough to tell if there were any changes or not. All I know is it sounds great.

[AmpRX]

After checking the resistors to see if they have drifted, try changing the trem tube cathode bypass cap.

[TxTeleman]

I did a little more testing.

The Depth control is pretty much working, but the Speed control goes from real slow (~1HZ) at "1" to about 2Hz at "6". If you turn it past "6" through "12" it kills the tremolo completely. I added a series resistor to the 68K (the one before the Speed control) and it seemed to speed up the tremolo slightly, but the problem remains.

I'll take a look a the bypass cap. Thanks.

Here's a schematic.

[AmpRX]

The cap values look too high (and the schematic says modified). I've worked on many of these and similar amps. None have three 0.047s in series. If you look at a schematic for the GA16T it has the same exact trem circuit and uses 0.03, 0.01 and 0.01 from left to right. The higher values will slow the speed down. Others usesome combination of 0.01 and 0.02. I would change the values to those of the GA16t.

[TxTeleman]

So, I tweaked the circuit a bit, tried different resistor values, and ended up putting them back to what they were, and then tweaked a couple of the cap values, and now it sounds great.

Here's a link. http://www.facebook.com/v/2396439040105 Red Telecaster, black amp. He loves the tremolo.

Thanks