Gibson GA5T Tremelo Intensity

[plexi50]

The tremelo on my Gibson GA5T 1967 model Skylark is too intense. The rate is good but it is so intense that notes depth is too much

Does the cathode cap on V4 determine the depth of the tremelo? Cant see it too good but it looks like a 10uf

Here is the schematic below:

[Fresh_Start]

No, the cathode bypass cap value won't weaken the relative strength of the tremolo signal.

I'm not really familiar with this tremolo circuit, but there are two resistors you could play with to reduce the signal coming out of the tremolo triode.  Try a 100K plate resistor instead of the 150K.  Alternatively, you might try reducing the value of the grid return resistor (currently 1.5meg, I think).  Might even be able to put a 1 meg pot in that spot actually and get variable intensity, but I'm not certain about that. 

In either case, you can tack a second resistor in parallel with the resistor in question to experiment. 

150K || 300K = 100K

I'd just use whatever resistor value I had handy to see if it made the kind of change you're looking for first, then worry about dialing in the best value.

Hope this helps,

Chip

[tubeswell]

You need to change the voltage divider on the grid load resistor for the output stage. The stock value is 220k/220k. try something like 330k/120k (with 120k being the leg), or replace the bottom 220k with a 220k-250k pot (the wiper lug comes from the LFO circuit, the pot input lug is connected to the 220k, and the other (output) lug goes to ground)

[jjasilli]

Ditto to Freshstart.  Or maybe disconnect the cathode bypass cap.  I'm guessing that will cut the gain of the tube in half; but that might make the trem too weak.  There's more control over the the outcome with Freshstart's suggestions.

[plexi50]

Ok i will your tips and see what happens.

[Fresh_Start]

The schematic Tubeswell posted is completely different in terms of how the tremolo signal is injected.  His suggestion is right on for that circuit. 

Does the "Skylark" schem originally posted match the amp?

Chip

[plexi50]

No the schematic Tubeswell is different than the amp i have. They made a bunch of GA5T amps all with some differences to them.

The amp was recovered in blue something or other vinyl and has the weird looking cab with the wings

The schematic i posted is the one i have /  Does anyone know where i can find a replacemant on/off polarity switch for this?

[Fresh_Start]

I don't understand how that switch is drawn in the schematic and can't see the front of the amp clearly enough to know what you need.

It looks like a double-pole switch in the schematic, but I can't tell whether you need an on-off-on switch or an off-on-on switch.  You can find on-off-on toggle switches from Carling or NKK at Mouser or other places.

Don't know if that helps.

Chip

[sluckey]

I'd just use Doug's 3 Way rotary impedance switch and forget about the death cap. And use a 3-prong power cord.

[rafe]

Tubeswell, where did you find that schematic? I've been looking high and low for it.That is the early (61 in my case) "skylark" with tremolo......I love this amp, but my tremolo won't fire up. I diddled around in it once or twice now I'm giving it a rest(tremolo) eventually I'll solve it ...When I turn it on it modulates once or twice and stops.....I havent gotten around to ordering trem caps....Thanks for the schematic

[plexi50]

Yeah the death cap thing is not really needed. Yes Freshstart it has a ON-OFF-ON pole switch on it now. This is the only part not original to the amp and would be nice to have a rotary on-off switch rather than that huge pole switch on the front of this panel. Just being PICKY

[plexi50]

Tubeswell, where did you find that schematic? I've been looking high and low for it.That is the early (61 in my case) "skylark" with tremolo......I love this amp, but my tremolo won't fire up. I diddled around in it once or twice now I'm giving it a rest(tremolo) eventually I'll solve it ...When I turn it on it modulates once or twice and stops.....I havent gotten around to ordering trem caps....Thanks for the schematic

Lots of Gibson schematics here:  http://www.0rigami.com/gg/amps.html

[rafe]

 GA-5T Skylark (two-tone) 

 5W 1x8" Guitar Combo with Tremolo
Speakers: 1x8" (Jensen 'Special Design'?)
Inputs (instr. + mic.): 2
Channels: 1
Volume Controls: 1
Tone Controls: No
Tremolo: Yes
Tubes: 3? (12AX7, 6V6, 5Y3 or 1x12AX7 (pre amp), 1x6BM8 (power amp and tremolo), 1x5Y3 (rectifier)?)
Watts Output: 4 1/2
Shipping Totals: 1960: 123, 1961: 2094
Harmony Central Review GA-5T Combo
Harmony Central Review GA5T Skylark
Harmony Central Review Skylark T (GA-5T?)
 
Not the right one schematic.....2217 total built in 2 year run, that would be kinda rare. I doubt they all survived. When you go to the reviews only 2 or 3 are about the correct amp. (12AX7, 6V6, 5Y3 or 1x12AX7 (pre amp), that is for the ga-5 and is a mistake! Thanks Plexi that's a good site

[plexi50]

Yeah Harmony Central isnt what it used to be. People are padding there reviews just to sell stuff on Epay

[tubeswell]

Ooops wrong GA5T schematic! - sorry 'bout that.

[jjasilli]

Another idea:  your circuit seems similar to the trem in the VibroChamp AA764:  Trem cathode  > cathode of Tone Recovery stage.  EXCEPT, in your amp these 2 cathodes share the same cathode resistor and bypass cap.  You could rebuild these cathode circuits separately, as in the VibroChamp schematic.  For your amp each section would then have its own 10K cathode resistor (as a starting point) & 10uF bypass cap.  Then a pot, trimmer pot, or fixed voltage divider circuit could be added to the bottom of your trem cathode, as in the VibroChamp schematic, to control or re-set Intensity.   

To do this it will be necessary to separate your 2 cathode circuits, otherwise the Intensity circuit will also affect the bias-gain of your Tone Recovery stage. 

[plexi50]

Great thinking jjasilli!  That is simple enough to do. I may just have some free time tommorrow. I really hate making changes to untouched amps these days. I was just the opposite a few years ago. I was gun ho (Amp Ho) to modifiy everything under the sun. Im thinking a little differently these days. But this isnt a major change and would be for the better. This trem is so deep it sucks the guitar notes to 0

[sluckey]

Try this...

[plexi50]

I used your method Steve. It worked wonderfully.  

I frist tried 2.7K and the trem dissapeared all together

I next tried a 1.5K and the trem was present again and  better but still too deep

I next tried a 2.2K and hit pay dirt! / Perfect. I also changed the V1 cathode from 4.7K to 1.5K for a bit more gain

This brought out some real nice shine and harmonics that were not present before

The schematic says that V1 cathode resistor is a 2.7K but it was a factory 4.7K

Very nice and short job. Steve your the man. Thanks all!

[zachass]

Sorry to resurrect an old thread, but what if I wanted to add a depth control? Using Steve's method, would I add a potentiometer (10k?) wired as a variable resistor instead of the 2.2k resistor in Steve's diagram?

[sluckey]

Should work. Just connect the pot as a variable resistor, ie, connect one outer lug to the wiper lug. 5K may be more usable.

[zachass]

Thanks Steve! Any thoughts on taper?

[sluckey]

I would use whatever I had on hand. If it works then I would experiment with taper, maybe.   

[zachass]

  Sounds good, I will try it out and report back!