Gibson GA18T build.

[GR1950]

Hello all, from a very Wintery NZ West Coast.

Having successfully built a Fender Champ clone, I thought I'd take a shot at a GA18.
I should first explain that I'm a very green greenhorn in the electronics world and had a very good friend and mentor draw me up a layout sketch. I've transcribed it and offer it below for perusal.

Now, I have a problem. The design calls for RA pots. for the frequency and depth functions. Since I'd have to source these from the USA, with consequential wait-time, I decided that I'd "roll my own", in accordance with this information from Tubeswell ......http://music-electronics-forum.com/t3084/

Do I have this correctly in my layout, and do I have to mount the pots. in order for them to operate counter-clockwise ?

Any replies, and critique of my layout would be hugely appreciated.

Thank you.



Oh yes, here's the GA18 schematic. (Yes; in spite of what it's titled, this is a GA18)

[loudthud]

What you have drawn won't work properly because the maximum resistance between the two wires going to the Frequency pot is 83K. To get the same frequency range as the GA18 you would like the resistance from the junction of the .03 and .01 caps to be:

Clockwise......68K....(fast)
Center..........118K
Counter CW...568K...(slow)

I don't think there is any way to get there with a linear pot and a resistor to "bend" the taper. To change the layout to work with a normal log pot, remove the 100K resistor and move the wire that goes to the footswitch jack to the un-used terminal of the pot. That will make the control operate reverse.

[GR1950]

Thank you loudthud; I'm mightily obliged to you.

Do you have any advice re. the depth control please ?

[loudthud]

I think the depth control will work the way you have drawn it. Because the load on the LFO (Low Frequency Oscillator) will vary with the position of the Depth control, the LFO may stop oscillating in which case you'll need to change the pot to a reverse log, a log connected backwards, or just use a linear pot.

One short-comming of the bias wiggle tremolo is that the output tubes need to be biased a little on the cold side or there won't be very much depth. You will need to experiment with the 250 ohm cathode resistor for the 6V6s to find a compromise. The value of the resistor will need to be increased, perhaps as high as 330 ohms.

[GR1950]

Loudthud, my apologies. I responded to your suggestion, but somehow failed to post it. Thank you for your invaluable assistance.

I had also intended, and forgotten, to ask one other thing. I remember once, a friend had a Gibson amp. I haven't a clue what model it was. It was quite a novelty encountering a Gibson amp, they're pretty thin on the ground in NZ. At least, in my experience.
In any event, I recall thinking that, even on its slowest setting the tremelo could have done with further slowing. Is there a way that one could do that, if one thought that was the case with the GA18 ?

[loudthud]

There are three capacitors and three resistors that control the frequency. One of the resistors is actually the 68K in series with the pot, it acts like one resistor but is variable. 510K and 2.2Meg are the other two. The caps are the 0.03uF and a pair of 0.01uF. Making any of these larger will lower the frequency.

You might want to try obtaining the part Fender uses, a 3meg reverse log pot. Available from Weber, Hoffman and others. That change alone might do the trick.

Once you get the circuit working, try this: set the pot at max resistance and swap caps in and out until you get a low frequency that you like. Changing the .01s to .022 might do it. You can connect caps in parallel to get in-between values, .01 // .005 = .015 etc. You can use 500V ceramic disc caps to save space.

Then change the 68K so the fast frequency is not rediculus and the higher frequencies are not too close together on the pot. If operation at the low frequency has problems, try increasing the 20uF cap at the tube's cathode to 100uF.

[GR1950]

Loud, I'm thoroughly indebted to you; thank you for your excellent assistance.

Gary.

[loudthud]

Gary, I learn almost as much from threads like this as if I had built the amp myself. Let us know how everything turns out, what worked and what didn't. There's a trick you see on many Fender amps I call a kick-starter. The tremolo on an amp like this might start slowly, that is, it might not come up to full intensity immediately when you switch it on. It's usually just a slight re-arrangement of the components that does the trick. But try it as is first to see if that is a problem.

[GR1950]

It's pleasing to hear that LT. I'll keep you posted ..... just hope I've drawn the layout correctly !!

Cheers, Gary.

[TxTeleman]

Wow. This is something. I'm starting to build my own GA-18 and I ran across this. What a great place!