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Old 9th December 2011, 12:28 AM   #1
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Default Gibson guitar amp tremolo oscillator - nada

I have built up a clone of a Gibson guitar amp, GA-1RT(1). It uses a 6BM8/ECL86 (V2) for power and for the tremolo oscillator, plus a 12AX7 (V1) for preamp. The amp works, but the tremolo doesn't. I've used a red LED for cathode bias on the triode section of V2, and can "see" that the oscillator is working, and is controllable with the potentiometer. Everything's connected as shown in the schematic, but there's simply no tremolo effect on the audio signal.

This is a link to the schematic I've used (although there are other models made by Gibson with an almost identical topology):

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http://www.0rigami.com/gg/ga-1rt.gif
I had to increase the plate resistor from that shown, possibly because my DC voltages are somewhat less than those shown in the table. (I'm using a different transformer with solid-state rectifier). With the stock plate load resistor, the LED would glow but not oscillate.

I'm baffled.
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Old 9th December 2011, 05:47 AM   #2
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the top 220k from the grid of V2b to ground should be a potentiometer with the coupling cap of the tremolo circuit on the wiper.

also try connecting the coupling cap to the cathode (pin3) of V1b .




another option is the tremolo signal too close to dc ground, try increasing the bottom resistor to somewhere say 680k instead of 220K and give the output more drive headroom by adding a 1k grid stop resistor too.



About 90% of the guitar schematics are different slightly from the finish product. There just typically used as guidelines.

When I hand tweak someone's guitar amp, I usually enclose the schematic, note the resistor/capacitor-ology and key voltage measurements ( and also the wall voltage at time of measurement)

Last edited by DavesNotHere; 9th December 2011 at 05:54 AM.
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Old 9th December 2011, 06:40 AM   #3
Enzo is offline Enzo  United States
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What is the amplitude of your oscillator signal at the 0.1uf cap?

On these amps, only the speed was adjustable, the amount of trem was not. You could make it variable by changing to a pot, but the circuit in the diagram should work as is. I suggest finding out why it doesn;t before changing it.

And since this is something we built, we cannot just assume the wiring was correct. Make sure the two powr tube grid resistaors are indeed in series and that the 0.1uf cap indeed joins their junction.

Last edited by Enzo; 9th December 2011 at 06:43 AM.
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Old 9th December 2011, 12:27 PM   #4
TheGimp is offline TheGimp  United States
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If you used the triode from the 6BM8 as the termelo oscillator, you might not have enough gain to insure oscillation. The triode section has a mu of 70 vs 100 for the normal selection of a 12AX7 for tremelo circuits.

Look at it with a scope and see if it is producing a dampened oscillation when the switch is closed.
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Old 9th December 2011, 06:34 PM   #5
Cassiel is offline Cassiel  Libya
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Quote:
Gibson guitar amp tremolo oscillator - nada
Nada is a cool word but if you want to be cooler try "nada de nada", which means nothing at all. Yes, it is also correct to say "Nada, nada de nada". Nothing, nothing at all. How cool is that? Existentialism in its purest form.

Apart from some Spanish lesson I can't help you out with this one, but what I can do is to tell you that something is wrong in your circuit because that tremolo works. I've built it. Double check everything.
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Old 9th December 2011, 09:41 PM   #6
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Thanks everyone for the replies.

Increasing the bottom resistor didn't do much, except add some oscillator hash - I could now "hear" the oscillator circuit, but it still wasn't modulating the audio output. Just some tremolo noise. Same result by connecting oscillator output to cathode of V1b.

I don't have a scope but my DMM measures around 0.04 mA (max) at the output of the oscillator circuit. With tremolo switched off, there is a steady 0.004 mA on the output of the oscillator circuit.

Any last thoughts before I resort to Plan B (tone control instead of tremolo)?
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Old 9th December 2011, 10:25 PM   #7
TheGimp is offline TheGimp  United States
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Increase the bypass cap to 470uF or 2200uf if you have it, that will increase the gain at low frequency where you are trying to oscillate. 20uF is not enough at 2Hz.
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Old 9th December 2011, 10:52 PM   #8
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How long does it take for a thread to be banished to the outskirts of town if it has Gibson and guitar in the title?
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Old 10th December 2011, 12:26 AM   #9
Enzo is offline Enzo  United States
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0.04 MILLIAMPS???

Put your voltmeter on AC volts and how much oscillator signal level is there at either end of that 0.1uf cap. We need to see a number of whole volts there, not milli-anything.
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Old 11th December 2011, 09:46 PM   #10
jjman is offline jjman  United States
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Using a DMM to measure ac voltage at such low frequency may not be accurate. A scope would be much more informative if available.

Have you double checked the value of the 270k plate resistor? What are the dc voltages on both sided of it with the trem turned off?
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