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):
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.
This is a link to the schematic I've used (although there are other models made by Gibson with an almost identical topology):
HTML:
http://www.0rigami.com/gg/ga-1rt.gifI 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.
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)
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)
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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.
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.
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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.
Look at it with a scope and see if it is producing a dampened oscillation when the switch is closed.
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.
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)?
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)?
Moved to the I&A forum per forum policy.It is the same schematic as the original poser,
http://www.tremolo.pl/Firmowe/GIBSON/GA-1RT.gif
I won't be able to get the exact voltages until Saturday, as the amp and I are in different locations right now, but my memory is that they were all close to spec, except the cathode on the 6BM8 was more like 5V.
http://www.tremolo.pl/Firmowe/GIBSON/GA-1RT.gif
I won't be able to get the exact voltages until Saturday, as the amp and I are in different locations right now, but my memory is that they were all close to spec, except the cathode on the 6BM8 was more like 5V.
I am running pretty hot. B+ is 352 (spec is 285). The pentode plate is 312 (spec is 275). I've got 187 to the oscillator plate (hard to read, but I think the schematic says 160). Oscillator cathode is 2.7 (2.2).
I scoped it, and it is not oscillating. Double checked the connections and values, and had someone else do the same. I bypassed the pot and it still didn't oscillate. Any ideas will be much appreciated.
I scoped it, and it is not oscillating. Double checked the connections and values, and had someone else do the same. I bypassed the pot and it still didn't oscillate. Any ideas will be much appreciated.
I swapped out my drop resistor (I was using a very hot, but cheap PT) with a smaller one. Now I have 254 at the rectifier, 244 at the output plate, 134 at the triode plate, and 1.9 at the triode cathode. I again went over the wiring with another person present. I checked the grounds, and checked for DC across the caps. Found nothing. Still does not oscillate on the scope.
New build. Yes, I tried two. to clarify. I had one drop resistor, removed it andy took the first set of voltages, then swapped in another and took the second. Even though I found no DC across the caps, I replaced them. No change. The oscillator on this is pretty orthodox. Similar to Merlin's. I can't imagine why it isn't working.
OK. Others have given good advice in the original part of the thread. Nobody else is chiming in now so here is my 2c. To help troubleshoot I would replace the tremolo cathode RC circuit with a LED. This will show immediately if the triode is conducting/oscillating without having to scope it.
From memory, Merlin has opinions on which of the phase shift RC circuits should control the frequency &/or be switched.
He also recommends the on/off switch be connected to the cathode to kickstart the oscillations.
From memory, Merlin has opinions on which of the phase shift RC circuits should control the frequency &/or be switched.
He also recommends the on/off switch be connected to the cathode to kickstart the oscillations.
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