Gibson Tremolo circuit

[EETStudent]

In the tremolo circuit in the schematic, what is going on with the cathode of the 6C4 tube? Is that just a pot wired as a variable resistor? If so, for what?

[EETStudent]

heres the schematic

[Enzo]

That is exactly what it is. it sets the operating point for the bulb in the LDR. Think of it as a bias control. You don't want the bulb on all the time solid, and you don't want the bulb to just peek on a little on the peaks of the LFO signal. This is just like a power tube where you don't want it so cold it is cut off except for peaks, nor do you want it running so hot it melts. This control covers for the fact that LDRs vary as do the tubes and components in the circuit.

Once the trem works, you can set it by ear. Set it to one end and the signal is squashed constantly, like one long continuous trem pulse off. Turn it from there and the signal will come in and the pulsations grow. Turn to far and the pulsations become more like on-off pulses.

[EETStudent]

Well right now my the tremolo is VERY weak, and I have a fixed 470k resistor in its place. Would adjusting this result in a stronger trem?

[EETStudent]

Wow i feel stupid. I did have a 470k, and the schem calls for a 500, not 500k.

[EETStudent]

Ok no another thing, the depth control has a major effect on the volume control. With the depth about halfway, the volume is significantly lower when maxed. With the depth at 10 the volume seems about where it should be. Is this a design flaw?

[Enzo]

Right now you have a 500 ohm resistor in there instead of a control that varies from zero up to 500 ohms. I would be surprised if that adjustment was most effective at the extreme. Put a pot in there and try dialing in a better performance.

Reread the second paragraph on post #3.

When a commercial circuit is not working very well, and expecially one you don't quite understand, your first thought should not be "design flaw."

[EETStudent]

i had a nice time adjusting the setting with a decade resistor, but regardless if the cathode saw 1ohm or 100kohms the volume was majorly affected. With the footswitch disengaged and the tremolo off, the volume was still terribly low. Having thought about it, when I got this amp a long time ago I had the same problem with it. The tremolo channel was much quieter than the normal channel.

[Enzo]

OK, then isolate the problem.

Turn off the trem with the footswitch - or just yank V3 - that stops the LFO, but the photocell is still in circuit. Now, do you get voltages at V2 that even remotely resemble the 144v and 3v on the schematic? Does turning that control make the plate voltage change? Or the voltage across the pot? We have to know the tube circuit works.

Look at pin 6 of V2, is there ever any +DC on that pin under any circumstances - ie any setting of the depth control. It should be zeroDC or even a tiny little negative voltage. Especially with the depth set to zero.

Diosconnect the LDR resistove side from the loudness control wiper at R9. Measure the photocell resistance with a meter. Does the resistance vary as the cathode pot is turned? If you turn the trem back on does that resistance go up and down with it? What resistance do you get there? It is in parallel with the loudness control output and acts as a variable shunt resistance.

With the LDR still disconnected, did the volume of that channel come up to something similar to the other channel. The two channels are very similar. Now we find out if the LDR circuit is the whole problem if if there was something else wrong with that channel all along.

[EETStudent]

ok ok cool, i figured out the volume problem. Sounds neat, but the only problem now is that there is a consistent 'chirping' sound at higher settings. With the depth around 5 it is gone, but of course the trem isnt very strong then. Around 6 it picks up and is consistent throughout. And it does coincide with the speed of the tremolo.

[EETStudent]

AND I should have mentioned that instead of using a 6C4 tube, i went ahead and used the second half of that 6EU7.

[Enzo]

6EU7? What is this, something you built? It is not an actual Gibson amp?

Look up the 6C4 and the 6EU7 data sheets in the tube manual. The 6C4 is more like half a 12AU7, while the 6EU7 is more or less a 12AX7 with different pinout. The 6C4 has an amplification factor of 17 and can dissipate 3.5 watts driving that bulb. They even call it a power tube. Compare the 10ma plate current of the 6C4 to the 1ma of the 6EU7. That right there ought to explain most of the dificulties with this stage.

[EETStudent]

Yeh its an actual Gibson amp, but i dont have a working 6C4 at the moment so instead I used the other half of split-load phase inverter tube.

The only difficulty im having now is that slight chirping sound. Its fairly subtle, probably something nobody in a crowd would notice. The tremolo sounds great, and were I to rid of that chirp it would sound awesome

[EETStudent]

Ok heres what I did. I subbed V2 of the trem circuit with a 12AT7 instead of the 6EU7, while the AT7 has similar characteristics to a 6C4. The trem did sound a little better, but I still got that chirping with the depth around 7 and on.

[Marc]

Ok heres what I did. I subbed V2 of the trem circuit with a 12AT7 instead of the 6EU7, while the AT7 has similar characteristics to a 6C4. The trem did sound a little better, but I still got that chirping with the depth around 7 and on.

Surely somebody in Columbia can bail this guy out with a 6C4!!

A 12AT7 is closer but it simply ain't a 6C4. The chirping just may go away with the correct tube.

Just curious, have you replaced the electrolytics in the cathodes and power supply of this amp and checked for leakage on the coupling caps?

Marc