Gibson Tremolo circuit

[Daver]

I'm working on the same model of amp. The trem channel also has low volume compared to the other. What did you do to get your volume corrected? Sorry to glom onto your thread. When the 6C4 warms up the resistance of the LDR drops to the point where the measured resistance across the loudness pot is about 5K. This should be close to 250K with the pot turned all the way up. Mine has a new LDR. The trem never worked before and at least it does now. The only problem is the volume.
Dave

[EETStudent]

maybe i should invest in a 6C4. Marc, I have replaced EVERYTHING in the amp a while back with new Weber parts, and i hacked the tremolo circuit because that channel just wasnt useful due to the lack of volume.

Daver, the only circumstance under which I did NOT have a volume problem was when i was experimenting with a 6EU7 instead of the 6C4. I then tried a 12AT7 and that resulted in yet again a volume problem. I dont have much experience at all with trem circuits so i dont really know what to think. I would recommend experimenting with that 500ohm pot on the cathode of the 6C4. Try raising that and see what you get.

I bought this amp in its original condition and the trem channel was so quiet. Both the old LDR and new LDR produce the same results. I hate to exclude the idea that the problem is in the design.

[EETStudent]

I meant to mention that im getting about 20k across pot with the tremolo engaged and the loudness pot maxed. With the tremolo disconnected I get 230k as expected.

[EETStudent]

Ok i know im posting excessively but i have figured something else out. I found a 12AU7 lying around, and those are pretty close to 6C4s. I subbed it in and the maxed out the depth control and....no chirping. However, the tremolo was pretty weak especially for maxed out settings and the volume was still pretty low for that being maxed out. I had the Rk value at 100k, and anything above provided more volume but weaker trem and some chirping, anything below provided less volume but stronger trem. Still reading about
20k across the loudness pot when maxed.

So apparently my problem with the chirping was the tube, but the volume issue is still a mystery to me.

[Daver]

EET,
Ah, OK, I though you'd stumbled onto the real cause of the problem. Measuring across my loudness pot at max volume, I get about 5K when the amp is on and off standby. When the amp is on but standby is also on, I get about 50K. That's with no B+ to the 6C4. Very odd. Looking at the schematic, it seems like any voltage drop across the LDR turns it on and drops the signal across the loudness pot. Maybe it is a design flaw or Gibson used some "special" LDR. I've tried a Fender style ( weakish trem and still low volume ) and the current Vactrol I have in there now. Maybe Enzo can save us both. Thanks.
Dave

[EETStudent]

Maybe he will save us! Otherwise I may go with a Fender style trem

[Rick Erickson]

Which Vactrol are you using? Do you still have the driver and the oscillator in the same tube? Your chirping could be caused by the two haves of the tube "talking" to each other. (or not) Try splitting them per the schematic into two different tubes. Layout can be critical in a trem oscillator/driver circuit. just having the wires or components too close to each other can cause interaction problems. I'm assuming you are doing a scratch build and not working with a factory layout.

RE

[Daver]

Rick,
In my case it's a VTL5C3 Vactrol that was suggested to me in this thread....
http://music-electronics-forum.com/s...ead.php?t=8017
My amp is a stock Gibson Mercury that came in for a tune up and the trem didn't work. The stock opto was dead. It has a 6C4. Now the trem works but the volume from that channel is very low.
Sorry to tack onto this thread, but EET and I seem to have a similar problem.
Dave

[Enzo]

Put a goddam pot in the cathode circuit and adjust the thing, sheesh.

If the bulb is always sorta on, then the LDR resistance will stay sorta low, and the circuit will be sorta shunted, and the level will be sorta weak.

The circuit was designed to work as it was. Putting different parts in it will make it not work the same way.

[EETStudent]

i HAVE put a goddam pot in the cathode. The only thing that did(after finding the correct resistance) was prevent the depth control from affecting the volume. As it stands, the trem works great and depth functions properly, but as you mentioned the overall signal in the amp is incredibly weak.

[Daver]

Well, I tried the stock pot, the goddam pot and a 50K pot. In my case nothing works with either the Vactrol or Fender style trem roach to provide acceptable volume and/or trem. So, I'm back to square one. What light source is correct for this circuit? Thanks, and try to relax Enzo. We need you around here.
Dave

[Enzo]

Oh I am pretty calm, really.

WHat I seem to be seeing is a pot was used when the wrong tube and different opto were used. But I didn't recall seeing that a 500 ohm pot was used once the tube was made correct and a proper bulb opto was installed. The LED version that is a Vactrol is a perfectly legitimate thing to use, but it reacts to the particular circuit differently than the incandescent bulb in the original. It is not a drop-in replacement.

SO what I am yelling about is that we seem to be wondering why a different tube with a different LDR bulb and switched resistors doesn't work like it should.

I wouldn't expect a neon based bulb to work right in this circuit. The circuit relies upon our ability to adjust its standing brightness.

In the stock circuit, the cathode pot should adjust from signal squashed almost off through trem effect, and on to mostly steady signal with maybe little tiny pulses on the LFO peaks.

[Daver]

Enzo,
I think I may have caused some confusion jumping in on this thread. I thought I explained it well enough, but I guess something got lost. My amp is separate from EET's. Mine has the correct 6C4 tube and stock 500 ohm cathode pot. The only thing missing from mine is the opto. So now that we're up to speed, why can't I get a good volume level with the bloody Vactrol! I have the correct B+ voltages and have adjusted the 500 ohm pot. I've subbed in a 10K pot and 50K pot. I have a 470 ohm resitor in line with the LED as was suggested in another thread. Jumping this doesn't change anything. I've tried 3 different 6C4's. Is the adjustment for the cathode so fine that I need a smaller pot to try? Thanks for all the help and sorry for any confusion.
Dave

[EETStudent]

Well, i have reworked a different solution to much satisfaction. Im using a
12AT7 instead of the 6C4, and Im also using a Fender style roach optoisolator. Ive installed a 50k pot on the cathode, and that little 4.7Meg resistor I turned into a 10Meg pot making the Depth MUCH deeper and giving me a very broad range of speeds I ripped out the Gibson tone stack(didnt like that one anyway) and installed a Volume/Tone fender brownface style tone stack, and I applied to output of the trem circuit into the wiper of the tone control. This resulted in only a slight volume loss and a GREAT sounding trem. I can now crank the amp and utilize trem. Of course this was a fair amount of work, but I couldnt be happier. The thread is all yours

[Daver]

Thanks EET,
I, too, have come up with a solution using my Vactrol. I needed to put a 15K resistor in the cathode of the 6C4. That left the volume the same as the non-trem channel. The next problem was the trem was a bit weak and the Depth control did nothing from 0 to 5. I put a 10uf cap across the 6C4 cathode, changed the oscillator plate resistor to 220K and added a 120K resistor between the Depth control and ground. Now everything works great. It was a bit of screwing around. I wish I had the stock opto, but that's life. Thanks to all who chimed in.
Dave