AC30CC VOX ticking tremolo

[Pedro Vecino]

Hello. I have found an Vox AC30CC with a big problem in the tremolo: It produces a very strong tick accompanying to every oscillation (a low frecuency noise) accentuated on having raised the lows in the amp. I have read something (it is a repeated problem) in other forums but without a diagnosis.
This amp uses a tremolo circuit with a mosfet and I have tried to filter better the circuit (adding a 22uF/450V cap) in the power line of the mosfet but without results. The mosfet is called LND150 and the schematic is here: http://www.bymcomunicacion.com/pveci...30CC2_CC2X.pdf
Any help is welcome. Best regards and sorry for my poor languaje

[Tom Phillips]

Pedro,
I think you may find some useful information at http://www.lynx.bc.ca/~jc/tremoloCircuits.html
Look at "Note 3" at the very end of the article.
Regards,
Tom

[Pedro Vecino]

Thanks, Tom. The EL84 quartet is not perfect (45/38/43/42 mA across the plates) but replacing it with another (tolerance 1 mA) the effect is the same. I have reduced the noise to half installing a 22uF capacitor paralleled with the first capacitor and another (47uF) paralleled with the screen grid filter capacitor.
I cannot raise the filtered of the first step due to the use of GZ34 rectifier.
The frequency content of the oscillation (woump-woump) is very low and produces a severe movement in the speaker with the tremolo depht to the max. With master volume to 6/7 is acceptable, but not to 10.
Due to the fact that it is not the first time that happens in the new AC30 version, it is possible that the failure has a more concrete origin that deficient filtered. Thanks again

[wizard333]

This is a common problem on Fenders. Fender recommends the following, and I've found it to work:

Lead dress: Make sure the wires leading to the vibrato controls are not close to your channel pots or other audio wires, or your filter cap wiring where it returns into the amp.

Bunch the wires to your vibrato tube. I unsolder them and twist them; they are probably short enough that each side of the tube will have to be bunched separately; i.e. 1,2,3; 6,7,8.

There is also a .01 cap to install but I couldnt tell you exactly where on a Vox; needs to go between one leg of your photocell and ground assuming the vibrato on a Vox is of similar design and uses that photocell.

Every single Fender I've seen tics to some degree without doing that stuff; doing it has never failed to eliminate it.

[Alex/Tubewonder]

The MOSFET circuit is a low frequency phase shift oscillator. Its output should be a sine shaped signal. Because of low oscillation frequency it is sensitive to any dc-leakage in C28,29 and 30 of the phase shifting chain. DC leakage in any of these caps will result in generated signal being pulse shaped rather than sine and thus producing ticking/clicking sound. Same applies to tube based tremolo oscillator.

[Rick Erickson]

Every single Fender I've seen tics to some degree without doing that stuff; doing it has never failed to eliminate it.

I've seen ticking Fenders that still ticked or thumped after doing all that. Usually caused by DC leakage between eyelets.

RE

[Rick Erickson]

C24 (22nF/400V) is a smoothing cap for the LFO. Try bridging it with a larger cap, maybe a .047 or .1uF and see if the ticking reduces or goes away. I would make a solid connection before powering up the amp. MOS-FET devices don't take kindly to voltage spikes, as I've learned the hard way.

RE

[Pedro Vecino]

This is a common problem on Fenders. Fender recommends the following, and I've found it to work:
Lead dress:
Bunch the wires to your vibrato tube.
There is also a .01 cap to install
Every single Fender I've seen tics to some degree without doing that stuff; doing it has never failed to eliminate it.

Thanks, Wizard. The circuit is mosfet based with print board and multiconectors. Itīs not possible to manipulate it.

The MOSFET circuit is a low frequency phase shift oscillator. Its output should be a sine shaped signal. Because of low oscillation frequency it is sensitive to any dc-leakage in C28,29 and 30 of the phase shifting chain. DC leakage in any of these caps will result in generated signal being pulse shaped rather than sine and thus producing ticking/clicking sound. Same applies to tube based tremolo oscillator.

Thank, Alex. This group of capacitors was the first thing that I tried to replace but without results. The original capacitors are fresh and dīont have leakage problems

C24 (22nF/400V) is a smoothing cap for the LFO. Try bridging it with a larger cap, maybe a .047 or .1uF and see if the ticking reduces or goes away. I would make a solid connection before powering up the amp. MOS-FET devices don't take kindly to voltage spikes, as I've learned the hard way.

Rick, this was the final solution that I applied. Initially I tried reducing the value of C27 (100n) to 27n to block the way of the lowest frequencies in the modulation.Finally (of similar form, because both condensers work forming a set) I raised the value of C24 (22n) to 220n to absorb a part of the low frequencies. The result is the disappearance of the woump, but with two small (and acceptable) effects: the intensity of the tremolo is not linear in all frequencies. Good final intensity but not complete (not 100 % silence in an side of the waveform for "hipnotic tremolo") and not linear with alls speeds: with fast speed the intensity decreases due to the slowest response of the capacitor (due to his big capacity). This latter effect is very natural.

The final solution that I have applied is:
* Better filtering in HT line: +22uF in the first step (OT center tap) and +47uF screen grids node
* Raise C24 (22n) to 220n)

All that is strange in a new amplifier. My idea is that it comes from an bad layout. I suppose that with the time we will know it. A million thanks to all.
Best regards

[TD_Madden]

Isn't this thing still under warranty....I thought this was a new amp.

[Robmosis]

Is this the same problem I have with my new AC30CC2. If I have the tremelo on with the volume in any position and I don't play anything on the guitar I can hear the trem oscillation in the speakers. Very annoying to the point where I don't even use it.
Thanks Rob