Comparing Vibrato Circuits

[TIMBO]

Hi guys, I having a few problems with the vibrato in the moody build.
First off,it DOES work as the schem and Sluckey's pickup on a misplaced component, and it does work with the cathodes connected as per the schem.
The problem I am having is thumping when the intensity is turn up and what I have found from the guys this side of the pond is that the low frequency's are not being filtered out, also it is said (on the net) that this type of vibrato is prone to this problem.
As per my schem I can only get it to 2 on the intensity dial before its starts to thump. The net showed a .1 cap connected to the top of the intensity pot to ground, with this I can get the intensity up higher but it is also amplified when the master is turned up.
Thanks to Willabe, posting Doug's REVIBE has a similar circuit with some extra bits.
I've marked some components that are extra compared to my schem, so if anyone can explain there use and if any of these bits could tame the thump. Thanks
 

[sluckey]

As per my schem I can only get it to 2 on the intensity dial before its starts to thump.

Do you have enough intensity with the dial set on 2? If so, set the int. pot full CW and increase the 1M resistor on V3 pin 1 to prevent thumping. Adding some filter caps on the trem signal will help too. I'm not very optimistic about this suggestion.

Here's an explanation of the components you asked about...

A... sets the fast end of the trem speed. Your circuit has this also but it's on top of the speed pot.

B... sets the low end of the trem speed. I needed this to prevent oscillator drop out when the slow switch is engaged.

C... filter cap to help keep the trem signal looking like a smooth sine wave.

D... P/O voltage divider to knock down the level of the trem signal. Also provides isolation between oscillator and intensity control. Your circuit has this also.

E... DC blocking cap. Your circuit has this also.

F... Another filter. Same as C.

G... Mixing resistor used to combine the trem and guitar signals.

H... Eye candy. Increases the amplitude of the trem signal considerably. Not really needed in this application.

I don't think you'll have much luck eliminating the thumping by doing anything to the oscillator circuit (unless the 1M trick fixes it). The thump needs to be dealt with after the trem is combined with the guitar signal. Your amp has a filter network on the plate of V2A that is supposed to remove the trem signal. Vox uses a much better filter. Look in the upper right hand corner on page 3 of this pdf...

     http://home.comcast.net/~seluckey/amps/VAC15/Vox_AC15.pdf

The Vox and "Hoffman" circuit both use a push/pull modulator that cancels most of the trem signal due to common mode rejection. Vox applies that filter after the modulator to futher insure the trem signal disappears. But your circuit uses a single ended modulator so it must rely on the filter alone to remove the trem signal. You could try that Vox filter and see if that gets rid of the thumping.

Are you wanting to make this circuit work better? Or, are you open to trying bias modulation on the power tubes?

[TIMBO]

Thanks Sluckey, I was going to try to recreate the original Moody amp as close as possible but the lack of info and an unreliable trace has meant that I can't do that.
I have built BIAS vibrato and they have work well, but at this stage the Moody is cathode bias but that can be changed.
I try some of your suggestions. Thanks

[sluckey]

I have built BIAS vibrato and they have work well, but at this stage the Moody is cathode bias but that can be changed.

Bias vary trem also works on cathode biased amps. Plenty of examples. I can vouch for this one...

     http://home.comcast.net/~seluckey/amps/RCA/Ampeg_J12B.pdf

It would be simple and easy to try on your amp. Just lift the ground from the grid leak resistors and connect the wiper of the intensity pot to the junction of those resistors. That'll let you know if it's worth pursuing.

[TIMBO]

Thanks Sluckey, Good info.