Bias vary Tremelo problem

[Toxophilite]

Some ideas here!


Baguette;



I changed the circuit to be more like the 6G16
there's an extra.1 cap to ground and where the 220K cap was i swapped in a 360K (I tried the 470 but it was little too little tremelo) Also the .05 cap on the pot is now disconnected
THis all helped some. As you said it seems to be about achieving some sort of compromise/balance


trobbins;

Are you referring to the negative feedback circuit?

 and by high pass cap do you mean something like a 500pf? or a .01 like my Super Reverb (different amp)has in parallel with the 820 ohm resister?
I'm not sure how I should check if the feedback circuit was providing too much feedback
Can I check that without a scope??


I believe I have a star arrangement as you suggested


I have .068 coupling caps  rather than the fender AB763 deluxe schematic's .1(I had them for something else and ended up using them in this build)
The grid stoppers are the 1.5k s on the 6V6 grids yes?. It might be messy to increase them..they're pretty tucked in there

[trobbins]

Yes, the negative feedback from speaker to PI.  For example, a 100Hz high pass filter would be 1 decade from 10Hz tremolo, so approx 15dB attenuation of 10Hz back to the PI.  RC=1/(2.pi.100Hz) , so 820 ohm would need 2uF in series with it.  Higher frequencies pass through the 2uF as if it wasn't there (ie. normal negative feedback conditions).


To my mind, taming what seems to be serious blocking distortion during the positive swing of the trem LFO signal being applied to the bias voltage is the initial target.

[Toxophilite]

Sorry if I seem dim but of the options you suggested initially which would you think would best accomplish that? Or are all effective to a similar degree, just different approaches?

[Toxophilite]

Okay I found this article which helps me understand a bit
http://www.aikenamps.com/index.php/what-is-blocking-distortion

[trobbins]

Yes, Randall's article is excellent.  Coupling cap value change is definitely worthwhile if it doesn't adversely affect low frequency roll-off for you. 


Raising grid stopper is trickier......   6V6GT should preferably have no more than 100k resistance back to a 'solid' bias voltage (ie. the bias filter capacitor), which is way less than what you presently have - especially for full intensity pot setting (ie. 1k5 + 220k + 250k = 471k).


Life, and circuit design, is never easy!

[Toxophilite]

I know this post is ancient but i think I solved this problem
Part way through this thread I mentioned that:


"THe other changes I have made within the last few months are:-I swapped in a different OT to try it, It's from a baldwin amp that was running 2 EL84s, it's physically a little bigger than the original AO-43 one and it's 27-1 ratio. It sounds great with no tremelo..I can't remember if the tremelo was a big problem prior.- I monkeyed with the dropping resistors after installing the reverb circuit. there is now 200vdc on the phase splitter which is about 20 v more than the original deluxe schematic."


I recently decided to swap the old OPT back in .It's the original one that was on the AO-43 and it's 30-1 turns ratio,
With the old OPT in the  tremelo works great NO nasty distortion.

as you can see from the quote the baldwin one I was trying in place of the original (cause it was bigger) had a different turns ratio annnnd (not mentioned) the blue and brown primary leads only worked properly when they were attached opposite to the original one . Blue to the 6V6 closest to the power supply and brown to the 6V6 closest to the preamp

I'm curious if this might just have been a bum OPT ?(maybe rattling)
or could it be the impedance mismatch as a result of 27-1 turns ratio?
Or the backwards primary hookup ?

[shooter]

only worked properly when they were attached opposite to the original one

just curious, what happened when they were *the other way*?

[Toxophilite]

I think it squealed until I reversed them ..it was a bunch of months and at least an amp or two ago so my recollection isn't perfect.
Now I have to go back and read this thread to check the mods I did to the tremelo while troubleshooting!. 

[shooter]

tremolo works great NO nasty distortion

to check the mods I did to the tremolo

sometimes it's better to let sleeping dogs lay