Slow tremolo mod

[valvetones]

Hello friends, just got done with a build as some of you know. I deviated in a few places, one of them, I have an issue with.
It’s a 6G16 vibroverb. Like the Princeton, it uses tube bias modulation for the vibrato.
I looked up how to slow the vibrato online and the suggestion was to replace the .01 caps with .02 caps in the oscillation circuit.
So I’ve got four .02 caps where the layout calls for two .02 and two .01.

The issues are this, The intensity control is way to intense. I can’t turn it past 2 on the dial or it becomes much to intense.

The other issue is that even with it all the way off, or at 0, you can still hear it.

So my question is, which one of the two caps should I change back to .01 in hopes to tame the intensity.

Personal thoughts
1. probably whichever one is connected to the intensity pot.

2. Possibly the effect won’t stop because the pot is out of spec and doesn’t actually taper all the way to 0 ohms to shut off the effect.

What say you friends?
Any suggestions?

[uki]

schematic

[sluckey]

Have you added an adjustable bias mod? If so, show us your actual circuit. Bias caps can be a factor.

[valvetones]

Yes I did add adjustable bias. The bias cap is a 25mfd cap at 150 volts.
Another thing is I took the power for the bias off rectifier pin 4, then to a 220k resistor, then to the diode, the other side of the diode connects to the cap and a lead that goes to the pot, then the other side of the pot goes to an 82k resistor to ground.
Pots value is 10k.
I put the 25mfd cap on the opposite side of the pot to make it filter straight to the intensity control rather than be in between the 82k resistor and the 10k of the pot.
I didn’t want the filtering effect of the cap to be in between resistances.
I wouldn’t think the placement of the cap is the issue because in the original the filtering goes straight to the intensity control with 33k to ground. This build goes straight to the intensity control with somewhere between 82k and 92k to ground.
Maybe it’s that 82k resistor?
Other than the resistor value and cap being moved to the other side of the pot, it’s wired as seen it the picture of below.
Thanks for taking the time to help.

[valvetones]

Actually yes, it’s wired exactly like the picture , except the 27k is 82k and the 100k is 220k and the 50mfd cap is 25mfd

[sluckey]

I would add another bias cap as shown in the attached pic. The INT pot relies on the bias cap that's connected to it's right terminal to provide an AC ground to be able to turn the trem signal all the way down. But is this really a problem? Most people use the footswitch to totally kill the tremolo.

It's much more common to not have enough tremolo intensity when using 6L6s. It's rare to hear "I have too much tremolo". Bias has a big effect on how intense the tremolo is. You may be able to get less tremolo just by tweaking the bias. Since the bias affects the tremolo it's always a juggling act to get good tremolo and proper bias. I suspect this is the reason Leo abandoned the bias vary tremolo circuit for 6L6 amps.

You can also lower the tremolo by increasing that 470K resistor between the cathode follower and the intensity pot (not the one from cathode to ground!). 1M might just be your magic number. This is probably the easiest thing to do. Another trick is to use a 250K log pot. The log pot doesn't actually decrease the tremolo but it spreads it out so you can use more of the pot rotation.

[valvetones]

Thank you Slucky, 

I’ll give it a try. So last night I had EL34s in the amp.
Tonight I put 6L6GC in and biased up perfectly using the pot.
To my great surprise, the intensity pot does indeed shut off all the way now, no changes, just the tubes and bias.
I still think the bias “pulses” in and out to deeply. Or to much intensity.
But at least 0 is actually 0 now.

I think I’ll try changing that 470k to 1 Meg as you suggest.

I discovered another issue. High frequency oscillation on the vibrato channel. It manifests when the volume control is at 8 and above.
I suspect getting rid of the bright cap on the volume control may take care of this. It’s a 47pf.

I think the (and I hope this is the right term) snubber cap in the PI is another place to look. The schematic calls for a 100pf cap for the snubber, I didn’t have one so I used 220pf.
To my understanding (which may be wrong) changing the value from 100 to 220pf should in fact roll off more high end, is that correct?

Once I hit 8 on the volume control she goes SQEEK!!!
Really bothers the ear. Not a pleasant sound. A real chirpy pierce the hell out of your eardrum sqeek.
No good.
Was hoping I nearly had this one in the bag. Still a few issues it seems.

I am extremely pleased with the tone, I just need to get it working right at all settings. Very tedious work.
I guess that’s what I get for trying to reinvent the wheel every time I build an amp, lol
Oh well… Build and learn

[PRR]

> when the volume control is at 8 and above. I suspect getting rid of the bright cap on the volume control may take care of this.

The brite cap does nothing at full volume. And probably almost nothing at "8".

Those values worked for Fender. If not for you, then likely layout.

[sluckey]

Once I hit 8 on the volume control she goes SQEEK!!!

Try swapping the OT plate leads.