Reverb Tone Shifting with Dwell knob

[Lectroid]

I've never played with Dwell much, so I added a Q&D Dwell pot to my AB763 DR clone just to see if I like it (Fender std. AB763 reverb).  I replaced the dwell resistor with a 1M pot. It seems to work fine and gives me a lot more 'depth' to the reverb sounds.

But, if I turn the Dwell up or down, it changes the tone of the reverb signal as well. Less pot resistance gives more Dwell effect, AND it also makes the sound more treble.  That makes sense since I'm changing the -3dB point of the cap/resistor HP filter.  I guess this is just a known effect of the Dwell control and I just never knew it?  Or is mine doing something new?

I know that the 1M/500pF values reduce some of the highs in the reverb signal by design, and the IM resistor is a good grid leak value for the 12AT7 reverb driver tubes.  I had even reduced that 1M resistor down to 470K to get some hard-wired dwell, but that 470K was enough to provide a good grid leak.

1.  When I turn the Dwell all the way up, then there is almost no grid-leak resistance on the reverb driver tubes at all (~3Ω).  Is that a problem?

2.  Can those standard values in the cap/resistor HP filter be adjusted to maybe not shift quite so far into the treble?  Has anyone played with this?

Thanks!

[stratomaster]

The 500pF/1M forms a passive high pass filter, not a low pass. This is to limit the lows into the tank to prevent it from getting overwhelmed.

How did you wire the pot? If you wired it as a variable resistor then you'll get a change in the corner frequency of the filter as you turn it. If you wired it as a voltage divider (like a volume pot) then you should not notice a change in tone.

[Lectroid]

The 500pF/1M forms a passive high pass filter, not a low pass.

Too right, I know better.  Sloppy proof-reading gets me again.

How did you wire the pot?

I replaced the standard 1M resistor with a 1M pot.  As to wiring, I think it's a voltage divider.  I copied the wiring from sluckey's web page.  I'll add the schematic.

Thanks!

[Merlin]

Sounds like you have note wired the pot the way you think you have

[Lectroid]

I agree that it's not the behavior I expected either.     I just checked and sketched it again and this is what I see.  As I turn the knob CW, Dwell increases.

[Merlin]

1.  When I turn the Dwell all the way up, then there is almost no grid-leak resistance on the reverb driver tubes at all (~3Ω).  Is that a problem?

With the pot turned all the way down the grid would be grounded (no reverb at all), so the grid leak would be ~zero ohms, which is fine.

2.  Can those standard values in the cap/resistor HP filter be adjusted to maybe not shift quite so far into the treble?  Has anyone played with this?

With a standard pot there will be no significant change in tone with rotation, since the cap always sees the same 1M pot resistance at all settings. So I'm not sure what's going on, assuming you have it wired correctly.

[ac427v]

Unrelated to your dwell issue: 160 volts on pin 1 triggers my troubleshooting radar

[Lectroid]

Unrelated to your dwell issue: 160 volts on pin 1 triggers my troubleshooting radar

Good catch.  That voltage is actually ~398V on the reverb driver., 8.55V cathode.   I'm known for sloppy cutting and pasting.   :facepalm:

Otherwise, I agree I've likely mis-wired something but I don't know yet what's going on. 

As I understand it, the two outer pins of the pot are put in place of the former dwell resistor.  One side from the 500pF reverb cap, and the other side connected to ground.  The pot's wiper then dials in the desired level of signal (voltage) that goes forward into the tube driver grid.  Somebody holler if that's wrong.

[stratomaster]

That's correct, but I'm wondering if you have an internal short from wiper to one of the outer lugs or a tin whisker/solder bridge.

Dial the pot to approximately mid point and check resistance to the two outer lugs.

[Williamblake]

If you are playing with dwell i think adjusting the signal with a pot is a good idea. But adjusting the high path filter by for example switching that 500pF cap to a different value on a switch is just as good. The lows in the reverb tank make a big difference.

[ac427v]

If you are playing with dwell i think adjusting the signal with a pot is a good idea. But adjusting the high path filter by for example switching that 500pF cap to a different value on a switch is just as good. The lows in the reverb tank make a big difference.
[/quote
Very interesting idea. Can you share more about this?

[Williamblake]

These are two reverb units i build with that in mind. The variable 500 pf capacitor is found in tube radios and needs to be isolated from the chassis. The switch in the other unit is behind the EF86 (on rubber) and i would put all the associated caps on a separate board if i would be using a switch again.

[Williamblake]

Some background i found interesting: "Proc. of the 12th Int. Conference on Digital Audio Effects (DAFx-09), Como, Italy, September 1-4, 2009"

PDF is too large to upload. https://www.dafx.de/paper-archive/2009/papers/paper_84.pdf

[ac427v]

Much thanks WilliamBlake. A radio tuner for a variable 500pf reverb input filter. Now that is out of the box thinking!

The standard Fender AB763 reverb circuit sounds good to me at low to medium volume. But not when cranked. Your work has me wondering if I might prefer a variable reverb input filter, not a traditional variable dwell control.

Related idea:  Use a 1 meg dual pot to replace the Volume control. Connect he wiper of the new pot to the existing 1 meg dwell resistor/12AT7 grid. Two different value reverb input caps connect the preamp signal to each of the outer lugs on the pot. Say 100pf and 1000pf. At low volume, the 1000pf would allow more low frequencies to go to the reverb springs. At high volume settings the 100pf cap would allow less bass frequencies to be 'verbed.

I am interested in thoughts from forum members. Both of my Fender clones are on loan now so no experimenting soon. But I'm always planning one mod or another and this idea intrigues me

[stratomaster]

I think your observation about not liking the reverb when cranked is valid as there are documented headroom issues with the 400v+ HT and 2.2k cathode. The input signal isn't attenuated in stock form, and it's downstream of the volume pot.  The higher the volume, the harder the signal hits the relatively low headroom driver stage.

A dwell pot should take care of that, though. No need for heroics with stacked pots.

You can always do Merlin's fix of dropping the HT to 350V and rebiasing the driver with a 1.5k too.