Reverb potentiometer questions

[d l x r e v e r b]

A few questions about reverb potentiometers :

(a) Is the potentiometer normally wired for maximum resistance when the knob is turned to "0" or "10" ?

(b) If wanting to use a 1 meg ohm potentiometer as described in the reverb modification in one of Gerald Weber's books ( wire the reverb to use channel one of a dual channel Fender amp), if the closest available specifications are 2 meg ohm linear taper or 1 meg ohm audio/log taper, which would be most the most convenient to use when turning the knob ?

(c) What is the affect of using a 1 meg ohm pot rather than the original 100K ohm pot ?

Thanks

[sdgvintage]

(a) on 0 you have a 0 resistance to ground.
(b) 2 megs would probably be too much, even with 1 meg you probably wouldn't be able to get past 2 without too much reverb.

[Wakarusa]

Something to noodle before you tinker with your Fender... consider replacing the 1Meg resistor from reverb drive grid to ground with your 1 Meg Audio taper pot. Now you've got a dwell control which is much more useful (least I think so) than mucking with the mix pot on the return.

[d l x r e v e r b]

I should have been more thorough in providing details in my original question. This mod is described on pages 40-41 of Gerald Weber's "Tube Map Talk For The Guitarist And Tech" book and involves removing the 1 meg resistor , and replacing the 100k -L reverb pot with a 1 meg pot.

The internet parts sources only have a 1 meg audio/log taper rather than a 1 meg linear taper which I assume is Weber's recommended replacement for the reverb pot.

How to perform this mod without the recommended pot ?

Would the audio/log pot be convenient enough for dialing in the dwell and then leaving it in place rarely touched again, or would adding a resistor to a 250K or 500L linear taper pot the better work around ?

Thanks

[d l x r e v e r b]

Typo in final sentence of recent post should read "250K or 500K linear taper pot".

[d l x r e v e r b]

Wakarusa ,

Is your recommendation to replace the 1 meg resistor with a 1 meg potentiometer and leave the original 100K-L reverb pot in place , meaning the circuit would have two knobs to turn ?

The total modification I would like to try is using the blackface Deluxe Reverb channel one to power the reverb circuit. Is your resistor replacement recommendation in addition to or instead of this mod ?

Thanks

[Wakarusa]

The internet parts sources only have a 1 meg audio/log taper rather than a 1 meg linear taper which I assume is Weber's recommended replacement for the reverb pot.


http://www.mouser.com (http://www.mouser.com/) part number 31VA601-F, $1.40 each.

[Wakarusa]

Wakarusa ,

Is your recommendation to replace the 1 meg resistor with a 1 meg potentiometer and leave the original 100K-L reverb pot in place , meaning the circuit would have two knobs to turn ?

The total modification I would like to try is using the blackface Deluxe Reverb channel one to power the reverb circuit. Is your resistor replacement recommendation in addition to or instead of this mod ?

Thanks

I don't have Weber's book, so don't have the suggested mod procedure in front of me so to speak. There's something missing from your description of the mod...

Assuming channel 1 is the "normal" channel, it's electrically separate from the reverb circuit. Is the idea to move the reverb to the normal channel, or to have it available on both normal and "vibrato" channels?

Next, "remove the 1 Meg resistor" -- if this refers to the 1 Meg from grid to ground on the reverb drive tube, it's advice I wouldn't follow. I can't think of any reason to remove the ground reference from the grid of that tube.

Finally, swapping the 100K "reverb" pot for a 1 meg part. The reverb pot is just a mix control that determines how much wet (reverb effected) signal to mix back into the dry path. Downstream of the mix pot is a 470K/220K voltage divider with the wet return picked off between the two. From an AC analysis, the 100K pot is in parallel with both the 220K to ground and the 100K plate load. Changing for a 1 Meg will alter how all of this works out and ultimately yield a bigger reverb return signal for the pot to work with. Next, because both the 100K and 1 Meg only have 270 degrees of rotation to work with, I'd think the change in wet/dry mix will probably be more difficult to control because a) it's a larger return signal and b) the resistance of the 1 Meg changes a lot more for a given rotation of the knob.

But then I haven't seen Weber's write up, so everything I just wrote could be hooey ;)

My suggestion to change out the 1 Meg for a pot on the drive side does indeed get you two controls for reverb -- dwell and mix. With the pot all the way at 10 it will act like the stock configuration. Dwell controls how hard you're driving the reverb tank and the drive signal for these amps is relatively strong (one of the reasons a 3 spring tank is just over the top for most people). Dwell's biggest influence is on the reverb's decay time. By messing with both dwell and mix you can really tailor how the 'verb sounds in the final output.

[d l x r e v e r b]

Wakarusa ,

Gerald Weber book write-up on reverb is included below.

Your advice is much appreciated because I am a clean tone player and the amp's reverb is usually my only effect. Any change in reverb tone will be directly heard rather than lost in a mix of other effects.

Thanks for Mouser part number for the proper potentiometer. Ten minutes on phone yesterday with Mouser technician found no part number for the proper physical and impedance size. Using your part number I checked the Mouser site and the number is indeed the correct pot. Thanks !


Here's the write up from pages 39 to 41 of the Gerald Weber "Tube Amp Talk" book which is further credited to Finland's Sampo Kolkki :


The channel one (Normal Channel) volume knob becomes the Mix control of the reverb and the bass and treble knobs become the reverb tone controls. The front panel's "Reverb" knob becomes a Dwell control .

(Note : Although not mentioned in the write-up , I assume the guitar is plugged into the Vibrato channel and the Reverb pan is connected to the Normal Channel ).

(1) Connecting reverb to Channel One (Normal Channel)
Run a wire internally from the hot lead of the Reverb Output jack to pin 2 of the first preamp tube . The wire that is already on pin 2 must be removed.

(2) Dwell control
Remove the center lead from the stock reverb pot and ground it.
Now remove the pot and replace it with 1 meg ohm pot, but don't solder anything yet. There are three leads on the back of the 1 meg ohm pot. Looking at the back of the 1 meg ohm pot, the left lead gets grounded.
Now look at the reverb driver tube. Pins 2 and 7 are jumped together and there is a wire running from one of these leads to the circuit board. Leave pins 2 and 7 jumped but disconnect the lead that connects them to the board. Now run a wire from either pin 2 or pin 7 to the middle lead of the 1 meg ohm pot. On the board notice the 1 meg ohm resistor that we will remove. Originally this resistor was grounded at one end and connected to a 500 picofarad cap at the other end. From the 500 picofarad cap lead, run a wire to the right lead of the 1 meg ohm pot. (This wire connects to the lead of the cap that originally went to the 1 meg ohm resistor that was removed). You're done.

The front panel "Reverb" knob is now a Dwell control. If you turn it up all the way it is the stock reverb . As you turn it down , it will make the pan less driven and alter the sustain characteristics of your reverb pan.

(END OF BOOK WRITE UP )


How would you approach this modification ?

[Wakarusa]

Heh. I get it now.

They're disconnecting the reverb output from channel 2 and instead running it through channel one (where it will eventually mix back in just before the PI). Included in the bargain is that the reverb signal goes through what was channel 1's tone stack, so you get tone controls. The cost is that you give up channel 1. The 1 Meg pot in both schemes goes (electrically) in the same place.

The Weber scheme will certainly work, but strikes me as wasteful. You can get everything but the tone control on the reverb by installing the "extra" 1 Meg pot for a dwell control. You'd either drill a hole or, more likely, chuck it in an existing hole. One way to use the existing hole is to put the dwell control where the reverb send RCA jack is now and then replace the reverb return jack with a stereo jack (since send/return share a common ground anyhow). This way you get to keep channel 1 for the later mod where you link the two channels together or rebuild the "normal" channel to do mad crunch ;)

[d l x r e v e r b]

Wakarusa ,

Thanks for detailed replies.

A few questions involving the "phase" of channels one and two which normally are designed out of phase with each other :

a) With the Gerald Weber book running the reverb through the amp's channel one (Normal Channel), is it "in phase" with the signal of channel two (vibrato channel) ? If yes, please explain.

b) With the unrelated modification of bridging channel one and channel two together as mentioned in your reply and on page 133 of Weber's "Tube Amp Talk" book, are the channels "in phase" with each other and, if yes, is this sound any different from plugging a guitar into both channels' input jacks at the same time when using a Barber Launchpad ABY pedal which adjusts the phasing so both channels are in sync ?

Thanks

[Wakarusa]

1) Interestingly, in phase. The reason is that the suggested mod connects the reverb pan output to the normal channel, not the output of the reverb recovery amp to the normal channel. The reverb recover amp becomes disconnected and unused in Weber's scheme. If you count, you'll see each path (dry and wet) has three inverting stages, so they end up (essentially) in phase.

2) Again, I don't have Weber's book so don't know what he suggests for bridging the channels. There are reasonable ways to do it that leave them in phase. For added fun, you can cascade the channels (so one drives the other). This requires a bit more thought because you actually end up having to tame things (too much gain from a straight cascade).