Adding Dwell control to overly aggresive reverb in Crate V18?

[Steve A.]

I already toned down the output from the reverb springs by tacking a 33k resistor on the RCA jack, which helps a lot. But when you do crank the amp up, there is a *lot* of signal going into the springs (this is solid state- TL072's I believe).

One fix on the other forums is tacking a 680 ohm resistor across the 4k7 NFB resistor on the IC. But I do like the way the reverb works at lower volumes, and even at higher volumes I do like that "To Infinity and Beyond" amount of Dwell some of the time.

My first thought was a switch to cut the signal to the 75 ohm reverb springs by 75% (while keeping the load on the IC the same). Wire it up like a 4x12 cab: a 33 ohm series resistor going to the 75 ohm springs in parallel with a 75 ohm resistor.

But I like the idea of a variable dwell control. The signal passes through a 470pF cap to the + input of the IC, which is loaded down with a 1M resistor. If it is necessary to keep a 1M resistance on the IC input then a 1M pot could be wired up with the wiper going to the 470pF cap and the CW terminal going to the + terminal. (If it is okay to ground out the input then the pot could be wired up normally, with the wiper going to the + terminal).

Now I don't want the Dwell control to be able to turn off the reverb completely, but merely attenuate it a bit. So I was thinking of wiring up a 500k pot with a 510k resistor in series from the CCW terminal to ground. So that would be like a 1M linear pot that could only go from ~5 to 10. Or maybe put a 100k resistor on the "tail" and then that should be like setting an 1M audio pot from ~5 to 10.

Any thoughts on all of that would be appreciated! It's been a long time since I dabbled in solid state...

Steve Ahola

P.S. I have rewired the preamp circuitry on my V18 so that it is like the Clean channel of the V33 (which is rated very highly by some folks)- only better! Yes, I think that some compromises were made to the tone stack and to the shared tube stages to accommodate the Clean and the OD channel. But I was not limited by that restriction since there is no OD channel in my amp. I call it a poor man's Maz 18- put a nice pedal or two in front of it and you can get all of the dirty sounds you want as well...

Here is a link to the V18 main board schematic:

http://www.blueguitar.org/new/schem/...main_board.pdf


And here is a link to the *corrected* V18 drawings - there were many errors in the factory schematics (these are also attached to this post):

http://www.blueguitar.org/new/schem/...d_drawings.pdf

[Enzo]

That was my first thought, turn R37 into a 1M pot, or a pot and a resistor if you want limits.

SInce the reverb transducer is in parallel with that 4k7 resistor already, swamping it with a 680 ohm resistor sems to me like a brute force method. I would try increasing R35. What happens if it becomes 470 ohms instead of 47? Or make it a 1k (or 200 ohm or whatever works) pot wired as a variable resistor.

[Steve A.]

That was my first thought, turn R37 into a 1M pot, or a pot and a resistor if you want limits.

SInce the reverb transducer is in parallel with that 4k7 resistor already, swamping it with a 680 ohm resistor sems to me like a brute force method. I would try increasing R35. What happens if it becomes 470 ohms instead of 47? Or make it a 1k (or 200 ohm or whatever works) pot wired as a variable resistor.

So the cable shield on the lead going to the reverb tank is lifted from ground by R35 and C25? I hadn't noticed that before...

So if I stick with the idea of replacing the 1M R37 resistor with a pot, is there a problem if I wire the wiper to the input of the IC? In other words could I ground the + input of the IC without having it go into fits?

Thanks!

Steve

P.S. It seems like a lot of those mods that you find on the web have nothing to do with electronic theory and design but just happened to be something that worked for someone who tried it out... Some of that stuff is absolutely frightening!

[Enzo]

We often see things people do with zero understanding - some of them even work. Or it works well enough, and they invent some elaborate rationalization for how it must work, but which really has nothing to do with how it works.

Imagine someone deciding to invent his own speaker attenuator by just slapping a 1/2 ohm resistor across the output. Certainly would knock the volume down.

I don't know why the chip would care if you grounded its input pin. Try it and see. If you have to wire the pot "backwards." guitar style, then oh well. And if we burn up the IC, they cost about 22 cents.

Come to think of it, DON"T wire the pot backwards. In that situation turning to zero grounds C24, which in turn would roll off your highs coming out of IC1B, heading for the PI.

[Steve A.]

... I don't know why the chip would care if you grounded its input pin. Try it and see. If you have to wire the pot "backwards." guitar style, then oh well. And if we burn up the IC, they cost about 22 cents.

Come to think of it, DON"T wire the pot backwards. In that situation turning to zero grounds C24, which in turn would roll off your highs coming out of IC1B, heading for the PI.

Good point! I think floating a 1M audio pot on top of a 100k resistor to ground might be the best solution since I don't want the Dwell control to turn off the reverb completely. Speaking of which, if I wanted to add a footswitch to turn off the reverb it would be easiest to just ground out J21 at the RCA jack (like Fender did in their BF/SF amps) although it might be better to ground out the wiper of the reverb pot CW6 (since you would be muting any noise from IC3A).

At this point I'm still trying to decide between putting the Dwell control on the top panel, where the LED indicator light is, or on the bottom of the chassis right next to the reverb jacks.

Steve

P.S. I've been very pleased with the Eminence Red, White and Blues speaker that I kinda picked out at random for this amp out of the various offerings at Musicians Friend (mainly to take advantage of the free shipping on my order). A friend was telling me that the response curve of this speaker is very similar to the EV speakers from the 60's and 70's, and that Glaswerks is using them in some of their cabs (Glaswerks is arguably making the best D'clones these days, with about 8 different internal relays to switch in post-OD EQ, among other things. Definitely not just another "me too" clone!).


Although Eminence seems to make some of the crappiest OEM speakers for the different manufacturers, their own lines of speakers are top notch. (I've been using their Beta 8" bass speaker in my Kustom Tube 12, which gives it a surprisingly big tone, and I've been gigging with it at some of the local bars. The speaker is rated at 225 watts!)

[Enzo]

Just as an experiement - if you feel like it - try changing R35 to 470 or something.

I have never liked the Fender approach - grounding the reverb return. This requires the signal path beiong connected to a shielded wire out to the FS. This is just begging to add noise and hum to the system. That return is a high gain stage. I'd much prefer to see that pot wiper grounded or somehitn like that.

Totally not my call, but I'd not stick the dwell on the panel. In my experience with this sort of mod, most guys wind up turning the control to max and leaving it after a week or so of diddling around with their new control. It is kinda like that cathode/fixed bias switch on the C30 - pretty cool, but after a few days it pretty much stays on one setting forever.

[Steve A.]

... Totally not my call, but I'd not stick the dwell on the panel. In my experience with this sort of mod, most guys wind up turning the control to max and leaving it after a week or so of diddling around with their new control. It is kinda like that cathode/fixed bias switch on the C30 - pretty cool, but after a few days it pretty much stays on one setting forever.

I decided to put the dwell control on the bottom panel, but mainly for cosmetic reasons. Unlike many of the amps that use variations of chicken head or Fender skirted knobs, this amp uses knobs that look like they came off of a Fisher receiver from the 70's... I don't have anything that matches them and I really don't want to use one of my remaining progressive Off-Standby-Toggle switches for this amp.

I would normally agree with your point, especially when added to a Fender reverb circuit, but with this design you get a lot more reverb drive signal as you crank up the amp. So I think I would adjust the dwell to match the overall volume level. Although I may want to turn it up on a slow blues to get that insane Ronnie Earl reverb sound...

So why does that not seem to be a problem with the Fender Reverb amps? Heck if I know...

Thanks!

Steve

P.S. I will use your suggestion on the reverb footswitch. BTW I was thinking of adding something else to the footswitch, to use the second button. Maybe add a 25uF cap in parallel with C6 on the first stage for something like a Fender Blues Jr Fat switch...? I think that I have seen some boutique amps that would use a footswitch for the cathode resistor bypass cap. Since there is no audio signal at that point of the circuit, it shouldn't add any hum or noise, but it might not like sharing the same ground point with the reverb pot. If it doesn't work right I could just disconnect the added cap...

EDIT I have a better idea for the two button footswitch: how about having "reverb"and "more reverb"? I think I might need to get a special switching jack- or else modify a footpedal to be used exclusively with my modded V18. One button would completely mute the reverb signal tapped from the 25k reverb pot wiper while the other button would just attenuate it a bit... maybe running the signal to ground through a 10k resistor.

[Steve A.]

Here's the write-up on the V18 reverb mod that I posted on the 18watt.com sister site, ppwatt.com (heh, heh- he said peepee! ):


There seem to be a whole sh*tload of reverb mods on the internet- often with no basis in electronic theory and in many cases lacking common sense. I would like to present the solutions that I have come up with myself.

One very simple mod that gives you a better range for the Reverb Control involves soldering a 33k resistor across the two terminals of the white RCA jack at the bottom of the chassis- you can do that without removing the circuit board. What it does is increase the grid load on the input of the reverb recovery circuit from 100k to roughly 25k. (If you already have the circuit board out you can get the same result by replacing R40 with a 22k or 27k resistor.)

At low volumes this works pretty well but once you crank up the preamp the stronger signal really swamps the reverb springs. With my V18 morphed into the Clean channel of the V33 you can get some really insane Ronnie Earl reverb sounds which I wanted to be able to use some of the time. So rather than permanently attenuate the input into the reverb driver circuit I decided to rewire the 1M load resistor as a potentiometer. Since I did not want to have two controls which could turn off the reverb (too confusing for us old timers!) I inserted a 100k series resistor to ground on the "tail" of the 1M audio taper pot. Now with an 1M audio taper pot, the 5 position roughly corresponds to a 100k load to ground so its like this control can only be set from 5 to 10.

To accomplish this (adding a Dwell control to the reverb), you need to remove C24 (a 470pf cap) and R37 (a 1M resistor). Since the layout does not show traces you need to use an ohmmeter to determine which copper pad connects to R26 and R27 (erroneously listed on factory schematic as R270) and which copper pad connects to ground. Whenever I get around to writing up an article I will show which pad is which on an annotated layout drawing- this is just a preliminary explanation.

With the two components removed you will wire up 2 conductor shielded cable to 3 of the pads. If you only have single conductor shielded cable you can run two cables. I found no place on the board to mount the 470pF cap so I used a mica cap (w/ thick leads) and soldered it to the CW (hot) terminal of the new Dwell pot. I soldered a 100k resistor from the CCW (cold) terminal of the pot to the back of the pot, which I used as ground. The third terminal- the wiper- will be connected to the input of IC2-B.

Looking at the component side of the circuit board, with the pot shafts facing down, here are the specific instructions. The left copper pad vacated by removing C24 goes to the 470pF cap on the CW terminal of the Dwell pot. The corresponding right copper pad goes to the middle terminal- the wiper. And I picked up the ground from the furthermost copper pad vacated by removing R37.

I mounted the Dwell pot on the bottom of the chassis next to the reverb jacks.

So far so good? Okay, I thought it would be cool to have a footswitch turn the reverb on and off. It would very easy to tap the reverb signal right at the return from the springs and ground it there, but if you do that it is very easy to pick up noise which is amplified by the reverb return circuit. So I tapped the signal from the wiper of the Reverb control and ran a shielded cable to a jack for the footswitch.

Warning: I figured that I'd solder the wires to the little round hole in the pot terminals "to save time". I think I've done that before and never did learn my lesson. Doing that can screw up the pot, so I'll be hunting around the local parts houses tomorrow looking for a 15mm chassis mount 25k linear pot. I will probably track down the terminal on the C31 .0068uF cap that is in common with the wiper and use that for my next attempt.

Refinement: Most of my footswitches have two buttons and as a matter of the fact the jack I used was a stereo jack. I was wondering how I might use the second button- perhaps switch in a 25uF Ck cap on the first stage- and then it occurred to me: how about "Reverb" and "More Reverb"??? I'm always reaching over to turn up the reverb on those slow blues solos so why not wire that up to the footswitch?

To get "Reverb" and "More Reverb", run a 8.2k resistor from the Tip terminal to the Ring terminal (I had the lead from the wiper connected to the Tip terminal so that the jack would work with a single or two button footswitch). My first attempt was a 10k resistor which worked well at the lower settings but not as well at the higher settings.

Caveat: With a typical non-switching stereo jack, plugging in a mono footswitch will engage the "Reverb" mode, not the "More Reverb" mode so that you will need to adjust the top panel Reverb control accordingly.

I've been very happy with how all of this works on my Clean-ified V18- except for the intermittent problems caused by me screwing up the Reverb pot. Well, actually if I could find a 25k audio pot I think that taper would be an upgrade from the linear taper.

Steve Ahola

P.S. These mods should work for the V33 and V50 as well, although the reference numbers would be different. Be sure to download the corrected schematics for the V-18 I put together to get the correct schematic reference numbers:

http://www.blueguitar.org/new/schem/...rected_drawing...

P.P.S. Many thanks to Enzo for all of his excellent advice on these mods!

[Mr. Droopy-Drawers]

Here are a few V18 'mods' I have every intention of trying. They may help someone else. Even now, it is a freakin' awesome deal on a cool tube amplifier. But can I leave anything alone?

• Change R3 to 100K, and add a 2.2uF capacitor across R5 from the foil side.
• Change R61 to 470K, and add a 470pF capacitor in parallel, which can be tacked on the reverse side of the board.
• Short out R17, and make R15 1Meg.
• Make R17 56K.
• Short out R22/C16, or just make it 10K or something a lot lower than what it is now.
• Lift the end of C24 where it goes to the Reverb driver IC. Add a 470K resistor. If you want to drop the Reverb drive even more, make it 220K and change R37 to 220K.
• Remove R42 and C31.
• Change R32 to 10K.

I plan to try and add NF, but that's not written in stone. If anyone tries any of these mods, let me know how they work for you. You can save me some time and headache if these mods sound like poop .

[Steve A.]

Here are a few V18 'mods' I have every intention of trying. They may help someone else. Even now, it is a freakin' awesome deal on a cool tube amplifier. But can I leave anything alone?

• Change R3 to 100K, and add a 2.2uF capacitor across R5 from the foil side.
• Change R61 to 470K, and add a 470pF capacitor in parallel, which can be tacked on the reverse side of the board.
• Short out R17, and make R15 1Meg.
• Make R17 56K.
• Short out R22/C16, or just make it 10K or something a lot lower than what it is now.
• Lift the end of C24 where it goes to the Reverb driver IC. Add a 470K resistor. If you want to drop the Reverb drive even more, make it 220K and change R37 to 220K.
• Remove R42 and C31.
• Change R32 to 10K.

I plan to try and add NF, but that's not written in stone. If anyone tries any of these mods, let me know how they work for you. You can save me some time and headache if these mods sound like poop .

I've posted more of my mods over at ppwatt.com (at the 18watt.com site). For my first child, er, V18 I morphed it into the Clean channel of the V33, bypassing the second stage (V1b) and doing many of the mods you just proposed. Basically just "normalizing" the circuitry- making it more like a typical Fender or Marshall amp.

The V18's at Musician Friend are long gone but I have another one coming in from a different source. I wanted to rework the OD channel on that one- I've been looking at the schematic for the preamp and PI section of an AC30/Trainwreck Rocket and it should go in there very easily. That still might not have enough gain and distortion (my first V18 is absolutely clean- no grind even if you dime all of the controls- but very powerful- kinda like a Music Man especially with the Eminence Red, White and Blues speaker in there). So I may be getting a V33H head to play around with some higher gain circuits...

Steve