Changing tone of the reverb on a Pro Reverb

[DanD]

I would like to change the tone of the reverb in my Pro-Reverb. I need to darken it a bit.
Any suggestions?

DanD

[Gene]

A common mod is to change the fixed resistor for the midrange on Pro Reverbs. Blackface, right? You can make it darker by changing the value.

[jack anderson]

The mixer/recovery tube , # 4 , I think, amplifies and mixes the reverb and dry signals. You could try different 12AX7's; one might be darker than what's in there now. But it would also darken the dry signal.

There's a 500 pico capacitor that feeds the reverb driver ; it can be changed to , say, a 1000 pico.

Magic Parts has a reverb pan that's a little darker, but it has a longer decay. It's part # RRVL2AB2C1B.

[tremolux]

If Im understanding this correctly, you want to make the "reverb signal" darker, in other words make the reverb wash sitting on the note less splashy/bright?

You can start by hanging small value caps across the 220k resistor (parallel) that ties the reverb pedal/switch to ground.....start with a .001 or .0015 (500 vdc rating or more) and work up towards .003 until you roll off enuff high end to make you happy. .0015 will get you in the ballpark pretty quick...

For the opposite effect, if there is a cap already there and you want the reverb signal to be brighter etc remove it (on the ungrounded end of the 220k only in case you want to put it back). If theres one there and you still want it to be darker, just increase the cap size by 500 to 1000 pf in increments...

....might leave that 500pf cap at 500pf...going up in value can cause rumble as the drive signal feeds into the tank/transducers etc....

[DanD]

Tremolux,
That is exactly what I need.
Thanks a bunch.
DanD

[Tomo]

Good thread indeed!

Tomo

[tremolux]

One other area of the reverb return circuit where a capacitor value change can make a huge impact on the tonality of a Fender (reverb) amp is the 10pf caps that sits on the 3.3Meg (parallel).....Its already a super small cap because it doesnt take much to magnify the treble balance of the amp in that spot...a very reactive and dynamic circuit area for adjusting the "overall" treble of the amp....just going down to 5pf or up to 15pf can make a big difference in the upper treble range of the amps response...

If I get a super bright Fender thats icepick hot I'll sometimes clip that cap out...... or vice versa, if its nature is too dark and swampy, jumping it up to 20pf can add some needed treble....Once its adjusted to suit the amp and the speakers upper freq range, it makes the treble pot more user friendly (eg: on a blackface/sf with bright speakers...)

To me, its better than hanging a "brite cap" on the vol pot or increasing the brite switch cap from 120pf upwards, because making those changes can let more than treble freqs thru and sometimes have the effect of increasing upper mids too much.

....silver mica (500vdc) caps sound best to me but some prefer the sound of a ceramic cap there....ymmv....

[DanD]

tremolux,
Thanks again, great stuff.

I recall a schematic that had the 3.3meg a different value (slips my mind which amp it was).
How critical is that resistor to the tone of the amp?
What happens if the value is increased or decreased?
Is it best to use a carbon comp resistor?
Is the wattage rating critical?
I've seen other post indicating that composition and physical size of some parts play a roll in the tone of the amp.

DanD

[tremolux]

Some of the proto schematics of the AA763 circuit show it as 2.7Meg, but it was later changed to 3.3Meg when the first blackface amps went into full production using the AB763 circuit(I think).....if you reduce the resistors value you'll get more gain in the return circuit and to my ears its not a very good sounding source for distortion created by additional gain. Usually the thing to do in the return circuit is to reduce the gain a little to reduce noise and clutter in the output signal after the reverb is mixed in.

Separating the cathodes helps get better control of the gain structure in V4 because you can lower the gain slightly with larger bias resistors and use different bypass caps to adjust the bass roll-off points on each side of the twin triode.

Ive never tried increasing the 3.3meg, but Im guessing it would lower the non-reverb signal being returned more. The 470K resistor plays a role here as well, so balance is a factor to consider in making changes in the return circuit and Leos designers did a good job.

I personally like a CC resistor for the 220k, 470k and 3.3M and realize Ill have to change them more often than a MF to minimize noise...its up to your ears there...I just stick with 1/2watts and 1 watts CC or CFs depending on the (dropping) voltage and the placement. Preamps 1/2 watt and power sections 1 watt.

Just for fun, you can try soldering a 470k 1 watt resistor into two RCA plugs (tips only!) and plugging it in where the reverb pan plugs in... for instant overdrive...try different 12A tubes in V3 until you hear something you like...you wont have reverb but you'll have a footswitchable extra "gain channel" in your bf/sf rev amp. Try a 12AY7 there if you have one...