reverb return jack resistor change to 270k to reduce reverb?

[mguitarworks]

hi has anyone increased the 220k reverb return jack to ground resistor to tame/reduce the reverb on a AB763? say to 270k...will it work...anydangers in doing this?
cheers

[LooseChange]

I'm assuming this is a typical Fender type circuit...
Nope. Won't hurt it but not going to do much either. You might as well just turn the control down.
It's the drive (send) side that needs modification. Start by trying a lower gain tube for the driver tube.

[mguitarworks]

hi dan..tried ay7 au7 didnt do it for me..changed the 3.3k mixer to 2.2 helped a bit. changing the reverb return jack 220 to 270 did help a bit and warmed it up too ,so i will keep it in. changed the control resistor from 470 to 670.. helped a tiny bit but still not happy. probably closest thing was a dw7 in the return, but didnt like the character of the ax7 half... the au7  half was was great. so thats where i determined that the place to look was the reverb return portion of the circut.

cheers

[mguitarworks]

yes sorry.... AB763 twin reverb clone (single channel no vibrato)

[tubeswell]

In the BF fender reverb circuit, the 3M3/470k is a voltage divider that cuts down the dry signal to a level whereby when the 100k reverb level pot is turned up, the wet signal is far stronger than the dry signal.

If you lower the 3M3 to 2M2 (or whatever), you are changing the voltage divider to allow more dry signal (because the voltage output at the junction of the 3M3/470k is otherwise increased relative to the voltage input). Depending on how small you make that ('3M3') resistor, this may/will have the (side) effect of increasing the amount of clipping in the dry signal (which you may or may not like).

Conversely, if you increase the lower (470k) resistor in this voltage divider, you will also have the effect of increasing the dry signal (because you are again effectively increasing the dry signal output voltage at the junction of the two resistors).

If you want to increase the strength of the wet signal without affecting the dry side's voltage divider strength, them the best thing to try is to increase the size of the (100k) reverb level pot (to 250k or 500k or...)

[sluckey]

There are several simple mods for the reverb circuit that are aimed at 'taming' the reverb.

1. Replace the reverb pot with a 100K log taper pot. This doesn't actually change the amount of reverb, but it foes let you have better control of it when the knob is between 1 and 5.

2. Reduce the value or totally remove the 25µF cap on the cathode of the 12AT7 reverb driver tube. This will actually reduce the amount of reverb.

3. Replace the 1meg grid resistor on the 12AT7 with a 1meg log pot. Now you can set the input level for the reverb circuit. If you can't or don't want to add another pot, then replace the single 1meg with  two resistors set up as a voltage divider to reduce the input level a fixed amount.

[mguitarworks]

hi guys, ive tried most of those ideas and didnt really get what i wanted. i will try to remove the 25uf bypass on the AT7. (wont this increase the operating noise though?)

im happy with the 2.2 mixer resistor ive put in. so that stays.im happy with increase in the retrn jack resistor from 220 to 270 (might try even higher).

how did fender come up with the 220k resistor on the reverb return jack to ground? is there any maths involved in it or just what they thought sounded best? isnt that the quickest and easiest way to tame reverb, just reduce they level of the signal being reamplified by the r-return valve?

[Fresh_Start]

sluckey's got good ideas.  I like a 10uf cap on the reverb driver cathode in Blackface reverbs - not as boomy.  An audio taper Reverb pot makes a nice change, and using 1 meg "Dwell" pot on the grid of the reverb driver stage is a neat addition too.

FWIW the OP and tubeswell weren't talking about the same 220K resistor.

The 220K resistor connected to the reverb return jack and pedal jack also is connected to the grid of the reverb recovery stage.  In fact, it's the grid return (or grid leak if you prefer) resistor for that stage.  Increasing the value of the grid return resistor increases the gain of a common cathode triode stage IIRC.  Going from 220K to 270K should be barely audible though.

The 220K resistor tubeswell was referring to is in parallel with a 470K resistor connected to the wiper of the Reverb pot on one end and to the 3.3meg mixing resistor on the other.  If you increase the value of that resistor a LOT, it would increase the "dry" signal relative to the "wet" reverb signal.  The Princeton Reverb circuit does not include that 220K resistor at all, so you might just try cutting it's ground connection to see what that does.  My guess is that may boost the dry signal too much and reduce your clean headroom, but a bigger resistor in place of that 220K (like 500K?) might be the ticket.

You also could try a 3.3K cathode resistor on the driver stage.  That would reduce the signal going to the tank.  Lowering the plate resistor on the reverb recovery stage (82K) would reduce the return signal but might not sound right due to the "colder" bias point.

Just a few thoughts.

Cheers,

Chip

[sluckey]

how did fender come up with the 220k resistor on the reverb return jack to ground? is there any maths involved in it or just what they thought sounded best? isnt that the quickest and easiest way to tame reverb, just reduce they level of the signal being reamplified by the r-return valve?

That particular resistor does very little when the reverb tank is connected, maybe a little damping. It's purpose is to provide a dedicated grid return for the reverb recovery amp to prevent the tube's bias from going wonky whenever the tank is unplugged. You can put any value you want within reason (100K to 1meg) and it's not gonna make much difference, if any at all. That resistor is in parallel with the tank transducer winding which is about 200Ω dc resistance and about 2250Ω impedance @ 1KHz.

[RicharD]

Changing that grid lead resistor doesn't seem like a good solution.  The dwell pot is the universal fix all for that reverb circuit.  To me, it sounds like you may actually wanna experiment with a different spring tank, or it's dampening baggie.  Perhaps even the tanks location with respect to the speaker could be changed.  Mechanical tone tweaks can be more important that circuitry tone tweaks.

[mguitarworks]

hi, changing the return jack resistor to 270k has Definently changed the reverb (as sluckey described it as "dampening") is a good description. i changed the 470k reverb pot wiper to 670 and it did increase the dry and gain as well.... didnt like it so it dropped it down to 520.better.

I tried the 3.3k cathode resistor for the AT7 didnt like it too "sterile" 2.2k is best for me

will try to remove the 25uf bypass cap and or increase the 220k (parallel to 470) up a bit, see what happens.

thought about lowering the plate resistor, sorry increasing the reverb return side only  (AB763 are 100k as are mine) to reduce its output but didnt know if it was ok too (balance).
 lower or higher resistor will reduce its output ...can u  only do half?

cheers thanks guys

[mguitarworks]

hi  chip...can i use a 10uf 100v for the bypass cap  for the AT7 as i cant find a 10uf 50v

[tubeswell]

So you tried a 250k reverb level pot?

[sluckey]

So you tried a 250k reverb level pot?

I thought he wanted to decrease the reverb?

[mguitarworks]

yes def decrease is 10uf 100v ok for bypass cap?

[tubeswell]

Sorry I misread the initial post. Well then, try a 50k or 25k reverb level pot

[Fresh_Start]

hi  chip...can i use a 10uf 100v for the bypass cap  for the AT7 as i cant find a 10uf 50v

Sure!  You can always use a cap with a higher voltage rating, just not lower.

I've been struggling with moving my ISP today... what a nightmare!

Chip

[mguitarworks]

Hi chip..found and tried a 10uf 50v.. did help a little bit but not enough.

so i tried it with the bypass cap removed....f..k... awesome. love it i now have a 100% fully usable reverb knob it kicks in around 2 and sounds great all the way full as well.Overall its mild round and warm LOVE IT!!

Thanks a lot.

P.S is there any problems running it with no bypass cap...100% safe?

i cant imagin why it wouldn't be.

Cheers

[tubeswell]

P.S is there any problems running it with no bypass cap...100% safe?

None whatsoever. The bypass cap just boosts certain frequencies (depending on the amount of capacitance). With no bypass cap, the stage has less gain overall, and the resulting cathode-current-feedback makes the signal 'smoother'.

[mguitarworks]

cheers thanks .... a great help.....

cheers

[mguitarworks]

its interesting though that its a complete different sound (no bypass cap) as aposed to putting a 3.3k cathode resistor in and leaving the bypass cap.

 ..both  changes have reduced the gain but the 2 results are completely different.

[sluckey]

Without the bypass cap the driver amp has local negative (degenerative) feedback across the entire frequency spectrum. Amplification is the same for all frequencies. Adding the cap makes the driver's gain change based on frequency. Higher freq. signals will be amplified more than lower freq. signals. Making the cap larger will bring up the gain of the lower freq. signals. Making the cap really big (like 25µF) will practically remove all degenerative feedback for frequencies we can hear, so the gain of everything we hear will increase. That's why the tone changes.

[carausius]

Reading this all with interest 

Obviously most will perhaps already know that the same 25uf bypass cap to the reverb drive tube referenced here is not present on the Master Volume Silverface layouts for the Twin Reverb. I'm assuming that reverb sounds different to earlier models because of this change. I think I'll try these changes myself!!! 

[mguitarworks]

yes... i much prefer the sound of the reverb without the bypass cap on ..no question about it!! more smoother less "cavernous" less in your face..subtler a bit duller darker too.

cheers

[Rev D]

 If it works and you like it, excellent job! It got cavernous because music was what it was at the time, surf ruled I guess, for whatever reason that circuit wasn't changed with the times. People instead started using amps with effects loops and using they're rack gear and pedals which is why I think a lot of that gear became orphaned until people played them again and went, you know with a few tweaks this old amp sounds GOOD (in this case the old CIRCUITS)!   Sounds like you got the best of both now, something useful and thats all most of us guitar players want (Gibson are you listening? Its not the Firebird X we want, with all the knobs, dials and patches, its a smooth useful sound... sheesh to much to ask for?  ).

Regards,

Don