7ender Blackface reverb circuit mods

[Colossal]

There are lots of mods out there that seek to tame what might be for some a bit too much of a good thing in the 7ender blackface reverb circuit.

I love the sound of the circuit (lush reverb that fades into analog space!) however, as most have found, the Reverb control is very touchy with pretty much from about 1-2.5 being the sweet spot. Anything more and it takes off into Dick Dale territory. I am looking for Eric Johnson/Alex Lifeson space not Zuma beach (no offense meant to surf cats).

As I'm sure this is well-traveled ground, is there one or two specific mods that will get a much more usable range out of the Reverb knob? Could it be as simple as decreasing the Reverb pot value from 100kB to say 50kB or even 25kB? I was also thinking that lowering or making the 1M grid leak resistor variable (e.g. the Dwell mod) at the entrance of the 12AT7 driver and/or changing the driver to a 12AU7 might be useful, but as I said, my intention is not to change the fundamental sound and reverb quality; only get the knob to be useful, not hair-trigger.

Thanks in advance for any comments, observations/experience, and suggestions.

[Firestorm]

I'm a fan of the 12AU7 driver, but some people find it alters the tonal character of the reverb. Simple enough to try. Adding a Dwell control may be useful, but you have to find a place for the pot if you want to make it user-adjustable. (You could also mount it internally and find a setting that does what you want; then leave it set or replace with a divider of the right values). Accutronics always recommended shaking the springs as hard as you can, which is pretty much what the BF circuit does, and 12AU7 or Dwell control will change that some. You could try changing the 100KB Reverb pot to 100KA which will add useable range at the bottom of the pot's travel, but make it jumpier at the other end. You could also try changing the tank itself; lots of variation among the different choices available (even some between "identical" tanks).

[Colossal]

I'm a fan of the 12AU7 driver, but some people find it alters the tonal character of the reverb. Simple enough to try.

Well, I'll have to give that a try. As you say, easy enough!

Adding a Dwell control may be useful, but you have to find a place for the pot if you want to make it user-adjustable. (You could also mount it internally and find a setting that does what you want; then leave it set or replace with a divider of the right values).

Yes, I was considering adding it as a 1M trimmer for now, just to see if it is useful in context with either altering the driver tube, Reverb pot, both, or on it's own. If it warrants front panel real estate, that certainly could be implemented in a permanent build.

Accutronics always recommended shaking the springs as hard as you can, which is pretty much what the BF circuit does, and 12AU7 or Dwell control will change that some.

That's good to know. I think the best bet might be just to swap in a 12AU7 just to see if that's a direction to go in.

You could try changing the 100KB Reverb pot to 100KA which will add useable range at the bottom of the pot's travel, but make it jumpier at the other end.

Ah! Changing from linear to log, that's a good idea. The jumpiness at the other end might be an acceptable trade as I would guess few users have the reverb cranked.

You could also try changing the tank itself; lots of variation among the different choices available (even some between "identical" tanks).

Have you a favorite of the new production tanks? There's the MOD and Belton lines, each have their differences but neither is supposed to be a dead ringer for the original Accuntronics tanks. I have used a medium tank 8 series before (but that was with the Prosonic reverb circuit). It was more subtle. I like that blackface circuit

Thanks for the reply Firestorm. Always appreciate your comments/XP on 7ender threads.

[tubeswell]

Increasing the reverb pot size will increase the reverb level (not decrease it)

Changing the reverb mixer/voltage divider can reduce the reverb and boost the dry gain, depending on how you change the divider. (Reducing the 3M3 to (say) 2M2 will increase the dry gain - at the expense of the wet gain. Increasing the 470k to (say) 1M will increase the dry gain a bit but not reduce the wet gain as much.

[Colossal]

Increasing the reverb pot size will increase the reverb level (not decrease it).

Whoops! Thanks Tubeswell (went back and edited). I was originally thinking the other way round, lowering the pot to 50k or even 25kB since this seems to be where the usable range is.

Yes, I have seen that 3M3 resistor targeted for mods before. Allen Amps for instance uses 2M2 only (no 10pF cap) on the Sweet Spot amp. My overall goal however is just to make the pot usable so was hoping to avoid having to affect anything upstream or downstream of the reverb circuit. No free lunch...

[roknroll]

8AB2D1A Reverb Tank, Accutronics
AB'd the stock tank to these numerous times. The short tank always wins. I have installed these tanks in more than 25 different Black and Silverface Fenders for many customers. All agreed it was an upgrade in the reverb.

[Colossal]

AB'd the stock tank to these numerous times. The short tank always wins. I have installed these tanks in more than 25 different Black and Silverface Fenders for many customers. All agreed it was an upgrade in the reverb.

Roknroll,

Thanks for the tip man, that's great. Good to know you've had success. And n=25 is a good sample.

So to clarify, you left the blackface reverb circuit unchanged, changing only the reverb tank? What would you say was the sonic effect? Same lush reverb but more useful range out of the Reverb control?

8AB2D1A Reverb Tank, Accutronics

Accutronics/Korea, Belton/Korea, MOD versions?

[roknroll]

AB'd the stock tank to these numerous times. The short tank always wins. I have installed these tanks in more than 25 different Black and Silverface Fenders for many customers. All agreed it was an upgrade in the reverb.

Roknroll,

Thanks for the tip man, that's great. Good to know you've had success. And n=25 is a good sample.

So to clarify, you left the blackface reverb circuit unchanged, changing only the reverb tank? What would you say was the sonic effect? Same lush reverb but more useful range out of the Reverb control?

8AB2D1A Reverb Tank, Accutronics

Accutronics/Korea, Belton/Korea, MOD versions?

The reverb is not as swampy or overpowering. Better range of use from the stock Reverb control. Accutronics

[Colossal]

The reverb is not as swampy or overpowering. Better range of use from the stock Reverb control. Accutronics

Nice! Thanks Jack, very helpful, much appreciated.

Regards,
Dave

[Mark]

I wouldn't recommend changing the 3.3M resistor with 10pF in parallel. The 2.2M with a 20pF cap in parallel sounds like a Mesa Boogie Mark 1 amp.

I don't think it has the charm of the 3.3M//10pF combination. Mind you HAD uses a 2.2M resistor, so I'm in the minority on this one.

[Colossal]

I wouldn't recommend changing the 3.3M resistor with 10pF in parallel. The 2.2M with a 20pF cap in parallel sounds like a Mesa Boogie Mark 1 amp.

I don't think it has the charm of the 3.3M//10pF combination. Mind you HAD uses a 2.2M resistor, so I'm in the minority on this one.

Hi Mark,

Thanks for the tip. My goal was to leave the circuit alone as much as possible. I don't want to change the fundamental qualities of the reverb, just extend the usable range of the knob so that Surf mode kicks in at 7, not 3!

[Colossal]

An interesting addendum to this story: The amp I've been working on for a client has a classic Fender reverb circuit with what I thought was a 2-spring, 17" 4AB3C1B reverb tank. I removed the tank from the bag and found it was actually a 3-spring 17" 9AB2C1B! I replaced this with an 8AB2A1B (9", 3-spring, medium delay). The result, as Jack said, was reverb that is less deep and fuzzy. I did not find that changing the tank resulted in any increase of the usable range of the knob however. The sweet spot still is about 2-2.5 but the reverb is more clear and the client found it to be more usable for his purposes (tested at a gig last night). I personally love the sound of the larger tank but it had to be kept around 2-2.5 as well. I will compare the 3-spring 8A-series to a 4A 2-spring at some point.

I also found that with the 8A series tank (short tank) with the Reverb knob at about noon, the reverb is almost completely gated and metallic sounding. I think the next time the amp is on my bench, I am going to put a resistor across the 100kB Reverb pot to lower the value and see what that does for the range.

Thanks for all of the comments guys.

[Super_Reverb]

The reverb circuits I am looking at have the 12AX7 recovery stage driving a 100K pot with a 220K/470K divider feeding the third gain stage in the preamp.

This divider sends ~ 220/(220+470) = 0.32 of the recovered reverb signal to 3rd gain stage. If you replace the 220K with a 150K, then this drops to ~ 0.24. A 100K makes it ~ 0.18

You can change one resistor in the circuit and easily tailor the amount of wet signal mixed back into signal chain.

Changing the 220K resistor has a secondary effect in that you are changing the load placed on the 100K pot wiper, so you could increase the 470K, while decreasing the 220K to maintain the same load on pot (220K+470K). This would require you to change two resistors.


rob

[Colossal]

The reverb circuits I am looking at have the 12AX7 recovery stage driving a 100K pot with a 220K/470K divider feeding the third gain stage in the preamp.

This divider sends ~ 220/(220+470) = 0.32 of the recovered reverb signal to 3rd gain stage. If you replace the 220K with a 150K, then this drops to ~ 0.24. A 100K makes it ~ 0.18

You can change one resistor in the circuit and easily tailor the amount of wet signal mixed back into signal chain.

Changing the 220K resistor has a secondary effect in that you are changing the load placed on the 100K pot wiper, so you could increase the 470K, while decreasing the 220K to maintain the same load on pot (220K+470K). This would require you to change two resistors.


rob

Thanks a lot Rob, that's great. Of particular note is that if you compare the AB763 (Deluxe Reverb) and AA1164 (Princeton) schematics, both are operating at almost identical preamp and reverb circuit voltages but the Princeton lacks the 220k to ground. I failed to mention that for the sake of this discussion, I am working with a Princeton amp Regardless, the discussion is valuable and timely as I also have a AB763 in the works and plan to address this same issue there.

Dave

[groovtubin]

Try some LOCAL feedback, say anywhere from .01-.1 in series with say anywhere from a 22meg, down to a 3.3meg from plate to grid, say @ the reverb send tube, or the recovery, or both, experiment and see what works best in the amp, this came from some of HAD`s amps..