Deluxe Reverb II Mods and REVERB circuit question

[proaudioguy]

https://www.dropbox.com/scl/fi/zxs6uyi9 ... g0z5e&dl=0


EDIT: Updated the schematic to 18K tail resister which is what it needs to be with this presence/ NFB circuit. Prescence now works right and the NFB mod also works. FYI, I used the ground switch to engage the 8µF NFB bypass mod.

Love the way this sounds with these mods. Night and day difference to me.

Mods include:
switchable bright at the DR value of 47pF. Not show is the 10 meg is still across the bright switch, but the rest is acurate. Not surprisingly, I leave it off mostly.
Pi nearly completely reqired to DR AB763
When I recapped it, I used an 80 40 30 20 can. The 80 is disconnected.
Changed first dropping resister to 10K.

I have a question about the Presence/ NFB control. If I replace the 100K in the NFB circuit with 820 and switch the NFB switch on, and have the 25K pot fully engaged, that gives me stock NFB? Does the presence woek differently when the 100K is swapped to the DR stock 820?

What would be the affect of getting rid of the 47K after the Volume pot? I wasn’t sure if that 47K needed to be there for the vactrol, but if its cutting level, then I’m losing gain compared to a stock DR which is 100% what I’m going for on the clean channel. If it doesn’t need to be there, I’d like to ditch it. If it isn’t really taking any gain (1/2dB or less), then its fine where it is.

[martin manning]

I have a question about the Presence/ NFB control. If I replace the 100K in the NFB circuit with 820 and switch the NFB switch on, and have the 25K pot fully engaged, that gives me stock NFB? Does the presence woek differently when the 100K is swapped to the DR stock 820?

The feedback resistor (your 100k) and the PI tail make a voltage divider. AB763 has 820 and 100, 10.8%, and DRII has 100k and the parallel combination of the 4k7 and the 25k pot, 3.8%. AB763 has 2.8x the speaker voltage feedback that the DRII has. That is tempered by the increase in loop gain that the DRII's 7025 brings wrt the AB763's 12AT7, say 20%, reducing the AB763 to ~2.4x relative to the PI input. If you want to go up to something like the BF NFB, reduce the 100k to 100k/2.4 = 42k, or put a 75k across the 100k. The presence effect will be essentially unchanged as long as you keep the PI tail resistance and the presence cap the same.

[proaudioguy]

Thank you for the response! I just logged in here to tell you all that I made an error with the 22K tail resister, which needed to be 18K to go with the 4.7K. When I changed the PI to BF standard, I just made all of it BF including the 820, but then I decided I wanted the presence control to work since that’s how it is labelled so I put the 100K back and re-attached, but I did not take into account the 22K being wrong for that circuit with 4.7K instead of 47Ohm. I happened to be looking at the ROB ROB pages as I often do when I am looking to post an answer to someone else’s question and stumbled on that entire section on making presence also variable NFB. The circuit here is identical to what he shows as an example, so that was fortunate. I changed from 22K to 18K, and the presence now works as designed! When presence is all the way down and the NFB bypass is NOT engaged, it sounds like a normal BF deluxe reverb with a GZ34 and the voltages are very close to that now. I have a BF’d Silverface in the room here with it that is a little different because I’m dropping more voltage with a 5U4. When the NFB bypass is engaged it tightens up the amp a bit, and then as I increase the control it gets nastier in a very raw way. In any case, YOU were right on the money. ALso, I have a 12AT7 in the PI. I will update the schematic now. Thanks for reminding me!

[pdf64]

The 47k resistors aren’t dropping any gain.

[proaudioguy]

The 47k resistors aren’t dropping any gain.

That is part of the original circuit. I changed that to 100k. Thanks for pointing out that I forgot to fix that. I’ll make the adjustmnt on the schematic. - ok, fixed. https://www.dropbox.com/scl/fi/zxs6uyi9 ... g0z5e&dl=0

[proaudioguy]

Could someone tell me how to calculate the frequency response of the reverb driver? It has a 2200µF cathod resister with a 25/70 (what I had on hand, but should be a 25/25) in parallel. It has a 220K resister in series with a 2200pF cap going to the grid. Perhaps I should change that to the DR stock of 500pF and no 220K (looking for the DR sound)? It also has a 560pF snubber.

Then it has a bright cap .0022µF on the return jack. The recovering tube has 1500 instead of 820 Ohm so I guess that means the 100K reverb knob needs to be at a higher setting to get the same level. It seems strange to me they are killing highs going to the reverb tank then trying to accentuate them on the recovery. I would expect the opposite if I was trying to reduce noise.

Any thoughts? Should I remove the 220K, change the 2200pF to 500pF, get rid of the snubber and get rid of the bright cap? If yes, should I also change the 1500Ohm to 820 Ohm? The tone of the clean is so close to my Deluxe Reverb at this point, I’m just wondering if this will get it another 3%. I left the snubbers on my DR ont he 6v6s when I blackfaced it. Its for all intents and purposes an AB868. My point is, I see those snubbers as an improvement so I left them. If these changes to the reverb in the DRII are actually an improvement, or if SOME of them are an improvement, I’m OK with that, but if they were simply trying to deal with other things they messed up (which I “fixed” with my mods that revert back to the old days), then I’d like to change that stuff back.

Additionally, does anyone have a suggestion on how to REDUCE the reverb level when using the lead channel? I’d love it to drop 3-6dB when switched, but I don’t want to tear the entire circuit apart to get it there.

Back to question one, how do I calculate the frequency response of the reverb driver? What about the return? I’d like to do the calculations for the original AB763 and also for the DRII, and compare.

[martin manning]

Should I remove the 220K, change the 2200pF to 500pF, get rid of the snubber and get rid of the bright cap? If yes, should I also change the 1500Ohm to 820 Ohm?

I would try those one at a time, except leave the 1k5 cathode resistor on the recovery stage. The value has been doubled since it is no longer shared with the mixer stage, and it's still fully bypassed by the 25u.

[proaudioguy]

Should I remove the 220K, change the 2200pF to 500pF, get rid of the snubber and get rid of the bright cap? If yes, should I also change the 1500Ohm to 820 Ohm?

I would try those one at a time, except leave the 1k5 cathode resistor on the recovery stage. The value has been doubled since it is no longer shared with the mixer stage, and it's still fully bypassed by the 25u.

Good catch!