Deluxe Reverb II mods - for robrob and the brain trust

proaudioguy

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I am attempting to get the DRII to pretend its a DR at least on the clean channel.

I am working on the clean channel and the power amp.
I have changed the tone stack to match.
I have changed the PI to nearly match, only keeping the presence, but that may change.

I am looking for compression that does not exist in this amp currently. It sounds pretty good, but doesn’t FEEL right. I am well aware of how compression works as a sound engineer with over 40 years experience and 30 of them as a professional. There is attack time, release time, ratios, etc. I’m wondering what I can do to get as close as possible to those parameters as they would be in a DR, but in my amp.

The amp sounds and feels like a "solid state" blackface right now. The power supply is very stiff and the attack of the notes is very strong. Its actually more direct in your face than my Twin, which is missing the usual Tremolo circuit. It does have a decent OD if you crank it up but I’d like to get more of that at a lower volume if possible, but at least more compression.

I have not split the plates of the preamp tube (yet). What will this do to get me closer? Seems like removing the 47K, splitting the plates, and using a pair of 100K to each plate would further isolate the tone stacks from the 2 channels, if I am looking at is correctly. Other than that, I’m not sure. Would this change the tone further toward my goal? The time difference between the before and after will certainly be too long for a fair comparison by my ears, even though they are well trained. I will only be able to say I like it, or I don’t like it, but I’m interested in ya’lls input in exactly how this would affect the tone.

The filtering is quite stout as 120µF for the first filter. If I cut the 80µF out, that would get me closer (or close enough) to the 32 16 16 16 of a DR? It would be 40, 20, 20, 20 instead of 120, 20 20 20. What is the downside? Will the hum increase?

If I were to use a resister inline with the B+, how do I figure the value to approxamate a GZ34, and a 5U4GB (more sag I presume), the latter probably being what I’m looking for. Also where in the PSU does the resister go? After the bridge, before the first filter, or further down the line?

My only direct experience with a Deluxe Reverb was a 67BF I played in 2014 in Sam Ash (Manny’s) NY that I regretfully did not buy on the spot and have shipped home. I have been chasing that dragon for 25 years, and I had it in my hand and let it go. I have the DRII, and I almost like it. I also want to fiddle with the lead channel once I have the clean doing exactly what I want. I see a lot of potential versatility in the ablity to switch channels. The speaker I am using is my favorite and it sounds stellar in 2 other amps but they have much more power. The Concert is for sale just due to the fact, its a stellar stage amp, and I am unlikely to ever play anywhere I need 60Watts of beast.
 

sds1

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The power supply is very stiff and the attack of the notes is very strong...
The filtering is quite stout as 120µF for the first filter.
Well there you go. Anyone would feel that compared to a DR, more than the difference between diodes and the GZ34 even. Definitely start here, then try adding some rectifier sag if you don't think you've gone far enough.

If I cut the 80µF out, that would get me closer (or close enough) to the 32 16 16 16 of a DR?
Yes close enough.

It would be 40, 20, 20, 20 instead of 120, 20 20 20. What is the downside? Will the hum increase?
No downside, 40 20 20 20 is still plenty of filtering.
 

proaudioguy

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SDS1, This is such an easy thing to try, I may just clip the 80µF out when I get home. I only have a day off to do all the laundry and repack for the next gig.

Anyone else?
 

Snfoilhat

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If I were to use a resister inline with the B+, how do I figure the value to approxamate a GZ34, and a 5U4GB (more sag I presume), the latter probably being what I’m looking for. Also where in the PSU does the resister go? After the bridge, before the first filter, or further down the line?


The AC15C1 has a reputation for emulating tube power supply sag well.
Vox_AC15C1_rectifier_sag_resistors.png


This kind of rectifier has a little less ripple current between the power transformer secondary and the reservoir capacitor than the rectifier in the Deluxe Reverb II, so for the same load and the same amount of sag I suppose each resistor should be a little smaller than these 180R resistors, but this should get you into the ballpark.

Deluxe_Reverb_II_HT.png
 

Lowerleftcoast

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I have changed the PI to nearly match
If you did not *up* the value the .022uF coupling caps between the PI and the power tubes, that should help to get more girth.

printer2 s suggestion for 1k screen grid stoppers will help with compression.

Maybe it won't help enough to be worth the journey, but finding a choke with lower resistance helps bass frequencies.
 

proaudioguy

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Try 1k or greater screen resistors.
OK what wattage rating should those be? Not sure what the 470s are.

The AC15C1 has a reputation for emulating tube power supply sag well.
View attachment 1047889

This kind of rectifier has a little less ripple current between the power transformer secondary and the reservoir capacitor than the rectifier in the Deluxe Reverb II, so for the same load and the same amount of sag I suppose each resistor should be a little smaller than these 180R resistors, but this should get you into the ballpark.

View attachment 1047890
Ah, so the resisters go between the transformer and the rectifier? THANKS


If you did not *up* the value the .022uF coupling caps between the PI and the power tubes, that should help to get more girth.

printer2 s suggestion for 1k screen grid stoppers will help with compression.

Maybe it won't help enough to be worth the journey, but finding a choke with lower resistance helps bass frequencies.

Everything in the audio circuit from the PI forward is stock DR AB763 except for the Presence control. I did try to make it a variable negative feedback and I didn’t like it so I put it back. I honestly do not use the presence at all though. I have never liked that control on any amp I have ever owned that had it. I only left it becuase the silk screening is there, and some people do like it. Someday I will be gone and my kids will sell the amp and even if its not stock, I’d like it to be useable and sellable. Hopefully it will be better than new. The power supply I suspect is the real missing link here. The choke is the same part number that is sold for the DR.
 

Lowerleftcoast

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There are a couple schools of thought concerning the wattage rating for the (470R) screen grid resistors.
The older schematics typically show a 1W rating. One must understand these resistors were Carbon Comp in those days. CC have a strength over their modern counterparts. They could survive longer in over-current situations. Now that we are using modern resistors, we need to pay attention to the ratings of the type of resistor we choose. (This holds true for the resistors in the power section as well.)

A school of thought is these should have a low wattage rating so the resistor can act as a fuse. (It is very possible the designers had this in mind.) Acting as a fuse can possibly save the resistors upstream and even save the PT. Opening up an old amp, many times we find these CC resistors have failed. Did they act as a fuse and save the rest of the circuit???

Another school of thought is to never use a resistor as a fuse. We place a resistor rated for the job at hand. Since we usually have high voltage on these resistors, ~5W resistors fit this criteria.

So one group will say 2W to 3W and the other group will say 5W and up.

Oh and this should have a fire resistant type resistor.
 

printer2

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OK what wattage rating should those be? Not sure what the 470s are.


Ah, so the resisters go between the transformer and the rectifier? THANKS
2W

As they are in series the resistor will drop voltage whether it is before or after the rectifier. Before the capacitor though.
 

proaudioguy

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How do I figure out the actual voltage drop for a given resister in series with the B+ before the Caps? Lets say the tube rectifier would have dropped 40 volts, how do I figure out what resistor value to use to get the same voltage drop? Is there anything reactive or non-linear about the tube rectifier? I assume the resistor is linear. Is it also save to assume the full wave bridge puts out more voltage to start with than a tube rectifier, even if the tube had no losses, just due to the style of the rectifier? In that case, how many volts difference? Then how do I figure that into the resistance necessary in Ohms for the dropping resister?

ALso of note. I remembered the replacement cap can I used is 80 40 30 20, as they did not have what was original to the amp. I cut the 80 out and just tacked the wire in. First impressions, not much different, but seems bassier, which doesn’t really make sense to me. This is the problem with AB comparisons that are 20 minutes apart. I have a 40 20 20 20 in the cart. I don’t see a cap can with 5 16µF sections. However, would I be better off just putting a doghouse on it and using inline caps or is the 40 20 20 20 so close, I would never hear/ feel the difference. I suspect the latter to be the case and using a cap can from C-E is way way more convenient and less physical modifications to the amp.

Also, I just put this 80 40 30 20 in last year (2021) and the amp probably has 3-5 hours of time on it so if anyone has one of these, or the PRII, and needs a cap can, PM me.
 




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