Tube Reverb, Part 2: Transducers, Tanks, Tone, and Signal Mixing 

As well, the solid wire supporting the transducer is a non-ferrous (Brass or Bronze) wire.. The magnetic part of the transducer has a small hole that the Brass wire supports the magnet.. the magnet being glued to the brass wire.. (Better seen than described).. The Reverb Tanks being supplied by a Company called Acutronics?? in Chicago.. Originally supplied Reverb Tanks to Hammond Organ Company.. I believe they are still in Business..

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I agree, James. He definitely ran the show. You're welcome.

Thanks so much, Sim :)

You're welcome, SL :)

Thanks for your very kind words, Stephen.

It does make a nice combo, SS. Thanks :)

He is one stealthy little rascal, DL :)

Thanks so much, Jorge. Best of luck with your project.

Thanks for joining us, Barry. We enjoy your company :)

We've got all the bases covered, FW ;)

We will, Ivan :)

You're welcome, CB. Thanks !!

You're welcome, JJ :)

My pleasure, Nick.

You're welcome, Colin. I'm glad to hear that the info was helpful. Thanks for your nice comments :)

You're welcome, Pedro....Thanks !!

Thanks so much, Peder. We will :)

Thanks, Rob :)

Thanks for your nice comments, Jorma. As you discovered, by carefully analyzing the circuit, all questions can be answered.

Glad you enjoy the videos, M.

I have found that if you can find a nearly completed car/truck for a reasonable price, and then complete it without spending too much......you actually can make a few dollars, but it's both a challenge and a gamble.

Thanks, Andrew. Yes, the crazy old guy has been all over the country in the #8 roadster.....and I give him major props for it.

Yes, he was such a character......I really miss him, particularly when making videos. Jack has a very interesting personality and is doing his best.

Thanks, Frank. It was Jack's idea :)

You're welcome, Sluggo. Yeah, I'm not a fan of Conestoga wheels with O-ring tires.....just like all the silly fads that come and go, I can't wait for it to fade away into obscurity. Give me some slotted 15" Ansen's or five-spoke American's with fat tires and I'm a happy guy:)

I have to say I agree with both of you about what looks good on cars & trucks. I think that the wagon wheels with O-rings stretched to the point of breaking looks extremely disproportional/disproportionate to the vehicle. Like Slugg0matic, I'm showing my age too. I wonder how many of those wagon wheels have to be replaced early because they ran over a rock or similar? My CJ-2A looks like it still has its original wheels after 72 years of not damaging the wheels by running over rocks! I too feel that Jack is doing a good job with Rusty watching him with pride.

It's never too late to learn, Russell. I was around 60 years old when I started studying tube amplification.

I agree completely, 50G, and if you met and talked with the old guy who built it, you would be even more convinced. It exemplifies the old "build it with what you got" approach, and then he actually drives it.......everywhere.

Thanks, Mark. I believe the wave transmission of energy in the springs is best defined as being longitudinal rather than transverse. Your old truck sounds wonderful, especially if you were terrorizing (fairly nearby) Van Horn, Texas :) I have a pair of identical (but 1966) heads currently in my store room, completely overhauled and waiting for installation on a worthy 327 short block.....if I can find one.

That's a great idea, T. Think of all the mistakes we could avoid that way :)

Yep :) You're welcome, KB.

You will see it in detail in Part 3, Tom. I incorporated a sere locking mechanism to keep it in place, rather than the standard, rather crude method employed by Fender.

I hope it's worth the wait, BB :)

Absolutely worth it as always. Rusty is certainly looking down with pride. We had to put our St. Bernard down one year ago. Ufffffda....that is tough stuff. Looking forward to Part 3.

Thanks, BB. Yes, it's just like losing a (close) family member.

Uncle Doug Yes, absolutely.

You're welcome, Ice :) We'll keep trying t make it better.

Thanks so much, Stephen. I'm glad the videos are helpful. Before you start building your effects unit, check out the ReVibe circuit developed by Gerald Weber.

I will for sure! Btw... I've looked for the chart you use to identify the reverb tank, but cant seem to find the same one. would you have a link? or a search string I can use. Thanks again. :)

How about this one: www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared

Thank you sir. I had seen that page actually, I just didn't scroll down far enough to see the chart. Thanks again, much appreciated. :)

Thanks, JJ.

You're welcome, MB. I guess cats are all alike :)

Thanks so much, PW.

The reverb tank itself is better to buy than build, K. The electronics are shown in schematics for the Fender 6G15.

@@UncleDoug thank you again!

@@UncleDoug what model oscilloscope are you using

Thanks so much, Al. I think the slight "dephasing" of the reverb signal is what provides the echo effect when it is mixed with the dry signal.

Greetings, Richard. I have to be careful with movies like that. I watched "My Dog, Skip" a few years ago.....and still haven't gotten over it. Re used reverb tank units, I think they're much like any used electronic device; you take your chances, relying on your own intuition and on-site evaluation of the condition. The tanks themselves are generally the first parts to fail, and they are inexpensive and easy to replace. As far as the headers go, I doubt that any math, other than the overall length of each pipe being equal, would matter. You start out, usually, with four equal-length pieces of pipe (per side), a tubing bender, and a TIG welder.......and the rest is Voo Doo :)

You're right, DK. I've heard good things about them, and you can pick and choose the parts you want.....chassis, components, transformers, tubes, cabinet, etc. I'll try to remember to mention this in Part 3.

Their kit has a 3 spring belton tank included with it, and my goodness does it produce a spacious sounding reverb.

You're welcome, David. Glad you enjoyed it :) We haven't had any measurable rain in over three months. It's lousy for plants and trees.....but great for preserving old cars and trucks.

You're welcome, AQ, and thanks for the input on the transducers. I feel fortunate to have the assistance of experts in the field to help out from time to time. I will keep your suggestion in mind for future videos.

Yes, the 6G15 is the only amp I know of that utilizes the front panel mount.....and it does have a glorious reverb tone. I designed a special damping mechanism for my clone and will demonstrate it in Part 3. I was a gunsmith for several years and borrowed the idea of a sere to lock the damper in place. Rusty has been gone for quite a while, but I still mourn his loss......especially with regard to our RU-vid channel. I always felt that he made our videos special.

Fender used a backwards "J" shaped bar to lock the pan in place. Instead of a sear, Fender used a pair of holes fitted with screws, and one of the holes had a second slot that acted to lock the bar firmly against the reverb pan. Rusty was a good and loyal friend... he indeed made your videos special.

Thanks, Bill.

Thanks so much, Steven. I like your analysis and find that your conclusions seem quite reasonable.

You're welcome, John. I hope so too. The purpose of the videos is to provide the knowledge to understand, and hopefully be able to repair, amp circuits.

A very logical observation, J1. Thanks !!!

De nada, Jorge. Foi um prazer.

Good observation, Jeff. I wasn't sure if anyone would make the connection :)

Thanks, Glen. Yes, the V3b triode is a cathode follower. No, I don't think that rectifying the 6.3VAC filament supply is necessary. You can use an LED bulb if you wish, but the filament supply has no problem powering an incandescent pilot light.

Thanks, James. Another viewer came up with a similar idea: The cathode follower, due to its high impedance and lack of gain serves to couple the guitar input to the dry output, so that when the "wet" mix is off, the guitar signal goes straight to the amp as if the reverb unit was not even present.

Thanks so much for your input, Roiderien. Having just analyzed the wet and dry circuits with an oscilloscope, I can vouch for the unity gain of the cathode follower. The dry output signal is absolutely identical to the input signal......so you are most likely correct as to the purpose of the cathode follower. With regard to the resistor suggestion, every amp I have ever seen has a grid leak resistor for every pre-amp tube, particularly the first one.......so wouldn't that solve the problem?

Yes, it would solve the problem except if the reverb unit was turned on before being plugged into the amp, and a charge built up on the output. OTOH, an amp in which the input jack was shorted when not plugged in should fix it as well.

Glad you liked it, Jeff.

Any tank from a SS circuit would require SS drive, Marty.

You're right, Mr. H. The value of the cathode follower was not related to phase. This was discussed in some detail in previous comments. Thanks :)

Thanks for the input, Jerome

Amen, Dave.

They call that "bus steering" due to its resemblance to the old horizontal-steering-wheel busses. Thanks for your nice comment regarding Rusty :)

I can't help but thinking of where that vertical steering column would end up in case of an accident--very scary!

Thanks so much, YT :) I really appreciate it.

@@UncleDoug i really appreciate the knowledge your sharing with the world. There isnt many people left who are educated in the ways of tube technology and most of them including your self are older people, who in turn dont really use social media or make videos, especially to the extent and depth you go in to. I remember a year ago when i was just getting interested in tubes and you stopped posting. Honestly i feared the worst in that we would never get another video from you again! Im glad life is treating such a wise man well :)

You are most kind, YT. Thanks for your very nice comments. After Rusty passed away, I took a sabbatical from YT, but returned rested and ready for more videos :)

I'm not sure why you want this modification, HH. Reverb tanks do work best with higher frequencies, but the elimination of low frequencies is generally built into the circuit. If you still want additional filtering, perhaps you might try applying the high pass filter before the 12AX7 that drives the tank.

Uncle Doug I guess I’ve just always found my Accutronics tank a little muddy relative to my clean signal. Hoping for more clarity. Thank you for the guidance.

@@rootvalue I prefer the MOD tanks, HH. They have a more "vintage" sound. Good luck.

We'll find out in Part 3, Jeff. I intend to evaluate the effect of each control.

I've tried it both ways, Dennis, and tend to favor the rivets. Let's face it, how often do you need to replace a tube socket?......and even then, rivets are probably easier to drill out than it is to remove fiddly little nuts and screws.

You should build it and see, Malcolm :)

That's an interesting suggestion, Sean. Instead of leaking treble to ground, leak bass frequencies instead. It would make more sense.

I have never heard of that use for an EL-84, Dennis, but can't imagine why it wouldn't work. If you try it, let us know how it turns out.

A grounded partition will block EM fields produced by the power supply from interacting with the very sensitive signal circuit in the pre-amp section, Ed.

@@UncleDoug OK, I understand, I tried this in a SS practice 5W amp Im building (no theory, hehe just experimenting) and it worked to some point, I believe many other factors contribute to noise. Just a square metal barrier connected to chassis one point ground.... thanks again

Perhaps it's a grid blocker, Chris.

You're welcome, Z. I too am wondering about the delaying effect of the reverb tank on the signal phasing......perhaps the wet and dry signals simply blend together regardless of phase (?)

The springs are more of a "continuation",if you will accept the term, than a delay,of the original signal. I think the charm we love about it is the utter mish-mash of harmonic overtones ringing on. There is certainly in and out of phase sounds happening within that sound, but definitely having opposing phase wet and dry would give the most dramatic cancellation of the primary signal.

I just completed an oscilloscope analysis of the wet and dry signals for Part 3. There appears to be no delaying effect of the wet signal by the tank and both signals arrive at the mixing control in near perfect phase alignment.

I actually prefer a slightly-higher-output 6V6 to drive the tank, Foo. My guess is that in the AB763, you have two stages of amplification prior to the 12AT7, while in the 6G15, you have none. Perhaps the higher output tube compensates for this lack of pre-amplification.

Yes, indeed. Thanks, Craig. You can find the chart on several different sites, but the one you listed is identical to the one I used.

I don't blame you, Eugene. Just remember that "input" to the tank comes from the "output" of the amp, and vice-versa. Also, nothing much will happen if you reverse them......just no reverb......so you can test it and correct if necessary.

All that's missing is a fuse connected to the tank, so it can be lit in case the rear-end collision wasn't sufficient to ignite it ;)

It would be fine as long as you obtain the proper tank. They are constructed differently for different mounting positions.

Watch my video on how to cover amp cabinets with Tolex or vinyl, LHR.

I have never compared the 2 and 3-spring tanks side-by-side, Joseph. Apparently, the tanks with 3 long springs have the longest dwell. Original Fender tanks generally had 2 long springs......so this is probably the most "original" sounding reverb. It would be interesting if you could rig a way to toggle between them.....and maybe have them both operate at once, for some truly massive reverb.

P.S.All your fans ,believe me miss old Rusty.What a pal.👍

Thanks for your input and kind words for Rusty, Bob. We appreciate them.

P.S.after review it is a 6-71.my mistake

I believe the 3-spring tanks provide longer duration of the reverb effect, D, however, unless you intend to do a lot of surf tunes, the 2-spring tanks are just fine.....particularly the long ones.

Would I damage a tube amp by plugging in a reverb tank instead of a speaker?

I see your tank here has an output impedance of 2250 ohm. If I used a tube amp with 8ohm output into your tank, could I then plug the output into a mic level input? E.g. run a line level into a reamp box, into a guitar amp with a 8ohm tank in the speaker output, then plug the tank output into a mic pre?

Lots of questions, Brendan......most of which I have no answer for. I believe the output from your Bassman would quickly fry the tank transducer, which was never meant to cope with 100W of power. My question for you would be: why would you want to do any of this? If you want reverb for your Bassman, buy a pedal or build a 6G15 Reverb unit.

@@UncleDoug hi Doug, the reason for this would be to have a spring reverb effect for music production/dub mixing. I have made a simple unit with an ART Tube MP powering a 600 ohm tank and then I run the output directly into a mic input which works but I think it could be improved. Your answer doesn't surprise me but I had to ask since you were talking about replacing tanks with speakers in the video. I'd really like to try to make your 6G15 one day to replace the crude set up I have here. What I have made was a lot cheaper than buying a unit by Vermona or other manufacturer. The dub mixing artists of yesteryear used the Fisher Space Expander and Roland Space Echo units. I have since bought a Roland SRE-555 but keep my homemade spring setup as another effect. Thanks so much for taking the time to reply. I am learning a lot from you since discovering your channel and I enjoy the snippets of humour and home life you cut into the videos. I have plenty left to watch on the channel and I appreciate you taking the time to share your knowledge and experiences with the world. It's a fantastic resource.

No. It would require significant alteration of the circuit, additional tubes, a reverb tank, increase of current supply by a larger power transformer, and in most cases, a transformer to drive the tank.

Thank you, Jennifer.

Uncle Doug Enjoying Jack’s cat scans these days, thanks for including your 4 legged family in the vids.

You're welcome, Jennifer. Jack has his own quirky personality and I think he adds an interesting dimension to the videos.

Uncle Doug As do the cars also... Really enjoy your humor and the side interest content too!

Thanks so much, Jennifer :)

There seems to be no impedance problems at all, Dennis. I have used the 6G15 with all sorts of amps, from a Champ to a 70W National......and it works perfectly. Apparently, the voltage of the mixed reverb output is not much greater than that of guitar pickups, so it suits the input of virtually all amps.

I didn't think there would be any problem with any amplifier. More like how it's done in general, perhaps at this particular one. But never mind. I'll dig through that!

Dennis, the reverb amp output delivers voltage not current. The main PA amp will be looking for voltage and not current. There can be no matching problems with voltage (except for levels) only with current. Hence the matching transformer to drive the current input of the reverb unit, but a voltage coil unmatched output direct to the next tube grid which is voltage driven

I agree that the 12AX7 triode serves to amplify the wet signal, but also inverts its phase since the output is from the plate. The cathode follower in the dry signal, however, does not invert the signal, since it already matches the phase of the wet signal.

I don't know the usual output of a reverb tank, but if it's in the same range of a microphone output, it will definitly need some amplification to reach the same level of a guitar pickup output

The reverb tank is a passive device, with output strength based entirely on the strength of the input signal minus losses within the tank. Apparently, the tank output in this circuit still requires additional amplification (by the 12AX7 triode) to properly drive the input stage of the amplifier.

Good point, Clint. This has been addressed elsewhere in various comments. I sincerely appreciate your input and assistance.

Thanks for your patronage, Hand. Diodes are cheap. If you can triple the current capability of your power supply for free, it seemed silly not to do so. Here is a discussion of the purpose for C14 in my schematic: electronics.stackexchange.com/questions/204073/what-is-the-thought-process-for-specifying-an-emi-bypass-cap-on-line-transformer

One follow up question (please don't take away my press credentials): Why reduce the power supply filter caps to 22uf? It seems like the idea here is to make this device as hum free as possible and reduce every source of noise. Isn't this taking us in the wrong direction?

Both Fender and I agree that three 40uf filter caps is excessive for this circuit, especially with a full-wave-bridge-rectifier, as evidenced by their alteration of the circuit to two 20uf and one 47uf filter cap in their reissue. This is the same rectification/filter configuration used in my version of the circuit. Excessive capacitance merely strains the rectifier tube without providing any additional hum reduction.

Thanks for your input, UV, but the lower triode is wired as a cathode follower, and the two output tubes are in phase.....so they can't function in that manner.

Doug is correct This is push-push not push-pull.

Greetings, Lux. Repairing a damaged transducer is a tricky process, and one that cannot be explained in words.....other than: find the two ends of the broken wire and reconnect them.....which is not very helpful. New reverb tanks are cheap, around $20, and replacing yours would be the best idea. As far as somehow merging the reverb circuit into the amplification circuit for additional gain, this is waaaaay beyond a verbal explanation and possibly not practical or feasible. Replace you tank.

Thanks so much, Andrew. Your appreciation for vintage jukeboxes and music may even exceed my own.....which isn't easy to do. Jukeboxes (and their music) were my obsession for about 12 years, during which I avidly restored, sold, and collected them. To me, they are, along with 1957 Chevy's and diner decor, a quintessential part of Americana.

Not really feasible, HP......it would require several different reverb tanks, and the difference would probably not be worth the effort.

They are uncommon, Chris.

I'll hunt down some old photos and post them on my Facebook page: Uncle Doug's Vintage Amps.

You can try. I have never done this, so I can't really speak from experience, but darned near everything is possible if you try hard enough. If you find that you can't do it, replacement tanks are readily available and are rather inexpensive.

@@UncleDoug okay I appreciate this thanks I think I'm going to give it a shot

@@rodneylarue9512 Good luck, Rod. You have nothing to lose by trying......and everything to gain, including experience.

In theory such a substitution would be feasible, but the only way to really find out is to try it and see.

@@UncleDoug yea I've seen a few other people talk about replacing theirs in their Monoprice 15w tube amp so I went ahead and ordered one and got it installed. Unfortunately, at the levels I practice at, I can't tell one bit of difference. It's only on the 1w setting and usually less than 5 on the gain & vol knobs so that might be why. I really need to crank it up and record them both somehow so I can get a good comparison. Luckily I've still got my Fender Mustang II combo and it does hellacious reverb even at low volume levels so I can supplement the 15w amp with a dual out pedal and the Mustang if I need to. It actually matches the sound of the Monoprice quite well at a few of the stock presets. I wanted to order a long tank but it won't fit inside and I wasn't sure if I could mount it outside the cab like some of those spring reverb pedals that have the external spring tank connected to the pedal in an enclosure and not in the amp. If so, I may order one and try it out. I also stuck with medium decay. Wondering if long decay may have given me the lush sound I'm looking for. Now if Amazon would hurry up and send that Allman Brothers 'Eat A Peach' poster I just ordered for my Dad. It's the center mural from the album with the mushrooms and fairies and all. 24x36 inches. I can't wait! If it looks really good, I plan on buying one for myself, as well, to go on the wall between my guitar hangers. It'll finish that wall PERFECTLY. Been looking for the perfect Allman Brothers poster for that spot. I think this'll do it. Either this or 1 of 2 album covers. The Anthology, of Duane fishing, or the Fillmore East cover shot. My favorite album of all-time.

The identification code involves several letters and numbers, Thomas, and will fully describe the characteristics of the tank. I have no idea what the "L" would mean by itself.

@@UncleDoug I was referring to the schematic and it labels it 'Accutronics Type "4L"'. Also says the DC resistance of input and output coils is approx 180 ohms (hard to read could be 150 or 160) which doesn't really match up to the chart. Ill just have to pull the back and get in there and find a part number. The amp is very fender inspired. All plate coupled. But Ive always wondered about the reverb as it sounds different. Ill let you know what the actual code is. Thanks for all your videos.

@@UncleDoug The code is very faint. Its 4FBxx1B. I can no longer read the 4th and 5th characters.

@@cigarettesmokingman9471 You could go to the Amplified Parts site and look up their translation table. It is full of helpful information. www.amplifiedparts.com/tech-articles/accutronics-products-and-specifications

@@cigarettesmokingman9471 See my other reply for a helpful site.

Thanks, Andras. I have never dealt with one, but will keep an eye out in the future.

Thanks for sharing your very amusing recollections, RD. I can just picture the scene in that recording studio :):) By coincidence, I have just lately become interested in plate reverb, and found this site: theproaudiofiles.com/plate-reverb/ which offers a very interesting comparison between plate and spring reverb. Plate seems to be almost pure echo, while spring sounds just like what it is......twangy, springy, and not nearly as clean as the plate, especially with vocals. I am going to continue reading up on the subject and may surprise you all in the future. Thanks again.

www.triodeel.com/studio.htm look at emt schems here UD. Yes I can think of no better man for the job of building a classic all tube Plate reverb. You need equal parts skill in welding/steel fabrication/Tube amps/electronics. Look forward to your version of a classic all tube plate reverb. The sound is incredible from a high quality plate reverb...otherworldly when you hear it in a studio. It's part of what used defined a studio's sound. Capitol records had one the best..why those old sinatra recordings sound so great. Mostly used on vocals but can be used on snare drum or whatever you like. In the studio they blend in as much/or little as required with the original track via send and return on mixers.

Very interesting, RD. Thanks again :)

You must admit that the phase reversals all add up, Craig.....but I'm anxious to hear what you come up with. It's an interesting question.

Here's my thought, without the dry cathode follower, the mixer pot and the wet's tone control will lower the circuit's overall input impedance, loading the guitar pickup. With the follower, the dry circuit loads the pickup with a high 2.5Mohm/2 resistance.

Andres, Exactly, you beat me to it, cuz my dogs aren't as fast as they used to be. Or is it me that's slower. Probably me.

An interesting theory, Craig, but since the guitar input is to the grid of V3b, why would it matter if the output was from the tube's plate or cathode? Wouldn't the input impedance of the dry circuit still be determined by the two parallel 2.2Meg resistors?

If the cathode follower was not there, say it was just a straight wire, you would have the reverb signal being sent back to the input stage of the unit. Probably not what you want. The follower is mostly a one-way signal valve.

You're welcome, Carl. I guess it's just us "old timers" who prefer 15" wheels with fat tires. The youngsters seem to favor the Conestoga wagon look.....but then, they also seem to prefer 4-day beard stubble, suits that are 3 sizes too small, and trips to the beauty parlor for facials and manicures..... ;)

Thanks for the info, Daniel.