designing a spring reverb

a fine line between stupid and clever

designing a spring reverb

Postby ChrisAlbertson » Sun Aug 02, 2009 3:30 pm

I want to add a spring reverb to small tube amp. The amp is very conventional with a couple 12AX7 based gain stages, tone stack, phase inverter then power tube. (Based very closely on the AX84 "High Octane")

Where is the best place to pick up the signal? Early after the first triode stage or later after the tone stack? Either way I think I need to mix it back with the "dry" signal at the same place.

Also, Is there any difference in the sound of a high vs. low impedance reverb tank? Some are a couple K ohms others are 8. If there is no difference I'll just get the one that matches one of the transformers I happen to have.
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Postby Gingertube » Sun Aug 02, 2009 10:44 pm

Chris,
Lots of ways to skin a cat.
Some reference material for you:
http://www.accutronicsreverb.com/index.htm

One of the easiest ways to add reverb to a small amp is to actually drive the reverb spring unit off the speaker feed (in parallel with the speaker) and then just add a reberb recovery stage (SS or tube) which couples back into the amp via a Reverb level potentiometer. Another quick and dirty trick here is that there will normally be a grid to 0V resistor after the volume control. Replace that resistor with a pot of the same value (wire to the ends of the pot) and then feed the output of the reverb recovery amplifier to the wiper of that pot. That prevents the reverb level control loading down the output of the volume control.
This would require a nominally 8 Ohm drive spring unit.

Of-course you can do a bit better with dedicated drive circuits and proper mixing stages etc. but that requires a fair amount of rework to your amp.
You can also just modify your amp to have line in/out sockets on the back and build a standalone reverb unit which means you can then do a no compromise design with the 19"ish long tank. I have done this on one of my amps. I found that in that case the 3 Spring Long Delay Accutronics Unit was best (I tried medium and long delays in 2 and 3 spring variants). I've never really liked any of the short (the 9 or 10 inches or so) reverb tanks.

Cheers.
Ian
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Postby ChrisAlbertson » Mon Aug 03, 2009 11:22 am

Gingertube wrote:Chris,
Lots of ways to skin a cat.
Some reference material for you:
http://www.accutronicsreverb.com/index.htm

One of the easiest ways to add reverb to a small amp is to actually drive the reverb spring unit off the speaker feed....



Thanks.

Maybe I could have been more clear. The amp is not yet built. I'm trying to think of the best way to add reverb to an existing design, not to an already built amp.

I'm building this amp head and speaker cab from scratch so I can make the box long enough to fit a "full size" reverb tank. Given that the box needs to be so large I have pleney of room for more 12AX or 12AU tubes for driving, recovery and mixing.

What I wondering is how reverb plays with overdrive. if the reverb needs to go before or after any preamp stage that might be overdriven.
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Postby jonnyeye » Tue Aug 04, 2009 12:43 pm

ChrisAlbertson wrote:What I wondering is how reverb plays with overdrive. if the reverb needs to go before or after any preamp stage that might be overdriven.

Needs to? Not really. But many people (myself included) think it sounds more natural - after all, if you hear an echo of a distorted guitar it would also be distorted. By this reasoning, the best place to take a signal for a reverb tank would be at the speaker outs, because you not only get preamp distortion but also the power amp distortion. The issue then becomes where do you mix this reverb signal back in - feeding it back into the amp will distort it further, which will sound less natural (but might sound good anyway!). Or you could build two small amps on one chassis and use one to amplify the reverb signal (this is referred to as foldback reverb, and it has been done commercially before - it just never really caught on).

The next place back in line to take the reverb from is the phase splitter. This is not as bad of an idea as it may seem - if your phase splitter is an AC long tail pair (like the Marshall type). See the attached schematic. The biggest advantage to this setup in my opinion is the increased isolation between the preamp and the reverb circuitry - they can't load each other down, and there's no need for mixing resistors. I don't know what phase inverter the AX84 uses, but I'd wager this would give great results.

Image

Of course, you could look at any Fender schematic and get some ideas, too - but the general consensus is that later (after tone stacks and distortion) is better.

As for what reverb driver schematic you should use... check out http://www.freewebs.com/valvewizard2/reverbdriver.html for some options you may not have considered. (Or don't, if you already know what you want. Sometimes too many choices are a bigger hurdle than help - and we all know and love the sound of a transformer driven reverb using a 6K6 or 12AT7, don't we?)
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Postby ChrisAlbertson » Sun Aug 30, 2009 2:04 pm

jonnyeye wrote:
ChrisAlbertson wrote:...Or you could build two small amps on one chassis and use one to amplify the reverb signal (this is referred to as foldback reverb, and it has been done commercially before - it just never really caught on)


Thanks. That is the way Hammond did it with their organs. The reverb had it's own speaker drivers. The dry sound went to it's own speaker (A pair of 12") and the wet sound went through an identical amp to it's own pair of speakers. (there was a bass amp too and two 15" speakers) All three amps and all six drivers where in the same cabinet. But these sold for two or three times as much as a new car sold for in the same year.

I don't plan to use much output tube distortion so it looks like the tap is going at the input to the power section via an "effects loop" type circuit.
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Postby Normster » Sun Aug 30, 2009 9:56 pm

The simplest reverb circuit I know of is the one Mesa used on their Mk I. Single tube and actually sounds pretty decent. This is also the circuit Gary Johnson used on his early Glaswerks amps.

Image

I typically pick up the signal just before it enters the PI and return it via a 220K voltage divider. If the amp doesn't use NFB, you can also return the reverb to the bottom side of the PI (like Matchless did). Here's an implementation I did on a Trainwrech Rocket.

Image
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Re: designing a spring reverb

Postby ChrisAlbertson » Thu Sep 03, 2009 10:41 am

Replying to my own question, after some research.

I found a schematic for the old Fender 6G15 reverb. Then looking around more found a scematic for the "re-issue" version of the 6G15. Fender changed a few things. They replace the 6K6GT tube with a 6V6GT and some other minor thing having to do with modern safety regulations. But the re-issue is very much like the old 6G15.

What's better about the re-issue is that there is a service manual with a very detailed schematic and a "theory of operation" where they describe what all the parts do and why. This higher level of documentation make building the re-issue easier

After studying many reverb schematics it looks like Fender is the only company who is taking Acutronic's (Acutronics is the company who makes the reverb tanks) advice and "driving the tank hard, nearly to saturation". Fender is actually using a 6V6 tube as a driver. Fender's tank driver looks just like a slightly modified Fender "champ" guitar amp.

OK there was one other company that used a higher power driver. Hammond. They typically used two EL84 tubes in push-pull to drive the tank. Back them Hammond has the company that made the tanks before that business was spun off to Acutronic.

Anyone who wants to build a stand alone reverb unit really should read the fender service manual which includes a schematic
http://www.schematicheaven.com/fenderam ... manual.pdf

If anyone does build one of these you'd want a user manual too
http://www.fender.com/support/manuals/p ... Reverb.pdf
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Postby Geek » Thu Sep 03, 2009 1:32 pm

Maybe my "test jig" can be of use? :)

http://geek.scorpiorising.ca/GeeK_ZonE/ ... pic=3551.0

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby jonnyeye » Fri Sep 04, 2009 1:08 pm

Driving the tank hard? Yes.
Nearly to saturation? No.

The accutronics website gives a nominal drive of 3.5 amp-turns and (for the 8/10 ohm tanks) 124 turns on the driver transducer. This corresponds to a nominal drive current of 28.2mA (RMS, of course).

At 50mVac input to the Fender 6G15 circuit, it shows 19.8Vac output. Into a 7K load (the nominal impedance of that transformer) this is 2.82mA on the primary, which gets transformed to 82mA on the secondary (minus losses). This is about a 9dB overload. Accutronics gives the overload margins for all of their tanks except the 8/10 ohm (natch!) but if the vertical scale on their graph is linear in dB, then we can guess it's only about 5dB.

Furthermore, guitar signals can be larger than 50mV - (I need to measure them, though. I used to just take 100mV as a typical struck string, but I'm not sure that's accurate, even if it comes from R.G. Keen.) If you use a stompbox to drive the reverb, you may have levels well over 1Vac going in, which can make things very muddy indeed. However, it does have a dwell control that allows you to scale back the gain if the whole thing starts sounding like a belch in a bandstand.

That all said, it's the right sound, isn't it? I don't think anyone who likes reverb doesn't like Fender reverb (and if they don't, they're wrong, anyway). If that's the sound you like, clone away. Just be aware that you may be slapping that tank silly.

And Geek, I've been looking at those 9DX-based tubes (6JT8, 6EB8/6GN8, 6LY8, etc.) for a while now. My idea was to use four of them as outputs for a Leslie simulator (just need a quadrature generator for the volume controls!), but this is cool too ;). Nice design. One note is that any transformer with a primary from 2K to 4K should work, which makes it easy to build from a junkbox. To use a 6EB8/6GN8 (which are common as dirt, it seems - that's the one I have, anyway) change the screen resistor to 4.7k 1W and the cathode resistor to 68 ohms.
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Postby ChrisAlbertson » Tue Sep 08, 2009 3:49 pm

jonnyeye wrote:Driving the tank hard? Yes.
Nearly to saturation? No.


I was just reading this from the Accutronics web site:

1....The driver, at maximum expected input level, should drive the coil to near saturation. This is of utmost importance in applications where a speaker is mounted in the same enclosure as the reverb, such as a guitar amp or organ.


I think your calculations are right. Fender does go well past saturation and leaves it the user to turn the dwell down. If you look at the newer "65 re-issue" reverb from Fender. The reissue hits the reverb tank even harder. Fender has gone to a 6V6 driver tube now. I think people like the overdriven sound.
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Normster!

Postby mcleodjoe » Fri Jan 22, 2010 8:55 am

That layout diagram you supplied above is by far the best I have ever seen! I can see the glow off those orange drops. I might hook some flying leads up to that board and try firing it up! How did you do that? What application did you use? Amazing...
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Re: Normster!

Postby MashBill » Fri Jan 22, 2010 7:07 pm

mcleodjoe wrote:That layout diagram you supplied above is by far the best I have ever seen! I can see the glow off those orange drops. I might hook some flying leads up to that board and try firing it up! How did you do that? What application did you use? Amazing...


I was going to ask the same thing! That is the best graphic quality I have ever seen!
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Postby Normster » Sat Jan 23, 2010 1:05 pm

I use Visio 2003 for layouts. Occasionally I get really bored and spend way too much time creating shapes. ;)

If you have Visio, you're welcome to use the shape library:
http://thenormster.com/Jelle/
Grab the Tube Amp Components.vss file
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Postby nyazzip » Sat Jan 23, 2010 3:41 pm

i have a reissue Fender reverb unit, and it is the most useful thing i have aside from my guitar, my car, and maybe my bed. i will probably try to build a copy because as you say it is well documented, and then i'll sell the Fender for too much money
but...if you like to play with a lot of preamp distortion, the reverb really needs to be inserted after the preamp or it sounds pretty bad.
i do have an old Traynor head that uses a longer tank than the Fender, and an el84 as a driver, and it sounds even better to my ears. maybe someday if i live long enough i'll be able to figure out how to build a stand-alone reverb box using the Traynor design...
by the way there are plans and kits that convert Epiphone Valve Junior heads into reverb units, which i also may try. $60 isn't too bad for a power transformer, SE OPT, a 12ax7, an el84, a chassis, a beautiful box, and a bunch of jacks and capacitors...
<i>the poor craftsman blames his tools</i>
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