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Old 22nd June 2011, 06:21 AM   #1
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Default Passive or Active? For Wet/Dry mix on valve driven reverb.

Hi, I am new to this site and an electronics novice, however I am having a crack at designing a Valve Driven Spring reverb for guitar, using 12AU7/ECC82 tubes and would like to put in a mono wet/dry mix control knob. I have been very careful to keep the signal path as simple as possible, keeping within the Audiophile school of thought.

The questions are; Should it be passive or active? Why? Should it go into the beginning or the end of the circuit?

If anyone has a design for such devices that would be much appreciated.

I have included a pdf of the reverb design as it stands so far. note both tubes are 12AU7, bias values have not been worked out yet and the circuit itself has not been tested yet.

Cheers.
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File Type: pdf spring reverb.pdf (16.6 KB, 66 views)
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Old 22nd June 2011, 12:38 PM   #2
SY is offline SY  United States
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With that pot on the input, the circuit is likely to be pretty noisy. If you stack the input tube like that, you'll need to bias up the heater supply so as not to abuse the heater-cathode voltage rating. With the output coupling cap, you can direct couple the second voltage amp to the cathode follower, eliminating C8.

If it were me, I'd mix actively following the reverb. It will allow everything to be done more cleanly.
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Old 22nd June 2011, 01:57 PM   #3
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Check out this thread. Connecting a tube reverb to an AO 28 There are links to schematics for a couple of aftermarket tube reverbs for Hammond organs in this.
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Old 23rd June 2011, 01:39 AM   #4
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by magnetman View Post
Hi, I am new to this site and an electronics novice, however I am having a crack at designing a Valve Driven Spring reverb for guitar,
The "classic" is the Fender 6G15 reverb unit. There are also many examples of reverb built into amps. Some things that are different between your design and what "everyone" does

1) Every tube driven reverb I've seen uses a transformer to match the tube's output impedance to that of the reverb pan. If you really want to eliminate the transformer then use the pan with the highest impedance you can find. The goal is a very high signal to noise going in, this means maximum power transfer

2) You really need to filter the input and NOT send any bass to the reverb. Reason is simply that it sounds bad and also quickly overdrives the pan. Reverb pan makers have a recommendation for the filter. 6 db per octave is about right

3) The typical tube reverb uses a parallel triode 12au7. Use both sections in parallel to drive a single ended transformer then then drives the pan. Any less then that and S/N is not great. Your driver is very weak. The Finder 6G15 uses a 6K6 and transformer the Fender re-issue uses 6V6 tube. Basically you need a 1W to 3W amplifier that is high pass filtered

4) The signal comming out of the pan is weakerthan you might think. Most designs use a 12AX7 with a gain of abot 60 to recover the signal

5) I think the mixer are all active. But really is just means a resistive mixer with a gain state.

6) the king of all tube driven guitar reverbs is this. It is "standard issue" to every surf band.
http://www.kbapps.com/audio/schemati...reverb6g15.gif
the key to the above's tone is the filter at the input and the very powerful driver tube. If you are not doing surf or country steel guitar then you can sub a paralleled 12au7 for the 6k6.

One more thing: Read everything on this web page under "how reverb works" and "applications". These guys are THE experts.
http://www.accutronicsreverb.com/

Ok, two more things. Why are your volume controls wired like they are? The input independence of the unit will change with the setting going all the way to zero at one point. The normal way is to connect the "wiper" to the tubes' input grid. Use about a 1M pot. Remember you source is a guitar, but some active device a guitar can drive a 1M load but not much more. Same on the recovery the wiper feeds the grid.

Last edited by ChrisA; 23rd June 2011 at 01:59 AM.
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Old 23rd June 2011, 03:15 AM   #5
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well your close,

usually, reverb tanks in tube world are coupled with an inter stage transformer. you could cheat a little with a otl headphone amp in the begenning, to get around it.

like this one:

HeadWize - Project: An OTL Tube Headphone Amplifier by Kurt Strain

split the signal comming out of this amp, one goes to reverb unit, the other on one side of a 50k pot. the output of the reverb unit's amp goes on the other side of the 50k pot. the wiper goes to the final output line stage's input.

here is a classic design:
Attached Files
File Type: pdf deluxe_reverb_boost.pdf (101.3 KB, 23 views)

Last edited by DavesNotHere; 23rd June 2011 at 03:25 AM.
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Old 23rd June 2011, 04:14 AM   #6
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There were a few capacitively coupled tube circuits in the past, notably from Ampeg.This is a good start.

Eric Barbour's 6BM8 One-Tube Reverb Imagined...

http://www.schematicx.com/view-schem...ier-schematic/
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Old 23rd June 2011, 05:58 AM   #7
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by DavesNotHere View Post
well your close,

here is a classic design:
OK, I was wrong about were I said a "parallel 12AU7" the above uses a parallel 12AT7. if you do get to look at the transformer Fender used, it, itself is a bass-cut device. The thing is tiny

With guitar projects unlike stereo, you get to decide on the kind of sound you like. After all you are producing music, not reproducing it. So you get to decide if the reverb should sound like the echo you get in a hard surface tile bathroom or in a wood paneled large hall. Is the sound bright or warm? Your choice.

One other thing is that the reverb pan is very microphonic. It will pick up footsteps. The best reverb units go to some effort to mechanically isolate the pan

BTW here is an example of that 6G15 sound that was recorded in this century. Dick Dale keeps evolving the surf genre sound.
YouTube - ‪Dick Dale - Nitro‬‏
Notice the few times he over drives the springs and gets the spring crash sound. Wimpy driver tubes don't do that.

Last edited by ChrisA; 23rd June 2011 at 06:20 AM.
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Old 23rd June 2011, 10:39 PM   #8
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Thank you everyone,

I have got a few things to think about now,

SY I have completely neglected to think about the heater biasing, Im not sure how to do this thus I will consult the documentation I have been using. I will also remove the pot on the input.

ChrisA I like the note on filtering the input of the tank, I didn't think of this either. I thought i might put in a pot so I can "tune the reverb" a little. What do you think?

I would like to eliminate the transformer before the tank if possible. The tank that I have is an Accutronics 9BB2C1B. its input impedance is 190 Ohms. The driver stage of this design it taken directly from

http://www.freewebs.com/valvewizard2/reverbdriver.html

I has not been tested, I am hoping that the impedance match will be ok. However there is the issue of drive.

As far as mechanical isolation is concerned, I have discovered a company that makes mounting bolts that have a silicone pad roughly 10mm thick, they make them in various orientations. I was thinking of using them as feet for the box also.

Generally I have taken all your comments onboard, and reviewing the schematics supplied. Undoubtably I will have more questions.

If anyone has more to add please do, I need all the info. I can get.

Cheers
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Old 24th June 2011, 12:09 AM   #9
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The tank you have is a low impedance tank (relatively speaking) and the circuit from the valve wizard is for a high impedance one. You will not be able to use a capacitive coupled circuit, it is intended for a F tank.

Accutronics Reverb Tanks
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