Questions revolving one tube reverb circuit

[J M Fahey]

For some other posters: Sorry if I disturbed the Kumbaya vibe here in MEF with my reverb driver questions ................. I guess I was naive when I thought I'd get more than "don't try to reinvent the wheel" :-)

OK, both established wheel designs (transformer and transformerless) were discussed and examples shown; I added a workable one which is not currently used ; basically because of practical reasons: if you already got a magnetic core and painstakingly wound a few thousand turns of very fine enamelled wire (thinner than a human hair) , most reasonable is to wind a few dozen turns of thicker (read as easier to handle) wire and get a suitable secondary which is both efficient and powerful.

Almost forgot: also suggested a hybrid solution

Please feel free to invent a solution *different* to those just mentioned ; you are welcome to post it here.

Won´t get into dick length and thickness competitions though

[tubeswell]

Firefly variation on a 6G15 circuit maybe

[Chuck H]

I wasn't originally going to share this because it's transformer coupled and uses the typical low Z tank and the OP was discussing cap coupling with a high Z tank. But since everyone else has been posting reverb circuits, some that really don't address the OP at all I thought I may as well share my one tube reverb that sounds VERY good. Notice that an additional mix stage is required, sort of making it a 1-1/2 tube reverb. But I think this should always be the case since there simply is never going to be enough output from the tank recovery stage to do much without a mix or second recovery stage. JM2C on that matter.

Please excuse any errors. I just cut and pasted this out of another schem quickly. The Tank is a typical three spring shorty at 10 ohms. I like the three spring shorty in combos because they sound good and are less prone to microphonics. The transformer is a typical "Fender" type replacement.

Notice the local NFB mixing stage. It achieves low coloration and unity gain for the dry signal while allowing for additional amplification of the reverb signal. So there's no actual "padding" of the dry signal. The unusual dual ganged pot reverb volume circuit keeps the relative circuit loads intact.

[J M Fahey]

I might suggest using/developing/adapting a tube and transformer driven reverb which does neither need an extra drive tube nor buying a reverb drive transformer
Enter he mighty Champ 12:


The low impedance reverb tank is driven straight from the speaker out tap, through a 47 ohm 3W resistor.
Mind you, it´s driven hard.

The recovery stage uses a FET, not a tube but, hey, quite close, to boot it´s self "cathode biased" and drives a "plate" load resistor, as close as can be.
This output drives a triode which really isn´t needed in your circuit, it´s there to add a CD/MP3/Auxiliary input , you might either use a full double triode for bothstages (no-silicon-in-my-amps !!!! capisce?) *or* design a higher gain recovery stage, say using 2 Fets or even an ...... Op Amp so recoverysignal is loud enough to be mixed with PI drive signal.

Only caveat with this "free" reverb circuit is that since signal is fed back backwards (from speaker to an earlier stage) and there is significant gain involved, it´s easy to get howling feedback , as if the tank were touching the speaker frame or something.

That´s why there is, besides the regular reverb control, an extra internal "reverb trim" one to limit gain: you set panel reverb control to 10, lightly tap the tank until it starts howling, then lower trim control until it stps, for safety lower it an extra notch and you are set up.

Being a minimalistic guy (polite way of saying cheapskate) , I used a similar system a lot to add reverb to some of my own amps which didn´t originally have any, or to mod other amps, same thing.

Another system which works very well, and in fact was my first "commercial" product, way back in 1969, was this reverb circuit, straight from Popular Electronics, works well both in SS and Tube amps (read article text which explains how to), can be fed from existing supply, either Tube or SS , only problem is that it uses passive mixing (like what Fender uses to mix both channels: 2 resistors) so you need a little more drive than usual:


for the full article:
Popular Electronics January 1968

transistors and FET mentioned are obsolete but modern replacements are easily available.

FWIW even one of my personal hero designers: Daniel Meyer, based his design on one of the Accutronics suggested drive circuits.

FWIW2: this is what Accutronics themselves suggest:
Untitled Document

notice that the first suggested circuit is so old (dated 1981 and clearly coming from an earlier age) that the LM307 drive Op Amp is obsolete and has been so for decades, its heir LM308 (of RAT fame) is also obsolete, and Mouser does not even know them.

Just showing that not even Accutronics themselves reinvent wheels but keep the old ones rolling.

[Chuck H]

That's an idea I've kicked around in my head a lot. I even tried something similar once with less than satisfactory results. The hitch is, of course, the fact that it's a feedback loop. To keep it stable requires concessions and I've read (no personal experience) that the Champ 12 reverb is thin and weak and that attempts to improve it have resulted in feedback.

[uneumann]

Since SS and Tube is in the mix here... This is a simple (cheap) external FX-loop reverb I built for the Flexi-50 amp. A ~24v supply is required - I used an LED supply (< $10). It has unity gain (with a follower driving the dry end of the mix pot) and carefully preserves the dry signal. The mix pot is a dual-gang linear pot - but it achieves a log-like taper for the wet signal.

It's only my opinion, but I have to say it... I'm a tubes "believer" for the signal chain, but when it comes to reverb - it seems that the expense and difficulties of using tubes tilt (strongly) a new design/build decision towards SS or at least hybrid circuits. Some greats examples are in this thread already.

[66 Kicks]

For the tank output circuit with a capacitor and a resistor across the coil, L and R form a low pass circuit with the high frequency cutoff proportional to R and inversely proportional to L. L is constant for the output coil, so we can only vary R. L and C have a resonant frequency which is independent of R. The coil puts out its maximum output at the resonant frequency. The high frequency phenomenon and the resonant frequency phenomenon interact.

The function of C in the tank output circuit is to form a resonant peak with L that will increase output as the frequencies increase toward that peak. It makes the output brighter which is the opposite of what many people think when they see a capacitor going to ground. A 0.0022uf capacitor and a 0.358H coil have a resonant frequency of 5.67KHz. This will progressively increase the magnitude of the frequencies above 1KHz.

If R is too small, the high frequency roll-off phenomenon will decrease the resonant frequency phenomenon. 220Kohms is sufficient to prevent this.

[66 Kicks]

I have heard that an emitter follower running directly through the input coil is poor form because the input coil is not gapped for DC.

[Tubenit]

There is quite a bit of information on "one tube reverbs" here: Adding 1-tube reverb

With respect, Tubenit

[nickb]

I have heard that an emitter follower running directly through the input coil is poor form because the input coil is not gapped for DC.

The gap it actually quite large and the magnets sit in it. I suppose, therefore, the current will be a lot higher than we might expect.




It's a fairly simple magnetic circuit. I can get the dimensions and make a guess at the iron but I don't have the number of turns otherwise we could estimate the current. OTOH I should be able to cobble something together to allow me to measure the inductance vs DC current and find out what the number actually is. Need to scratch head for a bit.

[nickb]

There is quite a bit of information on "one tube reverbs" here: Adding 1-tube reverb

With respect, Tubenit

Are you site owner of hoffman amps / el34 world or closely affiliated with them? You wouldn't just be promoting your site by adding cross links, would you?

[66 Kicks]

The gap it actually quite large and the magnets sit in it. I suppose, therefore, the current will be a lot higher than we might expect...

Do you think DC across the coil will cause the core to saturate earlier?

I believe there are 1700 turns in a 4F input coil.

[66 Kicks]

Are you site owner of hoffman amps / el34 world or closely affiliated with them? You wouldn't just be promoting your site by adding cross links, would you?

I don't know the gentleman, but from reading his posts on EL34 I would say that he is motivated by the desire to help people.

[Tubenit]

No, I am not the owner & have no benefit at any level from the link or forum. I've never received a penny for any help I've done. I have no part of Hoffman's business. Never even gotten a discount from Hoffman on parts.

I have another profession totally unrelated to music, amps or guitars. I am just a hobbyist like most of us.

Just sharing information that others have found useful. Occasionally, someone will say thank you and convey that the information was useful to them. Over 26,000 views were on that Hoffman thread .......... so I thought there might be a chance that someone here would also find it useful to them. Just trying to give back a small portion of the help that so many others have extended to me. Google "one tube reverb" and see what comes up.

It's that straight forward.

With respect, Tubenit

[Chuck H]

Just to keep the thread limping along ...

I think the "one tube" stipulation is a little misleading because there just isn't enough signal at the end of a single recovery stage to drive the PI satisfactorily (YMMV). So there needs to be at least one more stage of amplification of the reverb signal (and often recovering the padded dry signal too). To keep the discussion clear and the circuits accessible I'm interpreting the term "one tube" to mean a typical dual triode of a type typically used in guitar amps. Not some obscure three amplifier valve that needs to be hunted down NOS.

Thoughts?