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

[dchang0]

Repost of the 6BM8 pentode-strapped-as-triode curves that is no longer at the link I posted back in post #16.

ecl82pentode_as_triode.pdf

Also, a scanned copy of the curves and lines I drew on them in pencil.
I selected the line labeled "5K" in the end.

[Chuck H]

Well since this thread is being perused by the OP I think I'll take this opportunity to mention that the "one tube reverb" moniker is a little misleading. The output of a reverb tank is about 5mV. A recovery stage with an AF of 70 won't even get you up to a quarter of a volt. So you really do need to insert this where another amplifier stage can do some heavy lifting before the PI. And the trouble with that is then you need to modify the stock circuit for signal separation/isolation and that nearly always means padding the dry signal. In other words, you can't just jam this thing into an amp and splice it in at points X and Y. Some modification and most likely a gain reduction of the amps stock circuit will be required. Because of this I don't like the "one tube reverb" claim. I've seen guys here try one tube reverbs where they either pad the dry signal badly and then try to defend their effort because it does "work" and they're proud. The worst example will take that tiny recovered output and plug it into the other side of a LTP PI. And I've even seen guys say that it works great. No it doesn't. The PI behavior is changed compared to the stock amp and the actual reverb signal going into the other side of the PI is only a quarter of a volt or less. Again, it may "work", but it's not much reverb at all and your dry signal tone has changed.

Rant over

[Niquel]

"Rant over" ha,ha.
Chuck H, you must be right saying the title is misleading, as I ended using a mixing stage after the reverb recovery to get the one tube reverb working nice, but I used a cathodyne PI so I had one stage unused (I initially thought I could use it as oscilator for a bias tremolo).
Anyway, this design let me add a tube reverb with minimum added components and it saved a lot of space (and money!) in the chassis amplifier, otherwise I had to use a driver transformer or other topologies I don't understand.
Do you guys know any tube reverb circuit that does not use a stage to mix dry with reberb signal? All designs I recall have it.
Cheers.

[dchang0]

this design let me add a tube reverb with minimum added components and it saved a lot of space (and money!) in the chassis amplifier

^This is how most people who search for "one tube reverb" are thinking about it.

Perhaps Chuck would be okay with the title if I changed it to "I-only-have-space-to-add-one-tube-to-my-amp reverb" because that's how most people will approach it.

In other words, they know from the get-go that they're facing a compromise of some sort, and the glee/pride they feel comes from having managed to cram a "working" reverb into relatively tight constraints.

I'll leave the title as-is, though, because most people who would be looking for this type of circuit will be searching using that phrase/keywords.

[Chuck H]

Do you guys know any tube reverb circuit that does not use a stage to mix dry with reberb signal? All designs I recall have it.

There are a few different reverb designs that don't have a mix stage. Magnatone and Gibson both used a system where the power amp drives the tank and signal is fed to a separate amplifier. There was a later Champ model that also drove the tank from the power amp and then injected the signal back into the amp somehow. Though I'm not very familiar with it and I don't think it's recognized as a very good circuit. I've also seen designs where only the recovered reverb signal is reamplified and then sent to the "other" side of a LTP PI.

I'll leave the title as-is, though, because most people who would be looking for this type of circuit will be searching using that phrase/keywords.

Oh yes. That's just the name of the circuit now. So of course we'll keep using it. Though I wouldn't say that most people understand it's limitations because I've seen several attempts here to mix signal from the recovery stage right to the PI without additional amplification to the reverb. One even reported that "it works pretty good". (???) Probably a case of pride in effort preventing an honest evaluation? If, for example, you "only have room to add one tube" but your amp doesn't have a topography to modify the circuit for a mixing stage that amplifies the recovered reverb signal, well then one finds themselves in a pickle with holes already punched and drilled.

FWIW I've done my own version of this circuit using an ordinary 12dw7 tube. It works great. The au7 half is ample for driving the tank and the ax7 half has more amplification than the 6bm8 triode.

[dchang0]

If, for example, you "only have room to add one tube" but your amp doesn't have a topography to modify the circuit for a mixing stage that amplifies the recovered reverb signal, well then one finds themselves in a pickle with holes already punched and drilled.

I'd say that that would be a mis-applicataion of the circuit, not the fault of the circuit itself. If inserted properly into an amp that could offer a mixing stage, it does become an "add-one-tube reverb." Your objections would serve as the warning to future builders to pay attention to how they're using this (hopefully they scroll down far enough to read your rant).

FWIW I've done my own version of this circuit using an ordinary 12dw7 tube. It works great. The au7 half is ample for driving the tank and the ax7 half has more amplification than the 6bm8 triode.

Is the 6BM8 triode that much weaker than the AX7 half of the 12DW7? Could it be cranked up to approach the threshold of what you consider to be acceptable recovery gain?

[Chuck H]

Well, from my post above:

The output of a reverb tank is about 5mV. A recovery stage with an AF of 70 won't even get you up to a quarter of a volt.

The 6bm8 triode has a gain of 70. And as we all know the 12ax7 has a gain of 100. At that low signal level you won't get a lot more with a gain of 100. Maybe .35V or so. So to answer the question, no. A 12ax7 as the recovery stage won't amplify enough on it's own without a more amplification of a mixing stage either.

[dchang0]

Probably a case of pride in effort preventing an honest evaluation?

That could certainly be true--overweening pride.

However, I think it's important to remember that pride has positive aspects too.

I assume you've raised a kid, as have I. You've probably seen your kid run up to you bursting with pride about something they just learned to do while also seeing that what they did has some flaws that you, as an adult, can see but that they, as a total novice, cannot.

Do you deflate their pride by pointing out all the flaws immediately, or do you let them ride the wave of excitement as they delve deeper into this new subject they're learning (your plan is to teach them the details of their mistakes later, when they are ready for it)?

A brand new amp-modder might unjustifiably feel pride at having incorrectly inserted a one-tube reverb into an amp, but that undeserved pride might motivate to them continue to learn the "sport."

I've been thinking about this in terms of why I didn't continue to play baseball while young. The criticism I got at the time, while true, was intensely de-motivating, so I dropped out of baseball and didn't pick up any other sports until after college. Now, I'm not saying we should coddle every newbie and hand out participation trophies--that's too far to the other extreme; what I am saying is that pride has a positive aspect in that it can be used to motivate a person to improve and that criticism has to be delivered at appropriate times in the person's development process. I've got a buddy that coaches little league, and he's really good at letting the boys feel proud while still criticizing and teaching them how to play baseball properly.

I figure that the times when some amp-modders doubled-down in their pride and resisted criticism, it was because it was too early for them to hear that criticism. Later, as they mature as an amp or pedal builder, they may think back and realize that what they got was good advice.

[dchang0]

Thanks!

So, perhaps it is appropriate to put a warning/note that this circuit should never be used in an amp topology that doesn't offer a mixing stage.
I can add that to the schematic if you like.

[Chuck H]

Thanks!

So, perhaps it is appropriate to put a warning/note that this circuit should never be used in an amp topology that doesn't offer a mixing stage.
I can add that to the schematic if you like.

Naw. It's not like people are lined up to do it "wrong" In fact the last guy I helped with a reverb is a little green but well of the gain criteria.

[Chuck H]

That could certainly be true--overweening pride.

However, I think it's important to remember that pride has positive aspects too.

I assume you've raised a kid, as have I. You've probably seen your kid run up to you bursting with pride about something they just learned to do while also seeing that what they did has some flaws that you, as an adult, can see but that they, as a total novice, cannot.

Do you deflate their pride by pointing out all the flaws immediately, or do you let them ride the wave of excitement as they delve deeper into this new subject they're learning (your plan is to teach them the details of their mistakes later, when they are ready for it)?

A brand new amp-modder might unjustifiably feel pride at having incorrectly inserted a one-tube reverb into an amp, but that undeserved pride might motivate to them continue to learn the "sport."

I've been thinking about this in terms of why I didn't continue to play baseball while young. The criticism I got at the time, while true, was intensely de-motivating, so I dropped out of baseball and didn't pick up any other sports until after college. Now, I'm not saying we should coddle every newbie and hand out participation trophies--that's too far to the other extreme; what I am saying is that pride has a positive aspect in that it can be used to motivate a person to improve and that criticism has to be delivered at appropriate times in the person's development process. I've got a buddy that coaches little league, and he's really good at letting the boys feel proud while still criticizing and teaching them how to play baseball properly.

I figure that the times when some amp-modders doubled-down in their pride and resisted criticism, it was because it was too early for them to hear that criticism. Later, as they mature as an amp or pedal builder, they may think back and realize that what they got was good advice.

Excellent points. I admit I'm never patient enough with adults. With my daughter I probably wasn't as good as I could have been, but she still loves me and she makes me proud every day. I didn't point out mistakes or criticize, but very often said something like "You know how you could make that even better?" or maybe "Since you're doing that you could also..." That sort of thing.

[dchang0]

Naw. It's not like people are lined up to do it "wrong" In fact the last guy I helped with a reverb is a little green but well of the gain criteria.

Hmm. Well, what would be a good, concise note to put in the schematic to help someone avoid the pitfalls you've seen?
Maybe "never" is too strong a word?

[dchang0]

she still loves me and she makes me proud every day.

Excellent!

[Niquel]

Thanks Chuck H for refreshing my memory. I didn't remember the reamplified recovered reverb signal into PI, I think I saw some Soldano amp using this. I'm not familiar with Magnatones.
The gibson GA-1RT-1 reverb (that uses a 6bm8, but as power tube) is one of the weirdest things I've ever seen.
Would you say the 12AU7 would be able to drive an E tank? I am afraid your design was the main reason why I bought a 12DW7 years ago, I have to try it too (I have an F tank as well).
dchang0 that warning could be OK for non-cautious tweakers (as I sometimes am), I see your efforts for keeping versions updated.

[dchang0]

dchang0 that warning could be OK for non-cautious tweakers (as I sometimes am), I see your efforts for keeping versions updated.

Perhaps a new, better version would use the triode half--probably with a transformer--to drive the tank and the pentode half as a pentode to recover... I am not sure if this is possible, but it might be.

Eric Barbour used the triode as the first gain stage from the guitar, then the pentode to drive the tank through a transformer. He did not address the recovery or mixing. In a way, that solves what Chuck was talking about. By leaving out the other side, it forces the amp modder to come up with a solution for recovery and mixing on his own. In other words, Barbour's "one tube reverb" isn't a complete reverb--it's a "one-tube half-reverb" or "one-tube reverb driver."