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

[überfuzz]

Just curious, what bright cap are you going to use in the reverb feed?

[Steve Conner]

I'd love to keep my Princeton Reverb clone (sounds amazing), but it doesn't fit into our modular shelving like a Champ does.

I've carefully read through this thread, and I think the overall best solution might be a new GF...

[TheTinMan]

I've carefully read through this thread, and I think the overall best solution might be a new GF...

+1 on that conclusion.

The reverb pot (on the return side) needs to be wired with the signal going into the wiper, 1 end of the pot grounded, and the other end of the pot connected back into your circuit. Wiring it like a normal volume pot will have unintended consequences for the entire signal when you turn the reverb pot down.

Search here for tubenit's 1-tube reverb circuit. It works. Using a 12DW7 instead of an 12AX7 is an interesting idea, particularly with only one triode ahead of the reverb driver (pentode or triode).

Here's an example with an SCH file you can play with: Champ (&5F2 Princeton) w/reverb variations

Cheers,

Chip

[dchang0]

Hey, you helped quite a bit, BiBi.

Optimum load came from both the Valve Wizard and the Radiotron Designer's Handbook, although they same somewhat conflicting things. VW says that for maximum power, the load should be 2 x ra (anode resistance). RDH says that it should be AT LEAST 2 x ra. So, we merely need to calculate ra, which changes based on the chosen bias point. It's too long to explain here, but The Valve Wizard has it on page 22 of Chapter 1 "The Common Gain Stage," which he gives away for free on his website in PDF form.

When you see 20k as the internal resistance of the tube, you are probably looking at the MAXIMUM or AVERAGE value, not the value calculated for your chosen bias point. There's nothing wrong with using the max or average. I only bothered to go through the calculations because I wanted to re-learn the math behind it all. It was also nice to be able to choose a bias point that I like (cleanest, least distortion) rather than say, what someone else likes (more crunch, more whatever).

Hey, thanks for the schem! I'll use that to double-check what I've drawn up.

My plan is:

guitar -> 12ax7 triode (Fender 5F1 values) -> 0.022uF coupling cap -> reverb (470k bridge) ->master volume pot -> 12ax7 triode -> 5F1 power section

Hmm. It may be time to start drawing up the new, complete schematic for the 5F1 with the reverb inserted...


Edit: DOUBLE THANKS for the 500 R2 schematic. I just noticed that there is NO resistor used in the triode strapping. Though that is inadvisable, if the tube isn't being driven near any cutoff, it could be a way to save some space on the eyelet board layout, if I need the space.

Unfortunately, Tom Schlangen, the guy who compiled the curves for the t.s.p 6BM8 didn't specify the way in which he strapped the pentode. I think I might reverse-engineer the values on the 500 R2 schem to see what the designer did there.

[dchang0]

I don't know yet. Thought I might start with 500pF and experiment from there.... At this point, I'm so burnt out on calculations that I don't want to go back in and figure out which is the "best" value...

[dchang0]

Ah yes, Chuck H mentioned that in the other thread... I'll fix the schematic.

LOL--tubenit's 1-tube reverb is the bug that got in my ear to start this whole mess! I only deviated because so many people claimed the 12AX7 is too weak and because tubenit himself said that his design is not meant to emulate the surf-type reverb I'm going for.

Of course, I'm probably not going to get Fender surf now that I've swiped ideas from Ampeg's design, but at least the tank will be fully-driven.

Thanks for the tip and the extra schems! They are proving very handy in helping me decide how to insert the reverb into the 5F1.

Here's the final schem for this thread, Rev 2.0d:



I'll start a totally separate thread for the schems of the new 5F1 with 6BM8 reverb...

[dchang0]

LOL--that's likely the best solution to many a relationship problem!

[imaradiostar]

I'm going to throw a giant wrench into the works. The best two tube reverb is a dual triode or pentode for verb drive and an LND150 for recovery. Don't hate me!!

I figure a pair of 12au7's in parallel can create more drive current than a single triode and the LND150 has more gain than any triode without added filament current gain. Using Merlin's methods, a pair of triodes can make a nice SRPP verb driver, once again leaving the LND150 for recovery.

Feel free ignore me.

jamie

[dchang0]

Hey, nothin' but love here for my fellow amp builders.

You're welcome to post up schematics, etc., for your reverb, so that others who stumble across this thread might get ideas/help for their own situations. (Might help to start a brand new thread instead, though.)

Personally, I cannot use two tubes because of the cramped quarters I'm currently building in (5F1 chassis), but that doesn't mean I won't try a two-tube reverb later on. If this 6BM8 circuit works well without a transformer, though, I'll never use a transformer again.

[imaradiostar]

I probably wasn't clear- I'm sorry. Use a dual triode such as a 12au7 to build a drive circuit as shown on Merlin's web site and use an LND150 mosfet to recover the verb- they can easily have a gain of over 100 and use very little current, not to mention they use a tiny amount of current. They bias up more or less like a triode.

If you'd like I can draw up an example.

jamie

[dchang0]

Well, it's not for me, as I've already built the 6BM8 circuit. But for other readers, it would be useful if you did draw it up. Thanks!

[dchang0]

Newest schematic. The old one had a too-gainy recovery stage. I reverse-engineered the biasing on the Ampeg G-12 and FBT 500 R2 to figure out these new settings--the biggest change is the use of a much lower 27K grid leak resistor, but I also forced a lower load line with the 270K plate resistor and chose a warmer bias point by keeping the 1.5K cathode resistor.

Also changed the pot and send resistor to lower values, but I have not confirmed that these work yet.

This is Rev. 2.0e.

[dchang0]

Final schematic. This has been built and confirmed to work nicely. The reverb is dark and woody and very quiet. Can barely reach surf levels as is--increase the 22K grid leak resistor on the recovery side to 220K or as high as 1M to get more gain and reach surf levels (at the cost of more noise and microphonics).

Rev. 2.0f:

[imaradiostar]

have you tried higher value pots for the verb level pot? I usually would use a 1 meg in that location.

jamie

[dchang0]

500KA worked terribly, so I didn't go up to 1MA. Stepping down to 100KA solved all sort of wet/dry mix and gain problems.