6BM8 One Tube Reverb Help

[dchang0]

talbany posted this intriguing schematic that has so far been the only direct example of a one-tube 6BM8 reverb circuit I've seen. (Many references to such circuits have been made all over the net, but this is the only schem I could find.)

https://tubeamparchive.com/download/file.php?id=10953

Problem is, the pinouts on the 6BM8 are wrong (not talbany's doing, as someone else drew it). structo provided a pinout diagram:

[img:238:285]https://tubeamparchive.com/files/6bm8_pinout_197.jpg[/img]

I've relabeled the pins and would like your help confirming it.

[img1230]https://tubeamparchive.com/files/6bm8_108.jpg[/img]

Does this look all right? I'll be inserting this reverb circuit into a 5F1 Champ next, and this is the first time I've attempted adding on reverb and the first time dealing with a pentode...

Thanks in advance!

[Bob-I]

Yes, that pinout looks correct.

I've done 2 amps with a pentode/triode reverb but I used an ECL86. They sound great, not surf level reverb but a nice warm fat reverb.

In fact, not that I look at this it might be the modified JTM45 I did a few years ago. It's a great sounding amp and the reverb is excellent.

[dchang0]

Thanks, Bob! Especially your confirmation of the tone.

Speaking of which, what does it take, you think, to get a surf-level reverb? Is it even possible to pull off in a one tube design?

I do like the over-the-top slap-back on my Princeton Reverb clone. But since this is a Champ chassis, only one tube and minimal circuitry is possible...

[dchang0]

Oh, one question about the schematic.

The front of the reverb circuit uses the V1/"plate" voltage, whereas the back uses the V4/"preamp" voltage.

Since I'm inserting this circuit into a 5F1 Champ, the V1 voltage would be a too-high 330V, so I'm thinking I need to tap the V3 voltage of 250V instead.

Even then, it may be high for this particular tube. This thread makes it sound like 200V is about right:

http://www.ax84.com/bbs/dm.php?thread=150935

What do you guys think?

[Bob-I]

To get more reverb increase the value of the balance resistor, 470K in this schematic. Try doubling that to 1M, the downside is that you'll lose some overall gain and a little loss of highs.


EDIT, I just noticed that the 500pF cap is in the wrong location. It should be from the left side of the 470K resistor to the grid of the reverb tube, not in parallel with the 470k

[dchang0]

Thanks, Bill!

You mean like this?

There are some other things that might be wrong, like the V5 and Vb junctions that don't seem to connect anywhere....

[dchang0]

And for anyone who happens across this thread later on, here is the "more surf" version.

(Updated with high pass cap.)

[Bob-I]

Yes, that's the correct connection for the 500pF.

The Vb connection should be to the 10K trimmer. The V5 connection is harder to explain. You could take another 2.2K resistor off of V2 and another 47uF filter, so a parallel B+ rail. Or you could use V3 like Fender does on the AB763. It appears like the Fender doesn't have enough nodes on the B+rail to eliminate crosstalk, but you can't deny that's a great sounding circuit.

Also, on the 1M resistor, you might consider a small cap in parallel, 10 to 47pF. Fender did this to prevent loss of highs. Also you can increase that resistor if 1M doesn't give you what you want. Fender went as high as 4.7M on some amps.

Good luck with the project.

[dchang0]

Ah, I see.. Thanks, Bob, for all your help! I updated the "more surf" version per your suggestions.

Maybe someday I'll build one (after I get more theory under my belt).