Two Tube 'Verb for Tweed Bassman Amps

[tubeswell]

(I'm so happy with this I've just got to put this on a new thread)

This reverb circuit has been prototyped by me in a (modified) 5F6A build and works amazingly well (but it would probably work well in any tweed amp).

It does a fairly wide range of spring reverb sounds and can go from very subtle to very deep and intense, and all for only an extra 600mA draw on your heater winding, the space for two pre-amp bottles, and three control knobs (dwell, tone, mix).

The voltages on the schematic are what I got in the tweed bassman I prototyped it in. I added an extra filter/decoupling cap and 10k supply resistor in the power rail for the extra two pre-amp tubes. The Cathode Follower bypass stage and mixer functions like the same sort of setup in a standalone reverb, providing a unity-gain buffer, which on the 'dry' side, otherwise preserves the original mojo of the tweed amp without the need for interfering noticeably with any signal quality.

Many inestimable thanks are due to members of this forum for their perseverance with me in working this experiment up, but I feel I have to particularly acknowledge loudthud, MWJB, pdf64, Chuck H, txstrat, Steve Conner, tubenit, Merlinb, simcha (who doesn't post very often but was nevertheless very helpful at the last minute) who all provided really helpful suggestions at one stage or another that helped shape the final working item and Victor Denance (whose question a few months ago stimulated the original suggestion). (Appreciation also due to Leo, wherever you are looking down from )

[loudthud]

Tubeswell, you put a lot of effort into this amp. Makes me wonder, do you have a day job? Where do you find the time?

I'm kind of wondering about the tank drive. I saw that other thread but it doesn't look like you have much drive at all.

[GibsonLover]

I know you said it was 'final', and it probably works well, but how about a few questions for curiosity?
Since you have a Dwell control, why not feed it a constant input by feeding it from the top of the volume pots rather than from the wipers?
Had you ever considered a P-P drive? It still takes two triodes and one transformer, but with P-P benefits. Maybe Leo might even smile a little...
Last, it looks like you hit the driver with full bandwidth. Do you find the bass frequencies crashing the tank a little? Leo fed his tank post EQ (scooped mids) and rolled off lows (500pF/1M). Again, just curious if you have negociated any of this during your development process.

[tubeswell]

Hi loudthud I wish I didn't have a day job - then I would have time to build heaps more amps .

Anyway now there is plenty of drive. In fact there is so much drive, that you can almost hear the reverb with the mixer control hard on the dry side. A 6BM8 would probably be more intense, but that's a 700mA heater on its own, and while the PT could probably tolerate it, the drive is good enough with the cascaded 12AT7. So it is a more economical way to get a decent reverb. However I am not averse to trying a 6BM8

Simcha Delft was right on the button about 1/2 a 12AT7 having enough to power the tank (although her suggestion was to have that 'power' stage unbypassed, but she said it didn't matter driving that 'power' stage of the 12AT7 as hard as you could). Anyhow it doesn't make any unpleasant sounds the way it is. The cathode resistor on the driver stage (in front of the 'power' stage) is set (happens to be 1k5) so that the anode is just over about 1/2 the HT at that point, which gives a good clean signal going into the 'power' stage. The 'power' stage is about 3-4mA, and that is enough to make the tank come alive, when combined with the extra gain from having the stages cascaded.

The problem before was that I wasn't hitting the paralleled 12AT7 with enough input signal. I had too much attenuation what with the vol controls and the mixing resistors and the earlier dwell control, which combined with the 1M input impedance of the CF (because it was in parallel with all the other stuff), meant that the bridging wasn't sufficient. Okay so the bridging hasn't changed, but the extra gain has overcome that.

Gibson Lover, you have seen enough of my posts to realise that I have 'finality' issues. . I'm curious now about taking the 12AT7 input from the top of the Vol controls (but I wouldn't be able to turn down the amp properly tho' would I?) - altho' some of the others here may end up cursing you for suggesting it to me . The way it is now the dwell goes from dry (with the mixer on the CF wet side the input impedance is still high enough to pick up and pass the signal to the following DC Pair stage) to wet. The bass freqs don't appear to crash the tank (from what I can tell), but there is enough bass to make the reverb tone control really come alive in the recovery stage. PP driven reverb sounds like a phantasmagorical concept.

[Steve Conner]

Hey, I used PP reverb drive in my Ninja Deluxe amp. I accidentally trashed the reverb driver transformer I was using, and only had a PP one left, so I just used it. I drove it with the two sections of a 12AU7 in self-split.

Tubeswell looking at your schematic, I don't really see how the mix control works. It seems like there should still be lots of verb even with the control all the way to dry. Or is it the dwell control that adjusts the amount of reverb, and the mix control does something else?

Also, I don't think it matters if the reverb driver distorts and clips a lot. It just adds more high end to the reverb. I'm sure I remember Craig Anderton actually recommended clipping the drive to spring reverbs.

[tubeswell]

The mix control is the same as a 6G15. It is a blend control between the CF buffer and the reverb recovery - and it works purfect. If there is a wet signal (i.e. with the dwell maxed out) and the mixer is on the 'wet' side, it'll pick it up. Otherwise its dry. The dwell defines the amount of 'dingle' vs 'drip', and the mix determines whether this is more or less prevalent. (The tone makes the wet sound fatter or brighter). Its great fun to muck around with. (Now all I need is a whizzer to help with all these knobs)

[GibsonLover]

Hey Tubeswell,
I understand your issues - I suffer too. Take your Dwell and 0.01uF thru two more 270K resistors to the top of the volume pots for the verb input, and leave the the connection you have for the Dry mixer on the volume wipers. That way, your Dwell always sees the same drive level, instead of having to tweek everything every time you change volume. You probably will anyway ....:-) The advantage is better signal to noise, particularly at low volume settings, and your Dwell is constant, regardless of Volume. I think a strong signal is important, and with better efficiency of P-P drive you transfer more power with a cleaner signal. Voltage amplification is only half of the power equation. Hot is good, but I don't agree with clipping. Transformers hate square waves, and the transformer on the reverb input is about as small and fragile as you're gonna get. You can get high frequency response without feeding it square waves. Have Fun!