1 tube reverb?

[daz]

Since i realized I don't need the extra tube i installed, i was thinking of leaving it in and installing reverb. I have a pan already. Can anyone point me to a verb that uses 1 AX7, and also in the schematic below tell me where you would tap for it and return it to? I'm not sure there are any options with my design, but i gotta ask as i hate to leave a unused socket.

[jimboyogi]

Hi daz, i found this thread about 1 tube reverbs. Some really good options discussed. I have built one using a 12DW7, with the AU side driving a high impedance tank via a 0.47uF cap, with no transformer. But if your tank is a low impedance one, then the transformer is probably a better bet.

http://music-electronics-forum.com/t17972/

[Chuck H]

12dw7 is a better tube. It's half a 12ax7 and half a 12au7, basically. The reason is that half a 12ax7 doesn't make a good driver for a reverb pan whereas half a 12au7 can do it easily. So you can drive the pan with the au7 triode and use the hi mu ax7 side to re amplify the recovery side. There is still the issue of re amplifying the re amplified signal!!! The output of a reverb pan is only about 5mV to 7mV. Compare this to a guitar signal at 100mV to almost a volt. To get reverb signal up to a suitable level from the recovered reverb pan output is best done with two stages. If it takes one triode to drive the pan and two triodes for recovery that makes it impossible to have a one tube reverb. UNLESS you have a recovery stage that serves double duty as a second recovery triode while also doing another function in the amp. The only other option is to feed the PI with a reverb signal of about 1/4 of a volt. It's been done. It's usually disappointing.

Adding a digital reverb might be easier on an amp not originally equipped with reverb.

[loudthud]

What about using a MOSFET for drive?

[pdf64]

'To get reverb signal up to a suitable level from the recovered reverb pan output is best done with two stages. If it takes one triode to drive the pan and two triodes for recovery that makes it impossible to have a one tube reverb'
One method is to add a NPN transistor in cascade to the recovery triode to help things along a little. It can be integrated into the tube's cathode circuit quite neatly, such that the tube's cathode current supplies the transistor.
I got this idea from Elektor Electronics magazine decades ago, in which the article's author documented the 'improvements' made to his Vox AC30, including fixed bias, Si rectifier etc.
Pete

[daz]

Ok, you've convinced me. So what else might i do with that tube? Any good loops i could implement with the schematic i posted in mind?

[teemuk]

1. Drive the reverb tank from attenuated speaker output. Amplify output signal with the single tube stage.

or...

2. Use solid-state circuitry to perfrom functions to which you haven't got enough tubes.


or....


3. add more tubes.

[Chuck H]

1. Drive the reverb tank from attenuated speaker output. Amplify output signal with the single tube stage.

And inject it back into the same amp? Not a chance. It becomes a feedback loop. You'd think that the actual reverb itself would be phase ambiguous enough to make it work, but it doesn't. I've tried it.

or...

2. Use solid-state circuitry to perfrom functions to which you haven't got enough tubes.

Better. I'm especially intrigued by Pete's ideas.

or....


3. add more tubes.

Nope. Daz isn't likely to start cutting more holes in the amp. Maybe on a new build though.

[teemuk]

it doesn't [work]. I've tried it.

Ampeg made many amplifiers with such scheme. It works.

Probably not the best solution by all means but when you have only one tube stage and reverb tank practically requiring two then a compromise is bound to take place.

[jimboyogi]

Ok, you've convinced me. So what else might i do with that tube? Any good loops i could implement with the schematic i posted in mind?

Hi daz, I can't see any schematic. Can you try linking it again?

As many posters here have pointed out, you need more than one 12AX7 recovery stage to bring the reverb level back up before the PI. But you can possibly mix the reverb signal back in with the dry signal early enough in your amp that it will get plenty of gain by the PI.

[daz]

First of all guys, to those who seem to think i'm still using 1/2 the tube for the other issue, i have a single TUBE, not a single stage. So i have 2 stages. As i said, the one stage i was using to fix my level/tone issues at the PI is not longer needed ow that i discovered cbarrow7625's suggestion for a master volume worked even better than the extra tube. So i have a whole tube, but still from what you guys are saying it's not likely to work well so i think I might pass rather then create another mile long thread. But if you think it can be done w/o jumping thru hoops and wanna have a go with your ideas, be my guest. Sorry about the schematic. I had uploaded it but forgot to add it when i started the thread.

[nashvillebill]

Kevin Oconnor's The Ultimate Tone has quite a discussion on various methods of adding reverb stages, even including using the power amp's output to drive the reverb.

[Chuck H]

Ampeg made many amplifiers with such scheme. It works.

I can't find any representative circuits in any Ampeg schematic archives. Could you link one? I'm interested.

[luthierwnc]

These are three schematics that address (if not answer) your questions. I've built and still have all three. The T-Lite is a one-tube reverb designed for reinsertion near the inverter. You'd think it might be weak but I ended-up adding a lot more resistance and using a 12AT7 or 12AU7 (can't remember) so I wasn't channeling Dick Dale with the knob on 2. The Reverbalope is my own creation based on the reverb in a Gemini. More than a single tube but this works better (IMHO) than the Fender standalone tank. I've since folded it into a ReVibe but the circuit is still the same. I did use relay switching so I didn't have to have 10 feet of grid line to and from the pedal. The Power Prince is another one-tube verb and is very cool but you need some real estate on the board or somewhere else to handle the extra filter caps. Be prepared to do some tweaking to get the levels right. Hope this helps, sh

[teemuk]

I can't find any representative circuits in any Ampeg schematic archives. Could you link one? I'm interested.

Here's a better idea: Google search for speaker driven reverb.