hi, all--
I'm having a hard time coming up with a reverb circuit to fit a certain design I'm working on. I'm building this for my 2nd(Yay!) customer. We've decided to go with a Super Reverb-based circuit, but I would like to make the first stage a paralleled 12AX7. (This will be a single channel amplifier and will utilize a TMB tonestack, as well as a typical LTP driver)
For the sake of chassis space(as well as filament wiring headaches ) I would like to keep the preamp(including PI) to four tubes. The problem is, every classic circuit I've found has the first triode stage cascaded into the second triode, then into the reverb circuit. So:
Is there enough gain with a paralled 12AX7 to successfully drive the first stage of a reverb circuit?
In other words, is it necessary to have the cascaded "standalone" triode after the first pre stage?
Or, for the sake of keeping it to four tubes in the pre, is my only solution to go with a triple triode in the reverb circuit?
I apologize if my query isn't very clear. Any and all help/suggestions welcomed and appreciated. Thanks!
I know where you are coming from; I eventually bit the bullet and went for an extra pair of tubes (12AT7 and 12AX7) and a long spring Accutronics unit.
If you are not hung up on a tube driven verb, hop on over to AX84.com. There is a thread there about a SS reverb that was designed and installed into a California Dreamer amp - a blackface derivative. Includes a schematic.
The SS driven spring verb unit is not compatible w/typical tube driven circuits and vice versa - impedance mismatch between the two. You need to decide which drive to use before you pick the verb unit.
As little as 20mVrms, 30mV peak, should make full power.
The loudspeaker needs power tubes, most power tubes need 15V-45V peak to make full power. You need a gain of 300 to 1,600. One 12AX7 gives around 50, two give 2,500, so two triodes from input to power grid. Push-pull will need another triode to flip. Tone controls have losses, you need another triode for any heavy tone-stack.
The reverb needs a little power bottle, part-watt output. It tends to need 5V or 10v peak at its grid. So you need gain of 150 to 300 from jack to reverb driver grid. Two volt-amps. Much of this can be the main signal path, no extra tubes between 2nd main stage and reverb driver grid.
Reverb tank output is VERY low. Comparable to the guitar. So you need to find gain of 300 to 1,600 from tank pickup to speaker power tube grids. Much of this can be the main path, but not all of it.
You have to mix straight and reverb. Mixing is always lossy.
In short, you need total gain through reverb path to speaker, not counting power stages, of 100,000 to 2,000,000. Four volt-amp stages. Plus tonestack recovery, 5 stages. Say three twin-triodes, plus your reverb driver is four bottles.
That is, if you don't have odd demands like paralleling identical triodes which implies an extra half-bottle. There IS a double-fat version of the 12AX7 but the type number escapes me tonight.
The high-level view says it can barely work. The hard part is apportioning the gain.
It's only reverb. Put some sand in it. Sand recovery amp can even hiss a bit less.
> Is there enough gain with a paralled 12AX7 to successfully drive the first stage of a reverb circuit?
In other words, is it necessary to have the cascaded "standalone" triode after the first pre stage?
One 12AX7 gives gain around 50. Paralleled sections with the same load give gain about 55 or 60 tops. Two sections in cascade offer gain up to 2,500. It's not even close. A single AX7 stage, 2 or 3 or even 6 sections in parallel, will give very lame drive to most likely reverb drivers. It may work for "hint of spring", it won't be good for Surf.
> For the sake of chassis space
ALWAYS beware of cramming 10 pounds of stuff in a 5 pound bag. It takes longer to build smaller. Some people are good at it. I always regretted it in the end.
You need this much guts from pickup to speaker. If you want big power, you need another power bottle and another triode to flip polarity. If you want heavy tone yet with high sensitivity, you need another triode.
You need nearly this much guts from pickup to reverb tank input. You don't need quite so much power, but even a small driver will sill need nearly as much voltage.(*)
The path from tank output to speaker is again, like a Champ, with whatever power upgrade you like.
So draw three Champs. The first two stages overlap. The speaker output stages and their driver overlap. But you are always going to need two cascaded AX7 from jack to reverb driver, two cascaded AX7 from tank pickup to speaker bottles. And you pretty much need a tube to divide the reverb tap-off from the reverb return, or the reverb gets back into its own input and boings to infinity.
(*) A really good Video Amplifier Pentode will make a part-watt with far less drive than most audio power tubes, significantly less than the paralleled 12AT7 that Fender used a lot. But you only get one per bottle so you don't save a bottle. And you can't buy a replacement on a weekend.
> go with a triple triode
There's no such thing.
OK, there is, but again you won't find a spare on a weekend. And if it does not have shared electrodes which won't work here, then it will be on the odd 12-pin socket, which is big and rare. Long-run, a 5th twin-triode will probably be a better idea than tricky tube types.
thanks for the replies, fellas.
I do have a stash of NOS 6C10's and 6U10's, plus a bunch of new 12-pin sockets, so going the triple triode route is not necessarily out of the question. My Reverberocket uses one, and the circuit looks pretty straightforward.
rfgordon did suggest to me the One Tube Rvb circuit by Gary Glasman, found in the Dumble Files.
[img:761:337]http://www.glaswerks.com/schematics/reverbcir.jpg[/img]
I think perhaps that it may be my better option. Anybody here have experience with this circuit?
I'm also curious about the ECL86 rvb circuit, as well. http://ampgarage.com/forum/download/file.php?id=1123
I imagine substituting the more readily available ECL82 would not be much of a hassle, but my original question of 'would the gain of a paralleled 12AX7 be enough to drive it?' (either '86 or '82) would still apply.
Again, any help/suggestions welcomed and appreciated.
p.s. to PRR: I really enjoy reading your more philosophical(may I call it that?) approach to ampbuilding
The one tube reverb works great but it does not surf...... My guess is that the reverb tank doesn't get enough juice for this with this circuit. Maybe add another gainstage before the reverb tank? Or use a tube that drives the reverb tank a little more?
What about a foldback reverb circuit like in the Champ 12? I'm sure this circuit can be simplified from what is seen in the Champ 12. It uses only half a 12AX7, and pulls signal from the power tube cathode as apposed to preamp signal. I think some Ampegs and Hammond Organs used this style of reverb as well.
) I would like to keep the preamp(including PI) to four tubes. The problem is, every classic circuit I've found has the first triode stage cascaded into the second triode, then into the reverb circuit. So:
