RE-Vibe for line level use

[nateflanigan]

andrew_k
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Re: New project - Rack ReVibe
« Reply #11 on: January 24, 2012, 10:34:55 pm »
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Quote from: nateflanigan on January 24, 2012, 09:36:57 pm
Doug, or anyone for that matter, what would be involved in integrating a revibe into a mixing situation?  Meaning line level in, line level out.

a pair of 10k:150 or 10k:600 isolation transformers ($10 each from Ecdor) and a couple of resistors to tweak levels. One on the front end to go from balanced line level to unbalanced instrument level and another reversed on the other end to do the opposite.
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nateflanigan
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Re: New project - Rack ReVibe
« Reply #12 on: January 25, 2012, 06:52:57 am »
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Andrew, I think I'm missing something, please correct me if I'm wrong...

If we have a Low Z, line level signal and feed it into a 600:10k tranny, that will give us the impedance the circuit is looking for but will be stepping up the level even further.

[spacelabstudio]

The timing of this is impeccable.  I was just thinking about this last night.  I'm looking at this schematic:

http://www.el34world.com/Hoffman/images/revibeschematicToreT.gif

I'd think that all you'd really need to do is maybe just replace R6 with a 10k audio pot and feed the V2A and V1B grids with the wiper, so you could control the input level, and have your modern standard 10k input Z, although most modern equipment won't care if it's significantly higher, I don't think, unless it's a transformer output.  If you want balanced input and/or you anticipate feeding it from a transformer output, then I guess you need the input transformer in front of that.

I was fantasizing about a stereo version.  The reverb bits you'd just duplicate, but on the vibrato side you should only need one LFO.  It would be interesting to feed the LFO into a phase inverter so that L and R could be antiphase.  I'm not having such a good time making heads or tails of that schematic, though.  It's not at all clear to me what the function of V5B is or what's going on with V4.

Chris

[andrew_k]

Here's your input side - http://wiki.nimbleswitch.com/NewYorkDave_ReAmp_Box

[sluckey]

It would be interesting to feed the LFO into a phase inverter so that L and R could be antiphase.  I'm not having such a good time making heads or tails of that schematic, though.  It's not at all clear to me what the function of V5B is or what's going on with V4.

Here's a circuit description for the 6G4 trem circuit. Look on page 9.
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

The circuits are not identical but function exactly the same. V4 is the modulator. V5B just inverts the trem signal to modulate the treble signal at V4A.

[nateflanigan]

I was just looking for the NYD reamp, thanks!  I've built one of these before ( http://www.jensen-transformers.com/as/as092.pdf ) using an edcor transformer, it's a very useful little box.  That or many of the other "reamp" style boxes could go in front of the revibe pretty easily.  That much I get. But what to do with the output to bring it up to line level?  I realize that a sort of built in DI on the output could be fed into a spare channel on a board, that would be easy, but for the sake of science it'd be cool if we could think up a way for it to be placed between a send and return without any extra patching around.

  

[spacelabstudio]

I guess you could steal the output stage from an LA2A:

http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_1968.jpg

Looks like a cascode(?) into an output transformer.  If the output of that thing is close to regular guitar level, then it probably won't drive the 12bh7a really well, but mabye a 12au7?  I don't know, I'm still trying to learn, so take that as a stab in the dark with a hope of getting an expert to chime in.  600 ohm secondary impedance is an anachronism, but a lot of new vintage style equipment still uses it.  On my Electro-Harmonix NY-2A I have a 620-ish resistor strapped across each output so it can see the impedance it wants even driving modern 10k+ input gear. 

Chris

[spacelabstudio]

It would be interesting to feed the LFO into a phase inverter so that L and R could be antiphase.  I'm not having such a good time making heads or tails of that schematic, though.  It's not at all clear to me what the function of V5B is or what's going on with V4.

Here's a circuit description for the 6G4 trem circuit. Look on page 9.
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

The circuits are not identical but function exactly the same. V4 is the modulator. V5B just inverts the trem signal to modulate the treble signal at V4A.

Aha!  I didn't realize it was that kind of vibrato.  I have Catalinbread's Pareidolia pedal inspired by that circuit.  Really cool sound.  Now I'm even more interested!  So I guess the two halves of V5 are managing to oscillate out of phase from each other somehow.

Chris

[sluckey]

So I guess the two halves of V5 are managing to oscillate out of phase from each other somehow.

Yes. V5A is the actual oscillator. Call it's output 0º phase. It's output is sent thru the intensity pot to the grid of V4B to modulate the low freq guitar signal. But, the 0º trem signal is also sent thru R41 to the grid of V5B. V5B is a typical inverting amplifier. The trem signal is inverted at the plate. Call it 180º phase. This trem signal is applied to the grid of V4A to modulate the high freq guitar signal. The Revibe V4 is just like the modulator tube in the 6G4.

[nateflanigan]

Lot's of interesting ideas spacelab, and as always great insights from sluckey. 

What do you guys think of this...

"reamp" --> revibe --> NYD 1 bottle (http://www.groupdiy.com/index.php?topic=6711.0) skipping the input tranny of course.

The idea being to feed the "mic pre" circuit with a guitarish level signal straight to grid.

[spacelabstudio]

Well, you've essentially just built a reamp and a mic pre into the same case as the revibe.  Since those are standard tools you'll find in any studio, is it silly to build them into the same unit as the revibe?  They're certainly more flexible as standalone building blocks and then the revibe could be used with an actual guitar signal as well. 

Given that, if you want a revibe that can be used standalone with studio line levels without using a reamp and mic pre, I would maybe consider the simplest possible solution first.  On the input side, the main challenge is just to prevent overdriving the input circuitry that's expecting a small guitar signal.  Throwing a pot on the input should solve the level problem, and most modern gear isn't going to be too picky about the impedance.  Make it a 10k to conform to modern standards, and make it bypassable or have a seperate 1M input for guitar still. 

On the output side, we need to get the level back up to line level and make sure that whatever device is doing the driving is loaded properly. A 12au7 cascode stage seems to be pretty common for that kind of thing and can probably drive 10k unbalanced without a transformer, or balanced with a transformer.  Have to figure out how to couple and bias it.  I'm not entirely convinced it's worth the trouble, but I guess could be a good learning exercise.

[nateflanigan]

I'm not entirely convinced it's worth the trouble, but I guess could be a good learning exercise.

Totally, at this point the conversation is just for fun.  The theoretical purpose is, if one were to try and present a proaudio product based on the 6g15 you wouldn't want to say "in order to mix with this you need a reamp and a spare mic pre"

On the output side, we need to get the level back up to line level and make sure that whatever device is doing the driving is loaded properly. A 12au7 cascode stage seems to be pretty common for that kind of thing and can probably drive 10k unbalanced without a transformer, or balanced with a transformer

In my mind that's exactly what the 1 bottle would be doing.  We already have the B+ on hand, it's only adding one more tube (albeit a 12av7 instead of 12au7) and it's already designed to accept the levels we'd be sending it and output to a line level load.

In reality, when I get to building this I'll probably try to leave room in the box to add these sorts of things, but won't really worry about it. 

[spacelabstudio]

I'm not entirely convinced it's worth the trouble, but I guess could be a good learning exercise.

Totally, at this point the conversation is just for fun.  The theoretical purpose is, if one were to try and present a proaudio product based on the 6g15 you wouldn't want to say "in order to mix with this you need a reamp and a spare mic pre"

Ha, true!

On the output side, we need to get the level back up to line level and make sure that whatever device is doing the driving is loaded properly. A 12au7 cascode stage seems to be pretty common for that kind of thing and can probably drive 10k unbalanced without a transformer, or balanced with a transformer

In my mind that's exactly what the 1 bottle would be doing.  We already have the B+ on hand, it's only adding one more tube (albeit a 12av7 instead of 12au7) and it's already designed to accept the levels we'd be sending it and output to a line level load.

In reality, when I get to building this I'll probably try to leave room in the box to add these sorts of things, but won't really worry about it. 

I like forward to seeing what you come up with!

Chris