Reverb tank

[Mats]

I looked, in this forum, at the Topic about tremolo and Amp GA-1 RT-1.

Looks like something I would like to test.
I understand the Tremolo, but the Reverb and to hook it up to another amps speaker.
So it will work like one amp, dry signal and this amp with the reverb signal.
Am I right about this ?
Can you recommend a reverb tank that can work in this circuit ?

/Mats

[HotBluePlates]

I looked, in this forum, at the Topic about tremolo and Amp GA-1 RT-1.

We should look at the Gibson GA-1 RT-1 schematic.

I looked, in this forum, at the Topic about tremolo and Amp GA-1 RT-1.

Looks like something I would like to test.
I understand the Tremolo, but the Reverb and to hook it up to another amps speaker.
So it will work like one amp, dry signal and this amp with the reverb signal.
Am I right about this ?

I think you might be talking about the topic,    
On the Bench: Gibson GA-1RT-1 What the......? Is that correct?

This amp, its reverb, and everything is all one self-contained unit. The Red & Black inputs shown leading into the reverb tank are the + and - outputs from the output transformer secondary.

That is, the amp takes its speaker output (which is probably ~8Ω), feeds that into a reverb tank, and applies the reverb output to the 1st preamp stage.

Pretend it's 1962, and you're Gibson. Fender just came out with their outboard reverb unit, which plugs in between the guitar and amplifier input. Fender's unit takes a guitar signal and uses it to drive a 6K6 output tube, whose output transformer drives an 8Ω reverb tank. The resulting signal is applied to an amp input (after Dwell, Tone & Mix controls).

You want to one-up Fender, and your budget student amp has an 8Ω speaker output. Why not use that speaker out to drive a reverb pan, and mix that pan with the same amp's input signal? Sure you don't have all the controls of the Fender unit, but you "cheap amp" already has reverb Built- In!!

But wouldn't feeding the amp's output back to its input make it feed back? At low levels, the reverb output is different-enough so that the answer is No, but as the linked thread says, it will feedback at higher volume with the reverb on.

Can you recommend a reverb tank that can work in this circuit ?

The same 8Ω tank used by all old Fender amps.

Now if you want, you can build an amp that has no actual dry signal, but takes another amp's dry signal as input to the reverb, then amplifies that enough to drive a speaker. I suppose that's a way to add reverb to a distorted amp sound, where you reverb the distortion, rather than getting distorted reverb. It might be hard to match the Reverb Amp's power level to all desired dry amps...

Some old amps fed their dry output to one set of speakers (in the combo cabinet), and tapped that output to drive a reverb pan, then amplified through to a small single-ended output stage which drove other speakers (in the same combo cabinet). It was either THD or Budda that built a tweed Bassman copy that did exactly this, with 3 of the 4 speakers driven by the main amp, and 1 speaker driven by the reverb side-chain amp.

That it takes this much for me to explain the concept just goes to show the idea didn't really catch on, even the 2nd time around...

[eleventeen]

I'm going to place two questions regarding reverb tanks here, since someone else brought up the topic.

Q1: As one who likes to convert dead Peavey amps into toobers, the following question arises. These amps, when so equipped, have 600 ohm input reverb cans that are otherwise, well, regular reverb cans. So what would be the smart way to use these in a tube amp? I assume (but do not know for sure) the normal 12AT7 > little SE speaker transformer a la Fender would massively overload the input of the  reverb can in terms of signal level. Is there anyone who has developed or found a cathode-follower circuit that would allow one to directly drive a 600 ohm (input) reverb can from a tube stage w/no transformer? I imagine this might require a lot more tube than a 12AT7 but it would sure seem possible.

Q2: In a Fender amp, the dry signal is split off, amplified into the 12AT7 > transformer stage, and recombined on the far side of the 3.3 Meg/10 pf network. Clearly, there is some sort of phase relationship implied between the two signals. Has anyone tried to reverse the phase of the transformer to see what, if anything, would be the result?

[HotBluePlates]

Q1: As one who likes to convert dead Peavey amps into toobers, the following question arises. These amps, when so equipped, have 600 ohm input reverb cans that are otherwise, well, regular reverb cans. So what would be the smart way to use these in a tube amp?

I'd probably try to steal a MOSFET source follower, like Tubenit has successfully used in other applications, and plug it in here. Yeah, the MOSFET has more voltage & power capability than will be used, but no matter.

The thing to think about is that a solid-state circuit drove it before, and that's still probably the easiest way to drive it now.

... I assume (but do not know for sure) the normal 12AT7 > little SE speaker transformer a la Fender would massively overload the input of the  reverb can in terms of signal level. Is there anyone who has developed or found a cathode-follower circuit that would allow one to directly drive a 600 ohm (input) reverb can from a tube stage w/no transformer? I imagine this might require a lot more tube than a 12AT7 but it would sure seem possible. ...

I assume (and could be mistaken) that the input power needed for either impedance tank is the same. That is, that the product voltage*current is the same regardless of what tank you use.

Then the higher impedance tank expects more voltage for the same power input, because P = V²/R, and you made R (the 600Ω rather than 8Ω) bigger. You can then verify for yourself with Ohm's Law that current dropped for the same power input for the 600Ω tank case.

So then what about driving the 600Ω tank with the usual Fender transformer circuit? The usual small reverb transformer is like 22kΩ:8Ω; if you have a 600Ω tank, the transformer reflects 22k / 8 * 600 = 1.65MΩ to its primary, which will keep the 12AT7 from developing any power output.

What you need is a transformer with an impedance ratio of ~22kΩ/600Ω = 36.7:1, or a turns ratio of ~6:1. If you now remember that recording equipment used to run on 600Ω lines, and had transformer that matched tubes (usually 6SN7's/12AU7's) to those lines, you'll see you have options for a suitable transformer.

Because the sky's the limit on tube audio stuff mentioned in the last paragraph, I might call someplace like Edcor, tell them you need a transformer for ~22kΩ:600Ω (with the 22kΩ flexible), for tube use (push-pull use would be even better than SE), and that cost is key not wide frequency response, they may tell you they already have an inexpensive option on the shelf...

We could calculate the ideal plate-to-plate load impedance for using a push-pull 12AT7 stage, but it's probably faster to assume it will be numerically equal to the SE load commonly used (an approximation that usually works with bigger tubes).

[eleventeen]

Thanks for your replies, HBP.

Any idea about phasing the rev tank output?

The idea has me intrigued, not just the notion of flipping the reverb driver output phase by 180 degrees (easy, flip the reverb trannny output leads) but imagine a circuit that would flip/vary the phase of the reverb driver signal...potentially, it could generate a very, very subtle flanging effect that could be kind of interesting. And of course, waaaaay less drastic (and in the 21st century, corny) than flanging.

[HotBluePlates]

Thanks for your replies, HBP.

Any idea about phasing the rev tank output?

... imagine a circuit that would flip/vary the phase of the reverb driver signal...potentially, it could generate a very, very subtle flanging effect that could be kind of interesting.

Have you seen a phase meter, like the one below?

If you were monitoring a single mic pickup up snare drum hits, you would see a display that is a straight diagonal line from top-right to bottom-left. If you then ran that snare signal through an outboard reverb with just a little effect, you'd see the straight line but also a circle radiating out from the center of the meter graph.

That is, reverb adds so many delays that it blurs phase relationships. Further, all those phase relationships are constantly changing, so that the circle isn't uniformly round. Phasing or flanging are very much shorter delays, and usually varying at a single rate. If we think about all these (along with reverb) as time delays, the reverb is a delay hundreds of time longer than flanging/phasing, and maybe 40-50 times as many delays.

Actually, the fact that you have feedback (output back to input) that doesn't cause howling at low level but does at high levels, shows that the phase of the feedback signal is varying and uncertain. The manufacturer just hopes you won't turn the reverb and the amp up too loud (or buy one of their more expensive amps with better reverb implementation).

Bottom line, if you want to add flanging or phasing to a reverb signal, okay. Or if you want to reverb a phased/flanged signal, okay. But this circuit is the lowest-budget way to implement a reverb, and probably isn't a good candidate to base other effects on.

[jeff]

So it will work like one amp, dry signal and this amp with the reverb signal.
Am I right about this ?

I believe you are correct. I've done it and it sounds good.(8ohm tank don't forget the 33ohm resistor< some ampegs do this dry speaker/wet speaker)

The tank attaches to your main amp with no effects and this amp gives you reverb and trem out this amps speaker.


"This amp, its reverb, and everything is all one self-contained unit. The Red & Black inputs shown leading into the reverb tank are the + and - outputs from the output transformer secondary."
I'm not so sure about that

The reason I don't believe that this is foldback(amp's tank connected to amp's speaker) is the footswitch. Take a look at it again. If it were connected to its own speaker why would the foot switch switch between the tank or a 1M 3K3 voltage divider. Wouldn't it be tank/no tank? Why would you take the output and feed it back to the input?

If used as an outboard effect then it makes sence because the foot switch then switches between reverb comming out the secondary speaker or clean(to be tremed)

In other words if you're using two amps the footswitch switches between clean and reverb coming out the 2nd amp

If used as one amp folded back then the switch switches between reverb and feedback. Any reason to do that?

  ...I think...

Another point to support my arguement is that there is now way to adjust the level of reverb if the tank is connected to the speaker. If the speaker is connected to the tank then the reverb is always at full blast. If however this is to be used as a "wet" amp then the main amp sets the volume and this amp's volume adds the level or reverb/trem.

 

[HotBluePlates]

So I recently read a description on one website that claimed this amp was just reverb fora 2nd amp. I suppose it's possible, but I think the only way to prove this for certain is to read the catalog description of the amp. Gibson's intended use will probably be apparaent in the catalog copy.