Questions revolving one tube reverb circuit

[überfuzz]

I have an amplifier with no reverb. I plan to add a spring reverb, I have enough room in the chassis and enough juice from the PT. My idea is to source a high impedance reverb tank. Set up a gain stage with a 12AT7, or so, one triode for the driver and the other for the recovery. I have some questions.

1. Why is it common to use low impedance tanks in high impedance circuits, such as tube circuitry?
2. How do I match the dry signal path with the reverb circuit? I guess the question is how do I choose a resistor going in parallel with the reverb circuit?

I think that's all for now...

cheers!

[Chuck H]

It takes a lot of power to drive a reverb pan (speaking relatively to common preamp tubes). A transformer driven tube provides the most power. Since reverb pans of the day were commonly designed to operate off the power section of many reverb systems (before they started to be incorporated into preamps) I think this could be why the low ohm tanks were chosen in the first place. They were what was available. And since the tanks were low impedance, the transformer secondary was low impedance. Then this system was used on what ended up being, perhaps, the worlds most popular amp line, the BF Fender. The rest is history. Circuit design has a lot of memory and momentum. Why do we use high-ish impedance guitar pickups today when low impedance devices and preamps would be more efficient? For the same reason we still sometimes use low impedance reverb tanks. That's how we know to get the right tone. Getting outside the box requires a leap of faith on the part of both the designer and the consumer. Most people won't go there when their tone is at stake. I suppose for efficiency you COULD use a transformer to drive a higher impedance tank, if they made the right transformer at a reasonable cost. I don't remember ever seeing one. So the momentum may also be partly due to builders using the parts that were available like the 8 ohm secondary matching transformers for BF Fender type amps. Since available parts are the likely reason for the low impedance circuit in the first place the loop just keeps going round and round.

It's worth noting that there are a lot of modern reverb circuits that don't use the low impedance tanks. Getting a satisfactory result from a tube/capacitor driven reverb would depend a lot on circuit design. Since it has never been done to where it sounded as good as the transformer driven reverbs that Fender introduced perhaps that's another reason for adherence to the old design.

I don't think a one tube (one triode for driving the tank and one triode for recovery AND mixing) cap coupled reverb circuit would work well retrofitted into a vintage style amp. In fact I don't think one tube transformer coupled reverb circuits work that well unless there is some place in the amp circuit to mix and for another amplification stage. JM2C

[KevinOConnor]

Hi Guys

One-tube reverb loops are always a compromise and never sound that good, but.. it depends on how much you really care about reverb. For me, I would give up distortion before I gave up reverb so it is has to be a two-tube reverb circuit.

The mechanical delay line has 40dB of loss and that can be overcome using a 12AX7 stage for recovery. The input side needs a small amount of power, typically 10s to 100s of milliwatts regardless of the impedance. Traynor used a high-z input tank driven by a a 12At7 stage with resistive load - no transformer - and this worked okay for low reverb levels. PV did something similar as did other brands much earlier. Using a transformer to drive the tank provides better efficiency and then even a low-power tube can be used with good effect. Marshall used a 12AX7 for both drive and recovery using a high-z drive transformer.

The drive transformers most often used are SE and of lower primary impedance than needed. This is part of the lethargy of reverb design begun by Fender, who used a Champ OT driven by a 6V6, then later adapted things to use a parallel 12AT7.

The most basic loop is sort of like what Chuck described but not quite: one triode driving a transformer and another triode as dedicated recovery, with passive mixing of the dry and wet signals. This is compromised because the loop is often placed at the input of the power amp where a line-level signal is needed. The reverb loop does not have a good enough balance of wet tone to give a deep reverb sound. Driving the tank harder often makes the reverb sound too "boingy", so that is only a viable approach to a point.

With another tube bottle, the recovered sound can be boosted to any level you need. The standard Fender loop uses one whole bottle to drive the tank, with a dedicated recovery stage, passive mixing and another level recover stage. This is not an active mixer. overall loop gain is higher than unity. A variation of this is to have two dedicated recovery stages and passive mixing.

Have fun
.

[tubeswell]

A reverb pan input transducer requires lots of current to get the spring to twitch. So traditional designs use a transformer to convert the high voltage/low current output of the driver tube, into low voltage/high current to drive the transducer. You can drive a transducer with a higher input impedance directly from a driver tube (transformerless reverb), but you want to use a gain stage configuration that has as-low-an-output-impedance as possible, and as-high-a-current-output as possible. Merlin Blencowe has a suggestion on this topic at the bottom of the page on this link.

You also want a good strength of recovery signal because a reverb pan's output transducer typically puts out a pathetic signal. A single common ground inverting recovery stage will get you back upto a signal strength that can compete with most guitar pickups, but not with a dry signal that has already been boosted by one or more gain stages in the amp's signal path. Many amps use severe dry signal attenuation to get the balance 'in favour of wet', together with additional signal boosting of the reverb recovery side of the pan. Achieving copious amounts of reverb with a single recovery stage is challenging.

[überfuzz]

Okay thanks everyone!

I guess I can boil it down to setting up a gain stage that matches the input impedance of the tank.

Should I match the driver/tank to get a current driver or power driver?

[tubeswell]

Okay thanks everyone!

I guess I can boil it down to setting up a gain stage that matches the input impedance of the tank.

Should I match the driver/tank to get a current driver or power driver?

Current driver for a transformerless circuit or a power driver for a transformer circuit. (I think that's what you are asking?)

[überfuzz]

Current driver for a transformerless circuit or a power driver for a transformer circuit. (I think that's what you are asking?)

Hehe, I take it your reply comes out of knowledge. Thanks!

I sort of stumbled over a tube called ECC832. It has a 2.25W triode and a 'standard' 6AV6. It seems like a good idea to me. Should give me more current from the driver than a ECC83.

[J M Fahey]

Okay thanks everyone!

I guess I can boil it down to setting up a gain stage that matches the input impedance of the tank.

Should I match the driver/tank to get a current driver or power driver?

Yes, it´s basically that, but I suggest you do not reinvent the wheel, there´s already 3 or 4 versions of them, all solidly established, just pick one that suits you.
No need to design starting with a blank sheet of paper.

Please post the amp schematic, at least a simplified one, showing the channel where you want to insert the reverb up to the power amp phase inverter.

No need for extreme details; you can , say, draw a box where applicable and label it "Marshall tone stack", the idea is to roughly estimate signal level and gain at different points.

That said, the options run from the best lusciest reverb, the old Fender type , which requires 2 full tubes, a 12AX7 and a 12AT7, everybody forgets to count the extra triode that the reverb channel needs so that makes 4 of them, to ... gasp ... going SS
After all, many respected tube amplifiers (including Fender and Marshall) do that

[überfuzz]

Here's what I've got so far.

[66 Kicks]

An ECC832 is a 12DW7 and is frequently described as a 12AU7 triode and a 12AX7 triode in the same bottle. Sylvania gives 3.3W for the 12AU7 type and 1.2W for the 12AX7 type.

I think you should use a 4F tank with 235mH for a transformerless 12DW7 driver. Change R18 to 560ohms, C13 to 1.0uf, and insert about 3000ohms between C13 and L4. R16 will need to be rated at 5W. This would be pins 1, 2 and 3 on the 12DW7.

[überfuzz]

When I think of it, I've build a high impedance driver before, using a 9GB2C1B tank. I'll see what I'll can muster (simulate) with one of those.

[überfuzz]

An ECC832 is a 12DW7 and is frequently described as a 12AU7 triode and a 12AX7 triode in the same bottle. Sylvania gives 3.3W for the 12AU7 type and 1.2W for the 12AX7 type.

I think you should use a 4F tank with 235mH for a transformerless 12DW7 driver. Change R18 to 560ohms, C13 to 1.0uf, and insert about 3000ohms between C13 and L4. R16 will need to be rated at 5W. This would be pins 1, 2 and 3 on the 12DW7.

Hi!

I missed your post. Interesting points, but at the moment I don't know enough to just, from my point of view, ramdomly change stuff in the circuit. If you take the to elaborate I'm all ears. :-)

[66 Kicks]

How much the tank shakes the springs depends on how much AC current is flowing through the input coil. We would prefer that the springs shake the same at all of the frequencies that concern us, so we want constant current for those frequencies. In order to get constant current from your driver, the load presented by the coil circuit must be constant. This isn't going to happen with just a capacitor and a coil.

The impedance of a capacitor changes with frequency and so does a coil. The total impedance of a capacitor and a coil in series is the absolute value of the difference of the two impedance values at any given frequency. At low frequencies, the capacitor has a high impedance and the coil has a low impedance so that the total impedance is approximately the capacitor impedance. At high frequencies, the capacitor has a low impedance and the coil has a high impedance so that the total impedance is approximately the coil impedance.

In between the two impedance extremes the impedance drops. When the impedance of the capacitor equals the impedance of the coil, the theoretical impedance drops to zero. In practice there is still some impedance, but it is very low. This brings up the first problem with the 0.1uf capacitor. With a 4F tank, the very low impedance occurs at just over 1KHz. At and around this frequency the signal gets an ugly clip and the reverb sound is undesirable.

The second problem with a 0.1uf capacitor is that the higher impedance at lower frequencies causes less current to flow through the coil than at mid and high frequencies.

If you add a resistor between the capacitor and the coil, you have a series LRC circuit. The total impedance of this is the square root of the sum of the squared value of the resistor and the squared value of the total impedance of the capacitor in series with the coil. Obviously, if we make R very large, the total impedance is approximately R and all of the problems with a 0.1uf capacitor go away. The current will be constant, but there won't be much of it. So we hit the calculator to find an R that is much lower but that still gives us an acceptable range of total impedances at the frequencies that concern us.

After a few strokes on the calculator, the 0.1uf problem reappears. The high impedance at lower frequencies forces us to use an R that is higher than what we want. So we raise the value of the capacitor to decrease the impedance at lower frequencies. I don't want to use a huge electrolytic here because they are expensive at that voltage and they are bulky. I want the smallest one that causes the LRC equation to come out close to what I want.

So it is all a balancing act and 1.0uf, 3Kohms, and a 0.235H coil is acceptable to me.

[66 Kicks]

Do you know how to draw load lines?

[überfuzz]

Do you know how to draw load lines?

Yes I do, but I'm lazy, I use a spread sheet I found on the net. Why?