Transformerless reverb

[jeff]

I found this on Valvewizard. Anyone try this yet?


Cheap current source drivers:
 Low impedance reverb tanks seem to be persisting in valve amp design, which makes little sense since valves are not well suited to driving low impedance loads. By taking advantage of high-impedance tanks (usually intended for op-amp drivers) we can build much simpler valve drivers. A constant-current source can be approximated by using a voltage source and placing a large resistance in series with the coil. Provided the resistance is large compared to the coil's impedance (ten times, say), the coil's varying impedance will make almost no difference to the current flowing through the resistor and coil over a wide bandwidth. The circuit illustrates how this might work.

A paralleled pair of ECC82s has an internal anode resistance of about 5k, so for maximum power the load should be about 10k. In this case the load is formed by the parallel combination of Ra and Rl, which are both 22k, making 11k in total (ignoring the coil which is negligible and the output coupling capacitor which is large). Taking a bias point of roughly -6V, the AC loadline indicates that at full 12Vp-p input we obtain about 12mAp-p swing (shared between the two load resistors) making 6mA p-p or 2.1mArms through R2 which is about perfect for an Accutronics type F tank (1.4 to 1.9k)!

[kagliostro]

FBT 500R2

old italian amp with transformerless reverb circuit

Kagliostro

p.s.: in the pdf file the complete schematic

[phsyconoodler]

Look up Traynor amps in Hoffman's list of schematics.A few have transformerless reverb circuits.

[DummyLoad]

and magnatone...
and doric...
and now your next amp?    

keep us in the loop if you do decide to give it a go. 

[sluckey]

...don't forget Ampeg.

[andrew_k]

The Ampeg Gemini II drives its reverb pan with a 6CG7 (aka noval 6SN7) and sounds glorious.

[jeff]

The Ampeg Gemini II drives its reverb pan with a 6CG7 (aka noval 6SN7) and sounds glorious.

Does this use an Accutronic F type tank?(1K5 input). I do have a bunch of 6SN7s and it'd be cool to put the to use.

Thanks for all the suggestions of amps that use this type of reverb.

Anyone try this notice a difference in sound?
Is there an advantage/drawback in using a reverb transformer?
Next amp I build will have reverb for sure, but why pay more if a tran is unnecesary?
What's the benefit of using a transformer and an 8 ohm tank over just using a 1K5 tank.

Has anyone who tried this noticed any major problems with doing it this way?

Amp building was so much easier when I only had one schematic. Having so many options puts me in a state of "analysis paralysis".
I'd love to hear your opionions.

[tubeswell]

Amp building was so much easier when I only had one schematic. Having so many options puts me in a state of "analysis paralysis".
I'd love to hear your opionions.

You'll just have to build 'em all and report back (with soundbytes)

[jeff]

Will do. Can't be bad, just a different way to skin a fox.

[PRR]

> Low impedance reverb tanks seem to be persisting in valve amp design, which makes little sense

The transformer Fender used was VERY cheap at the time.

Resistance-coupling is at-most 4% effective at putting B+ power into a load.

Transformer coupling can be 50%. With a thin triode (12AT7) maybe 20%.

This means 5 times less B+ power to make and filter, or 5 times more power to a load (spring). I think Fender went for more drive to the spring (more than Hammond said to use).

True, in context of a 40 Watt amp, a Watt more or less in the reverb driver is not a killer problem, but more heat and weight is not wanted.

With a sufficiently thin triode, "Rl" may not be needed, as it might be with low-Z sources. It lowers THD a tad, but wastes nearly half the available power; it could be that a no-Rl plan would give the same spring-power and the same THD (and save a part).

The FBT 500R2 uses a pentode and works it at 5 times the current suggested in the first post.

Ampeg V-4 uses a "big 12AU7" with 13mA flowing. The one I had, except re-gigged around 8mA, did not have the La Jolla surf sound of a Fender (it wasn't a surf act so I didn't go digging for more).

Current-source drive is not essential. It naturally gives a useful frequency response; we can shape a rising voltage response other ways.

> different way to skin a fox.

I think that is the moral.

[octal]

I have built Merlin's / Valvewizard's  constant current reverb circuit with (IMO) excellent results. I think the advantage of a constant current circuit is that you're able to drive the reverb tank fully across the spectrum without saturating it, which makes for a better s/n ratio. I have never done a side to side comparison with a Fender type transformer circuit on the same amp though.

I did fiddle with his filter/divider network at the input of the circuit to get the results I wanted. I'm not sure where valvewizard thinks you're going to find 100V p-p in a preamp circuit used at normal volumes, but most of the amps I've built are WELL into overdrive before preamp signal levels get that high.

I really don't think the 'extra heat and weight' of this circuit are significant design considerations (doesn't the extra weight of reverb driver transformer almost offset any needed increase in the PT size in the cap coupled circuit?) and the layout advantage of not having another orientation sensitive transformer is a plus. It's also less expensive. The biggest disadvantage IMHO is that it's much harder to find high impedance input reverb tanks, so if you're a touring musician or something and need to be able to find a replacement tank in a pinch, you're probably better off using transformer driven low impedance tanks which can be readily obtained. 

Nathan