Spring reverb, calculating impedance

[suebutcher]

I have here a 1970s Japanese spring reverb I'd like to use in a music amp I'm constructing. The plan is to drive it via a 6DX8 (ECL84), so I need to choose a transformer to match it to the valve's pentode section. The reverb's drive winding is about 2.5 ohms measured with DC, but the inductance of the winding is about 3mH, which would be about 20 ohms at 1kHz. Can I conclude that I need a transformer to match 14kohm (the valve's approximate output impedance at the maximum plate voltage of 220V) to 20 ohms? Or is there some other factor I haven't considered?

[Radio Wrangler]

That impedance will be modified by mechanical resonances and across the audio band will have peaks and dips, some quite sharp.

This is not unlike driving a loudspeaker and the values seem fairly similar.

Forget the word 'matching' that applies when two things are made equal in impedance (or conjugate) to each other. Here, you are pondering how to transform the spring transducer to a load appropriate to the valve characteristics.

In a loudspeaker, drive voltage relates to voice coil velocity because velocity creates proportional back EMF If the velocity is too great, back EMF opposes the drive voltage and reduces current, reducing force and therefore acceleration. This is how speakers produce relatively flat responses despite their wildly wandering impedances.

So your drive amplifier needs to present a low output impedance to the transducer to damp its resonances, and the amplifier had to be capable of enough current and voltage swing to drive the transducer to the wanted level on both its impedance maxima and minima.

If you do a load-line on a set of characteristic curves for your chosen valve, the inductive component of the transducer (or loudspeaker) produces not a simple load line, but an elliptical one.

It all gets rather complicated and I think most get done on the suck it and see principle.

David

[lesmw0sec]

I am not familiar with that particular reverb unit, but seem to recall that in general, the power required to drive them is quite small, so my only comment would be that perhaps your choice of output stage might be on the high side.

[suebutcher]

OK, perhaps 10 ohm would be a good starting point.

[paulsherwin]

I agree with David, these things are usually less critical than people expect. A small output transformer from a scrap radio will probably be fine.

[suebutcher]

I've got a Australian Rola ECG87, a small 1950s speaker transformer with a 5000/3.7 ohm winding. That's about the right turns ratio I think.

[Jez1234]

I'm sure that will be close enough for jazz as they say... 12AT7 (ECC81) is quite a common driver for these and they don't need much level.

[Radio Wrangler]

Originally Posted by suebutcher

I've got a Australian Rola ECG87, a small 1950s speaker transformer with a 5000/3.7 ohm winding. That's about the right turns ratio I think.

That sounds like an ideal candidate.

ECL82 biased for lowish current has Ra of about 40kOhms by my square-counting. So with that transformer and no feedback, the transducer will think 30 Ohms is driving it. We're in the ball-park.

You could bring the impedance down with some feedback, you've got plenty of gai n to throw away, if you want to exert more damping on the spring line. If you want to try less damping then series resistance is possible as another way of throwing gain away.

So by dint of waving a wet finger around, that transformer would put you in a reasonable place, with room to move around in several directions.

David

[Valvepower]

Hello,

From memory the Hammond Accutronics reverb tray used in guitar amplifiers was 8Ω although there were higher impedances available.

The driver transformer was typically a single ended jobbie and said by Jez being driven by two triodes in parallel typically the two halves of an ECC81.

As an example here is link to a ‘replacement' Hammond part for Fender.
https://www.hammfg.com/part/1750AX?referer=1025

As previously said, it’s all quite noncritical.

Terry

[retailer]

I wound the reverb driver transformer for the Fender amp copy amp I made a few years back, link :- https://www.vintage-radio.net/forum/...d.php?t=159474

I used a small mains transformer as the donor, and wound it for 25K primary and 8ohms secondary, the E's and I's were assembled as for a SE transformer - the driver valve used in the Deluxe Reverb is a 12AT7 with both triodes connected in parallel.

[suebutcher]

Good-o. I picked the 6DX8 as a driver because it's cheap, I've got a couple of good ones, and Australian guitar amps used it for this purpose. Circuits here were often based on TV valves to keep costs down. The amp is loosely based on the U.S. Vox Cambridge V3, which has a paralleled 12AU7 as driver and a 12AX7 as pickup - two sockets and two expensive valves when one could do the job! I'm trying not to spend money on this in case it doesn't work. If it does, great, but if not it doesn't matter.

[suebutcher]

Originally Posted by retailer

I wound the reverb driver transformer for the Fender amp copy amp I made a few years back, link :- https://www.vintage-radio.net/forum/...d.php?t=159474

I used a small mains transformer as the donor, and wound it for 25K primary and 8ohms secondary, the E's and I's were assembled as for a SE transformer - the driver valve used in the Deluxe Reverb is a 12AT7 with both triodes connected in parallel.

I shall be reading this article very carefully! I do have a small mains transformer as an alternative to the Rola.

[Ed_Dinning]

Hi Sue, the turns ratio on a mains transformer are calculated at 50Hz. You can probably reduce turns if your lowest frequency/ 3dB point is much higher than this,

Ed

[Radio Wrangler]

Triode drivers will have appreciably lower Ra than your ECL82, and so present a lower driving impedance to your spring driving transducer.

The tetrode was invented as a solution to the low Ra of triodes limiting the maximum amount of gain which could be got from a single valve stage, and the pentode and beam-tetrode wer derived from the tetrode to improve linearity while retaining the gain.

There may well be a difference in sound between the amps with ECC81 and ECL82 doing the driving duty. If you notice a difference or have a preferred amplifier, then that's the best reason for one choice over the other.

David

[suebutcher]

I won't be using an ECL82, the spring reverb won't need 3 watts, and that valve has nearly twice the current requirement of the ECL84. As with the rest of it, the power transformer is a junk box item of just-sufficient capacity. I've already saved power by using silicon rectifiers rather than an EZ81.

[G6ONEDave]

If it helps, a company called Wilsic Music used to use the Altai (or was it Eagle) LT44 and LT700 audio transformers for coupling to and from their reverb spring lines. It might be worth checking the specifications out on these transformers, although I think they were using them with transistor circuits.

Dave

[retailer]

Not sure how far you've progressed with the reveb tank project, but if it is still in the pipeline this may interest you, it has some good info.

[Valvepower]

Hello,

Ooh, those Accutronics part numbers send me back with the memories of building guitar amplifiers with reverb - combo amps were fun with the risk of feedback between the speakers and reverb tray at high levels.

I recently revisited the reverb tray in a Clavioline Concert Reverb keyboard restoration which appears in a recent Bulletin. Although it used a driver transformer the input transducer was higher than normally seen with transformer driver at 1500 ohm, which was usually around 8 ohm - well on most amplifiers.

Terry.

[suebutcher]

Originally Posted by retailer

Not sure how far you've progressed with the reveb tank project, but if it is still in the pipeline this may interest you, it has some good info.

Thanks, that is useful. I had to put the project aside because of an art deadline, but I'll be on it again soon.

[Simondm]

For what it's worth, I salvaged a spring from an unusual BBC designed* sound desk of the 1960s, a while back. It's a commercial unit (Grampian, IIRC), and was simply driven by a line send amp, and the output was a mic amp - completely passive otherwise.

The spring unit itself has phono connectors, so I suspect balancing transformers were used somewhere (the desk was fully modular, so non-standard amps etc. would be bad, as they couldn'tbe substituted easily).

I have the manuals for the desk (somewhere!). If I can find them I'll see if it's documented.


* Jonnie Longden's design, but stereo. He told me the reverb was an afterthought. It’s strictly mono.