Spring reverb, calculating impedance

[knobtwiddler]

Be aware that the Grampian spring reverb units are highly collectable. Do a search for 'Grampian 636' (there is a recreation that costs about £3K) and 666. The 636 was used by the late, great Lee Scratch Perry and has cult status. Other models aren't as valuable, but they are still sought after.

NB - the 666 Grampian model uses Germanium transistors. I suspect the 636 uses similar. Germanium circuits are fancied by guitarists for their soft overdrive characteristic, which may well be a reason why Reggae producers such as Perry liked the 636.

[m0cemdave]

Originally Posted by knobtwiddler

NB - the 666 Grampian model uses Germanium transistors. I suspect the 636 uses similar. Germanium circuits are fancied by guitarists for their soft overdrive characteristic, which may well be a reason why Reggae producers such as Perry liked the 636.

The 666 also uses the preferred method of driving the spring with a current, rather than voltage, source. The spring drive coil is in the feedback loop of the amplifier.

I sold mine decades ago, before they became valuable, wish I had kept it...

[suebutcher]

Here's an image of my reverb, and possibly the same reverb in a 1976 Dick Smith catalogue. It looks practically the same, but if it is, the dimensions in the advert are all wrong! Mine is 235 x 55 x 30 mm. But 16 ohms impedance for the input is probably right, given the impedance table in the reverb tank PDF and the DC resistance of 2 ohms.

[Valvepower]

Hello,

Seeing this sent me down a rabbit hole and I remembered the F.C. Judd Reverberation unit in Practical Wireless in the early 1970s.

I realize its solid state, but still worthy of quick glance, if only for curiosity sake.

Link below…

https://www.worldradiohistory.com/UK...PW-1972-11.pdf

Terry

[Radio Wrangler]

Fred Judd was heavily involved in electronic music ("Space Patrol" for those who remember).

He also produced a book on 2m antennae and was responsible for the slim jim antenna. So he certainly knew about matching.

That article shows a 2-spring tank driven without any attempt at matching either the source or load. It seems he got away with it. He drives it with a low output Z voltage source in the shape of a feedback amplifier. The germanium transistors may have been to increase the voltage swing given a limited supply voltage. The load end is into a high-ish Z amplifier.

It may be that being reflective at both ends ads multi-pass echoes increasing the reverberation ambience.

David

[Jez1234]

I may be travelling this road myself at some point in the future as I'm fancying a guitar amp for doing mainly Billy Bragg covers! Small with overdrive and clean channels switched by a foot switch and of course reverb. Valved (or mainly valved. The drive and return to the reverb will probably be SS) and I have a Pye fenman 2 chassis which can donate the mains and output transformers I have a couple of Marshall guitar amps I can scrap for a reverb line (yes it would be really easy to just fix one of the Marshalls but that would be cheating and as they are not valved Marshalls they are probably worth nowt so not worth sorting out for sale). I have a vintage Ampeg SS guitar amp of only around 12" x 15" x 8", prob originally 10W or so, SS and from maybe '72 ish which can be the casework and speaker as it contains a Jensen alnico magnet 10" unit. It's just an idea I've been formulating recently for a project probably next year some time.

[suebutcher]

Originally Posted by Radio Wrangler

Fred Judd was heavily involved in electronic music ("Space Patrol" for those who remember).

I do remember that the music was one of the best things about that show.

[Radio Wrangler]

Some demos of different effects and a sample piece of Fred's music was given away on a floppy 45rpm disc with one month's Practical Electronics.

David ... Gamma rays, on. Yobba rays, on.

[Valvepower]

Hello,

Talking F.C. Judd, I found the attached F.C. Judd book at the Tonbridge Audiojumble back in March.

Terry

[dougietamson]

If you want to drive a higher impedance spring line unit usually found in solid state driven reverbs with a low cost valve eg EF80 and the bonus of not needing a transformer (it's anode output is coupled with a 1uF non polarised capacitor) then there is a good article here:

https://valveheaven.com/2016/10/

I've built the driver and it sounds great.

I've also driven low impedance (8?) tanks with small radio SE OTs and tried quite a few low value/low power pentodes with good results. This was part of junk bo bits n bobs build of a Fender 1963 reverb unit clone (surfs up...).

Here's the write up an some sound clips with the results:

https://groupdiy.com/threads/wind-a-...sformer.80908/

EF80 and retro mono synth:
https://on.soundcloud.com/DE4ar

EF80 and surf guitar:
https://on.soundcloud.com/eTUeF

My favourite was an old EL42 as the driver.

Doug

[vidjoman]

Around 1965 I built a spring reverb unit with germanium transistors driving a Garrard GCS10 stereo cartridge with another at the receiving end. It was very easy to replace the stylus shank with a slightly stretched spiral electric fire element. Don't remember any matching problems and the circuitry was very simple. I've still got it somewhere - must dig it out and see how I did it. Included a 3 or 4 channel mixer as well and operated from a PP9 battery.

[Valvepower]

Practical Wireless did similar reverb project using two crystal pick up cartridges in 1970.

Link to the July 1970 PW on the World radio history site…

https://www.worldradiohistory.com/UK...PW-1970-07.pdf

Terry

[dougietamson]

I've been planning a DIY plate reverb project, must get round to it.

https://www.youtube.com/watch?v=CZWAntOnrx4

[suebutcher]

Originally Posted by Valvepower

Practical Wireless did similar reverb project using two crystal pick up cartridges in 1970.

Link to the July 1970 PW on the World radio history site…

https://www.worldradiohistory.com/UK...PW-1970-07.pdf

Terry

Some low-end Japanese Teisco guitar amps used a ceramic one-spring reverb, a tiny thing about the size of a Cadbury Flake.