Reverb driver design?

[Steve Conner]

Hi all,

I recently had a setback on my new amp project, when I broke the reverb driver transformer. It was an O.T. from an old tube table radio, and the primary went open circuit.

So I guess I have two questions: Where am I going to get a Fender reverb driver trannie in the UK?

Also, I have one of those small Oxford Electronics universal OTs, but it's a good deal bigger than the Fender part or the table radio part I used.

It also has no airgap in the core, and a centre tapped primary, so I was thinking of wiring the reverb driver tube as a self-split push-pull, instead of paralleling the sections. Anyone know if this will work? My only concern would be that it might make too much power and drive the reverb tank too hard, especially with a 12AU7.

But I don't know how hard is too hard, and the Accutronics data sheets don't really help with that either. It just says "to avoid saturation do not drive the tank too hard" and nothing else.

Or of course I could save space by replacing the whole mess with a LM386 ;-)

steve

[Enzo]

Or even a 4558 ala Peavey. One dual op amp for both drive and recovery.

You could also convert your tube reverb drive to cap coupled from the plate instead of transformer drive. Transformer uses the low impedance reverb pan, while the cap drive wants the high inpedance pan. Look at old AMpeg circuits for examples of this.

Meanwhile a there no sources for Fender parts or Fener replacements over there?

Would not the OT from any small SE tube practice amp work? Like a Champ or something?

How do you parallel the sections of a center tapped primary?

If whatever you do will want to drive the pan "too hard," wouldn't it be siomple enough to pad the signal going into the drive tube with a couple resistors. The reverb equivalent of "turn it down if it is too loud."

[loudthud]

In the TUT books, O'Connor suggests using a push pull drive configuring the dual triode as a differential amp, sort of a long tailed phase inverter. Those reverb driver transformers are a fairly high impedance. I've seen 13K and 20K primarys stated. Hammond makes one now. Do they have European distribution?

[Steve Conner]

Hi guys, thanks for the replies...

Enzo, I meant paralleling the sections of the tube, not the transformer. Oxford Electronics claim you can use their transformer as either a P-P or a SE, but since it has no airgap I'm doubtful. They probably mean that it sucks about equally in both applications

I'm not so keen on the high impedance drive, just because I've already bought a pan with a 8 ohm input. I'm also going to be a little short of chassis space and heater current too, and I have a 12V DC rail, which is why I'm tempted to use a chip for the drive. The recovery is no problem, since I have a spare half 12AX7 anyway.

Loudthud, what transformer did Ko'C say to use with that circuit? A tiny P-P one, or did he have some way of connecting a single-ended primary to two tubes?

I didn't know that Hammond make a reverb transformer, and they do indeed have UK distribution, I'll check that out

On another note, I just figured out that it might have been a good idea to use undersized coupling caps to high-pass filter the reverb drive and recovery. My prototype used full-sized caps, so it would have been flat apart from whatever limit the driver OT put on it. I was picking up a lot of 50Hz hum and low-frequency feedback from the speaker, and the reverb sounded more like the inside of a church than Dick Dale.

[rexindigo]

Hello Steve.
check out this link:
http://www.tube-town.net/ttstore/pro...ZEIT-AUSV.html
its a german site they probably ship to the uk, but the site says that the transformer is currently out of stock ( at least You'll get the information about the Part Nr. etc)

[sjhust]

but he has another one, not from Hammond, that is in stock:
http://www.tube-town.net/ttstore/pro...---022921.html

steven

[Earl]

Never heard this one, but Fender used a transformerless reverb drive in the Champ 12 amplifier. They just pulled the drive signal off the speaker.

Check the schematic here - http://www.ampwares.com/ffg/champ_12.html

[Enzo]

Oh, that is a good trick, I had forgotten that one.

Hammond organs used a similar trick for theri reverb amps. They had a separate speaker for the reverberant sound. They tapped of the main amp speaker terminals for drive and sent that resulting signal to the input of the separate reverb amp.

[CarlZ]

Here's a thought that I've had kicking around in my head nearly forever but never bothered to try to implement...

You've got a whole tube for the reverb driver so what's wrong with the idea of possibly using a dc cathode follower or a bootstrapped follower? You'd get a nice low impedance output that should mate well with most tanks except the extremely low impedance versions, and a fairly simple circuit to boot. Dig through some old Orange schematics. I think they did something similar with one or two of their amps at one point.

-Carl

[Earl]

Enzo - That's exactly right. I worked at a Hammond Service Center back in the mid to late 70's, and installed a few of those satellite reverb systems on Hammonds. They sounded pretty dang good.

[Joe Bee]

CarlZ, do you mean the cathode follower should be directly connected to the 8Ohm tank, without any transformer? If so it would not work, since the CF only "fakes" a low impedance through negative feedback, but it cannot actually drive any real current into a load.

Earl and Enzo, the Fender design is considered very cheap and dirty by many, and I think it really invites oscillations. Think about it, you effectively add positive feedback to the amp.
The Hammond solution is more elegant, but the expense of adding an extra speaker is proabably not worth the effort. Where would you put the speaker, and would the result still look "champ"?

I prefer Steves original plan of running the tank with a self split PP stage. Too much power? Hey, I think you will notice when the reverb drowns out your original sound :-) And if the spring tank starts smoking, just turn it back a notch.

[Steve Conner]

What if the springs fly out and take someone's eye out at a gig?

Anyway, I finished the chassis metalwork on Sunday, and I've made holes for the Oxford Electronics transformer and a reverb driver tube, so it looks like I'm going to try the self-split approach :-) I fancied having a self-split output stage around the place for experiments anyway.

As for the cathode follower thing, I think reverb tanks like to be driven by a high impedance. They are basically an inductive load, so a high impedance drive will get more treble energy in and make the reverb sound brighter. I'm sure I remember Fender using current feedback in their solid-state reverb drive circuits to emulate this, and then hyping it as "authentic shimmering tone" or some similar buzzwords... So I wouldn't fancy using a cathode follower with its low output impedance.

[Kevin Kuehn]

Hi,
A little of topic here but...
I have modified my Boogie Mark III with a cathode reverb drive transformer. I have a single section of 12AT7 driving the stock reverb transformer primary, with only the second half of the
12AT7 as the recovery amp. I'm getting very strong bright reverb with more than enough volume. The stock Mark III (purple stripe) was known for having a pretty wimpy buzzy reverb return with only one 12AX7 section on the drive end. The stock reverb sounded nice, but it was so weak that it buzzed if you cranked it up to a decent level. I now have the level turned way down, and have good strong reverb.

[Edit] thought I had cathode drive reverb on my Super Reverb also, but it turns out that is cathode drive tremelo, driving an LED opto isolator. To many projects going on gets me very confused

Kevin

[Enzo]

Earl and Enzo, the Fender design is considered very cheap and dirty by many, and I think it really invites oscillations. Think about it, you effectively add positive feedback to the amp.

I am not sure how the reverb would add positive feedback. I can't imagine the computer power it would take to calculate the phase relation of the output of the spring unit to the input. It isn't a straight wire, so to speak.

For my money, I'd think the AMpeg approach was cheaper and dirtier - it required no transformer.

Of course to me the cheapest adn dirtiest was that horrid home grown piezo thing that Silvertone used.

[loudthud]

On the Champ12 schematic there is a procedure outlined where you set a trim pot to limit the amount of reverb to prevent oscillation.

I looked through my TUT books. On the London Power Standard, KOC uses the push pull drive and just calls the transformer "316". I couldn't find any Hammond part with that number. In another place he uses a 125A which IIRC is a 3W universal OT. In at least three other places he just uses the more or less standard Fender configuration.