replacing a crystal-driven reverb

[Alex R]

The early 60s Bird amps used a spring reverb tank in which the drive and pickup were handled by crystal record player cartridges. They soon died of course - I've never actually heard one but I can't imagine they were much good. I'm restoring one of these amps and I would like to put a normal type of electromagnetic tank in there. The amplification side will work ok, but the drive side, well I need some advice. it uses half a 12AX7 and drives the tank input from the plate through a coupling cap, which with the ultra-high impedance of a crystal would have worked ok i guess. I thought if I used a tank with a high-impedance coil as in cap-driven solid state reverbs it might drive the tank ok, but it doesn't seem to. The 30vAC signal that the 12AX7 generates through the cap seems to disappear when faced with the the 400 ohm(-ish) impedance of the tank's input coil. Any ideas or advice about how I might get this to work without a transformer?

[J M Fahey]

The *simple* solution is with a transformer
Ampeg, among others, avoided it but needed a much lower plate resistor (closer to tank impedance) , much higher current, and geberally a low power pentode to comply with these specs.
Check them.
But it implies adding a tube (or replacing 12AX7 with, say, a triode-pentode such as ECL82).
Or add a SS reverb circuit using a dual Op Amp like most do today.

[52 Bill]

I've not seen a Bird amp, but I have seen a few of the old AC-30s that used a similar system with the Sonovox ceramic phono carts. Even with NOS phono cartridges, they don't sound very good. The old Danelectro reverb units used a similar ceramic element and sounded just as bad.

I've tried using small pieces of Piezo transducers to replace the ceramic elements. The return side works very well, but the drive side lacks the power efficiency necessary to drive the spring. As Juan pointed out, most versions of capacitance coupled drive circuits used something more powerful than a half of a 12AX7. What is the other half of the 12AX7 driver tube being used for?

[Steve Conner]

MOSFET follies anyone? I think a source follower direct-coupled to the plate of that 12AX7 would give it some wallop. It may need a quite low value bias resistor and some sort of heatsinking.

[Enzo]

What pan are you using? A 4FBxxxx woul dbe the choice in this situation. That is 1400 ohms impedance.

AMpeg used a triode drive into a High Z pan from the plate:

http://www.webphix.com/schematic%20h..._vt22_vt24.pdf

http://www.webphix.com/schematic%20h...s/v2preamp.pdf

http://www.webphix.com/schematic%20h...rocket_12r.pdf

http://www.webphix.com/schematic%20h...2_gemini22.pdf

[Enzo]

What pan are you using? A 4FBxxxx woul dbe the choice in this situation. That is 1400 ohms impedance.

AMpeg used a triode drive into a High Z pan from the plate:

http://www.webphix.com/schematic%20h..._vt22_vt24.pdf

http://www.webphix.com/schematic%20h...s/v2preamp.pdf

http://www.webphix.com/schematic%20h...rocket_12r.pdf

http://www.webphix.com/schematic%20h...2_gemini22.pdf

[g1]

I'll second that 4FB series tank, driven through a cap. Traynor also did it that way a fair bit.
traynor_guitarmate_ygm3a.pdf

[Alex R]

It's a 4FB tank, yes, I just ordered the highest impedance input coil i could.

The other half of the 12AX7 is the trem oscillator, unfortunately. There is an unused half 12AX7 elsewhere, but I'm a bit leery about causing oscillation if I wire the plate across to it. Might give it a go though, it's the simplest idea. And a 0.1 (Traynor) or 0.47 (Ampeg) as the drive cap, it's a .001 currently, I think. Thanks for the advice, i'll let you know what works.

[J M Fahey]

Simplest/safest: 22921 Fender Reverb Driver Transformer MADE IN USA

All of U$16 solves your problem

[bluecat]

Luckily I got to read this thread! And yes, more than 10 years old, but very helpfull!
I´m working on a Teisco 10 tubeamp (7189A Pentode, 6X4, 12AX7, 12AX7), got it back to life and it is sounding great.
Sadly, this old crystal driven reverbspring is dead and gone. Help is needed.
So i´d like to ask AlexR., or anybody else, did you solve this problem, changing from crystal-driven to megnetic-driven reverb ?
Teisco10 uses half of a 12AX7. Did anyone try that solution using the Fender transformer ? What changes have to be made?

Greetings from northern Germany.

[Chuck H]

Ordinarily (and ideally) you want to start a new project in a new thread. Referencing this thread in the new one and include a schematic. Not that you've done anything wrong but that method seems to get more responses than resurrecting an old thread. That said...

The schematic I looked at doesn't indicate pin numbers for the tubes. If it's convenient I might suggest a replacing the reverb driver tube with a 12dw7. The lower mu triode is much better at handling current and though it's still not an ideal match for the standard reverb transformer it's better than the 12ax7. I've done fine in a couple of designs driving modern spring tanks with the low mu triode of the 12dw7 and the standard reverb transformer. The higher mu triode of the 12dw7 should be equal to the 12ax7 for whatever duty that triode serves in the amp. So I guess it depends on your layout and the pinout for the reverb driver triode in your amp whether you'd need to rewire the socket. The low mu triode for the 12dw7 is pins 1,2 and 3.