Reverb driver

[Leevi]

If a reverb tank is driven by a one half of 12AX7 and no reverb transformer
is used how much should be the input impedance of the tank?
/Leevi

[sluckey]

Get the highest input Z you can find.

[Leevi]

I know some tanks have even ~2K Ohm input impedance. Do you think
that is enough?
/Leevi

[HotBluePlates]

Not for a 12AX7, without a transformer. Think about some other circuit type... Where would you see a 12AX7 driving a 2k load without serious loading of the 12AX7 and weak output? 2k would be a severe load for a 12AX7 even used as a cathode follower and cap-coupled.

[Leevi]

That is a real case. I was repairing a Japanese amp called TEISCO where the tank is driven with 12AX7 and without transformer.
I saw even the schematic which confirmed that.

The schematic:
Go to the following site and download the schematic (pdf)

http://www.abeltronics.co.uk/download.php?manu=teisco&file=teisco-checkmate-18-amplifier-schematic.pdf

/Leevi

[HotBluePlates]

The linked schematic uses a symbol on the input and output of the reverb tank which looks like the symbol for a piezo element.

Let's guess it is a piezo element; if so, it looks like a small capacitance which looks like a fairly large impedance at lows and rolls off highs quickly. I think I've heard about piezo reverb tanks before, with unflattering comments about the dull sound.

[LooseChange]

Ampeg did a lot of amps with capacitively linked reverb pans.

[OldHouseScott]

A lot of the modern Carvin tube amps also use capacitor coupled reverbs with 12AX7 drivers. I'll try to check my Carvin Nomad's tank, but I'm guessing it's somewhere on the order of 2k input impedance. It doesn't have the soupy wash of reverb like a Fender, but there's enough of it.

Carvin Nomad schematic

[Leevi]

Ok guys, it really seems to be a piezo element. It's probably
difficult find that type of tank for a reasonable price? Maybe easiest solution
could be to add a new tank and reverb transformer?
Thanks
/Leevi

[kagliostro]

Maybe easiest solution
could be to add a new tank and reverb transformer?

I think so

or

you can use a cap coupled reverb tank

anyway I think is better to mod the reverb circuit

here in the forum see the VERY interesting experiences of Tubenit

http://www.el34world.com/Forum/index.php?topic=7957.0

Kagliostro

[tubesornothing]

Check out this thread:

http://www.el34world.com/Forum/index.php?topic=9129.msg82205#msg82205

[Leevi]

Thanks for the thread info Tubesornothing. This sounds a feasible solution.
/Leevi

[tubesornothing]

BTW, that is a 1 tube reverb using a 12DW7 or ECC832, which is 1/2 12AU7 and 1/2 12ax7

[imaradiostar]

I'm a big advocate of the single tube verb now after being introduced by folks here. I've used the 12ax7 into fender transformer/tank with single 12ax7 recovery circuit a few times and I'd liked it, mostly on very low powered (under 20 watt) amps.

If you need more recovery gain (say you're feeding the 2nd input of a LTPI that's driving big output tubes that need lots of voltage input), you can always use a 6gh8a or 6u8 or any of a bunch of TV tubes. They have lower plate resistance than a 12ax7 but gain approaching a 12at7- this means higher transconductance so they'll drive the reverb nicely. The pentode side can be set up for very high gain to recover the verb and boost it to a high enough level to blend it in right at the phase inverter.

I also like the single low mu triode into cap into high z tank, mosfet recovery thing. You can make a really great recovery amp on a square inch of perfboard with an LND150 and it'll squarely "outsurf" a 12ax7. It also uses less filament current! I like to save filament current for fun things that are in the signal path and complement an amp's tone like cathode followers and booster gain stages.

jamie

[tubesornothing]

I like to save filament current for fun things that are in the signal path and complement an amp's tone like cathode followers and booster gain stages.

The reverb recovery *is* in the signal path.

[imaradiostar]

Eh...I guess it is because you hear it but in the strictest sense it's a parallel circuit. You could remove the verb and the amp would still function- I would call it an effect path.

Either way- verb recovery amps are usually aiming for high gain but rarely are overloaded like other preamp stages are. An lnd150 might not sound good if overdriven- use as a booster at the front of a preamp or a verb recovery amp puts it in a place where its strengths shine and weaknesses are a non-issue.

JT

[tubesornothing]

Reflections of the notorious peavy SS reverb circuits.  Not me.  Tubes all the way. 

[Tiny_Daddy]

Maybe you could use a power FET as a follower after the 12AX7, to drive the tank.

[imaradiostar]

Reflections of the notorious peavy SS reverb circuits.  Not me.  Tubes all the way. 

Ah yes, so true. Once solid state devices start slipping into a design it's a slippery slope to toneless crap! I completely understand why some people choose not to use solid state devices and I would never argue that it's an even fight! I prefer tubes as well!

Let's be fair though- the Peavey implementation is hardly the idea way to use a silicon device. It's not bad and many people think it works great but it's not going to satisfy you in a Dick Dale cover band. I prefer a higher fidelity reverb sound and I've always added 1 meg pots to the grid of the driver tube on reverb amps I've built to that effect. Driven moderately and with the right frequencies a spring verb starts to leave surf territory and drifts toward plate like sounds. I tend to favor designs that take the verb send from before the volume control so the dwell can be set independently from the volume of the amp. I usually return the verb after the volume control which can be confusing because you can get all wet sound and not direct guitar sound if you desire it!

I think the majority of the tone of the reverb lies in proper drive. A tube is ideal for this because it likes to swing lots of voltage- dropped across a resistor that voltage starts to look like a current source to the coil it's driving. When the driver tube starts to distort it does so in a graceful tubey sort of way instead of abruptly adding all kinds of harmonic content like an op amp or transistor does. Assuming the tank is driven appropriately the mosfet recovery amp is essentially the solid state equivalent of a pentode recovery amp, something I've had success with in the past.

Anyway, for the amp in question there are a lot of options but two seem the most straightforward. If there is already a 12ax7 in place I'd start with a single tube reverb and buy an appropriate pan to match- low z if you intend to use a transformer, as high a Z as possible if you intend to drive it with a cap and use a 12dw7 in place of the 12ax7.

jamie