Spring reverb for amp question (OT, sorry)

[ElectricDruid]

This is slightly off topic since it's an amp question not a stompbox question, but you lot are the most likely group of people to know the answer, so here I am!

I've got a Carlsbro TR60 amp chassis in my workshop. It was originally a 2x12 combo amp, but it long ago lost the cabinet, the speakers, and the reverb spring. I'd like to get it back up and running and give it some TLC.

Now, building a cabinet and adding speakers I can do, but I'm not sure about the reverb spring. There seem to be loads of options, and how would I know what input/output impedance I should be looking for?

Here's the schematic for the amp:

https://el34world.com/charts/Schematics/files/Carlsbro/Carlsbro_60tr_60tr_twin.pdf

(Incidentally, this range were the last tube amps Carlsbro made before they went solid state, back in 1972, so this amp is basically fifty years old).

Thanks everyone!
Tom

[antonis]

how would I know what input/output impedance I should be looking for?

I've to admit you've puzzled me for a short time, Tom..

SFY51 is an Emitter follower of very low output impedance(r_e//470R) and BC207 is a grounded Emitter CE amp of also low input impedance (r_e x h_FE)

One could calculate both Collector currents for a more precice particular r_e estimation but I let you do the dirty job..

[ElectricDruid]

You might have to help me a bit with the dirty jobs I'm afraid, Antonis. I know next to nothing about transistor circuits. I grew up with the 741, and I've never done much with transistors. I'm very willing to do the work, but you'll have to explain what I need to do.

I suppose what I'm really asking is "how fussy are these stages likely to be?"

If the driving transistor is a low impedance output with a decent drive, then will I be ok into an input impedance on the spring of only 8ohms? (many are). Some are "as much" as 250 ohms input impedance. Would that be better?

The output is as difficult. Output impedance for the spring can be anywhere from 500 ohms to 10K. Can that CE amp cope with that? Which end of that range do I need to be at?

Here's some examples of the tanks and the amount of variation they have:

https://www.banzaimusic.com/Accutronics/

Sorry, but I'm feeling rather lost with this. There's too many options and I don't know how to decide which is best.

[antonis]

I give you my word for -almost- precice impedances calculation till tomorrow noon (or so), in case of no other well-intentioned guy did..

[PRR]

I don't feel comfortable with this plan. I really wish there was(??) a Carlsbro fan club with the secret knowledge.

That 20k 10 Watt resistor has nearly 10 Watts in it and should be burnt by now.

We never match a wound-driver reverb to a source. We never match a load to a cathode follower. These thoughts fight in opposite directions.

There's 20mA in the driver. The 4-10 Ohm tanks can take >38mA drive. This suggests 50 or 100 Ohm nominal.

The return tank is 5k-10k, as it always is, unless you have to drive very long lines.

[antonis]

Things are more clear when you deal with op-amps..



page 5.8, Audio/Radio Handbook, National Semiconductor, 1980

[ElectricDruid]

I don't feel comfortable with this plan. I really wish there was(??) a Carlsbro fan club with the secret knowledge.

Perhaps there is somewhere, and if anyone knows any good amp forums where they think this might be a more appropriate query, I'd be delighted to hear about them.

That 20k 10 Watt resistor has nearly 10 Watts in it and should be burnt by now.

Now, that I can check!

We never match a wound-driver reverb to a source. We never match a load to a cathode follower. These thoughts fight in opposite directions.

I'm sorry Paul, but I don't understand what you mean by this. "We never match a wound-driver reverb to a source" sounds like you mean "You don't match the reverb spring to the driving amp", but since I have the amp and not the spring, what else am I supposed to do?
"We never match a load to a cathode follower" I can't make sense of at all, unfortunately. Presumably you're talking about the other end of the spring, but in my head the spring is the output, and the transistor is the input, and I generally understand the input as the load for the output, not the reverse. As you can, I'm lost here and clutching at straws.

There's 20mA in the driver.

This would be coming out of the BFY51, right?

The 4-10 Ohm tanks can take >38mA drive.

Ok...I'll take your word for that. How do I know that?

This suggests 50 or 100 Ohm nominal.

It does?!? Why/How?

The return tank is 5k-10k, as it always is, unless you have to drive very long lines.

You mean the output coil on the reverb tank is going to be 5K-10K output impedance? There seem to be many that are less.  Is more better? Normally I'd want *low* output impedance on something that's going to drive a following stage. Is that not what's going on here?

Sorry for so many questions, but I'm not even understanding your answers just yet. I need more background.

Thanks,
Tom

[pinkjimiphoton]

tom,
read-ie linky bro

it should help you get something in the ballpark. on the ssguitar forum someone may be able to help...

but this demystifies reverb tanks somewhat... hope it helps brother

http://www.electricalfun.com/workbenchfun/reverb_tank.pdf

[Rob Strand]

What I have and a bit more.


Carlsbro Reverb Springs

Used on:
- Carlsbro amps
- Carlsbro stand-alone (solid-state) reverb unit
  (No schematic for but circuit looks identical to amps.
   Only difference is PSU uses lower voltage and lower series resistor (470R).
   Guess 18 to 20V on PSU rail.)

Solid state reverb circuit even on tube amps.
Seems like all units use a similar reverb circuit and spring.

Tank:
- Pressed steel.  Greyed finish.
- Large slots/holes in case above coils.
  A characteristic of tanks from Gibbs Manufacturing (around in early 60s).
  (otherwise looks quite similar to Accutronics)
- Spring is long, almost full width of stand-alone Reverb unit
  and 2/3 to 3/4 the width of the amps.
  (Possibly 16.75" tank but could be a little shorter.)
- RCA connectors

Output impedance:
- likely to be around the common 2k ohm nominal.

Input impedance:   
- unclear
- circuit 470 ohm emitter resistor on AC couple drive buffer.
- unlikely to be 300 ohm or less.
- most likely 600ohm or 2k ohm

Delay Times:
- unknown

Output jack likely to be isolated.
---------------------------------------------------------------
Web data:
- someone measured a Gibbs tank (not from a Calsbro)
  Input Coil DC resistance 170 ohm
  Output Coil DC resistance 170 ohm

That implies nominal impedances of 2k ohm input and 2k output.
(Impedance is inductive, Nominal impedance is a 1kHz.)
---------------------------------------------------------------
[Added 2022]
Amplifiers:
- 470V DC feeds 25k 10W resistor to reverb circuit.  50uF supply cap.
- If DC supply voltage on the reverb circuit is say 20V then the
  current from input supply is IDC = (470-20)/25k = 17.8mA.
  Say 17mA to Reverb buffer.
- DC voltage on buffer emitter = 17mA * 470 ohm = 8V.
  Look very reasonable for a reverb supply of 15V to 20V.
  (Also in same ball-park estimated for Carlsbro stand-alone Reverb)

Analysis:
- buffer drive level is about 17mA peak
- Reverb springs are typically driven at levels *somewhat* higher
  than the manufacturer's nominal (rms) current.
- This drive level is OK for 2k ohm impedance tanks and workable
  for 600 ohm impedance tanks.
- The output swing however is only about 8V peak which means
  only 8V/2k = 4mA into a 2k ohm tank.  While possible
  it's not normal to have such a low drive current.  The
  required voltage swing below 1kHz will be less, so 2k ohm
  is looking a little borderline.

Conclusion:
- 2k ohm input impedance:    borderline
- 600 ohm input impedance:   probably OK

Unfortunately the chosen impedance will affect the level of the reverb.
Analysing the reverb recovery amp gain might help narrow down
the impedance.

Note the 50uF cap is only just good enough for 600 ohm.
---------------------------------------------------------------

[PRR]

> 50uF cap is only just good enough for 600 ohm.

For High Fidelity? 6Hz?

In spring reverb we often want a bass-cut, 100Hz to 400Hz.

Or am I looking at the wrong place?

[Rob Strand]

For High Fidelity? 6Hz?

In spring reverb we often want a bass-cut, 100Hz to 400Hz.

Or am I looking at the wrong place?

Supply fluctuations under large drive currents (think of say 100Hz signal).

Keep in mind this my best guess based on the info I have.
Like any guesses one extra piece of info could nullify the whole lot!

[antonis]

What puzzles me is recovery configuration low input impedance (about 800 Ohms)..

Drive configuration output impedance is less than 2 Ohms, which is fine..

[PRR]

A good DIY reference is ESP:
https://sound-au.com/articles/reverb.htm

> output impedance is less than 2 Ohms, which is fine..

Nearly all reverb drivers are HIGH output impedance, just like tape record head drivers and some guitar amps.

[Rob Strand]

Nearly all reverb drivers are HIGH output impedance, just like tape record head drivers and some guitar amps.

What puzzles me is recovery configuration low input impedance (about 800 Ohms)..

Reverb drive and recovery stages are all over the place.

There's three drive cases:  voltage drive (low impedance), series resistor (low impedance amp with series resistor), and current drive (high impedance).    All types are out there.    I have a preference for current drive but if you want to use a simple power amp IC for low-z tanks then a series resistor is a good option. See this schematic for example,

https://www.diystompboxes.com/smfforum/index.php?topic=125320.msg1192790#msg1192790

Because the load is inductive and the spring responds to current the type of drive circuit (greatly) affects the frequency response.

I didn't sim the circuit but by eye-balling the circuit and bashing out the numbers on a calculator I actually get about 36k for the recovery stage input impedance.

[pinkjimiphoton]

check that link i posted. it has a plethora of info i think will help out a lot when it comes time to find a suitable substitute.

[danfrank]

Rob got it right... 600-800 ohms tank input and 2000 ohms tank output. The input impedance is the important one that should be followed, the output impedance can be fudged a lot...
Coincidentally, Acutronics/Belton/Mod makes the correct tank... The Fender Blues Jr uses the same spec tank.
Tank number:    8EB2C1B

[digi2t]

Tom,

Look this guy up, he still has the chassis and reverb tank. https://reverb.com/ca/item/29353228-carlsbro-tr60-chassis-from-1971-drake-transformer-set-for-marshall-jmp50-or-jtm45-custom-build

I messaged him about the reverb tank, and he just sent me back a message. He's in Belgium.