How to calculate impedance for reverb tank in a guitar amp

Question

Background:

I'm in the process of restoring a HIWATT HI-GAIN 50 COMBO Valve guitar amp and one of the issues it has is that the reverb dial has no effect on the sound.

Attempted troubleshooting:

To troubleshoot this I first replaced the 12AX7 tube responsible for the reverb recovery circuit and that made no difference. After this I disconnected the reverb tank and connected headphones to phono / RCA connector that was connected to the tanks input. As expected I could hear the guitar signal with no issues. The next test was tapping the phono / RCA connector that was connected to the output of the amplifier. Sure enough the tapping created the expected buzz on the amplifier confirming that the signal paths to and from the tank were working. The next test was to connect the input signal to the tank and connect headphones to the output of the tank, nothing could be heard other than the sound of the reverb springs when the tank was moved (this could be heard with or without input phono connected)

I removed the tank from the amp and inspected all solder / mechanical connections and all seemed fine. Measuring DC resistance the input transducer measured 30 Ohms and output transducer measured 69 Ohms (Obviously this wont be the impedance the tank is rated by, but at least it showed that the transducers were not a dead short or open circuit). To further test my reasoning I fed an audio signal into the input and probed with an oscilloscope. Signal could be seen up to the input of the input transducer and nothing at the output / springs of the input transducer hence the conclusion it must be dead

Replacement Part:

The code on the tank is 3CA3A1B

From what I've found on the internet this means:

• 3: reverb type = 3 springs
• C: input impedance = 240ohms
• A: output impedance = 600 ohms
• 3: decay time = long 2.75 to 4.0 sec
• A: connectors = input grounded/output grounded (Appears to be wrong as input is isolated output is grounded)
• 1: locking device = no lock
• B: mounting plane = horizontal open side down

However I cannot find any tank with these impedances for sale (specifically in or ship to the UK)

Getting to the point... The Question

I have a schematic for the amplifier and would like to find a way to calculate the allowable ranges of input and output impedance for the reverb tank (connected to CN5 / CN6). By doing this I may be able to find another tank that will work with this amp that I can buy in the UK

The tank is driven and recovered by JRC5532 OP AMPs before being passed to the 12AX7 amplifier circuit. What confuses me is that in the datasheet for this part the maximum drive output is given as 600 Ohm which is far higher than the 240 Ohm impedance required by the reverb tank however the 600 Ohm output drive is given at 10Vrms.

In Short

How can I calculate the range of input and output impedance that would be compatible with this amplifier?

Answer 1

The opamp buffers at the input and output of the reverb unit mean that the circuit performance is not at all sensitive to the specific impedances of the unit.

Feel free to try any replacement unit you can get your hands on — you won't break anything.

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The drive circuit (IC2A and IC2B) is functioning as a current source. First of all, IC2B is simply boosting the output current available from IC2A, and R94/R95 simply make sure that they share the current evenly.

The feedback is taken from the node between R96 and R97 (CN5 pin 2), which means that the signal voltage (IC2A "+" input) is also imposed across R97 (47 Ω). In order for this to happen, a current proportional to the signal voltage must flow through the reverb drive coil (connected across CN5).

The effective source resistance of this circuit is very high (it's actually limited by R96), which means that the actual impedance of the drive coil won't have any significant effect on the current that flows through it.