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Is it safe to change Reverb Impedances?
Is it safe to change Reverb Impedances?
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Old 3rd February 2013, 08:50 PM   #1
bmcf85 is offline bmcf85
diyAudio Member
Join Date: Feb 2013
Default Is it safe to change Reverb Impedances?

I have a Fender Hot Rod Deluxe PR246.

I've noticed recently that the reverb doesn't sound quite like it used too.

I pulled out the tank, and sure enough, one of the springs broke loose from the transducer.

It's an Accutronics 4EB3C1B, so 600 Ohm input impedance, 2250 output impedance.

I'm probably going to replace the whole tank.... but since there's no huge cost difference, I thought about swapping it out for a 3-spring model. Only problem is, I can't find a 3-spring with the same impedance.

So.... is it safe to use a tank with different impedance? i.e. will it fry the amp/driver circuit?

If so, is it better to go higher or lower for either side (in/out)?
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Old 4th February 2013, 12:43 AM   #2
Printer2 is offline Printer2  Canada
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Join Date: Apr 2010
If you use a different impedance tank the driving circuit you have will not be able to give it the proper voltage or current and the tank will not perform at its best.

Best to Google the topic and see what other people have done, like this one.

FenderŪ Forums • View topic - Replacing Stock Reverb Tank in Hot Rod Deluxe
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Old 4th February 2013, 06:52 PM   #3
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
I think whay he is finding is the 9 series (three spring) has sightly different impedances at each letter step compared to the 4 series (two spring).

All you need to do is change the 4 to a 9. SO 9EB2C1B. None of this is rocket science, it is just a guitar amp. NONE of these impedances are critical. The input of a 4EB pan is 600 ohms, the input of the 9EB is 800 ohms. This difference does not matter. Likewise the output of the 9EB is 2575 ohms instead of 2250. Again, this does not matter.

SO do not go higher or lower, just stick to EB. Any size EB pan will work.
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Old 4th February 2013, 07:57 PM   #4
cyclecamper is offline cyclecamper  United States
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Join Date: May 2010
Location: Chicago IL, Long Beach CA, Vienna VA
Is it driven by an op-amp or by tubes and a transformer? Does the stock reverb (when working correctly) have sufficient 'wet' reverb? Being a more recent model, I suspect it's driven by some limiter zeners and an op-amp configured for current drive.

A lot depends on how you did or didn't like it to start with.

A higher input impedance is harmless, but at a given drive it pushes the spring less hard, and with the 'receive/recovery' reverb mix turned more 'wet' to compensate and achieve the same level of 'wet' it will have a bit more background noise and sensitivity to spring noises from being bumped and could be a little more microphonic. Then again, maybe it won't drive it hard enough to make the springs jangle...it might end up a more sophisticated sound.

A lower input impedance will be harder to drive and push the springs harder. I would add a heat-sink to the op-amp, because people DO blow them out (then again, people plug the stupidest things into an amp). Maybe you could change the feedback resistor if you were a fanatic, but it's probably OK up to a 2:1 mismatch. But if I had to have a mismatch, I'd prefer too high rather than too low.

A lower output impedance will make it quieter, and again if you turn it more 'wet' to compensate you get a bit more noise.

A higher output impedance will make it louder and also a bit more difficult for it to drive the receive/recovery opamp, but that impedance match is usually way over-engineered anyway so it would probalby work OK.

There is one other consideration, which is that a tank with a higher-impedance output may be more electrically damped, and have a bit shorter decay and bounce fewer times from end to end before it becomes inaudible.

As I recall, I think the hot-rod had a lot of reverb, and if you had to turn it a bit more wet it wouldn't be any big deal.

I had a similar problem when I wanted to change from the factory-standard short-tank 2-spring medium decay in my Peavey Classic 50 to the long-tank 3-spring long-decay. I figured out wht the tank number would be from the chart on the accutronics site, but nobody had one, not even a 'mod' replacement.

But I managed to buy Accutronics direct from the new Korean mfgr. in any of their standard configs, even when not available in-stock from anyone here. They made it for me quite quickly. Shipped reasonably quickly, but the shipping cost approx. doubled the price.

Now I'm wondering whether I can drive and receive both tanks mixed or possibly switch between the two tanks. They use current drive, not voltage drive. With two tanks I should ideally add another identical drive op amp and add another identical receive/recover opamp and then mix them. But I might just put in a switch between old and new..

Long ago, in the 1970's when they were manufactured in Illinois, I upgraded my Tapco reverb's Accutronics tank from 2-spring to 3-spring. It was worthwhile for vocals, but in those days spring reverb and tape echo were about all we had (an analog flanger or digital dealy were rare).

If you want to special-order any standard model number direct from Korea, go to the 'contact' link and email the Korean international address.

Last edited by cyclecamper; 4th February 2013 at 08:01 PM.
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