Reverb Transformer impedance Measuring

[vintagekiki]

practical experience

From long years of practice, I can see that the output transformers of low power more important gear ratio primary / secondary from impedance values.
If known gear ratio primary / secondary, with the known value of the secondary load, there is a probability that the primary have the required impedance value.

For Fender Reverb transformer for Rp = 25 kohm and Rs = 8 ohm gear ratio is 55.9, or for Up = 230V~ with transmission ratio of 55.9, the secondary voltage should be the order of 4V~
If you do not have an adequate output transformer, main transformer low power <5W with appropriate transmission ratio will finish the job.

For the gear ratio 55.9 free can be used transformer from low buget universal AC-DC adapter whose secondary voltage between 3-12 V provides a sufficient range of transmission ratio.
Example https://www.maxiaids.com/battery-eliminator-universal-ac-dc-adapter

[g1]

Sorry folks, as I was unclear. I edited my post to show infinity instead of the mis-leading reading of 0 ohms.

Is this correct?

Yes, that is correct.
An infinity symbol on meters and keyboards would be very helpful as far as this common mis-communication.

[Enzo]

Yes, those numbers are probably correct. Here is the deal: the transformer is not going to change. The turns ratio will always be. The resistance you see published somewhere is going to be what they measured on that particular part. But there is no guarantee that the transformer made by Fender wil measure the same resistance as one made by MM or whomever, even with the same turns ratio. Point being, resistance is no indication of health. Pretty much the thing is open or not. If not open, and windings are not shorted to ground, then it is OK. If it has a turn or two shorted together, your meter will never know. Transformers usually don;t short windings end to end.

And of course intermittant arcs are like intermittant ANYTHING, tough to find.

It is just a little tiny power amp, impedance is not much of an issue as it is driving a spring, not a speaker. We don;t listen to it directly. But as a test you CAN connect a speaker to it, you can hear what would be going to the reverb pan. Pretty much anyone's reverb transformer is close enough. In fact if I needed to sub one in for tests, I could use an OT from a Champ or other small amp. I have a Fender Bassman OT on my bench as my universal test sub. I would wager that I could sub even that into a reverb circuit and get reverb.

[Tom Phillips]

An interesting fact about Fender Reverb drive transformers is that most techs leave them unloaded (I.e open circuit) when they bench and amp for testing. They seem to survive when a signal is run through the amp but it would be heresy to suggest that it is OK to apply dive to an output transformer that has no load on its secondary.

[52 Bill]

An interesting fact about Fender Reverb drive transformers is that most techs leave them unloaded (I.e open circuit) when they bench and amp for testing. They seem to survive when a signal is run through the amp but it would be heresy to suggest that it is OK to apply dive to an output transformer that has no load on its secondary.

It's not just techs that do this, owners that don't keep up with normal maintenance do this all the time. If the RCA plug on the drive side is dirty or the tank input wires are broken the reverb transformer is left unloaded. I've know guys that never use reverb and will pull the tank wires to kill the function. I try and get them to at least pull the reverb driver tube when they do this.

I suppose that the drive levels are so low that the transformers survive this mistreatment, at least until they don't.

[g1]

But I always turn the reverb knob to "0" when I have it disconnected.

[Tom Phillips]

But I always turn the reverb knob to "0" when I have it disconnected.

That will kill the reverb recovery signal but, if the tank load is disconnected, there is still an open circuit secondary at the reverb drive transformer. From historical experience this doesn't seem to be too stressful on the reverb transformer. Still, it's best practice to \put a load on the transformer when the amp is being bench tested. Especially when a signal is being applied to the input and the output power is being measured.

[g1]

I was kidding but it's good to have the info up here. I've had the same experience, never seen or heard of one getting damaged on the bench for this reason. But then again how would you know a bad tank connection was not the reason for a blown TX in actual use.
Agree with using a load as best practice or pulling the driver tube.

[J M Fahey]

They survive because power and voltage level is not that high, also a triode is a "softer" driver than a pentode , which is a constant current driver and will excite very high peaks.

[TimmyP1955]

I've worked on a boatload of Fenders with broken tank input wires. I have to assume that some of those amps were played with no load on the reverb transformer. I've not had to replace a transformer or a 12AT7 because of it.

[Enzo]

I have replaced maybe three of them in my career.

[52 Bill]

I have replaced maybe three of them in my career.

I think maybe a half dozen or so in the last 40 years.