Having trouble getting the reverb to work on a Fender Princeton Reverb amp - Reissue

[Bob M.]

Hi All,
I'm having trouble with the drive circuit of the reverb on a fairly new Princeton Reverb-RI amp. I'm wondering if anyone has ohmage readouts for the reverb transformer in this circuit (voltage readouts on the primary and secondary would also be helpful).

I've replaced the driver tube (V2 - 12AT7), the reverb pan, the cables, and the 4 components in the drive circuit (coupling cap, grid load resistor and 2x cathode parts, resistor and cap). The voltages in and around V2 measure good, within spec, just about the same as every other Princeton Reverb I've seen. The only other things left are the printed circuit board traces (not so great on these reissue amps) and the reverb transformer.

I've done the 'spring test' , shaking the amp to see that the reverb is being recovered - working fine, lots of thunder. I've played guitar inputting through the RCA reverb return jack on the back panel, sounds fine so I know the recovery circuit is in good working order. This amp sound great but no reverb sound.

Any help on testing this reverb transformer is appreciated.

Thanks, Bob M.

[Jon Snell]

The transformer delivers at around 15 Ohm impedance so connect a test speaker to it and listen if you have not got access to an oscilloscope.

[Tom Phillips]

...Any help on testing this reverb transformer is appreciated.
Thanks, Bob M.

Jon Snell already gave good advice to test the driver circuity by connecting a speaker to the spring drive signal.

However, I've attached a set of data that I collected using a working Fender reverb drive transformer so you can see the typical measurements.

In addition, I recommend that you verify that the reverb driver transducer in the reverb tank is working property and is connected properly. I say this because both transducers in the spring tank will produce the crashing sound but only the drive (input) tank transducer will drive the springs hard enough to produce the normal reverb sound. The point is to make sure that the tank isn't hooked up backwards.

You can also measure the DC resistance of the tank transducers with the tank disconnected from the amp.
The Input (Drive Transducer) should measure a little less than 1 Ohm and the Output (Recovery) transducer should measure around 200 Ohms.
Tip: When measuring the Input transducer, make the measurement right at the tank connector and make sure that you subtract the resistance reading you get when you short the meter probes together.

You should also Ohm out the reverb return cable itself to verify that it's not the problem. Test for continuity end to end and verify the inner & outer conductors are not shorted together.

[Helmholtz]

However, I've attached a set of data that I collected using a working Fender reverb drive transformer so you can see the typical measurements.

Your table shows PT measurements.

I'd like to add that a transformer may have shorted turns not showing in DCR, but still killing the output.
So the speaker method is more reliable.

[Tom Phillips]

Your table shows PT measurements...

Well...that won't do!
Thanks for the heads up.
Fixed.

[Helmholtz]

1963, wow!
Must have been from a very early Fender Reverb amp, maybe even from a brown Vibroverb?

[Tom Phillips]

1963, wow! Must have been from a very early Fender Reverb amp, maybe even from a brown Vibroverb?

I have seen quite a few Fender amps that had reverb transformers that were much older than all the other dated coded parts in the amp. My theory is that Fender made a very large quantity buy of the Blackface era reverb driver transformers and it took a few years to use them all up. The transformer used for the data measurements that I posted was installed in a 1966 Princeton Reverb amp.

[Helmholtz]

I have seen quite a few Fender amps that had reverb transformers that were much older than all the other dated coded parts in the amp. My theory is that Fender made a very large quantity buy of the Blackface era reverb driver transformers and it took a few years to use them all up. The transformer used for the data measurements that I posted was installed in a 1966 Princeton Reverb amp.

Thanks.
But as Leo didn't have a reverb amp before 1963 (first one being the 1963 Vibroverb), it's unlikely to find an older than 1963 Rev. transformer.

[Tom Phillips]

... Leo didn't have a reverb amp before 1963 (first one being the 1963 Vibroverb), it's unlikely to find an older than 1963 Rev. transformer.

I concur but my post does not imply otherwise so I don't understand what you are getting at.

[g1]

I think he misunderstood the first sentence quoted.
("I have seen quite a few Fender amps that had reverb transformers that were much older than all the other dated coded parts in the amp.")

[Tom Phillips]

I think he misunderstood the first sentence quoted.
("I have seen quite a few Fender amps that had reverb transformers that were much older than all the other dated coded parts in the amp.")

Yep. That must be it.

[Helmholtz]

I concur but my post does not imply otherwise so I don't understand what you are getting at.

All I meant was, that this must have been one of the earliest Fender reverb transformers.

[g1]

Maybe this is one of those happy accidents where they bought so many of the initial supply, they didn't mess with the circuit for many years, establishing the classic Fender reverb sound.

[Bob M.]

Hi All,
Thanks for the replies. Tom, thanks for the reverb transformer data.

I keep a virgin reverb tank on my bench (Fender tube amp reverb spec), never been in an amp, just for testing situations like this. I also have a new set of RCA ended cables, again just for testing, and they did not make any difference in the reverb circuit of the PR-RI amp, still no reverb. But I went one better - I took out the PR-RI reverb tank and mounted in one of my numerous Fender amps w/ reverb where it worked perfectly fine. I did the same with the reverb cables in the PR-RI and they worked perfectly as well. And, of course, I did the ohmage measurements of the reverb tank and cables from the PR-RI and they measured as expected for working, good condition tank and cables. I eliminated these components as the source of the fault in the reverb circuit.

[Helmholtz]

Did you try connecting a speaker to the reverb send jack?
This is actually the 8 Ohm output of a little power amp.
So if you're getting speaker sound the reverb drive stage and transformer should be fine.