Transformer candidate for a reverb driver?

[Dingleberry]

Hi.

Started a scratch build once again... I have a recycled 2x12" combo cab and chassis and try to use as much stuff as possible I have on hand.
PT and OT are recycled from old amps and will make a perfect pair for a Blackface amp of my dreams.
I have an output transformer from an old tube radio that had SE EL41 power section.
I measured the impedance ratio and it turned out 2k5:1 ohms.
That equals 20k:8 ohms.
The transformer is like at least four times bigger (lamination core is about 5x6 cm.) than "normal" fender reverb driver so I probably have to cut lows and highs a fair bit due the broader bandwidth to get it sound the way I want, but that I'll do anyway with scratch builds. And I'm gonna make it a 2- or 3-knob reverb anyway.
Is there any reason not to use that transformer for driving reverb tank?
Primary Z is 5 kilo-ohms lower than standard reverb driver, and will probably stress the driver tube a little bit more, but... Close enough to boogie?
-T

[Phil_S]

I'm not that technical. If you have 20K, I think the standard is 22.5K. That's a little more than 10%. In this sort of work, I believe that is close enough.

[gingertube]

The transformer prersents an opportunity to use a "decent" reverb driver tube.
6BM8, 6GW8, 6M5, 6V6 , 6K6, 6Y6, EL84 etc. instead of crappy parallel 12AT7 triodes.
Even your old radio EL41 would be good.
Cheers,
Ian

[Dingleberry]

I'm gonna try 12AT7 first, if I'm not happy, I'll fiddle with nine pin pentodes.
It's gonna be sort of EZG-50 scratch build with the addition of power tube bias tremolo. The output transformer is ~5k:8/16ohms so should be just in the EZG-50 camp. Plate voltage will be ~435V, give or take few volts and I will make the bias supply able to bias 6L6, KT66 and EL34 tubes.
Can't wait to get started this project finally... it's been haunting me for years.

-T

[Stevem]

That size transformer is still not gonna do much below 80hz so I would try it first and then see what size blocking caps it may need on the input grid of its drive tube!

[Leo_Gnardo]

That size transformer is still not gonna do much below 80hz so I would try it first and then see what size blocking caps it may need on the input grid of its drive tube!

For the task of shaking reverb springs, bandwidth isn't much of an issue. Who needs 80 hz coming out of the reverb?

[TUBEDUDE]

Or...you could go xfmrless with a current driver directly into the tank.

[JMFahey]

I measured the impedance ratio and it turned out 2k5:1 ohms.
That equals 20k:8 ohms.

Maybe at 2kHz

The original transformer must have been designed for something like 100 or 200 Hz lower limit (remember, table radio or TV).

Since you are not touching inductance but rising working impedance 10X , low frequency limit will rise by the same amount, so now your transformer will die below 1000 or 2000 Hz .

There is a reason Fender (or Hammond/MM and 1000 others) took special pains to wind thousands of turns of hair fine wire (think pickup wire and you won't be far) and create special built reverb driver transformers, instead of using off the shelf "table radio" output transformers.

Never forget that primary inductance is in parallel with primary impedance, sucking away lows ... although with such extreme mismatch it will suck mids and mid-highs too.

[Dingleberry]

Or...you could go xfmrless with a current driver directly into the tank.

I'm panning to do that with another project.

[Dingleberry]

I measured the impedance ratio and it turned out 2k5:1 ohms.
That equals 20k:8 ohms.

Maybe at 2kHz

The original transformer must have been designed for something like 100 or 200 Hz lower limit (remember, table radio or TV).

Since you are not touching inductance but rising working impedance 10X , low frequency limit will rise by the same amount, so now your transformer will die below 1000 or 2000 Hz .

There is a reason Fender (or Hammond/MM and 1000 others) took special pains to wind thousands of turns of hair fine wire (think pickup wire and you won't be far) and create special built reverb driver transformers, instead of using off the shelf "table radio" output transformers.

Never forget that primary inductance is in parallel with primary impedance, sucking away lows ... although with such extreme mismatch it will suck mids and mid-highs too.

Is there something I don't understand?
The radio run the EL41 singele ended, speaker was 4 ohm, so in original state the transformer was working at 10K:4ohms.
Early fender 6G15 stand alone reverb units used something like champ OT for driving the tank and the sound isn't bad imho and don't lack low end or mids or highs. I've made few stand-alone reverb units following 6G15 topology, even one dead-on clone with recycled transformers (knock-offs from tube radios) and ended up with stellar sounding reveb.
-T