Deluxe Reverb Output Transformer in Modded PRRI (Primary Impedence)

Noobtoob

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Messages
42
A question for the technically minded, will an Output Transformer designed for a deluxe reverb (in this case a Hammond 1760H) work in a Princeton Reverb Reissue? My primary concern is that the primary impedence might be wrong? Please advise.
 

easyed

Member
Messages
2,561
It should; since the impedance for the tubes is the same, as is the impedance for the speaker.

I would get in touch with Dave Allen http://www.allenamps.com/trans.html. The TO20 or TO20B look like what you need, and the TO26 will fit a PR Chassis but is designed for a DR.

Mr. Allen is a delight to deal with and when it comes to transformers, he really knows his stuff.
 

zenas

Member
Messages
8,871
A DR OT is used with 6v6s and an 8 ohm speaker load, it'll work in a Princeton. Might have to drill different mounting holes.
 

Tony Bones

Member
Messages
1,212
It'll work fine. As Zenas said, you might have to drill a new mounting hole or two because the DRRI transformer is bigger, but it will function just fine, no worries.
 

HotBluePlates

Member
Messages
13,903
It should; since the impedance for the tubes is the same, as is the impedance for the speaker. ...

Far, far, far too much is made of "impedance matching" in guitar amp output transformers.

Thanks team, so does the primary impedance affect anything?

It affects output power.

The Princeton Reverb and Deluxe Reverb used approximately the same power supply voltage, and so had the same maximum output tube plate voltage swing available. From the equation Power = Voltage^2 / Impedance, if voltage stays constant and impedance is lowered, power is increased.

You can calculate peak output power from a peak voltage swing and the OT primary impedance. Since the amp is Class AB, use plate-to-plate impedance divided by 4, as this is what each side of the output stage will see during those peaks. Peak power (of a sine wave) = 2 * RMS power.

Assume peak plate voltage swing is 270v. Hammond's replacement transformers are 8.5kΩ for the Princeton Reverb and 6.6kΩ for the Deluxe Reverb. So that's 8.5kΩ / 4 = 2125Ω for the PR and 6.6kΩ / 4 = 1650Ω for the DR.

Princeton: Peak Power = 270v^2 / 2125Ω = 34.3w, and RMS Power = 34.3w / 2 = ~17.2 watts
Deluxe: Peak Power = 270v^2 / 1650Ω = 34.3w, and RMS Power = ~44.2w / 2 = ~22.1 watts

By changing just the OT primary impedance, the Deluxe Reverb can make more output power from the same tubes and the same supply voltage.

(Though changing OT primary impedance also changes the peak 6V6 plate current, and Fender switched to a long-tail inverter to better drive the 6V6's to those peaks.)
 

Noobtoob

Member
Messages
42
Thanks for that in depth explanation! I think the phase inverter in my prri has been modified too. Either way, having an OT with 6.6kohms will work? Thanks again, my RAM is full of accounting garbage so takes a bit for this kinda stuff to filter down.
 

Noobtoob

Member
Messages
42
And if the power supply and power supply can't, will they just work to their capacity or will they be damaged? Thanks again,
 

neteraser

Member
Messages
1,993
Have to add, that Princeton's 'around 8k' of the primary impedance, are there not for no reason. Better is to follow those specs.
 




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