Hammond 290-series Guitar amp PTs

[prosounddaryl]

Hi Daryl

That would be a Super Champ X2, rated at 15W using 2x 6V6.

The amp has a full and proper tube power amplifier with a proper output transformer that matches the push-pull tube stage to an 8R speaker. Why do you need to change it? There are lots of mods you can make to the PA, but changing the OT is not the first one that comes to mind - if at all. And as always, do not waste your money on Mercury Magnetics. Total hype and not as good as "generic" brands.

"Autoformers" are auto transformers )single winding with taps and a shared end for input and output)designed for audio, generally for low-quality public address systems. Note that such systems should NEVER have full bass response, let alone extended bass response, as this low-frequency content becomes more of a background "rumble" than sounding anyways musical. Background music systems are explicitly that - BACKGROUND and should therefore be unobtrusive.

In a PA system, the power amp may have a 70V line transformer built-in, or simply be rated at such high 8R power that it can produce 70Vrms output directly. The latter would have to be rated for close to 800W into 8R - not too common. In typical systems, the driving OT has a step-up voltage ration that matches the amp's nominal rated voltage output (what corresponds to its 8R output) to the 70V line. Then at each speaker location is a smaller autoformer to tap a portion of the power.

None of the 70V system transformers are applicable to your guitar unless you wish to distribute the sound through such a system, with notably reduced fidelity /altered tone compared to the combo speaker. Using an over-sized line AT may reduce the fidelity loss.

Have fun

Hey, Kevin, Thanks so much for the reply. The following may not be useful given your statement that changing the OT may not be worthwhile, but a couple of comments first.

The transformers I am speaking are not the simple voice only you are familiar with but instead designed for full range response. The Electrovoice AT100 for example is a 6lb monster of Iron and copper, I have over 100 of them which were taken out of the Indianapolis Hoosier Dome for their speech/music system, and the performance level is very high. (Spec attached) From that same installation I have numerous 300-watt Crown isolated transformers (not autoformers) so for me it would be a no-cost improvement, if in fact it is an improvement. The Crown ones have only a fixed 3:1 ratio, so while they are huge and hifi, this may noy be enough of a change in the reflected impedance. For example, attaching one of these Crown 300 watt Xformer to an 8 ohm speaker would change the load the amp sees to only 24 ohms. I expect the impedance load the amp side would want to be higher?

I also have a lot of 32 watt transformers, again designed for high performance audio. I'll Have to go take a look at one to get the make and model number, but I would suspect they are similar in performance to the Hammond stuff. They certainly weigh similar amounts. These 32 Watt models give me a few more taps than the 300 watt Crown, so perhaps I can arrange loads close to 50 or 100 ohm impedance.

Just as aside, my Crown Slide rule (and Crown's engineering documents) tell me that 625 watts @ 8 ohms = 70 Volts. Not that this info would be useful to you, but you never know.