Hammond 290-series Guitar amp PTs

[prosounddaryl]

Hi Daryl

Hammond has full-range autoformers up to 250W, so I am familiar wioth these, plus Plitron made custom units for Bryston's PA-oriented installation amplifiers.

None of these devices are appropriate replacements for the OT in a tube amplifier.

The amp you have has an OT with a primary impedance around 8k-aa, that is anode to anode. The load each tube sees is one-quarter of that, so 2k. The impedance ratio of primary to secondary is the square of the voltage ratio, and as you can see, all the numbers are quite high when matching tube plates to speakers.

Fender tended to use slightly under-sized OTs, as do most MI amps. This means that the bandwidth is wider at low output than at full output, which is only of concern if it bothers you. There are benefits to having rolled off bass with guitar inasmuch as the overdriven amplifier will be less "farty sounding". To optimise that operating mode further requires changing some capacitor values within the circuit. TUTs explain all of this, along with transformer specification methods.

For a line transformer (regardless of single or separate windings), the ratio is quite small.Say you have a 100W line match for 70V. The 70V impedance is 142R, just from using the power equations and Ohm's law. The taps on the device are usually given power ratings assuming 8R for the speaker. There may be an 8R tap rated at 100W that is used to match the PA output;  the 70V tap (full winding)connects to the other distribution transformers; the low-wattage taps go to specific local speakers.

Have fun

OK, got it, I think. Even if I were able to find taps on my other "hi-fi" transformers that would get me a 2K reflected impedance, it still wouldn't be great because in this case too much bass would get me a muddy signal that probably would not be responsive to where the tone controls are centered? Did I get that right?

So I guess I'll abandon that idea, which was planted by reading about OT upgrades from your favorite vender

But just curious since you raised the issue. If you were to perform the mods you mentioned above before worrying about the OT, what mods would those be?