Hammond 290-series Guitar amp PTs

[prosounddaryl]

Hi Guys

Hammond has reverse-engineered every useful guitar amp OT and PT and have a line of 1700-series output transformers and 290-series power transformers. These are all drop-in replacements, so if you have an amp with a dead OT or PT you can use these parts without having to drill any holes.

because the transformers are modelled after named amp models, hobbyists and builders can be assured of the tonal range possible assuming similar or identical circuits and tubes are used, too. In any case, if you want an amp with the power of the original Bassman, say, you can buy the matching OT and PT set. More adventurous builders may wish to mix and match these offerings with those of other brands. By this I mean, say, using the repro Bassman OT from Hammond but use a non-Hammond PT.

There is a critical point of interest regarding the PT specifications in the Hammond 290-series inasmuch as the plate winding uses an AC voltage but a DC current. This is the same as they did in the "Classic" transformer lines, and it will confuse most hobbyists and builders born in the sixties or later.

For example, the Bassman PT for a 50W amp has a 660Vct plate winding rated at 230mA. We can take this as 330Vac at 230mAdc given the half-bridge rectification.This results in about 470Vdc and 108W of power. The Bassman OT is 4kaa, representing a 1ka load. At 100Wpk audio power, there needs to be 316Vpk at 316mApk. Then we need about 60Vsat across the tube at full load and have a loaded supply of  about 380Vdc The sag is 90V, or almost 20%. If we look at the 316mA at 380V we have 120W, which is not too far off the 108W calculated previously, so we will say it is "close enough".

Most of Fender's amps evolved to use about the same supply voltages and a variation of OT impedances. Hammond has added the devices Fender should have switched to when tube rectifiers were dropped. That is, non-CT plate windings. This allows the use of a full bridge rectifier modules and requires a separate winding for bias. The latter helps to improve grounding, as TUT3 shows.

Power supply design defaults should always be:
full-wave rectification is always used
half-wave rectification is NEVER used especially for bias supplies
every supply has its own winding
dual primaries for world-wide use
electrostatic screen and/or core band for noise reduction

Have fun

Kevin, a question for you on output transformers. I bought a London pre kit from you a while back. Thank you very much, it is wonderful, and I appreciated the guidance received during assembly. If you don't recall (and I wouldn't expect you to) I used your kit in place of a DSP board that had failed in a Fender Champ X2. So the preamp controls are all stuffed into the X2 chassis, along with a Celestion neodymium 10" speaker.

It's a neat rig, but I was considering changing the output transformer.  As a consequence of my former business (used to provide voice/music audio for a lot or large temporary events like airshows, car races, World cuo ski events, etc) I have a tone of left overs of autoformers and isolation transformers.

My question is what sort of ratio do I need to match the output of the tube driven Fender X2 to the speaker I am using? For example, I have a bunch of EV AT100 autoformers.  Would these be a good match, and what info would you need to make such a determination? 

Or am I overthinking this? Should I just find a good transformer in my stockpile that is large enough to handle the output voltage and yet  presents the tube side with a somewhat higher load than 8 ohms? if so, 50 ohms? 100 ohms?

Thanks in advance for your reply.
Daryl