TO20 Low-Profile Output Transformer
This brilliant new output transformer, co-developed with Allen Amps’ David Allen, is a breakthrough for the Blues Junior, Pro Junior, Princeton Reverb Reissue, and Super Champ XD. Designed from the ground up to provide big-transformer performance in the least possible space, the TO20 is built by highly respected US manufacturer Heyboer, fits the stock mounting holes in all of these amps and has the same height as the stock transformer. The “widebody” core is made of premium M6 steel, which has superior magnetic properties, allowing maximum efficiency and power transfer from primary to secondary.
At 1.5 lb., the TO20 is more than half a pound heavier than the stock transformer for any of the above amps. The core size is carefully balanced against the wire gauge and turns ratios. The design increases the “Q” of the output circuit and provides a higher damping factor. The amp thus controls the speaker more accurately, which is especially beneficial for low frequencies. Flaws like woofy, flabby or farty tone are a thing of the past. Overdrive tones are sweeter, harmonics are richer. Frequency response extends well past the range of hearing, so nothing is left behind.
Output transformers seldom make an amp louder; they’re mostly about tone quality. But they do improve headroom by reducing distortion.
Below is a power spectrum sweep, using a stock Blues Junior OT:
Next is the same sweep, same amplifier, nothing changed but the OT:
You can see that the power curve has shifted up several dB; the increase in loudness is just noticeable, not overwhelming. The real difference is in the distortion-free bass and improved tone throughout the range.
The only “caution” is for Princeton Reverb users, who may like the classic grind of the stock, undersized output transformer, what Fender’s Shane Nicholas calls the “aaaaaat!” tone. But it’s the same tone that forces you turn down the bass as you turn up the volume. With the new OT, the bass comes through in a way that you’ve never heard—unless you’re one of the old-timers who tore out the stock PR OT and installed a Deluxe Reverb OT. The TO20 gives you all of the performance of the DRRI transformer or other oversized aftermarket transformers without having to drill holes, find and attach the right quick connects, or worry about interference with speakers. (The “20” designation is just to differentiate it from the TO22; either OT can easily handle 25 watts.)
The Billm TO20 kits fit the Blues Junior, Princeton Reverb, Pro Junior, and Super Champ XD (SCXD available end of October), with the correct wire lengths and the proper push-on quick connects for each amp.
For reference, here is the power spectrum for the TO22 transformer, which has a 4 ohm tap as well as an 8 ohm, and is preferable for maximum efficiency running two speakers or a 4 ohm cabinet. You can see that the TO22 is very similar to the TO20, and both are superior to the stock OT:
You can order either output transformer from the Mod Kits and Services page.