This new output transformer is a breakthrough for the Blues Junior and Pro Junior amplifiers.   Designed from the ground up to provide big-transformer performance in a smaller package, the BJT8336 is built in the USA.  The “widebody” core is made of premium M6 steel, which has superior magnetic properties allowing maximum efficiency and power transfer from primary to secondary.

The BJT8336 is a “deep stack” low-profile transformer with considerably more core than the transformers it replaces.  It is bifilar wound, with 6,600 ohm impedance into 8 ohms, a good match for EL84 tubes. It can easily handle 20-25 watts with essentially no thermal rise.

Yes, we are aware that we are not the first to develop an improved output transformer for the Blues Junior and Pro Junior.    The late Bill Macrone along with David Allen of Allen Amplification developed the first upgrade transformer almost 20 years ago.  This was called the TO20, and was available from both Billm Audio and Allen Amps.    Unfortunately, it is no longer available.    Mojotone and Mercury Magnetics sell their versions of improved Blues Junior/Pro Junior output transformers.   So what is special about the BJT8336 versus these other competitors?    Construction and cost.   Both the Mojotone and Mercury Transformers are open core kraft paper type, while the BJT8336 is completely sealed in a sturdy metal housing for improved shielding, ruggedness and reliability.   The Mojotone replacement sells for $77, while the Mercury replacement sells for $151.    The BJT8336 sells on this website for only $69.95.

At 1.5 lb., the BJT8336 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.   In addition, the bifilar winding method makes the two halves of the primary winding identical in both DC resistance and AC impedance.   The result is a transformer with improved response across the audio range, particularly in the bass region.

Below is a power spectrum sweep, using a stock Blues Junior OT:

Power spectrum, stock Blues Junior Output Transformer

Next is the same sweep, same amplifier, nothing changed but the output transformer:

Power spectrum, BJT8336 Blues Junior Output Transformer

You can see that the power curve has shifted up by approximately 8 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.

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