blues jr pt

[mark enger]

I have a fender blues jr pt i checked out the specks and its only a 68 ma pt how does fender get away with powering a pair of el84 with these? seems better suited for a se build, mark

[Phil_S]

In a push pull amp, in theory, each tube is conducting 50% of the time. This also means 50% of the time it requires 0mA (when it's off.) In real life, it isn't perfect.

The BJ has 3 12AX7's, and I'll guess they require 10mA for the 3 of them. One EL84 requires about 50mA. On that basis, it is barely enough.

It is also possible they have tweaked the operating point so the EL84's run at the lowest possible current draw. As I recall, there is some sand in that amp, too, and I don't know if that has something to do with it.

[martin manning]

From Antique Electronic Supply:

Fender Original Blues Junior Power Transformer. 15 watts. Equivalent to #049967.

Specifications
Audio 15 W (RMS)
Primary 117 V, 60 Hz
Secondary 330 VDC, 68 mA after rectification
Filament Winding 6.6 VAC, 2.6 A
Mounting Centers 2" x 2.5"
Mounting Type Horizontal
Weight 2.6 lbs

Depending on the OT primary impedance (which I believe is ~7k), 68mA looks barely adequate for two EL84 and 3 12AX7's. I'd rather see ~100. I suspect there is a lot of consideration given to keeping cost down here.

[mark enger]

Seems a little weak to me, although i remember using one years ago in
a spitfire like amp but its been along time ago, i just scored some ecl86 tubes ill have to look into them or it would make for a tmb se amp like a class5 still dont see how fender gets away with it mark