blackfacing instructions ?

[hebaton]

Looking for instructions on Blackfacing a Silverface Twin (actually AA270 Dual showman reverb)

Thank you !

[Roe]

no need to blackface the phase inverter

[hebaton]

no need to blackface the phase inverter

O.K. but, where do I get a full description of what needs to be done ?

[martin manning]

... but, where do I get a full description of what needs to be done ?

Just get an AB763 schematic, http://www.thevintagesound.com/ffg/sche ... _schem.gif and an AA270 schematic https://schematicheaven.net/fenderamps/ ... _schem.pdf and find the differences. Basically it's in the preamp voltages and resistor values in the PI (plate loads and bias resistor, plus the power tube grid leaks). Everything else looks to be the same. You can't do anything about the plate or screen voltage, but you can adjust the two resistors in the power supply to get the Blackface PI and preamp voltages. Starting with the AB763 values (1k and 4k7) might get you close. You would do that after changing the other parts, of course.

[sluckey]

The attached pdf contains all the info a competent tech needs to convert an AA270 TR to a blackface AB763 TR.

[martin manning]

If hebaton has a working bias circuit, I'd say leave it as is.

[hebaton]

Thank you so much ! Exactly what I was looking for !

[Ten Over]

If hebaton has a working bias circuit, I'd say leave it as is.

Absolutely.

The one-wire bias mod is actually superior to the AB763 bias circuit. The AB763 bias circuit defaults to the hottest bias voltage (i.e. 0.0Vdc) in the event of wiper failure. The one-wire mod defaults to the coldest bias voltage upon wiper failure. Of course, one could install a relatively large resistor between the diode side of the pot and the wiper in order to remedy the shortcoming of the AB763 circuit, but then it wouldn't be pure AB763.