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Super Reverb Blackface Mod Question

66.32.82.115
Posted on July 3, 2009 at 08:38:16
Thermionic27609
Audiophile

Posts: 97
Joined: March 29, 2009
 A friend of mine asked me to take a look at his 1970 Fender Super Reverb, AA270 circuit; he's had problems with an output tube going into thermal runaway. He told me (several times) that he'd had the amp converted to Blackface specs by a previous tech, which, as far as I've been able to discover means making the relatively minor, but sonically significant circuit changes to the AB763 circuit.

The amp had been converted to adjustable bias and had the 100k 6L6GC grid resistors changed to 220k, but the PI plate resistors were still both 47k rather than 82k/100k. To my mind, changing the phase inverter resistor values seems one of the more significant parts of the mod--other than snipping the shunt capacitors on the 6L6GC grids.

Since I'm ignorant of this, did my friend get what he paid for when he paid for a "Blackface Mod" if the tech left the 12AT7 phase inverter plate resistors at their AA270 circuit values? To change one circuit to another makes sense to me. To change it halfway, but not all the way leaves me puzzled.

David

RE: Super Reverb Blackface Mod Question, posted on July 17, 2009 at 03:41:53
FenderLover
Audiophile

Posts: 3308
Joined: July 31, 2007
Contributor
  Since: May 17, 2009
 David, there's some other changes going on. Prolly one of the more important one is the AA270 schemo seemed to run a 5U4GB rectifier where the AA763 uses a GZ34. There also seems to be a difference in plate voltage on the PI and the size of the coupling cap (0.001 versus 0.01 mfd).

I haven't modded an SF Super back to a BF, but YES, attention of the voicing of the PI is important. Maybe playing with the plate voltage and the coupling cap might revert the SF back to the BF a bit more. And that rectifier.

Good luck!

RE: Super Reverb Blackface Mod Question, posted on July 7, 2009 at 18:36:23
DMW
Distributor or Rep

Posts: 319
Location: Midwest USA
Joined: July 26, 2003
 Go to AMPAGE and post your question. Many pro amp repair guys post there. A tech offered free info to me to help me repair my Mesa amp some years back. What he told me to do was right on the money and, speaking of money, it saved me quite alot.

RE: Super Reverb Blackface Mod Question, posted on July 7, 2009 at 07:17:29
Pro Sound Guy
 Ask this guy

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