From larrys–(at)–ol.com Wed Oct 2 19:09:23 CDT 1996
Article: 23962 of alt.guitar.amps
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From: larrys–(at)–ol.com (LarrySB)
Newsgroups: alt.guitar.amps
Subject: Re: SF Deluxe Reverb mods?
Date: 2 Oct 1996 14:40:58 -0400
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<< Would there be any reason to skip this particular amp and wait for an early 70's model to pop up? >>
I don’t think so, I happen to own one myself.
The DR is one of the few to escape most of the horseshit design changes
Fender did in the 70’s. I bought a bone stock 78 DR (even the tubes were
original) for about what you’ve been offered. It’s real clean and works
great.
I did do a blackface conversion on it, and removed the “pull for boost”
nonsense. I also had to change the reverb pan.
The late 70’s DR can be blackfaced easily with two exceptions. The B+
voltage will still be a little too high; and the wiring quality utterly
sucks compared to the ones from the 60’s.
What you do is this:
Remove the suppresion caps from the 6V6 sockets (1200pf discs)
Revert the phase inverter schematic to the 65 circuit (just a few resistor
value changes)
Revert the reverb drive 12AT7 circuit to the blackface circuit
Remove all the wiring associated with the boost switch and pedal
Optionally change the power supply divider resistors to get the preamp and
phase invertor supplies to lower voltages. (I actually preffered the
Silverface values myself)
Option but recommend: rewire at least the output stages with short, neat
runs of wire, using the pictorial layouts from the early designs. You can
find a suitable layout in the Groove Tube book or Weber’s book.
optional: if you find the voltages more than 10% higher than on the BF
schematic, you can lower them neatly and easily by installing a stud mount
Zener (cathode case) between the power xfrmr center tap and the chassis
ground. Use a voltage that will drop the B+ by the desired amount,
usually around 20-30 volts. You can also change the power xfrmer or add a
bucking xfrmr, but the Zener diode is clean, cheap, easy and reversable.
For tubes, I really like NOS Sylvania 6V6GTA the best. Current Russian
ones will almost certainly self destruct in a matter of minutes. Even NOS
6V6 will sometime arc or carry on. Bias around 25ma per output tube.
Since I had to replace my reverb spring, I went with a 3 spring
replacement. The reverb is super lush now, sounds like a big church!
I still haven’t found the perfect speaker, but the stock ’78 unit is
pretty decent.
If you like to work on amps yourself, a 78 can be a bargain. But if you
are relying on a tech to do it for you, ask his opionion first. The poor
quality of wiring is enough to make most techs charge a little more for
the conversion.
—
Dr. Nuketopia
Technology Director of the World-Wide Monetary Conspiracy
All opinions strictly reflect the party line.
Read the Blue Glow in Tubes FAQ at http://www.persci.com/~larrysb
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