From mgarvi–(at)–anix.com Thu Sep 19 10:05:17 CDT 1996
Article: 22999 of alt.guitar.amps
Path: geraldo.cc.utexas.edu!cs.utexas.edu!howland.erols.net!EU.net!newsfeed.internetmci.com!panix!news.panix.com!not-for-mail
From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Advice on 69 Super Reverb
Date: 19 Sep 1996 02:36:29 -0400
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In <51pl59$t4--(at)--fw-ixnews4.ix.netcom.com> kahun–(at)–x.netcom.com(Joe) writes:

>I have a 69 Super Reverb that sounds great when played at
>low to medium volume but not so when really cranked. It does
>not distort in a satisfying way. Kind of a braaaapp farting
>sound when chording the low strings on my Strat with the neck
>pickup on. Bass control is only on 5 or so.

>The speakers are CTS alnico’s with date code 137 6909
>Fender special design PN. 064121. They seem to be in good

Check it with another speaker cab. Then try another brand of output
tube (just in case). It’s hard to tell exactly what you’re
describing, but the effect might be caused by ‘grid blocking’ (ref
Radiotron). This happens when your output tubes’ grids are driven
into conduction. When the PI output swings positive far enough, the
negative grid bias will not be sufficient to keep the voltage at the
output tubes negative. The grids conduct, sorta like a diode, and an
‘artificial’ negative charge develops across the coupling cap. That
charge keeps the tubes biased very cold (in class C) until the charge
can bleed off thru the bias feed resistors.

No guarantee that this is the cause, of course. Have a tech
monitor the *DC* level on the output tube grids as you play loud.
See if you notice the effect when the level gets close to ground.

The amp’s feedback loop also reacts badly to this condition and
will generate some nasty sounding spikey artifacts.

>The amp circuit is the AB 568 and differs from the AB 763
>in that it has 47K PI plate load resistors, on the output tubes
>100K bias feed resistors, 20000 pf bypass from grid to ground,
>150 ohm cathode resistors and a 5 uf cap connected between the
>cathodes (what the hell is this?). Also some minor differences
>in the bias supply. Rectifier tube is a 5U4 vs. GZ34. Did I miss
>anything?

The 5U4 should be OK. The cathoder circuit is (guessing) an attempt
to provide a bit more stability to the output section. Fender
has traditionally operated their output tubes outside of the
mfg’s spec for ‘dc path from grid to ground’. That can cause
excessive ionization which wears the output tubes. The spec calls
for around 100k or so for fixed-bias 6L6’s. That spec loosens
up a bit when cathode bias is used. My guess is that both design
changes were aimed at the same fix.

I don’t personally find much of a problem with mixing cathode
bias and fixed (Anyone who *does*, please explain why it should
make a difference). Yes, I’ve done A/B listening tests. And the
150 ohm res’s only develop a few volts.

I think most of the difference in sound is due to the lower value
resistors in the driver/bias circuit. Again, probably a ‘proper’ fix
for the res-to-ground problem.

>As far as I can tell everything is right, tubes, caps, voltages,
>bias (about 35 ma) etc. What difference in sound can I expect if I
>convert it to a blackface style AB763?

The voltages should be a tad *lower* due to the 5U4 rather than the
5AR4.

Expect a fuller, richer sound from the ’63. A bit more gain. Some
players like the cleaner sound of the new circuit. Most don’t.

That change (to AB763) is not guaranteed to fix the problem that
you’ve described, though.

How you guys been, anyway? Is Carl behaving himself?

Mark G.

 

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