Ampeg VL-503 guitar amp bias problem
William F. Zachmann
A week or so I'd posted a message looking for help resolving a problem
with an Ampeg VL-503 guitar combo amp I'd recently purchased used.
The amp has bias adjustment screws on the back panel and LEDs to
indicate correct bias (off for under bias, green for correct bias, red
for over bias).
The problem was that, at least with 6550 tubes installed (a selector
switch on the back panel allows use of either 6550s or EL-34s) the
amp, after being correctly biased initially (both LEDs green),
appeared to drift into an over biased condition (first one, then the
other LED would turn red and the output tubes would glow hot).
No one here seemed to be familiar with this particular amp (I don't
think there are a lot of 'em around) and other than suggestions that I
contact St. Louis Music (now owners of Ampeg) or Lee Jackson (original
designer of the amp) no one seemed to have any specific information
about it.
Lee was very kindly responsive but told me I'd need to get the
schematic from SLM since he did not have the rights to distribute it.
SLM was very helpful and very responsive. A fellow named Pat in their
customer service department promptly sent me a detailed schematic and
a copy of the owners manual as well as an engineering modification
memo from Feb. 18, 1994 that, as it happened, specifically dealt with
the primary problem I was having. And a fellow named Tom in their
tech support department was equally helpful in answering some further
questions.
I must say I was very favorably impressed by how helpful and
responsive the folks at SLM were. I must confess I've previously had
a somewhat dim view of Crate amps (another of SLMs brands). Given my
experience with the SLM folks, I'm far more inclined to favorably
consider Crate amps than I was previously. The SLM folks service
response equaled that of Mesa Boogie which has long impressed me as
the best of the amp companies in terms of service and support.
The point of this message, however, is to let folks know who might own
or who might encounter on of these amps in a service situation that
the bias drift problem on the Ampeg VL-503 is fundamental to the way
at least the early production (pre-February, 1994) models were built
and that there are specific engineering modifications to correct it.
These are addressed in a "VL-503 MODIFICATION Feb. 18, 1994" memo
available from SLM/Ampeg (1-800-738-7563).
The basic problem is that there is a metal bracket/shield over the
bias adjust and hum balance trim pots that comes very close to the
exposed connectors at the back of the pots. There is a redesigned
bracket part available from SLM/Ampeg. Also, two ground wires should
be added to connect R76 and R77 to the a/c ground screw on the
terminal board. These mods are described in greater detail in a May
30th, 1995 memo titled "SLM Electronics, Ampeg VL-503 Upgrade Items".
The Feb. 18, 1994 memo actually seems to me bit off base in the
diagnosis of the problem. It implies, at least, that the bracket is
actually shorting out the trim pots: "the bracket . . . can touch the
leads of the pots." It says, "Insulating tape is often not enough to
assure isolation of the bracket from the pots due to the sharp edge of
the bracket."
It also treats the addition of the ground wire as an unrelated aside:
"another possible cause of no bias or strange behavior is bad ground
connections from pcbs to the chassis". The Feb. 18, 1994 memo is not
specific about adding ground wires from R76 and R77 as the May 30,
1995 "upgrade items" memo is.
My own view is that the problem had nothing to do with the bracket
directly shorting out the pots (were that the case, I would have seen
evidence of arcing on the contacts) but, rather, that the conductive
metal bracket, being in fact chassis ground, in close proximity to the
bias voltages in the wire connectors at the back of the trim pots, was
creating some sort of capacitance/inductance linkage not only between
them and the chassis ground, but very possible among each other as
well (i.e. I think the bracket may have been functioning as, in
effect, a sort of transformer core linking them).
In any case, should you happen to own or run into an Ampeg VL-503
combo amp or see one come into your repair shop because of a problem
with bias drift (and therefore potentially prematurely blown output
tubes as well), don't give up on it in frustration. Instead, call SLM
and get the detailed documentation for the amp and the engineering
mods for it as well as a replacement bracket to fix the problem (I
don't yet have the replacement, by the way, but have bent the original
back far enough away from the leads to resolve the problem temporarily
until I get the replacement).
Although it won't satisfy terminal-strip-only, no-PCBs,
no-op-amps-in-my-guitar-amp purists, the Ampeg VL-503 (and, I suspect,
the other amps Lee Jackson designed for Ampeg and Crate, as well) is a
fantastic amp with a great range of excellent tone.
It matches or exceeds what I get out of my Mesa Boogies (a Mk. I, a
Mk. IIc, and a Studio Pre-Amp, 50/50 Power Amp rack rig). And unlike
my Boogie Mk. IV (which I sold long ago) it offers a three-channel amp
that not only sounds great in all sorts of different settings (I could
never get the Mk. IV to sound very good in ANY combination, which is
why I eventually sold it), it actually has a remarkable simplicity, at
least by comparison to the Mk. IV. And it is more versatile, offering
a wider range of sounds, than any other amp I've every seen.
All the best,
Will Zachmann
w...@CanopusResearch.com
will_z...@post.harvard.edu
http://www.CanopusResearch.com
On Compuserve: 76004.3657 (or GO CIS:CANOPUS)
Wilfred
The current capability of bias circuits is usually far to small to be able
to burn a visible arc. Their claim makes sense.
Wilfred
William F. Zachmann heeft geschreven in bericht
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