ASK AMP MAN
JEFF BOBER
Turn The Amp OFF … Or Else
Hi Jeff,
Last year I plugged into a Fender Vibro-King at my local used gear store and absolutely fell in love with the tone; the “fat”
boost sounds playing magic with the P-90s
on my Les Paul Jr. Anyway, I ended up trading three guitars and a handful of pedals
for it, and walked out of that store lugging
that behemoth of pure tone behind me.
For the next few months, I found that the
sound of my Vibro-King inspired me to
play every day. Sadly, I had to fly out of the
country on business for a month, and when
I returned home I realized that I had left
my Vibro-King on the entire time (I knew
I should have replaced that non-working
jewel bulb right away). When I plugged into
it, the sound fizzled and then stopped. I
replaced the slow blow fuse with another
and turned the amp back on. That glorious
sound returned, but it was short-lived as the
amp fizzled and then stopped working once
again. I have tried many different types of
fuses as prescribed by the Fender user manual, but the same scenario always occurs.
Can you help me? Can you me tell how I
can get my beloved Vibro-King working
dependably once again? I miss her so.
Thank you so much for any help you
can provide.
Gregory Fischer
Hi Greg,
I’ve seen this situation before. It sounds like your
amp is suffering from abandonment issues due
to your extended travel abroad, and this is its
attempt at payback. Oh wait … that theory was
from my psych class!
Okay, I’m kidding of course. While there’s no
way that I can be sure of what’s causing the fuse
to blow, here are a couple of possibilities. The
first—and probably only—user-repairable item
I would suspect would be the output tubes. I’m
going to break the following down into two
steps, as it will make the explanations of multiple
causes easier to define.
48 PREMIER GUITAR DECEMBER 2009
Step 1: Remove the output tubes (the 6L6/5881
tubes), replace the fuse and turn the power switch
on. Let the amp idle in this state for as long as a
typical time would be for the fuse to blow. If the
fuse has not blown, move on to the next step.
… I found that the
sound of my Vibro-King
inspired me to play every
day. Sadly, I had to fly out
of the country on business
for a month, and when I
returned home I realized
that I had left my Vibro-King
on the entire time …
Step 2: Turn the standby switch on, and again
let the amp idle for a while. If the fuse has not
blown in the typical amount of time then chances are that the output tubes are shorting after
they’ve been on for a while. This is really not an
atypical symptom of output tubes, and could
easily have happened regardless of whether or
not you left the amp on for all that time. Have
the tubes replaced and biased and you should
be back in business.
Now, if you did experience the fuse blowing at
any point in the above procedure then the amp
will need to be serviced by your local experienced tech, and here are the possible causes:
A fuse failure during Step 1 would more than
likely point to either the mains transformer itself,
or one or both of the primary capacitors (220uf 285V) in the power supply being electrically leaky, which would eventually cause an increase in current draw enough to cause the fuse to blow. In attempting to isolate one cause from
the other, I would first disconnect the 220uf caps
from the circuit. With the standby switch off,
turn the amp on and let it idle for a reasonable
amount of time. If the fuse continues to fail I
would be forced to suspect the mains transformer. While the transformer in these amps is
a pretty substantial unit, it is a possibility since
transformers in most consumer goods such as
these are manufactured for “intermittent use”
and not “constant use.” I’m pretty sure that
leaving the unit on for a month would fall into
the latter category. If the fuse did not fail during
this mains transformer test, I’d revert to suspecting the primary capacitors in the power supply. I
would definitely replace both caps since the malfunction of one can place a serious overvoltage
condition on the other.
If the fuse eventually failed during Step 2, I
would suspect either the output transformer
or any of the other six filter capacitors (22uf
500V) in the power supply. First, disconnect the
primary center tap lead (red) of the output transformer from the circuit. If the fuse does not fail,
this could point to an output transformer with
a winding that is eventually arcing and shorting
to another winding or to the case, which is at
ground potential. If the fuse does still fail, we’re
more than likely left with the diagnosis of failing
secondary filter caps. Unless one shows distinct
signs of being stressed (pressure relief vent protruding), or is physically blown (you can’t miss
that one!), I would replace all of them.
Hopefully this will return the amp to being the
“King of Your Tone.” If not, maybe you’ll have to
sweet talk it and ask for forgiveness … and buy
it a new light bulb!
… ‘Till next time.
Jeff Bober
Jeff Bober, Godfather of the low wattage amp
revolution, co-founded and was the principal
designer for Budda Amplification. He can be
reached at pgampman@gmail.com.
www.premierguitar.com
