MANUFACTURING ERROR on the Fender HotRod Deluxe.
We found this error
this week on a stock, untouched HotRod deluxe - a pretty severe error
as the amps were obviously shipped with this error built in.
Since we know the
circuit boards are loaded from "ammo pak" type parts batches this could
appear in a LOT OF AMPS.
We noticed the amp
sounded really ugly and weak. It obviously had crossover distortion,
even with new tubes and bias set correctly.
As we have learned
from experience the plate load resistors for the phase inverter burn
out - they are often 1/4 watt resistors, not up to the voltages present
on that circuit.
these resistors are on
the tube circuit board, R57 (82K) and R58 (100K) -
In checking these
parts in place I found that R57 was 82 ohms, not 82,000 ohms. - this
had NO GAIN and caused half of the phase inverter to not work at all -
this makes only one
power tube work giving terrible terrible crossover distortion. Both
both tubes should be working all the time.
This is why the amp
BUT, the real worry is
that this was a stock factory installation - machine inserted and wave
soldered in place.
This amp left the
factory, and later on left the store in this condition.
Why anyone would buy
an amp that sounded this bad is beyond me, couldn't they hear?
TEST FOR A BAD PHASE INVERTER - ALSO FOR A BAD POWER TUBE
your amp for
this problem, take out one of the power tubes - if it doesn't sound any
different, you have the bad phase inverter OR a bad power tube.
If you get
you obviously have the bad phase inverter and/or a bad power tube.
the tube affected by
the resistor error is the right hand one, the one next to the preamp
Check your amp to be
sure R57 is really 82k ohm (Gray/red/orange is 82k - 82 ohms is
and R58 is
really 100k ohms (brown/black/yellow).
carefully change them to the correct values so you have your
whole amp working right. If this is too difficult for you - and it is
pretty hard to get that board out of there, desolder from a printed
circuit board, and refit parts without destroying traces, then get a
pro shop like ours to fix if your you.
this also points out a
really horrid "internet trick" that is totally wrong. You CAN NOT
remove one power tube of a two power tube amp to get half power.
both tubes are needed
in the Push Pull circuit. they "trade off" amplifying the signal
milli-second to milli-second. Half the wave form on one tube, the other
half on the other - taking one tube out clips off half the wave form
creating a horrible sounding crossover distortion, and leaves half the
output transformer running its signal into no load - this
will burn the output transformer up in short order, the bias supply,
the phase inverter and WILL NOT give half the power, more
like 1/10th the power until eventual failure of many parts.
Of course we offer
diagnosis and repair of this problem in our shop every day and can fix
if for you.
final notes on web page
Legendary Book "Inside Tube Amps" is now
available on Amazon/Kindle
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