Fender preamp tube question

[gtto]

The never ending saga of trying to solve my Fender Supersonic 60 problems with not enough electronics knowledge continues....The problem is still that after playing for an hour or so the volume in the burn channel decreases a lot and the sound is very distorted. No problem in the vintage channel. I have changed preamp tubes but it hasn't helped. FWIW I just turned on the amp after it being off all night and it is fine. This is typical and after an hour or so it will show the same symptoms. Any suggestions of where to start looking? Thanks.

[52 Bill]

The never ending saga of trying to solve my Fender Supersonic 60 problems with not enough electronics knowledge continues....The problem is still that after playing for an hour or so the volume in the burn channel decreases a lot and the sound is very distorted. No problem in the vintage channel. I have changed preamp tubes but it hasn't helped. FWIW I just turned on the amp after it being off all night and it is fine. This is typical and after an hour or so it will show the same symptoms. Any suggestions of where to start looking? Thanks.

You need to start monitoring things to see what is changing after an hour or so, that could be causing your problem. The most likely thing is voltages. When something is heating up, it is changing the voltages to the circuit and causing the signal to reduce and distort.

What sorts of meters do you have and what kind of ability to work on live circuits do you have?

[gtto]

Bill,
I only have a cheap digital multi meter and a very basic ability to work on live circuits. I have learned much since this problem started and understand the potential for serious shock caused by the voltages present. If I had access to a tech I would take the amp there but I am currently living in a village in Thailand and a tech is hard to come by. Any suggestions where to start testing?

[mikeydee77]

I spent a while debugging one of these recently. My fault turned out to be sensitivity to the loud speaker lead being too close to the chassis on my bench and not a fault with the amp. Anyway I learned a few things about these as a result. Sounded great when all fixed up.


Sounds like you can can swap valves ok and it is possible that this is the cause of your fault. However there are plenty of other things around the valves and the power supply that could cause such a fault.

A couple of extra pieces of info that would be useful to know are:

1. When the fault occurs is the power amp OK? Can you inject a signal directly into the power amp in socket and is it the same as before the fault developed? IF OK THEN POWER AMP IS OK

2. When the fault occurs are the other channels OK, I think you already indicated yes to this but just want to be sure? SO IT IS DEFINITELY NOT THE POWER AMP AND ONLY THE BURN CHANNEL AT FAULT

Assuming it is just the BURN channel at fault see if you can follow the circuit description and work out which valves (e.g. V2-A or B) could be at fault. I am hoping that you can find a way of swapping ones from the "good" channel to the "bad" channel. Or taking suspect tube and putting it in a putting it in an always in use spot (such as V1).



PRE-AMPLIFIER
The guitar input signal flows through a tube gain
stage (V1-A) before it is split into two signal paths,
the “Vintage” path or the “Burn” path. The “Vintage”
channel, consisting of passive 2-band tone stack,
volume control and tube sections V2-A/B, can also
be split into two paths within the channel, known as
“Vibrolux” and “Bassman”. The path selection will
depend upon the selection of the
“Vibrolux/Bassman” front panel switch (or via Footswitch).
The “Vibrolux/Bassman” switch controls relays K2-
A/B thru K4-A/B in order to activate the desired
path. When the “Vibrolux” path is selected, relay
K2-A/B thru K4-A/B will be in the “normally closed”
state. When the “Bassman” path is selected, relay
K2-A/B thru K4-A/B will be in the “normally open”
state. The “Vibrolux” path provides a tone that is
similar to the ’65 Fender Vibrolux blackface amplifier. The “Bassman” path provides a tone that is
similar to the ’66 Fender Bassman Head blackface
amplifier.
The “Vintage” channel remains active until the “Vintage/Burn” front panel switch (or via Footswitch) is
selected. This will change relay K1-A/B to deactivate the “Vintage” channel and divert the signal to
the “Burn” channel. When the “Vintage” path is selected, relay K1-A/B will be in the “normally closed”
state. When the “Burn” path is selected, relay K1-
A/B will be in the “normally open” state.
The “Burn” channel provides a new tube distortion
tone that is more versatile to past Fender tube drive
circuits. The “Burn” channel consists of two Gain
controls, a passive 3-band tone stack, volume control and tube stages V1-B and V3-A/B Setting
“Gain 1” (R31) to “0” will provide no signal through
this channel. “Gain 1” provides the high gain to the
signal, while “Gain 2” (R35) provides harmonic
overtones to the signal, Using “Gain 1” and “Gain
2” in tandem can provide many distortion tones,
ranging from standard clean rock “crunch” tones, to
“Texas Blues” tones to “lo-fi” fizz tones, to “playsitself” ultra-high gain tones.
The signal is directed back to the summing point at
V4-A. From here, the signal is again split into two
paths: The “dry” path and the “Effects Loop” path.
This path selection is controlled only by the footswitch. When the “dry” path is selected, relay K5-
A/B is ”normally closed”. When the “Effects Loop”
path is selected, relay K5-A/B is ”normally open”.
When the footswitch is not used, K5-A/B will default
to “normally closed”. The “Effects Loop” will not be
in operation unless the “Send” and “Return” jacks
are connected to either an outboard Effects system
or if a jumper cable connects the two jacks together.
Connecting the jumper cable would allow the user
to use the Effects Loop as a switchable “boost”,
when the footswitch is used. The “Send” and “Return” signal levels can be adjusted by the “Send
Level” control (If a plug is connected only to the
“Return” jack, the signal of the Pre-Amp will be broken, thus not allowing signal to flow to the “Pre-Amp
Out” jack and the power amplifier.
The output from the desired path (“dry” or “Effects
Loop”) is directed through K5-A, pin 4. From here,
the signal is sent to a recovery amplifier stage at
V5-B and to the reverb circuit at V9 and V10. The
amount of reverb is controlled by potentiometer
R166. The reverb signal is summed at the input of
V5-B. The output from V5-B is split into two paths:
“Pre Amp Out” and “Pwr Amp In”. The “Pre-Amp
Out” path includes a buffer and differential op-amp
circuit (U4-A/B) to “telescope” the signal to outboard
gear, such as a mixing board, PA, etc., through the
“Pre Amp Out” jack, J4. Simultaneously, the signal
is fed to the shunt of the “Power Amp In” tip-shunt
jack, where the tip of the jack is connected to the
input of the phase splitter circuit at V6-A/B. When
the “Power Amp In” jack is used, this will “break” the
pre-amp signal from continuing to the phase splitter,
thus allowing the power amplifier to be “slaved” by
an external pre-amp.


POWER AMPLIFIER
The Power Amplifier is a discrete push-pull topology
with feedback. The basic amplifier sections include
a phase-splitter stage (12AT7 at V6) and a pentode
push-pull output stage (6L6GC’s at V7 and V8).
The output signal is directed through an output
transformer capable of driving 4, 8 or 16 ohm
speaker loads. The 4 and 8 ohm outputs are wired
to the External Speaker jack (J8). The output impedance of the output transformer is 8 ohms when
a plug is inserted into the Main Speaker jack (J9)
only (NOTE: An 8 ohm speaker load should only
be used when the output impedance of the output
transformer is at 8 ohms.). The output impedance
of the output transformer is switched to 4 ohms
when an additional plug is inserted into the External
Speaker jack (NOTE: A 4 ohm speaker load should
only be used when the output impedance of the
output transformer is at 4 ohms.). The 16 ohm tap
of the output transformer is not used.
The phase splitter circuit is a “long-tailed” configuration commonly found in Fender Tube amplifiers.
The output circuit consists of two 6L6GC tubes in a
push-pull configuration, again, commonly used in
Fender Tube amplifiers. The 1 ohm Resistors
(R140 and R141) are can be used by the technician
to properly measure the amount of bias current for
each output tube (V7 and V8). The Optimal bias
setting for each tube in this amplifier is 30mA-
33mA. The bias current for V7 can be derived by
measuring the voltage across R140, which in this
case will measure 30mV-33mV. The same procedure can be used for V8 by measuring the voltage
across R141. The bias current for both output
tubes is controlled by potentiometer R156, found on
the Main PCB assembly.
The Feedback network consists of feedback resistor R132, shunt resistor R133 and shunt capacitor
C70. The feedback configuration of the power amplifier in connected in a “non-inverting”
configuration. Capacitor C70 is used to shape the
frequency response of the power amplifier such that
up to 6 dB of feedback is applied to the response at
low frequencies (approximately 150Hz and below).
There is little to no feedback as frequencies are increased from the 150Hz breakpoint. This results in
greatly reducing nonlinear speaker response at low
frequencies, while maintaining the high frequency
“sparkle” of the speaker.


POWER SUPPLY
The Power Supply is typical of a Fender tube amplifier. The secondary side of the Power Transformer
consists of 3 windings: high-voltage winding, lowvoltage winding and heater winding. The high voltage winding is used to generate the high voltage
DC power supplies in the amplifier. The high voltage DC power supply consists of a standard bridge
rectifier configuration. The “voltage-drop” resistors
R148 thru R151 set the DC plate voltages used at
various tube stages throughout the design. The low
voltage winding is used to generate the low voltage
+/-16V DC power supply, as well as the output tube
bias supply. The low voltage DC power supply also
consists of a standard bridge rectifier configuration.
16V zener diodes (D49 and D50) and “voltagedrop” resistors R158 and R159 are used to regulate
the +/-16VDC power supplies. A voltage doubler
circuit (C85-C86 and D43-D44) is used on ½ of the
low voltage winding to generate a negative DC
power supply for the bias control potentiometer
R156. The heater winding produces a 6.3VAC
power supply that is used for the heater filament in
all of the tubes in the amplifier, as well as illuminating the pilot light.

[gtto]

funny my friend is Mickey Dee the former drummer in Motorhead......anyway, swapping tubes doesn't change anything. I am trying to get an amp tech to maybe help me online using a webcam or something and maybe they can help me diagnose the problem.

[mikeydee77]

Mickey Dee the former drummer in Motorhead.......

I wish it was me!

I think it is time for a tech to investigate. Good luck with the fix.