Fender Twin Preamp Modifications: First off a fresh set
of pre-amp tube, and good speakers will do wonders. Second, always
remember that you can kill yourself
around with an amplifier, even when it's unplugged. You need to discharge
the filter caps to make sure things are safe. If you don't know what I'm
about at this point, you shouldn't be trying this in the first place. Try
this at your own risk. I'm not responsible if you toast
your amp or yourself!
Here we are: the guts of any Fender(tm) Reverb equipped amp. This is the main eyelet board, and whether this is a Deluxe, a Super, a Twin, a Bandmaster, a Showman, a Pro, a Vibrolux or a Tremolux the eyelet board is the same. Many "Silver Face" amps used the same, or very similar topography as well so this may help owners of those model amps as well.
Output Circuit Coupling Capacitors
These two coupling capacitors are between the phase inverter output and the power tube grids. They block the DC at the phase inverter plates and allow the AC (audio signal) to go on to the power tubes.
(Partially hidden beneath the big orange .1uF capacitor)
Various Fender amp models used different feedback resistor values depending on a number of factors including the load to be driven and the desired response of the amp. More feedback tends to flatten frequency response, lower output section gain and also lowers the effective output impedence (eg: you get better speaker dampening). Negative feedback also affects how the amp responds as it transitions into overdrive. It is a simple matter to install a variable resistance in series with this resistor and have variable feedback control to allow the player to find the best amount. We'll go over this in the mods section.
This is where the tremolo action is created. A neon bulb is fired by the tremolo oscillator and the light it produces hits a photo-resistive cell which is mounted inside the rubber tubing with the bulb. The resistance of the photo-resistive cell changes with light, and in fact the resistance drops when it it illuminated. Since the photocell is connected from the signal line to ground, like a level control, it acts as a level control and the volume drops as the resistance of the photocell drops.
Reverb Driver Grid Resistor
This helps set the input signal level to the reverb driver tube (a 12AT7). You can replace this fixed resistor with a potentiometer and have a "dwell" control like the stand alone reverb units did. You can also reduce this resistor's value to find a better reverb level for the way you play.
Reverb Channel Tone Caps
These are the tone caps for the Reverb Channel of the amp. There is a set of tone caps for each channel in this kind of amp, one for each set of tone knobs. No big surprise there I hope. One way to get more variation in tone is to have different values in one channel. We'll go over some common values to try.
Reverb Ch. Cathode Resistor (Rk) and Cathode Capacitor (Ck)
These two components act on the pre-amp tube for the Reverb channel of the amp. Together they help control gain and frequency response for the stage.
Normal Channel Ck & Rk
These are the analogus parts to the Rk & Ck for the Reverb channel. (see above)
Normal Channel Tone Caps
These capacitors form the tone control network for the "Normal" (non-reverb) channel of the amp.
Slope Resistor (Normal Channel Tone circuit)
This is the "slope" resistor for the Normal channel tone circuit.
Plate resistors (Normal Channel. Abbrev Ra, a=anode)
These are the two plate resistors which help set the gain for the Normal channel pre-amp. In later (SF) amps you'll often see 1 watt resistors used here probably as an attempt to reduce noise in the pre-amp.
Slope Resistor (Reverb Channel Tone Circuit)
This is the "slope" resistor for the Reverb channel tone circuit.
Plate Resistors (Reverb Channel. Abbrev Ra)
These are the two plate resistors for the Reverb channel pre-amp.
Shared Rk/Ck (Both Pre-Amp channels)
This Rk / Ck combination is sharedby both channels of the pre-amp (Rvb & Norm). You may note that while the other pre-amp Rk's are 1.5K, this one is 820-ohm; since it has approximately twice the current flowing through it, you only need 1/2 the resistance to obtain the desired bias setting (Ohms law: Volts = Amperes X Resistance).
Reverb Driver Tube Rk & Ck
These two components set the bias and low frequency cut off for the Reverb Driver tube. As with any of the stages, raising the value of Rk will tend to decrease the gain, and this can be very effective at taming a reverb which is too "surfy". We'll detail how to play with this in the mods section.
Phase Inverter Input Cap
The Phase Inverter input cap couples the pre-amp to the power amp section. The value of this capacitor also helps set the low frequency cutoff for the power amp. Closed-back cabinet models or amps with 10" speakers (Tremolux and Vibrolux for example) tend to use smaller values here to roll off the lowest lows which would not reproduce well or damage their speakers. The larger more powerful amps use a larger capacitor here since they can better handle low frequencies. Some guitarists prefer the tone of a large cap here. We'll go over some common values.
Bias Splitter Resistors
These two resistors act as the output stage grid resistors as well as feeding the negative bias voltage to the output tubes. A good place to measure your bias voltage is at the point of the "V" where they join together. One of the common master volume circuits involves removing these altogether and replacing them both with a dual potentiometer.
Ok, now you understand the guts let's modify
1. Unplug the unit. Unplugging from the wall will not drain
the filter caps, it actually won't make a bit of difference, but again this
is for our own
safety. We unplug to keep the amp from accidently turning on while our hand
is inside the amp. Don't assume anything's impossible, as that's how people
die. Unplugging is the equivalent to "locking out" before working
on a downed machine. Everytime you turn the unit back on you recharge the
2. Standy switch: Turn the power switch OFF, and place the standby switch in the ON position. Note: This isn't the same as having your amp "on standby." We're hoping the filter caps will drain down some with the standby switch closed.
3. Draining the Caps It is absolutely vital that we drain the filter caps in the DC power supply before working on the amplifier! These often hold a charge of several hundred volts, which is potentially deadly. Caps can hold their charge for weeks or longer, even with the power off AND the amp unplugged. It's very important that we learn to properly "drain" this residual voltage so that we may safely work on the inside of our amplifier.
Place the amp on a sturdy table in a well lit room. Be sure to wear rubber soled shoes, and if you have a properly insulated floor mat (made of rubber) standing on it can help prevent the likelihood of electrocution. Remove the back panel of the amp.
The absolute easiest way to drain the capacitors is take a 'jumper cable' (2 alligator clips connected on a wire) and short (or attach) one end to the chassis on a 'lip', or other metal point that it will stay on, and the other end on the first preamp tube, either pin 1 or pin 6
Other Methods of Draining Caps
It seems to be widely believed that you can drain the filter caps by turning your amp off while strumming your guitar. I agree that it sounds like they're draining, but I've found that it does not make any difference. This method can not guaranty that the filter caps have drained to a safe level, therefore it can not guaranty your safety. The only way to know if you're safe is by checking each filter cap with a reliable multimeter.
4. Pull The Power Tubes
They do pull straight out. Really stubborn ones might need a little
rocking in a circular motion (not much). *DO NOT twist or turn.
Don't worry too much about breaking the glass... they are *much* more
sturdy than a light bulb.
I'll add that you should put your hand over the top of the tube.
Otherwise, you are likely to break it if it hits the cabinet.
I suggest placing your hand over the top because if it is kind of "sticky"
coming out, when it does release, you are likely to slam it into something
and break it.
5. You want to work off the AB 763 Black Face circuit. When you connect
the wire from the preamp out to the vibrato jack you'll lose the vibrato
and also the master volume (on silver face twins). Use the Vibrato jack as
your preamp out jack to your power amp.