Notes found around the interwebs that I want to keep track of.

Production years:

• 1964 -1967 “blackface” circuits AA763, AB763
• 1968 -1982 “silverface” circuits AC568, AA769, AA270

Tube layout:

• AB763 Tube layout (Seen from behind, V1 is to the right side)
• V1 12ax7 = Preamp normal channel
• V2 12ax7 = Preamp vibrato channel
• V3 12at7 = Reverb send
• V4 12ax7 = 1/2 Reverb recovery and 1/2 gain stage for vibrato channel
• V5 12ax7 = Vibrato
• V6 12at7 = Phase inverter
• V7 6L6 = Power tube #1
• V8 6L6 = Power tube #2
• V9 6L6 = Power tube #3
• V10 6L6 = Power tube #4

Half power mod – Pull out two 6L6 and disconnect 1 speaker.

Pull out the two inner 6L6 tubes and you have a ~40-50W amp. You should/can also disconnect one of the speakers to not have an impedance mismatch. The Twin reverb output transformer is designed to work most efficiently with the following impedances/loads and tube power configurations:

4 x 6L6 ->; 4 ohm speaker load. And then comes the standard Fender +/- 100% tolerance.
2 x 6L6 ->; 8 ohm speaker load. And then comes the standard Fender +/- 100% tolerance.
The plate voltage will increase a little when two of the tubes are removed. Ideally you should have the amp re-biased to not run the two remaining tubes too hard.

The voltage levels on the plus- and minus-sides of the output transformer’s primary circuit remains the same whith two and four 6L6 tubes. But the delivered current is doubled with four tubes. Hence, the power is also doubled. Power (Watts) = Current (Amperes) x Voltage (Volts).

This mod is a must-know survival trick for all Twin owners.

12AY7 or 12AT7 as preamp tubes – Less preamp gain.

If you want cleaner and spankier preamp distortion charcteristics, you may replace the V1 or V2 12AX7 preamp tube with 12AT7 and 12AY7. These tubes have different frequency responses than 12AX7, particurlarly when distorting. People describe these tubes to have less harsh and buzzy distortion. This mod does not alter the tone significantly when amp is played clean or when only the power amp section distorts. You’ll have to increase the volume setting to achieve a similar volume as before. The reason is that 12AX7 tube has a voltage gain factor = 100, while 12AT7 = 60 and 12AY7 = 45.

Pull out the V1 normal channel preamp tube – More preamp gain in vibrato channel.

We’ll start with saying that this is a must have mod. It is so easy to enable and disable that it can hardly be called a mod. If you are like most players and only use the Vibrato channel (reverb, tremolo, the brigth cap and the extra gain stage), you should pull out the V1 tube. This is the preamp tube for the normal channel which you are not using. Vice versa; If you’re using the Normal channel, you can pull out the V2 tube. All AB763-similar circuits (Deluxe Reverb, Super Reverb, Pro Reverb, Twin Reverb, Vibroverb, Vibrolux) are designed so that signal is leaking between the two channels. The amp will play louder at the same volume knob setting when pulling the V1/V2 tube that you’re not using. The stronger signal will push the second gain stage (V4 tube) harder and give you increased sustain, compression and harmomics. This mod does not change the amp’s clean headroom.

This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always did to Stevie’s amps.

Replace the 12AT7 PI tube with a 12AX7 or 12AU7 – Less clean headroom.

Very practical mod at practice and low volume environments. This mod reduces the amp’s clean headroom and you’ll achieve sweet spot at a lower volume. You’ll notice that the amp gets looser and with less attack. Tips: If you pulled the V1 12AX7 tube you may use it as V6 phase inverter.

12AX7 as phase inverer tube will give the most effect out of this mod. 12AU7 will be in between 12AT7 and 12AU7.

Replace the 12AT7 reverb driver with 12AU7 – Better reverb control.

Reverb is an important character with vintage amps, yet so individual and mysterious. We all know that speakers change their tonal character during age. So does the reverb. The reverb function sounds and behaves differently between “identical” vintage amps. Some amps have long, lush and soft reverb while others are mushy and overwhelming. We often find the reverb sweet spot around 2.5 on the reverb pot, varying from 2 to 4. Some amps are sensitive and difficult to control the reverb on. The whole dynamic area can be within a narrow interval, i.e. 2 and 2.5. These amps require a careful touch when dialling in the reverb, which irritates us.

The reverb circuitry consists of two tube sections (reverb driver V3 and reverb recovery V4) and the physical reverb tank. All these components will drift during age and minor differences in component values are noticeable to man’s ear.

If you replace the V3 12AT7 reverb driver with a 12AU7, you will reduce the effect of the reverb and it will be much easier to control with the reverb knob. So simple as that.

Use normal channel for reverb control – Adjust EQ and depth of reverb.

This mod is relevant only for two-channel amps with normal and vibrato channel. This trick is great for the reverb enthusiasts among us, and who is not? Plug your guitar into the vibrato channel, then unplug the reverb return cable on the back of the amp (the one that comes from the reverb tank output) and plug it into the normal channel input. You will need a converter to go from male phono/RCA jack to a 1/4″ male jack. You may now use the normal channel as a reverb control where you can adjust the depth and tone using the volume, bright switch, treble and bass knobs (and mid if you have a Twin Reverb). The reverb knob on the vibrato channel will have no effect any longer.

This mod is not applicable together with the Pull V1 mod, as you need the normal channel preamp tube.

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Since the issue is apparently limited to the reverb channel, you could try isolating the symptom by stages — one by one, pull V2, V3, and V4 to determine if there is any decrease in the hiss. That may help determine where the problem lies.