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There is just one slight problem. The V1A plate load resistor is R4. Voltages Using a Bucking Transformer. The lower value grid leaks also reduce the gain from the phase inverter which also reduces the possibility of blocking distortion. This is the most transparent of all the master volume types. This will allow you to use modern 3-prong accessories with the amp. Many AB763 players never use the Normal Channel since it has lower gain and no effects. The ratio of our silverface amps with a 47 ohm NFB Tail resistor is 820 / 47 = 17. At first blush I was going to recommend not using this mod on the first stage of amplification because if the cap fails as a short it could inject high voltage into the guitar circuitry but even if this does happen the large value feedback resistor will limit the current to 18 micro amps (. By setting the Mid slider (bottom left) to 68% you get 6800 ohms to equal the Deluxe Reverb's fixed 6800 ohm Mid resistor. Deluxe reverb tremolo not working properly. 1uF cap blocks DC flow through the pot to eliminate "static" when adjusting the control. The robrob Reverb Mod is also superior to the "Fritz Mod" and Fender Reverb shown in the next section because retaining both mixing resistors prevents the severe signal attenuation and/or channel interaction that occurs with the other reverb mods.
Another mod to consider for a Vibroverb conversion is to install a Tremolo Cut Switch which will boost the amp's gain to mimic the 3 preamp gain stage Vibroverb. 8M resistors on the Master Volume pot reduce the pot resistance to 220k (the stock value) and add a failsafe path for bias voltage. See this to measure and adjust the bias. I purchased a 50KRA pot with an off position when you turn the dial full down from Vintage Amp Repair's ebay store. This allows me to set the Mid control to minimum and get Fender "normal" 6. You can add this resistor to any AB763 by replacing the wire that runs between the Bass and Middle pots (or Middle resistor) with an 18k resistor. Replace V1 or V2 with a 12AY7. For the 65 DRRI you need to do the following: The Normal channel 220k mixing resistor is R12 on the circuit board. Deluxe reverb tremolo not working on guitar. 01 mfd 600 volt Mylar capacitor on the 10 meg ohm resistor in the vibrato circuit. The AB763 Deluxe Reverb and non-reverb Deluxe are infamous for their flabby low end that can lead to "farting out" at high volume. For the 68 CDR style mod simply connect the top of Resistors R23 & R11 with a jumper wire and clip or remove cap C5 (both mods upper left in layout above).
Simply replace the 15K resistor that's soldered to the original Bias Balance pot's back with a 10k 1/2 watt resistor and a 25k trim pot. These tend to have a very rich and soft pulsating sound. Question] Vibrato not working on my Fender Deluxe Reverb 65' Reissue. For a 16 ohm output transformer secondary change the resistors to a 16 ohm 25 watt and 32 ohm 25 watt. B+4 430 with V2 plates 290, V1 plates 285 (Schematic lists B+4 of 375v, plates 275v). You must clip or remove the left (Normal Channel) 220k channel mixing resistor at far left above. 022uF cap to lower the filter's cutoff freq so more mid-high freqs would be cut. Tremolo Cut Switch Mod Add preamp gain with this simple and very useful mod. Tremolo before or after reverb. Most cathode biased use pre-amp tube bias shifting trems but there is version that modulates the bias indirectly through the grid leak resistors of the power tubes. The pre-amp bias wiggle and other tremolo types are simply better for reliability. The C and D power resistors are located in the "doghouse" which is located outside the chassis next to the output transformer.
Vox Cut Control & Cut Control + Master Volume. When doing this mod be sure and sample the max volume distortion tone to ensure the power tubes aren't pushed too far. Early in-the-circuit tone controls affect the substance of the overdrive tone. Whenever I turn up the reverb a static noise starts. Vibrato stopped working on Deluxe Reverb Reissue. Trainwreck Type-2/Lar-Mar Master Volume. Keep an eye on your power transformer's temperature when running the big power tubes but be careful because the transformer can get hot enough to burn your fingers. You can simply replace that 6.
The schematic for the Deluxe shows only 170 idle volts on the preamp tube plates while the Twin Reverb shows 270v. For an amp with a 2 ohm secondary we would need an 8 ohm speaker, a 4 ohm secondary needs a 16 ohm speaker and an 8 ohm secondary would need 32 ohms of speaker load (two 16 ohm speakers wired in series). Note the two wire bundles that are wrapped with a reddish ground wire. 8k Mid Minimum resistor. Turning the trim pot to reduce resistance will increase the bias current and make the bias hotter. No matter how I turn the Speed and Intensity dials nothing changes. Tremolo is injected at point Y (center of layout) using the yellow wire from the Intensity Pot right terminal. This is an Ice Pick Cap on a 5F6A Bassman but it works the same way in the AB763. 3-Way Negative Feedback Switch Mod Make your amp more versatile. Fender 5G9 Tremolux.
There are many master volume options but the Trainwreck Type-3 Post Phase Inverter Master Volume (PPIMV) is very easy to install and it works almost as well as any other master volume I've tried in any of my amps. These work by cutting on and off the actual signal within the amp. The Presence Cap bleeds NFB high frequencies to ground and also blocks DC from flowing through the Presence pot. Insert V1, V3 and V5, one by one, until you discover the bad tube. The "763" in the model name comes from the circuit change date of 7-1963. You don't have to use 1k for the C resistor, you can chose any resistor value between the original value and 1k to tweak the tone change to your liking. You need V2, V4 and V6 present. 8M Safety resistors allow bias voltage to pass around the master volume pot in case of a wiper failure. Some actually prefer the Sigs to the more expensive 10F150 and 12F150. We experiement with different old and new tubes and select the tubes that results in the most volume and punch. Removing high frequencies from the NFB boosts high frequencies at the speaker. 470Ω Screen Resistors moved to switch input so they are only used in the 'Low Power' switch position.
047uF (400v or higher) cap. The Bandmaster Reverb uses a 4 ohm output transformer with 820 / 100 negative feedback circuit. It is needed to make the tremolo work and has no effect on the tone of the amp. The NFB switch fundamentally changes the clean and overdrive tone of the amp. Phase inverter tube. The trim pot is wired as a variable resistor with the trim pot's lower right terminal not connected to anything. 8k Mid resistor is at the bottom of the schematic. Remove the 1M resistor.
Running 6L6 tubes will stress the transformers so I don't recommend it but if you want to give it a try then you should make a change to the speaker load to compensate for the different power tube impedance. We start from right to left, amp seen from behind: V1 – 12ax7. You can also reduce the tremolo circuit load and boost gain by replacing the stock 50K-RA (reverse audio) Tremolo Intensity pot with a 100K-RA pot. B+3: 455 with 237, 240 on phase inverter plates (Schematic lists B+3 of 430v, plates 225v). This tube is only in use in the vibrato channel, but even so, it means nothing for the dry guitar tone of the vibrato channel or the normal channel. Just remove the resistor from the pot and connect the pot's left terminal to a Raw switch terminal with a new 6. Add a Presence Control.
I will also have a list of on the bottom of the page of some of the amps that use each type of tremolo. The 8 ohm 25w resistor is placed in series with the amplifier speaker and the 16 ohm 25 watt resistor is placed in parallel to bring the total speaker load back down to 8 ohms.