Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Perkinsman on September 11, 2020, 05:02:00 pm
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I've got a real, not the reissue SF drip edge VR, that someone had put in what he called a "Reverb mod with Dwell Control". It effectively disconnected both inputs of the Normal channel. The Normal channel Volume now adjusts the amount of reverb & the Reverb adjusts how hard the reverb driver pushes the tank, like the Dwell knob on a stand alone Reverb Box.
I want to return it back to original so both Normal inputs work and the Reverb works as originally designed but:
1. I want to make sure I'm using the correct schematic to reference
2. I don't read schematics that well yet so I was hoping for guidance if I post some pics
The label on this model is AA864, so it's supposedly wired just like the blackface.
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Look at AB568, layout and schematic :
https://el34world.com/charts/Schematics/files/Fender/Fender_vibroluxreverb_ab568_schem.pdf
Layout is better here:
https://cdn.shopify.com/s/files/1/2325/2729/files/Vibrolux-Reverb-AB568-Layout.pdf?5926397210715640943
Here the AA964 layout, AA864 not available
https://el34world.com/charts/Schematics/files/Fender/Fender_vibrolux_reverb_aa964_layout.pdf
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That's awesome Latole. I found the wires that had been attached, detached & moved. After spending a couple hours following the layout along with the actual wiring, I was able to restore it back to the way God intended it. Powered up and voila, the Normal channel inputs both worked, the Reverb worked on the Vibrato channel BUT:
1. there was still a hint of reverb still on the Normal channel
2. when the Reverb knob is dialed past 7 on either Normal or Vibrato, it makes a Bonking noise, I know...weird..(feedback maybe?)
3. theres somes differences in resistor sizes
4. It still sounds good (except for the Bonk)
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3 - Resistor size !!
Did you mean values in ohms or power in watts ?
If it is value, wich resistors are you talking about ?
Size ; most resistor are 1/2 watts and that is good, fews need to be more powerful
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2. when the Reverb knob is dialed past 7 on either Normal or Vibrato, it makes a Bonking noise, I know...weird..(feedback maybe?)
Sorry I don't know what you mean.
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As far as the resistors, I see a couple that are different values (not wattage) in several locations around the amp that are different than the AA 568 layout. I was just making a general statement, nothing specific to the reverb section.
As far as the bonk/flubby noise resulting from the reverb being turned up past seven, I was hoping that someone would be familiar with this problem and help me correct it. It’s probably correctly termed an oscillation. What can cause the Reverb to oscillate? The tubes are high quality, good tubes, the sockets have all been deox’d.
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> What can cause the Reverb to oscillate?
Usually output wires near input wires of a high-gain stage.
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Do you mean the output & input wires of the reverb located next to the high voltage 12at7 tube? Are you suggesting that the wires be shielded?
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Do you mean the output & input wires of the reverb located next to the high voltage 12at7 tube? Are you suggesting that the wires be shielded?
Good lead / wires dress is a must in a point-to-point amp. Wire may act like antenna......
Show us good pictures
We like amps porn :laugh:
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Regarding the weird sounds / performance, I guess the the electrolytic caps on the main board and in the doghouse are old? If so they may be failing.
Their intended operating life is usually only a few thousand hours.
Inadequate decoupling cause all sorts of mischief.
V4’s shared cathode is especially sensitive to its bypass cap going bad.
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Good call Mr pdf! I should have checked this sooner...the caps were installed 2017 so they weren’t suspect but the cathode cap that was supposed to be 25/25 was a 4.7/50, possibly due to the mod that was done that enabled Reverb on the Normal channel inputs. I installed a proper 22/50 & we were off to the races. Thanks so much for your advice.
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I “rewired” my stock 68 SF VR, which actually had cloth covered wires, but was poorly dressed, wires too long and sticking up in the air, to AA964 specs, shortened the long wires and layed them down, and replaced most caps and drifted resistors. I used a friend’s BF VR for a dress guide and overall visual reference.
The results were that my friend was mad that my SF VR sounded much better than his BF VR so he sold his, and bought another BF VR and again compared it to mine, and sold that one as well :w2: .
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:laugh:
I'd get a new friend, or suggest you work together on his next one :icon_biggrin:
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Shooter,
He quit coming around so it took care of itself :dontknow: :cussing: