Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on August 16, 2011, 08:42:48 am
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I'm about to pick up my drummer's Twin Reverb RI to fix the reverb which does not work at all. That's the only problem with the amp.
He has tried switching out reverb tanks and the send tube w/ no change.
I'm guessing there's something wrong with the recovery circuit.
But before I get started, has anyone else had this problem w/ the reissues?
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If you can smack the Reverb tank with the control turned up anywhere past 2 and not hear the sound, then you may have trouble in the recovery circuit. That's my quick check for Recovery. (Of course you have to rule out the tank and return cable before you blame the tube circuitry).
My quick check for the reverb driver section, isn't as quick but can be very tell-tale. I pull all of the 6L6 power tubes, then hook a speaker up to the RCA jack on the back of the amp that drives the reverb tank. You can play a guitar and listen for output.
It appears that you've already reversed the wires that lead to the tank. That's good. I'd say that 1/4 of my Fender Reverb "repairs" involve wires that were accidentally reversed. Usually there is a nasty hum and the tone of the amp suffers.
The wires are often the weakest part of the reverb circuit or most prone to failure, but it sounds as though you've ruled that out.
I find that most Fender reverb issues can be diagnosed or at least narrowed down without taking out the chassis. When the amp is in your possession, Try giving that tank a good swat and listen for a crash.
j.
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It was an open cathode resistor on the reverb send tube. It's weird because it was a 1 or 2 watt resistor, I would have thought it could handle the current.
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Gaaaah! The reverb works but the vibrato channel sounds lousy when the volume or treble are turned up to 8-10. Volume shuts down, awful distortion, altogether unpleasant. Also the 2 6L6s on the left redplate even at the lowest bias current setting. Do these things have problems with leaky coupling caps or something?
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No. But stuff breaks off the PC boards. Take some voltages starting with the power tubes. You can pull them to take the readings.
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last one I worked on had a short between B+and the grid of the PI. Was shorting on the PCB. Lot of things can happen on the boards that one would not think.
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Okay, I'll inspect the PCB REAL CLOSE. I figured since the noise starts with treble/ volume turned up all the way, the treble cap must be leaking. And the PI coupling cap must be leaking some DC to the 2 power tube grids. But nothing is that easy, right?
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Here are the voltages.
V1 1-273, 2-0, 3-1.9, 6-279, 7-0, 8-2.1
V2 1-277, 2-0, 3-1.9, 6-270, 7-0, 8-2.1
V3 1-1/6-447, 2/7- -24.8, 3/8-16.9 negative on the grid? is that right? :dontknow:
V4 1-298, 2-0, 3-2.5, 6-220, 7- -5 to -1 (depending on treble pot setting) :w2:, 8-2.5
v5 1-449, 2-7.9, 3-0.5, 6-400, 7-0, 8-0.5
V6 1-268v, 2-71, 3/8-87v, 6-248, 7-73
All 4 power tubes have plate 462, screen 460, grid -49.6, cathode 0, with tubes out.
I have no idea how negative voltage is getting on the v4b grid. I've tried tracing it back- the negative voltage is found on the V4 side of the 3.3m/10p, and is reduced on the v2 side of it. So the most negative voltage is found on the v4 side, if that makes any sense.
When I dial in the highest (or lowest) possible negative voltage, @ 8 on the treble pot, i can hear an audible high-pitched oscillation.
Also, why is the v3 grid so negative? Yesterday when I replaced the v3 cathode resistor, I found 10v on the cathode- correct nominal voltage. Today it is 16v. Something caused the stock resistor to blow, prob. from overcurrent.
Someone help, please. This PCB is breaking my brain.
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I see nothing that tells me you should have redd plating on two of the power tubes.
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Yeah, me neither. I'm trying to address the preamp noise issue right now, since I'm working with power tubes out. Any ideas how negative voltage is found on a preamp grid?
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Leaky cap?
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But negative voltage? where is that coming from?
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It's fixed now.
It was the V2 grid wires (not sure which) being too close to the plate wires. I separated them by @3/4" and magically everything was fixed. Including the redplating.
Maybe someone can explain what the h*** was happening there with the oscillation producing a negative voltage at the grid. And affecting the output stage!!? :dontknow: As far as I'm concerned, it sounds great, I'm done.