Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on September 18, 2024, 01:24:15 pm
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This is a weird one. I have not been through this particular issue before with quick success. I have NO Oscillation in the Vibrato circuit. Replaced the usual 3 trem caps and roach.The roach lights up but that's it. I am using the foot switch which is part of the grounding for the Vibrato & Reverb.The foot switch is good and has no issues. The Intensity pot is good and is grounded.The volume of the amp drops a bit when increasing the Intensity pot. The Optocoupler light just stays on with no intensity.
Their is an older posting of this exact issue with a Twin Reverb but their was no answer. V5 is a new tube as i have tried many others.
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Schematic? Voltages?
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Schematic? Voltages?
V5 Tube voltages. With Tremelo engaged. Pin 1 +410vdc - Pin 2 -37vdc - Pin 3 +3.26vdc - Pin 4&5 3.25VAC - Pin 6 +345vdc - Pin 7 +.035vdc - Pin 8 +10 vdc - Pin 9 3.25VAC. Optocoupler flashed weak 2 times and quit.
This model has the pull master switch.
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What does the plate resistor on the oscillator triode measure? Seems you're about 110v too high on the plate based on the schematic (if I'm seeing what I think I'm seeing--copy fidelity is poor.)
Also your cathode voltage is very high compared to schem. Would be a good idea to measure the cathode cap and confirm bypass cap health.
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My initial approach with trem issues is to replace the trem bias resistor with an LED, so you can at least see when the darn thing is oscillating!
Pin 2 -37vdc -
Does that sound normal to you? Because it shouldn't! Trem tube is stuck in 'off' mode. Your footswitch isn't working properly; maybe the footswitch jack has a bad ground connection. (Try shorting the hot wire to ground with an allegator lead to confirm)
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I got it working. Changed out 5uf -50vdc cathode cap parallel to 100 K resistor and it started working. Foot switch is fine. I always use a grounded RCA plug instead of a foot switch.