Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on January 25, 2013, 04:50:31 pm
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Hi !
This is THE month of Fender amps for me. I have spent nearly 2 days on this one. The reverb is feeding back with a high squeal when the reverb volume is set at 7 and up. The tremolo works fine but the voltage on the plates of the trem is WAY over what the schem shows. The B+ before the 220K is supposed to be 460V and 250V after, I get 436V after, so the drop is not enough for some obscure reason ! It seems this amp was modified. The power tubes grid leak resistors are 220K, PI plate resistors are 47K each, at least 2 caps were added with no report to the schem. Anyway, the voltages are good except on the trem tube .
Thanx ( and happy week-end ! )
Colas
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I wouldn't worry about the trem. I'm thinking the plate voltage is high because you don't have the footswitch plugged in and the trem engaged while taking that reading.
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The B+ before the 220K is supposed to be 460V and 250V after, I get 436V after, so the drop is not enough for some obscure reason!
That tube isn't drawing any current, so no voltage drop across the 220K plate R.
Could be, bad tube, heaters are not lighting up, bad scocket, dirty scoket/tube pins, cathode connection to ground or it's KR is bad.
Brad :icon_biggrin:
Edit; I was writing while Sluckey posted. I'd try what he wrote first.
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thanks guy , I will verify that and get back to you
I love you, bros.
colas
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Could be, bad tube, heaters are not lighting up, bad scocket, dirty scoket/tube pins, cathode connection to ground or it's KR is bad.
I wouldn't think so since the trem works fine !
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I wouldn't think so since the trem works fine !
You didn't say that. :laugh:
Then it's gotta be what Sluckey said.
Brad :icon_biggrin:
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If the trem is working, then the tube should be drawing current.
Could the 220K be more like 220?
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Could the 220K be more like 220?
no, I thought of that too so I unsoldered one leg of that resistor to make sure it had the right value
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Did you try what Sluckey said to try?
Brad :icon_biggrin:
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no, because I don't have no footswitch :w2:
I have though plugged in a rca connector to act as one
Colas
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no, because I don't have no footswitch :w2:
I have though plugged in a rca connector to act as one
Ok, and did you take a voltage measurement then?
Brad :laugh:
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Yes I had. THe voltage swings so fast I just can't measure it but my meter has an average function and it indicates 450 or so volts. One the LDR side, other plate is 100V or less volts, but still too much according to the schem. I have very poor understanding of a vibrato and LDR circuits. One thing is sure: the goddam voltage is too high. But the main issue is the reverb that starts to squeal / howl like a mic feedback when the reverb control knob is set past 7. And I am wandering if the trem tube HV and the reverb issue are not related.
Thanks bro.
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But the main issue is the reverb that starts to squeal / howl like a mic feedback when the reverb control knob is set past 7. And I am wandering if the trem tube HV and the reverb issue are not related.
Sorry. :dontknow:
Brad :icon_biggrin:
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no prob, thanx for help ( or some sort of....... :l2:)
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Well I'd try swapping out the verb tank with a known good one. I'd also try taking the verb tank out of the speaker cab and make sure the speakers are not causing the problem by shaking the verb tank.
I'd also try swapping the 12AT7 verb driver tube and the 12AX7 recovery tube with known good ones 1 at a time. If that don't fix it, I'd try chop sticking it to see if a wire is in the wrong place. Grid wire being most important, keep it away from plate wire.
I'd probable try swapping the tubes 1"st.
I didn't say these before cause I was thinking you already did these tests.
Brad :icon_biggrin:
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I did not swap the reverb tank because I didn't have a spare one. The tubes though were replaced at least twice with no effect.
I will order a new tank from Doug, since this one has no plastic cloth cover anyway and I suspect it to be defective.
Thanks, Brad Pitt
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Doubtful. Take a real close look at the reverb components on the board and make sure there are no burnt,open or severely drifted resistors.
Then check the wire from the reverb pot to the board and add a shielded one just to check the result.
Throwing parts at this amp will not fix the reverb.
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I am having same problem with Showman head.
456V before 220K resister in vibrato circuit, and 450V after, at pin #1 of V-3.
No reverb however.
But low volume on vibrato channel. Just ordered a tube socket for V-4 as pin #7 shows burn marks cant clean.
Does your amp have low power in that channel compared to the other?
If you resolve problem, please post and I will do same.
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Hi Cork, Yes, low volume when tremolo volume is raised. For sure I'll post the results when I find the cause of the issue.
Cheers
Colas