Just opened up a SF Vibro Champ which has very little output and odd behavior on the Speed and Intensity controls. Speed doesn't have appreciable effect, and Intensity acts like it's a gain stage. As far as I can see from matching parts to the schematic and layout (AA764), the amp hasn't been modded, but when it came in, it had a 5U4 rather than a 5Y3.
I'm thinking that I need to check the OT b/c of the incorrect rectifier tube and check voltages throughout....any thoughts on what might have been cooked other than the OT? I'm relatively new to amp debugging but am studying hard to improve my knowledge and processes.
Thanks
mh
Vibro Champ...
Moderators: pompeiisneaks, Colossal
Re: Vibro Champ...
probably time for new electrolytic caps.
test the tube they are probably low, rectifier tube it's probably fine as they age much slower then preamp and power tubes.
It was common to swap rec tubes to get less impedance and tighten up the amp.
The voltages will be slightly off due to the new rec tube. you will need to do the math based on the rec tube.
clean the pots with moving fader contact lube. Any ceramic disc caps should be replaced they are a constant issue for ticks. you can put in tested ceramics or orange drop caps will work.
If you continue to have problems try replacing the resistors in that section, as it's a know problem with vintage fenders to have noises directly related to resistors.
test the speaker and make sure it meters 3.4ohms on your meter = 4ohm speaker impedance. anything else will directly effect your volume.
test the tube they are probably low, rectifier tube it's probably fine as they age much slower then preamp and power tubes.
It was common to swap rec tubes to get less impedance and tighten up the amp.
The voltages will be slightly off due to the new rec tube. you will need to do the math based on the rec tube.
clean the pots with moving fader contact lube. Any ceramic disc caps should be replaced they are a constant issue for ticks. you can put in tested ceramics or orange drop caps will work.
If you continue to have problems try replacing the resistors in that section, as it's a know problem with vintage fenders to have noises directly related to resistors.
test the speaker and make sure it meters 3.4ohms on your meter = 4ohm speaker impedance. anything else will directly effect your volume.
My Daughter Build Stone Henge
Re: Vibro Champ...
I've been looking for the math to calculate the voltage after the rectifier tube. Would you mind posting the general rundown?you will need to do the math based on the rec tube
Re: Vibro Champ...
Clearly, the trem oscillator circuit has gone south in some way. Pull V2 (second 12AX7) and see how the amp behaves without it. Also give the speed pot a shot of moving contact cleaner and rotate it rapidly to clean it up. Also make sure the 47R is making good contact at both ends. Especially check to be sure the ground end hasn't pulled loose.
I can't imagine why anyone would put a 5U4 in this amp (or in ANY amp for that matter
), but it's usually believed that Fender PTs can deliver the extra amp the 5U4 needs. You can supposedly tell if the 5U4 is stressing the PT by measuring the 6.3V filaments. Measure AC volts from pins 4,5 to pin 9 on a preamp tube. If it's down around 6V, the PT doesn't like the 5U4. Probably, if the 5U4 damaged the PT, you wouldn't be guessing...
I can't imagine why anyone would put a 5U4 in this amp (or in ANY amp for that matter
), but it's usually believed that Fender PTs can deliver the extra amp the 5U4 needs. You can supposedly tell if the 5U4 is stressing the PT by measuring the 6.3V filaments. Measure AC volts from pins 4,5 to pin 9 on a preamp tube. If it's down around 6V, the PT doesn't like the 5U4. Probably, if the 5U4 damaged the PT, you wouldn't be guessing...-
hackerguitar
- Posts: 4
- Joined: Fri Jan 21, 2011 5:31 pm
Re: Vibro Champ...
Thanks for the responses!
I appreciate for the reassurances about the 5U4 - I did some math and it looked to me as though the OT was pretty stressed with a 5Y3, and the 5U4 would be too much current, period....if that's not the case, that's a relief.
I will pull the tube & test & then check the other items you've suggested. So far, I've discovered that the Vibrato Speed pot is loose as can be I'm going to check the connections to that pot after it's tightened up; I'm going to guess that they're broken. The pot's been wound round at least once and the lead dress needs fixing....
mh, slowly learning to fix these things...
I appreciate for the reassurances about the 5U4 - I did some math and it looked to me as though the OT was pretty stressed with a 5Y3, and the 5U4 would be too much current, period....if that's not the case, that's a relief.
I will pull the tube & test & then check the other items you've suggested. So far, I've discovered that the Vibrato Speed pot is loose as can be I'm going to check the connections to that pot after it's tightened up; I'm going to guess that they're broken. The pot's been wound round at least once and the lead dress needs fixing....
mh, slowly learning to fix these things...
Re: Vibro Champ...
The issue with rectifier tubes is filament current: 2 amps versus 3 amps. 5Y3 is 2A. No way it would stress a PT speced for it.