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Hey all,
I'm looking for help to find the source of a real bad buzz in a Deluxe Reverb Reissue circa 93-95.
It is not a noise or hum present without input or even with a 1kHz test signal. It used to be so subtle (a few years ago) that I thought it was rattling something in the cabinet, the speaker or something in the room. But it is now very prominent on top of a guitar signal at all but the lowest volume and pick attack. It begins at first attack and rides through the decay of the note(s), swirling a bit.
A few years ago I replaced the speaker. No change. I don't play out so I just lived with it. But now I have taken up this hobby of herding electrons for sound with the help of this forum, Randall Aiken's site, Merlin Blencowe's books and Doug Hoffman's site/forum/store. My circuit theory is way rusty but I have a 25 year old unused electrical engineering degree (I own/run a bakery/bar), a scope and multimeter and I know how to be safe. But I am very new in this fun game, so have mercy.
As the amp is over 20 years old, I thought changing the filter caps (then all electrolytics) could do it. No. Also not the tubes.
I have watched the signal with a scope through all stages (with 1kHz test sine and guitar) and it seems fine and predictable until the plates of the output tubes where the signal gets very jagged in a fractal-looking way as I increase the volume pot. Either channel, NFB connected or not.
Power supply seems okay though high. B+ (to OT tap) is at 437V with ballpark 1-2V p-p ripple. Z (to screen resistors) 437V. Y (to PI load resistors) 355V. X (to preamp load resistors) 300V. C- (to power tube bias pot) -50V. Bias test point set to -37 (schematic test conditions).
DC at 6v6s: Grids -36V. Screens 310/314V. Plates 437V. Cathodes 4mV.
Strange to me #1: Though I've read around here that the output transformers rarely just go bad without some user help (I don't believe that I have ever turned on without the speaker plugged in) I am hoping that is so I can replace it and solve this. I have not de-soldered any OT connections yet. The primarys to tap read 199 ohms, primary to primary 398. When I test any connection to ground the meter fluctuates, usually starting around 15M-20M then counting down all the way to about 50K then going back up to the high starting point. Secondary to secondary is near zero. Again, while connected.
Strange to me #2: I put in bias test points (from power tube cathodes to test point, through 1 ohm resistors, to ground test point, back to their pcb ground point) and the current draw of the 6v6s is at 4mA. Adjusting bias pot gets only to max of less than 8mA. Some old tubes that were run way long before I changed them read 6/7.8 mA with the bias test point at -37V).
Should there be any DC readable at the grids of the phase inverter? 55v? The waveforms out of the PI seem to look fine, with just a slight imbalance. DC at the plates 221/226V.
I hope something here can help someone flick on a lightbulb for me.
Thanks
I'm looking for help to find the source of a real bad buzz in a Deluxe Reverb Reissue circa 93-95.
It is not a noise or hum present without input or even with a 1kHz test signal. It used to be so subtle (a few years ago) that I thought it was rattling something in the cabinet, the speaker or something in the room. But it is now very prominent on top of a guitar signal at all but the lowest volume and pick attack. It begins at first attack and rides through the decay of the note(s), swirling a bit.
A few years ago I replaced the speaker. No change. I don't play out so I just lived with it. But now I have taken up this hobby of herding electrons for sound with the help of this forum, Randall Aiken's site, Merlin Blencowe's books and Doug Hoffman's site/forum/store. My circuit theory is way rusty but I have a 25 year old unused electrical engineering degree (I own/run a bakery/bar), a scope and multimeter and I know how to be safe. But I am very new in this fun game, so have mercy.
As the amp is over 20 years old, I thought changing the filter caps (then all electrolytics) could do it. No. Also not the tubes.
I have watched the signal with a scope through all stages (with 1kHz test sine and guitar) and it seems fine and predictable until the plates of the output tubes where the signal gets very jagged in a fractal-looking way as I increase the volume pot. Either channel, NFB connected or not.
Power supply seems okay though high. B+ (to OT tap) is at 437V with ballpark 1-2V p-p ripple. Z (to screen resistors) 437V. Y (to PI load resistors) 355V. X (to preamp load resistors) 300V. C- (to power tube bias pot) -50V. Bias test point set to -37 (schematic test conditions).
DC at 6v6s: Grids -36V. Screens 310/314V. Plates 437V. Cathodes 4mV.
Strange to me #1: Though I've read around here that the output transformers rarely just go bad without some user help (I don't believe that I have ever turned on without the speaker plugged in) I am hoping that is so I can replace it and solve this. I have not de-soldered any OT connections yet. The primarys to tap read 199 ohms, primary to primary 398. When I test any connection to ground the meter fluctuates, usually starting around 15M-20M then counting down all the way to about 50K then going back up to the high starting point. Secondary to secondary is near zero. Again, while connected.
Strange to me #2: I put in bias test points (from power tube cathodes to test point, through 1 ohm resistors, to ground test point, back to their pcb ground point) and the current draw of the 6v6s is at 4mA. Adjusting bias pot gets only to max of less than 8mA. Some old tubes that were run way long before I changed them read 6/7.8 mA with the bias test point at -37V).
Should there be any DC readable at the grids of the phase inverter? 55v? The waveforms out of the PI seem to look fine, with just a slight imbalance. DC at the plates 221/226V.
I hope something here can help someone flick on a lightbulb for me.
Thanks
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