I'm working on a generally noisy and sometimes weak-volume RI deluxe reverb (2002). Tubes are drawing 17-19mA at 425vdc plate voltage. Tubes test OK. I went through all 37 test points off the schematic (not because every one needed testing, but because I had never done it before and needed the practice). Everything looked good using the described procedure w/ 1K test signal at 37mV, using a 10ohm 10w resistor on the output (which I burned myself on...). So I haven't made any progress yet...
My tester said the 5AR4 was good (I typically see burned filaments in Chinese rectifiers in these amps, but this was not the case), but I'm still suspicious of a dirty power supply so I put the scope on the B+ supply (OT center tap). I'm seeing ~12vac p-p ripple with sharp points, but I'm down to less than 1vac ripple by the next filter stage. How much ripple should I be seeing at the B+? Any ideas what to check out next? I don't have a handy 5AR4 to swap.
Thanks!
adam
My tester said the 5AR4 was good (I typically see burned filaments in Chinese rectifiers in these amps, but this was not the case), but I'm still suspicious of a dirty power supply so I put the scope on the B+ supply (OT center tap). I'm seeing ~12vac p-p ripple with sharp points, but I'm down to less than 1vac ripple by the next filter stage. How much ripple should I be seeing at the B+? Any ideas what to check out next? I don't have a handy 5AR4 to swap.
Thanks!
adam
5ms/div. DVM reads 120hz on this first filter tap. Again, next filter stage is almost no ripple at all. 
Perhaps if you told us more what you were trying to fix, it might help. Are you getting hum? Or other noise? Hum is a noise, but I myself tend to use the term noise for other than hum. Ripple is only a problem if it makes hum in the amp. And hum in the amp can come from MANY places. Ripple can make hum, but it will have no effect on the strength of the amp. Too much ripple, and the amp will still be loud and strong, just more hummy.
So, for the record, the '65 Deluxe Reverb reissue?
425 is a trifle high, but within spec. On that you find 12v p-p ripple, that is 6v peak or 4.24v AC. That is 1% ripple, not bad at all, I'd be pleased. The next node is the screens node, and I expect very little ripple there. (and none farther downstream.) 120Hz is what we expect from a full wave rectified 60Hz AC voltage.
This is a push pull amp, and the opposing plates tend to cancel out ripple on the power tube plates. So unless the two sides are WAY out of balance, this small ripple should cancel out mostly.
Your rectifier tube is fine. It won;t usually make ripple anyway, and to be honest, your amp is a better rectifier tube tester than your tube tester. The amp is real world conditions, but your tube tester is not.
Does your noise continue with ALL controls at zero? And with nothing plugged into the amp?
So, for the record, the '65 Deluxe Reverb reissue?
425 is a trifle high, but within spec. On that you find 12v p-p ripple, that is 6v peak or 4.24v AC. That is 1% ripple, not bad at all, I'd be pleased. The next node is the screens node, and I expect very little ripple there. (and none farther downstream.) 120Hz is what we expect from a full wave rectified 60Hz AC voltage.
This is a push pull amp, and the opposing plates tend to cancel out ripple on the power tube plates. So unless the two sides are WAY out of balance, this small ripple should cancel out mostly.
Your rectifier tube is fine. It won;t usually make ripple anyway, and to be honest, your amp is a better rectifier tube tester than your tube tester. The amp is real world conditions, but your tube tester is not.
Does your noise continue with ALL controls at zero? And with nothing plugged into the amp?
Thanks for the reply, enzo
Yup it is a '65 deluxe reverb re-issue.
No hum/noise with all controls at 0 and nothing plugged into the amp. I guess the problem is that typically an amp hums as you bring up the volume (with or without anything plugged in), but there is a fair bit of hiss which sounds much like the old TV sets with no reception, or snow, on both channels.
Yup it is a '65 deluxe reverb re-issue.
No hum/noise with all controls at 0 and nothing plugged into the amp. I guess the problem is that typically an amp hums as you bring up the volume (with or without anything plugged in), but there is a fair bit of hiss which sounds much like the old TV sets with no reception, or snow, on both channels.
There is one problem which I haven't mentioned because after originally removing and re-seating the tubes it went away - or it was coincidence because that trick isn't working anymore. On both channels, the amp will appear to play fine for any length of time, but randomly the signal will experience minor crackling/static, followed by volume variations, complete loss of volume, and cycle through these symptoms at random intervals. When it transitions between these symptoms it does so smoothly, not abruptly (that is the volume will glide between interruption levels).
Tapping or shaking the chassis, wiggling the tubes did not help. However, cycling the standby switch seems to temporary alleviate the problem (sometimes up to 5-10 min). Under normal operation the standby switch will make a noticeable 'pop' when flipped into standby position and also make a momentary static noise when flipped out of standby. Not sure if that matters at this point.
Tapping or shaking the chassis, wiggling the tubes did not help. However, cycling the standby switch seems to temporary alleviate the problem (sometimes up to 5-10 min). Under normal operation the standby switch will make a noticeable 'pop' when flipped into standby position and also make a momentary static noise when flipped out of standby. Not sure if that matters at this point.
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