'65 Deluxe Reverb Amp Buzz

[Mick Bailey]

At TP1 the AC ripple is usually about 4v.

Most of these amps (on UK 240v) measure close or higher than the DC schematic voltage even when correctly configured for 240v (they come into the UK set for 230v), so your voltages are lower than I would expect. Also check the caps for any bulging or crusty dried up leakage where the positive lead exits.

[The Dude]

Check AC voltage at same points. If it is substantial, those caps may be bad. If not, it could be that the amp is idling way too hot (power tubes).
The method you described in post #8 is incorrect as far as I can tell.
With power off, you measure the resistance from standby switch (CP15) to pin3 of each power tube. Then with power on, in 'play' mode, you measure voltage at those same points. Voltage divided by resistance tells you the idle current for each tube.

^^^^^^ That. Check that both tubes are conducting. If one is not conducting, or conducting very little, the imbalance will cause hum.

[bananaguitar]

I just tested the Vac across those two points and they both read below 0.009Vac. While measuring these removing and placing the probe did affect the buzz.

I have checked the caps no sign of any damage to any.

[bananaguitar]

Check AC voltage at same points. If it is substantial, those caps may be bad. If not, it could be that the amp is idling way too hot (power tubes).
The method you described in post #8 is incorrect as far as I can tell.
With power off, you measure the resistance from standby switch (CP15) to pin3 of each power tube. Then with power on, in 'play' mode, you measure voltage at those same points. Voltage divided by resistance tells you the idle current for each tube.

Both B+ and CP15 are the same connection so I would get the same result.

[g1]

Both B+ and CP15 are the same connection so I would get the same result.

Ok, sorry I misunderstood your post #8. So R7 and R8 are resistance values for OT primary halves. Your power tubes are barely on at 2 and 3 mA. I'd expect something more in the range of 15 to 20 mA per tube. Check that 6V6 pin5 voltages are in the -37V ballpark. Check that the screen resistors R62 and R63 are 470 ohms.

As for the low B+, do you have another rectifier tube to try?

[bananaguitar]

Ok, sorry I misunderstood your post #8. So R7 and R8 are resistance values for OT primary halves. Your power tubes are barely on at 2 and 3 mA. I'd expect something more in the range of 15 to 20 mA per tube. Check that 6V6 pin5 voltages are in the -37V ballpark. Check that the screen resistors R62 and R63 are 470 ohms.

As for the low B+, do you have another rectifier tube to try?

Both tubes are sitting at -39V

Both R62 and R63 sit just above 460ohms, which is in the tolerance of the resistors

I don't have another rectifier tube, I am able to order one though.

[Mick Bailey]

What's the AC reading at TP1?

[bananaguitar]

What's the AC reading at TP1?

Both TP1 and TP2 read below 0.009Vac

[Mick Bailey]

Just to be sure - Is that with the amp operational and with respect to ground? The ripple at TP1 suggests very little current draw.

What are the DC and AC voltages at TP1 with both power tubes removed? With a correctly working amp the ripple should drop and the B+ go up noticeably.

[bananaguitar]

Just to be sure - Is that with the amp operational and with respect to ground? The ripple at TP1 suggests very little current draw.

What are the DC and AC voltages at TP1 with both power tubes removed? With a correctly working amp the ripple should drop and the B+ go up noticeably.

I just re-checked the ripple voltage, it is now showing around 75Vac from CP14 to Ground. Not sure what's different possibly didn't get a good contact on one of the ends.

[Mick Bailey]

75Vac ripple is extreme. Would you re-check your voltages for calculating the dissipation? You need to eliminate the power tubes as being an issue and the readings you've made suggest the measurement may be flawed. My thought is either (or both) C31 and C32 have gone high ESR or open if the tubes are not the problem. If they're the grey Illinois types then these can be prone to failure but you need to be certain they're faulty before replacing.

[bananaguitar]

I retested the voltages for dissipation the still showed low.
I also swapped power tubes with some from a known working amp and there was no difference to the buzz or any of the measurements.
I also checked the caps and they are grey Illinois