'65 Deluxe Reverb Amp Buzz

[bananaguitar]

Hey guys,

I've just turned on my '65 Deluxe Reverb and it has a very loud buzz to it now. Tried changing power source incase it was dirty power, but still was happening. Now turning it off or on standby causes a loud pop. Linked is a video of the amp buzz.
I've gone through and tapped on the tubes, there was no issue there and none where red plating. Moving any of the pots makes no difference to the noise as well. Any advice on where to start looking would be great.

https://www.youtube.com/watch?v=GCZG83BlHyU

Thanks,
Lachlan

[pdf64]

With the amp de-energised and its mains power cable pulled out, what’s the resistance between either of the heater feeds (black and white wires?) and the chassis metalwork?
About 50ohms is expected (R70//R71).
https://el34world.com/charts/Schemat...reissue_sm.pdf

[bananaguitar]

Hey,

Thanks for your reply, both heater feeds read 50ohms

[pdf64]

What anode or cathode current are the output valves idling at?

[Chuck H]

Did the amp sit for a while since you last used it?

Could be dirty/oxidized tube pin or jack contacts.

[Randall]

If you had an o-scope you could look at the hum at the speaker to get an idea of it's shape and frequency, but I would suspect it is a failed/failing filter cap in the power supply. If you had a fresh one, you could very carefully temporarily clip it in parallel with each original cap one by one. If the hum subsides, it is a good indication that it is time for filter cap replacements.

But, having said this, this is a potentially dangerous thing to do given the high voltages involved, even when the amp is switched off and unplugged from power, and really should be done by an experienced technician who knows how not to get killed doing it.

[pdf64]

Fair point, but to help clean up cruddy contacts, anyone can reseat each valve and plug a few times.

[bananaguitar]

What anode or cathode current are the output valves idling at?

Am I wrong in thinking that measure the resistance and voltage across B+ and pin 3 of the power tubes should give me this figure?? Because these are the values I got:

R8 195
R7 200

V8 .520
V7 .675

I8 0.002
I7 0.003

I've used the amp quite often and it gets gigged with regularly when not in lockdown, and have checked and reseated all the valves no difference and no damage.

[bananaguitar]

If you had an o-scope you could look at the hum at the speaker to get an idea of it's shape and frequency, but I would suspect it is a failed/failing filter cap in the power supply. If you had a fresh one, you could very carefully temporarily clip it in parallel with each original cap one by one. If the hum subsides, it is a good indication that it is time for filter cap replacements.

But, having said this, this is a potentially dangerous thing to do given the high voltages involved, even when the amp is switched off and unplugged from power, and really should be done by an experienced technician who knows how not to get killed doing it.

I don't have access to an o-scope sadly. Which caps are the filter ones on this amp?

I do have plenty of experience soldering and putting DIY projects together I just lack the experience in diagnosing issues, so I'm always looking for ways to gain it. So I'll try my best to do it myself, then send it off if I can't fix it. But thanks for your concern

[Mick Bailey]

With all the pots on zero does the amp still buzz? Try removing the PI tube and see if the buzz disappears.

[bananaguitar]

With all the pots on zero does the amp still buzz? Try removing the PI tube and see if the buzz disappears.

All pots to zero and PI tube removed buzz still continues unchanged in sound.

[Mick Bailey]

So, the problem lies either with the power supply or the output section itself. Take a look at the schematic check that TP1 and TP2 voltages correspond with the amp operational (not in standby). A low voltage at TP1 would indicate there's likely to be a problem with C31 or C32 (or the solder joints to those caps). It's also possible that the rectifier tube pins/socket are not making good contact.

Reasons for power amp buzz can include;
1. A failed PSU capacitor causing excessive B+ ripple
2. A bad solder joint to one of the capacitor legs
3. One defective output tube which will prevent the common-mode PSU ripple rejection
4. A failed 470R screen resistor on one 6V6 tube
5. Bad contact preventing one output tube from conducting.

Another reason for a hard buzz is a loose lamination or PT mounting. This can cause a loud buzz that can be felt through the chassis and can be difficult to distinguish whether its coming from the speaker or not. However, this won't cause the amp to pop when the standby switch is operated.

[bananaguitar]

So, both TP1 and TP2 are sitting around 342V. Do I now get to new caps to replace C31 and C32?

[Jazz P Bass]

You have tested for Vdc at the test points.
Now go back and for Vac. (set your meter to read ac voltage)
That reading is the 'ripple' voltage.

[g1]

So, both TP1 and TP2 are sitting around 342V. Do I now get to new caps to replace C31 and C32?

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. (edit, method is correct, R7 and R8 designate OT primary resistances).
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.