Debugging a nice 60/120 hum in a reverb circuit

[Tom Phillips]

Also remember that some rectifiers, such as the 5U4, have directly heated cathodes. Therefore, the output is always connected to the 5V AC heater supply.

[mikepukmel]

Sorry, Chuck, really sorry, didn't mean to be a pest. First amp build, thought I remembered everything in there, but had a few rewiring rounds and such. I though it was pretty much stock except for the extra cap+resistor for teh reverb but forgot about the 'immortal mod' diodes. I don't think anything else in there.

Thanks Leo, I will go back again and check the socket wiring. I went through the main board and all wires from preamp through to the reverb, think everything looks right, but will check again. I have another GZ34 for the next amp build, will pop that in and see what happens. I read a whole bunch before buying this rectifier tube and arrived at Ruby as well. Maybe I screwed it up somehow.

THanks for the tip, Nosaj, working up the nerve to try looking at some of the HV traces. Not sure it will lead anywhere, at this point.

Thanks Tom, I think that is what this amp has. One side is one leg of the heater + HV output, pin8, what Leo is talking about mis-wiring?

[Chuck H]

Sorry, Chuck, really sorry, didn't mean to be a pest. First amp build, thought I remembered everything in there, but had a few rewiring rounds and such. I though it was pretty much stock except for the extra cap+resistor for teh reverb but forgot about the 'immortal mod' diodes. I don't think anything else in there.

NBD. Really If it were I probably wouldn't even have posted. As it is now I'm probably just going to lurk unless a symptom that offers a clue (to me?) is given.

[mikepukmel]

Thanks Chuck, Id hate to wear out my welcome.

[Chuck H]

Thanks Chuck, Id hate to wear out my welcome.

You? Not a chance! There have been a few. I was probably one once upon a time You're easy comrade More important, you're clearly enjoying this stuff rather than being angry about it.

[nickb]

Heck of a time getting a photo of the scope trace. scoped hooked up to speaker output inside the chassis. Kinda stinky but this is it:

[ATTACH=CONFIG]49868[/ATTACH]


Peaks are about 17ms across, so about 59hz (within the calibration state of the old scope). Reverb knob on 10, about 150mv. Reverb knob on 1, just some noise.

This was taken with the short jumper soldered from pin 2 of the reverb recovery socket to nearby ground.

Maybe I screwed up the cap board wiring?

Could this be a leaking filter cap but the reverb recovery just picked it up and maybe its there on the other channels at a lower level?

That looks a bit like the current that flows to the primary filter caps. Here's a couple of suggestions. First move your wire to between the grid and cathode bypass cap ground of the reverb recovery. Keep your wire short. If this eliminates the hum then it coming from the ground. In that case a gut shot might help.

Les likely but should be checked if that didn't help, try disconnecting the wire to the plate of the reverb recovery tube. The objective being to leave the 100K connected to the supply and the coupling cap in place. This will tell us if it's coming from the supply. Have you checked that the 100K really does connect to node "D" of the power supply?

[mikepukmel]

You? Not a chance! There have been a few. I was probably one once upon a time You're easy comrade More important, you're clearly enjoying this stuff rather than being angry about it.

Chuck, the most enjoyable thing I've ever done!

[mikepukmel]

That looks a bit like the current that flows to the primary filter caps. Here's a couple of suggestions. First move your wire to between the grid and cathode bypass cap ground of the reverb recovery. Keep your wire short. If this eliminates the hum then it coming from the ground. In that case a gut shot might help.

Les likely but should be checked if that didn't help, try disconnecting the wire to the plate of the reverb recovery tube. The objective being to leave the 100K connected to the supply and the coupling cap in place. This will tell us if it's coming from the supply. Have you checked that the 100K really does connect to node "D" of the power supply?

Thanks Nick. When you write "move your wire" do you mean the shorting jumper? So, short the grid to the ground side of the bypass cap on that tube?

Ah, ok and second test, just turn off the tube. but this also keeps the supply out of the equation at that point in the circuit.

Re node D Yes, I checked a couple of times, think its OK. Will check again. I put 4 photos covering the amp guts viewed from the top, post #46, should be fairly high res, on the previous page. I didn't get shots of the cap board, though, will do that when I get home tonight. The photos show the extra cap and dropping resistor that I have removed while going through the suggested tests.

Todo:
- different routing of shorting jumper on reverb recovery grid.
- disconnect reverb recovery plate lead
- try a different rectifier tube

[mikepukmel]

In the photos, post #46 on the previous page, you can see my "jungle gym" attempt at a star ground. I took 4 lugs, sanded and put nokorode, bolted them together and soldered them all together so they would not lose contact. Then I bolted them to the chassis and put some more solder on so that they are soldered together and to the chassis.

It turned out much more ugly than I thought it would.

Two things that bothered me about the way it turned out were
1) the long wires that need to be routed from/to the ground lugs, to various points in the circuit
2) not sure if it mattered which wires should be soldered to the same lug, e.g. should power tube cathode and power transformer center tap be on the same lug? Its not really a 'star' if various pairs of wires are soldered to different lugs, so they are still separated.

[mikepukmel]

One more variable: I didn't solder grounds to the back of the pots. (many threads on MEF about why we should not do that). Instead, I ran a wire from the pot lug to the ground bus wire. I twisted all of the wires coming from pots to the main board. Should I have ran the ground wire separately, i.e. not twisted it in the bundle?

[ric]

The corrosion inhibitors used in house wiring have silicone in them I believe from the way they feel. Kind of a silicone caulk that doesn't set up. If that's what you're talking about using before you soldered your ground connections, that could be a concern.

(Nokorode on surfaces not letting solder bond)

[mikepukmel]

The corrosion inhibitors used in house wiring have silicone in them I believe from the way they feel. Kind of a silicone caulk that doesn't set up. If that's what you're talking about using before you soldered your ground connections, that could be a concern.

(Nokorode on surfaces not letting solder bond)

Thanks Ric! I checked with the gurus on MEF, and this is what they suggested for soldering to the steel chassis, like fender and clones. With a big soldering iron, works great. It's really a flux, rather than a corrosion inhibitor. I think the name means that it is not acid core, like some plumbing solder fluxes.

[loudthud]

So I have this old throw away TEK 7613 scope. It came with one set of 10x probes. If I use Ac coupling, can I probe the plates in the preamp? that would be 200v range dc, but curious what AC might be there, and what the trace would read (if it won't do something terrible like sparks or fire)

The TEK 7613 can use several different input "amplifiers" that plug into the scope below the screen. The 7A18 is a common dual trace plug-in. The AC coupling cap in that plug-in is rated at 600V.

[mikepukmel]

The TEK 7613 can use several different input "amplifiers" that plug into the scope below the screen. The 7A18 is a common dual trace plug-in. The AC coupling cap in that plug-in is rated at 600V.

Thanks! Yes this one has a 7A26 and a 7A18. So, with 10x probe, the cap would 'see' 1/10 of whatever the probe touches? The rectified output of the GZ34, with about 760vac rms input, (330 - 0 - 330) or about 330 *1.414 less 40 volts or so lost to the tube rect, something like 420v. So the scope end of the probe sees about 42v dc? So, I could probe that without blowing up the scope, or causing a fire, or other horrible thing? These are really old probes, should I clean off the wires with something maybe alcohol?

[loudthud]

Thanks! Yes this one has a 7A26 and a 7A18. So, with 10x probe, the cap would 'see' 1/10 of whatever the probe touches?

No, the cap sees the full DC Voltage at the probe tip.

The rectified output of the GZ34, with about 760vac rms input, (330 - 0 - 330) or about 330 *1.414 less 40 volts or so lost to the tube rect, something like 420v. So the scope end of the probe sees about 42v dc? So, I could probe that without blowing up the scope, or causing a fire, or other horrible thing? These are really old probes, should I clean off the wires with something maybe alcohol?

I don't recommend cleaning any part of the probe unless you know what you are doing. Parts of the probe might not be able to stand the liquid. Some plastics might absorb the liquid and change their high frequency characteristics.

All the circuitry that changes the Volts/Division (called the Deflection Factor) is accomplished by passive circuits before any active circuitry on the 7A18, slightly different on the 7A26. At the 5mV/Div position, the signal goes straight thru to the input JFET. The active circuitry is protected against excessive Voltage, but, like anything that might be subject to abuse by a customer, you can't protect against everything. One of the toughest things to protect against is the charging and discharging of that input coupling capacitor. It's no accident that the AC/DC switch has Ground as the center position. To protect the input, Switch that switch to GND, connect the probe to the energized circuit, then move the switch to the AC position. To remove the probe, switch to the GND position, disconnect the probe from the circuit and touch the tip of the probe to ground to discharge the cap. Then switch to DC and you can use probe as usual.