Author Topic: 100s hum troubleshooting  (Read 1603 times)

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Offline samcr

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100s hum troubleshooting
« on: June 12, 2024, 09:00:06 pm »
Hey all,
First post on the forum but great to be around some other enthusiasts.
I will try my best to give as much information upfront in terms of what I have tried and haven't tried.
I have a 1968 Sunn 100s (with tremlo/reverb circuit) that is in working order and otherwise sounds great except for some hum.


Hum characteristics (what I know):
  • Does not change with volume knob
  • Does not change with input plugged in or not, instrument in or not in
  • Only appears after tubes have warmed up (e.g. from cold start isn't immediate)
  • Appears to be 50hz (based on oscilloscope readings). I am based in Australia running it off a 240v ->110v stepdown transformer
  • Does not change with pulling or swapping preamp/reverb tubes (12ax7/12au7)
  • Disappears if the 6an8 tube is removed
  • Remains the same with different 6an8 tubes
  • Is mildly affected by tone controls
  • Audible through speaker when amp is on plus not on standby
  • No change with power supply B and C caps replaced
  • No change with post phase inverted 0.1uf caps replaced

Extra diagnostic information:
Voltages


Gut shot


Oscilloscope
I am conscious I might have misidentified 'no hum' areas because they are pre-gain stage and don't 'appear' yet, but even zooming in vertically they didn't seem to be there.

I may have mistakes in the signal tracing, but in the image on the right i am trying to show circuit paths in red (with hum) and those in green (no hum)


Example of cycle as shown on probe attached to speaker output


Oscilloscope searches for hum on some key points on the overall schematic


Hoping someone out there might have ideas or suggestions of what to look at or investigate next. Thanks for your time.

Offline loudthud

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Re: 100s hum troubleshooting
« Reply #1 on: June 12, 2024, 09:20:28 pm »
Note that the schematic you used has a mistake. Pin 2 of the 6AN8 is connected to pin 6 of the 6AN8

Do you have a Mains safety ground connected to the chassis ? Are you familiar with the old USA ground cap (sometimes called the DEATH cap ) ?

Offline samcr

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Re: 100s hum troubleshooting
« Reply #2 on: June 12, 2024, 09:55:00 pm »
@loudthud
I can't know for certain what was stock vs modded later as I don't know the full history of the amp. My understanding is at that some stage in the past a tech has added the 3rd prong earth (seen in the bottom right of the gutshot picture), but has left the old death cap present at the standby switch, but theoretically that doesn't need to be there anymore right? (My multimeter has continuity between the earth prong of the power chord and the chasis)

Re schematic vs wiring. Similar situation, That's the most prevalent version of the 100s schematic you can find online. With my level of expertise/knowledge, I can't be 100% certain when there might be a schematic mistake (OR i have a variant thats different to the diagram) vs a wiring mistake.

Offline loudthud

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Re: 100s hum troubleshooting
« Reply #3 on: June 13, 2024, 12:14:15 am »
Yes, once a third wire ground is added in a new Mains cable, the death cap can be removed. I can't see exactly where the green wire is grounded to the chassis though. Also, the ground for all the filter caps looks to be on the terminal strip where the rectifier for the bias supply is located. Sunn amps can develop grounding problems because those terminal strips are riveted to the chassis with "pop" rivets made of aluminum. These should be soldered to the chassis with a BIG soldering iron.

You said you were using a step-down "transformer". Are you 100% sure it is a "transformer" with an independent primary and a secondary windings, or simply an "Auto-transformer" which would have a single tapped winding with no isolation between the 240V and 110V circuits. In any case, the 110V connection should have a "neutral bond" where one side is tied to the Safety ground so it doesn't float, that will cause hum in some cases. You can check this with an Ohm meter when the transformer DISCONNECTED FROM THE MAINS. (I don't know if the Mains in your country are connected in this way with one side grounded.)

Offline samcr

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Re: 100s hum troubleshooting
« Reply #4 on: June 13, 2024, 01:08:29 am »
Sorry not the best photo but here's the grounding: https://i.imgur.com/vJ1AnxW.jpeg

Will double check the step down transformer soon when I can.

Edit/update: The step down I have is not isolated and is a toroidal autotransformer
« Last Edit: June 13, 2024, 06:31:41 am by samcr »

Offline loudthud

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Re: 100s hum troubleshooting
« Reply #5 on: June 13, 2024, 11:50:55 am »
First a WARNING: This amp and ones like it contain Voltages that are lethal and can cause injury or DEATH. If you do not have the training and experience working with dangerous Voltages, it is strongly suggested that you refer service to a qualified technician.

50Hz hum can only come from a couple of places, most hum in an amp is 100Hz. Check that each side of the 6.3VAC heater Voltage measures about 3.15VAC to ground. Next, check the two diodes on the rectifier socket with a DVM set to diode check with power off and caps discharged. If one is bad, this could cause 50Hz hum. Most 100Hz hum you find in the 6AN8 and KT88 circuits will cancel itself out in the Output Transformer. A matched pair of KT88 helps in this effort.

The Hot bias could cause some 100Hz hum. Check that the red wire from the output transformer connects to the power supply 'A' node. Some techs connect it to the Standby switch side of the choke like most Fender amps. This too can cause hum, but usually 100Hz. Check that you have high Voltage to pins 3 and 4 of the KT88s.

Looking a little closer at the Voltage readings you took, it would seem the Voltages around the 6AN8 are somewhat off, which is not unusual for one of these amps. The true test is to measure the power output of the amp into a dummy load. Bias to the KT88s looks like it is on the hot side, I would expect the bias Voltage (pin 5) to be more negative. Note that your meter may load down the Voltage if you measure right at pin 5. Better to measure at the bias adjust pot. Check that the plate structure (large metal pieces inside the tubes nearest the glass) of the KT88s are not glowing red (called red plating). This indicates the tubes are drawing too much current (a potential cause of 100Hz hum).

The first thing to check in the 6AN8 circuit is the resistance of the two 47K resistors connected to pins 1 and 3. These are carbon composition resistors which can drift in value with age. If they differ by more than about 5%, I would replace them. If the Voltage ACROSS one of them is off by say 20V, the Voltage ACROSS the other should be off by the same amount. Voltages to ground would be off, but in the opposite directions. Another problem which can effect that circuit is leakage of the two "Black Cat" capacitors that connect from the 47Ks to pin 5 of the KT88s. Check for Voltage ACROSS the two 100K resistors (should be near zero). You can take these measurements with the KT88s removed from the amp for safety. Use extreme caution and don't let the probe slip and short to ground.

6AN8s are getting hard to find, install a new or fresh one if you have it. If these and other components checkout, I would adjust the value of the 680 Ohm resistor (between 470 and 1K) connected to pin 9 to obtain the 80V on pin 3. Note the 75V indicated on the schematic for pin 9 is an error, Voltage should be more like 0.5V but depends on the tube. Adjusting the 680 Ohm should get the amp firing on all cylinders with full power output after a bias adjustment to the KT88s.

Offline samcr

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Re: 100s hum troubleshooting
« Reply #6 on: June 13, 2024, 07:32:57 pm »
Thanks for that detailed reply. A few things I can address right now, and a few I will need to go off and test/try.

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most hum in an amp is 100Hz
Yep, we gotta know what we're chasing. From the picture we see on the oscilloscope with divisions on the x axis being 5ms, its pretty much 4 divisions per cycle. 20ms -> 1000/20 = 50hz.

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diode check with power off
When I did this the other day I was getting with the DMM multimeter about 0.595v voltage drop and OL the reverse  direction, so functionally they seem correct

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Voltages around the 6AN8 are somewhat off,
This is something i'll need to take another look at this weekend when I'll have the chance to go over things again

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"Black Cat" capacitors that connect...
Yeah I was thinking it might be these but I replaced with a modern 0.1uf 600v rated cap and it made no difference. The resistance of the 47k and the voltage across 100k i'll have to go back and check.

Quote
install a new or fresh one if you have it
I have a backup and noticed no difference in hum between the two of them.

Quote
Adjust the value of the 680 Ohm resistor (between 470 and 1K) connected to pin 9 to obtain the 80V
This is something I will get to if the above does not reveal any issues.

Offline loudthud

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Re: 100s hum troubleshooting
« Reply #7 on: June 13, 2024, 08:36:48 pm »
You never said where the scope was connected when you took the picture above. I see what looks like random noise spikes. Is that 50mV per division on channel 1 across the speaker ? Do you have an 8 Ohm dummy load ?

There is always going to be some hum. Where is the red wire with the yellow stripe grounded in relation to where all the filter caps are grounded ?

Offline loudthud

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Re: 100s hum troubleshooting
« Reply #8 on: June 14, 2024, 10:36:55 pm »
OK, I see where you said the scope pic was across the speaker, but is the 50mV per div accurate taking into account any probe (x10) attenuation ?