2002 UK AC30TB 60HZ Buzz

[garytoosweet]

Amp is a 2002 AC30TB.

I have a 60HZ hum/buzz that I am having trouble tracking down.

Confirmed 60HZ by measuring frequency on a scope.

With the volume at half or noon, and a guitar plugged in, there is 330mV AC of hum at speaker out with me close to the amp. If I move the guitar about 1.5m away, this quiets down to 13mV but it is still audible and annoying.

The hum is largely controlled by the normal channel volume control, but also by the brilliant control to a slightly lesser degree. The Vib channel volume does not effect the hum. If the cut control is all the way clockwise it can filter out most of it. It can become more apparent with the treble at 10(max).

Removing all preamp tubes except for the PI tube, the hum is still present. When the PI tube is pulled, the hum disappears. I have tried swapping a fresh matched quartet of EL84's with no difference.

Scoping the PI tube, hum is present on the plate of both sides, pins 6 and 1. I don't see it on the grid of either side. There is no AC signal there).

At C45 reservoir cap I have 12.5VAC, is this a bit high?

At the junction of the choke and C46 filter cap I have average 200mvAC, it is fluctuating between 150 and 400mv.

B+ around 350vDC, all power tubes have correct voltages.

I got this amp in non-functioning and neglected condition. After bringing it back to stock condition, it was very noisy. After some troubleshooting and research I realized it was just the way these amps were from the factory, but it was still not an acceptable level of noise.

I performed a couple noise/reliability modifications developed by Lyle Caldwell (shown in this video) that essentially involved:

• Cutting the ground trace on the PCB and modifying into 3 separate ground sections (1 output + filter caps, 1 for input section (connected to chassis near inputs) and 1 for reservoir cap.
• Deleting power tube heater trace on PCB, running standalone power tube heater wiring, as well as new preamp tube heater wiring with tighter twists
• Disconnecting heater centre tap from ground, and installing an elevated artificial centre tap referencing output cathode (I later tried reversing this mod to see if grounding the stock transformer CT helped get rid of buzz - it didn't)
• Running shielded cable from input to grid of first preamp tube

I also replaced all electrolytic caps in the amp while I was in there. After completing these modifications I am left with the constant 60hz buzz. I've went over all my work and re-checked many many times and cannot find any error.

I've attached the schematic.

Any help in tracking this down is appreciated.

[Jon Snell]

Try humbucker pickups.
"With the volume at half or noon, and a guitar plugged in, there is 330mV AC of hum at speaker out with me close to the amp. If I move the guitar about 1.5m away, this quiets down to 13mV but it is still audible and annoying".
A ripple of 12 volts on C45 is about right but then, that doesn't matter as it is 120HZ on the HT rail.

[pdf64]

60Hz buzz seems to indicate heater voltage contaminating the signal path.
For the purposes of troubleshooting, would it be feasuto power one / some / all of the 12A_7 from a DC bench supply?

[Helmholtz]

Ideas:

1) Make sure the chassis has very low resistance continuity to the mains plug safety ground contact.
2) Connecting a guitar is not helpful, because this adds external (PU and other) noise, which cannot be mitigated by circuit modification.
3) I agree the 60Hz point to heater noise. A scope picture of the hum could help (no guitar, amp inputs shorted).
4) The effect of the cut control on the noise indicates the presence of spikes on the hum signal.
5) In a similar (?) case of heater noise with an AC30, the OP achieved improvement with a humdinger.
6) Susceptibilty regarding heater noise varies from tube to tube. So try different ones, especially in PI position.

[garytoosweet]

Try humbucker pickups.

A ripple of 12 volts on C45 is about right but then, that doesn't matter as it is 120HZ on the HT rail.

Tried a humbucker guitar with no noticeable difference. Right re: ripple. Good point, thank you.

60Hz buzz seems to indicate heater voltage contaminating the signal path.
For the purposes of troubleshooting, would it be feasuto power one / some / all of the 12A_7 from a DC bench supply?

My bench DC supply is a measly 1A. I pulled V9-11, leaving only V6-8 and it was able to power those 3 tubes @ 6.3VDC. No improvement in hum, possible worse. 600mvAC at speaker out with guitar plugged in and me sitting in front of the amp. Power supply used is cheap and minimally filtered so this may not be a definitive test. I am looking for a better bench supply to use for heaters.

I also checked to make sure heaters were balanced, they are.

[Enzo]

Forget the guitar. Fix whatever hum the amp makes on its own first. If you need to put a shorting plug into the input, do so.

[garytoosweet]

1) Make sure the chassis has very low resistance continuity to the mains plug safety ground contact.
2) Connecting a guitar is not helpful, because this adds external (PU) noise, for which the amp is not responsible.
3) I agree the 60Hz point to heater noise. A scope picture of the hum could help.
4) The effect of the cut control on the noise indicates the presence of spikes on the hum signal.
5) In a similar case of heater noise with an AC30, the OP achieved improvement with a humdinger.
6) Susceptibilty regarding heater noise varies from tube to tube. So try different ones, especially in PI position.

1) 0ohms, connection is good.
2) Got it, thank you.
3) Scope photo of hum attached here. .2v/div 2ms/div
4) Interesting, I don't understand that relation but will attempt to learn more.
5) I don't have a typical humdinger pot. I do have a Bourns 2W 5k 10 turn precision pot, any reason I couldn't use this just for testing?
6) I pulled all preamp tubes except PI (as buzz is still present) and swapped it with 10 different tubes, no change.

[garytoosweet]

Forget the guitar. Fix whatever hum the amp makes on its own first. If you need to put a shorting plug into the input, do so.

Got it, good advice. Thank you.

[Helmholtz]

3) Scope photo of hum attached here. .2v/div 2ms/div

Where was the probe connected?

I see a distorted 60Hz signal with a lot of HF or even RF hash. Not a typical heater noise shape. The hash might even indicate some instability.
Or a strong nearby HF/RF noise source.

4) Interesting, I don't understand that relation but will attempt to learn more.

The cut control only attenuates HF content, not the 60Hz fundamental.

5) I don't have a typical humdinger pot. I do have a Bourns 2W 5k 10 turn precision pot, any reason I couldn't use this just for testing?

Might work, but a lower resistance value (500R) trimpot is preferable.

[garytoosweet]

Where was the probe connected?

I see a distorted 60Hz signal with a lot of HF or even RF hash. Not a typical heater noise shape. The hash might even indicate some instability.
Or a strong nearby HF/RF noise source.

I believe the probe was connected across speaker out, going from memory.

Over the weekend I picked up a proper 10V 10A power supply (HP 6282A). Powered the heaters from it and the amp didn't like it. Worse buzz than before. The power supply is new to me but was sold tested from a fairly reputable source and appears to be working correctly with no AC ripple present. Doesn't make sense to me but I will have to investigate further.

Then, I attached the Bourns 2W 5k 10turn pot as a humdinger using alligator clips, and was able to dial out the hum almost completely! Now a very reasonable level, even with amp/volume EQ maxed.

The humdinger works best when the wiper is attached to the 'dirty' ground with the reservoir cap and HV centre tap. I tried the wiper to output tube cathode and to 'clean' ground and the results were not as good.

I am going to order some 100R 5W pots and permanently install one on the rear of the chassis. Hopefully that will sort it for good.

Thank you very much for your help.

In hindsight, when I identified the buzz as 60hz, I should have done more investigation into heater noise. I had just done so much work re-wiring them, routing away from any audio signal, I had some sort of mental block and thought "it can't be the heaters"! I was literally looking at everything else first before the most likely suspect. Lesson learned.

[Helmholtz]

Just out of curiosity: Can you hear some speaker hum with amp in standby?

[garytoosweet]

Just out of curiosity: Can you hear some speaker hum with amp in standby?

I re-wired the standby into a mute on this amp. Shorting R50 & R53 just before power tube grid stoppers.

[garytoosweet]

Just out of curiosity: Can you hear some speaker hum with amp in standby?

Come to think of it I believe I did notice a very slight hum with the amp on standby prior to beginning work. Why do you ask?

[Helmholtz]

Come to think of it I believe I did notice a very slight hum with the amp on standby prior to beginning work. Why do you ask?

Because I've seen AC 30s that directly couple mains frequency hum from the PT into the OT due to unfavorable transformer orientation and insufficient magnetic shielding.
In this case some hum showed in standby mode.
A humdinger could at least partly compensate the mains hum.

[nevetslab]

One trick (in use of the amp) I've often made is with nothing plugged into the Normal Channel, or either of the other two channels, the amount of hum heard in the Bright Channel can be somewhat nulled by turning up the Normal Ch volume control. It only helps a little bit, but does lower the hum. I haven't yet installed a humdinger pot on the heater supplies of any of our AC30-6TB amps, but did on one occasion install a separate heater xfmr, disconnecting the heater wiring from the main power xfmr. While that did lower the hum significantly, the down side was sit increased the plate voltage of the EL-84 power tubes, which are already running way too hot. We have a number of AC30-6TB in our rental inventory, so I will be visiting this humdinger pot before too long.