Tubelab SE buzzing

[Evenharmonics]

I built Tubelab SE mono as discussed here. It's been fine until recently. I hear buzz from the speaker. It remains even when IC cable is disconnected (only amp and speaker connected). I went through the "check out" process and all the voltages are normal. I connected the speaker cable to other speakers and the buzz followed. What could possibly be causing this?

[rknize]

What happens if you short the input connection?

[Evenharmonics]

I tried it and it still buzzes.

[rknize]

Then it's not picking it up through the input wiring and it is less likely to be a ground issue (but still could be). If it's a 120Hz buzz and not a 60 hum, then it is probably a power supply related problem. Do you have a scope with high voltage probes? If so, take a look at the B+ and B-.

[tubelab.com]

Is the buzz from one channel or both? If it is a single channel, try swapping the 5842's. I have seen oscillation cause a buzz in one channel.

[Evenharmonics]

Originally Posted by rknize

Do you have a scope with high voltage probes?

No.

It's more of 120Hz buzz.

Originally Posted by tubelab.com

Is the buzz from one channel or both? If it is a single channel, try swapping the 5842's. I have seen oscillation cause a buzz in one channel.

It's a monoblock amp. I swapped 5842 with another TSE but it does not follow the tube. What puzzles me is, it buzzes one afternoon, then I use it again later that evening and it won't buzz. I'm wondering if it has to do with the power from the outlet. I use TrippLite surge protector.

[rknize]

Do you use CFLs? Turn off the lights.

[Evenharmonics]

I have a dimmer in other wall outlet but it was off when the buzzing came back. My other TSE (stereo) doesn't have any of this problem. I'm wondering if it's the power supply circuit. Could any caps or resistor going bad cause such noise? It was working fine for 2 years.

[tubelab.com]

Could any caps or resistor going bad cause such noise? It was working fine for 2 years.

It is posible that a capacitor could be developing a high ESR condition.

Watch your line voltage to see if the buzz is prevalent under low voltage conditions. Also are there any new additions in your house that tend to distort the line voltage? Flat panel TV's and computers without PFC are big offenders.

The filament regulator feeding your output tubes may be going into dropout during low line conditions. Dirty power makes things worse. Several years ago I measured the distortion on my line voltage at 4%. Now it varies from 8 to 14% with flat topping very obvious in the evenings. This makes the B+ voltage vary even more than the line voltage does.

If this is the case adding more capacitance may help.

[Evenharmonics]

Originally Posted by tubelab.com

Watch your line voltage to see if the buzz is prevalent under low voltage conditions. Also are there any new additions in your house that tend to distort the line voltage? Flat panel TV's and computers without PFC are big offenders.

I haven't added anything electronic for a year. AC from the power socket is at 123 to 125 VAC when this amp buzzes.

If this is the case adding more capacitance may help.

Higher C4 value for higher B+?