Hi All,
Just finished a new stereo amp build using a 6F5P (ECL85) triode strapped with a beefed up split rail power supply and some oversized Musical Power Supplies output transformers. The amp sounds amazing. Despite taking great care in power supply design and layout there is still a minute amount of 120 cycle hum ? The left channel has a bit more hum than the right. The right is nearly dead silent. Speakers are 95db efficient.
The heaters are connected to a virtual ground using two 100R resistors that are connected to the cathode of one of the output tubes. Roughly 30 volts.
I am using a ground bus that only connects to the chassis in one place. The safety ground also connects to this point.
I'm getting about 100uv of ripple on all the B+ lines.
Swapping tubes does not change the hum.
The strange thing is that with both tubes removed I get equal audible 120 cycle hum in each channel. Still have to be very close to the speaker to hear it.
Replacing the tubes the hum nearly disappears as the tubes warm up.
All testing has been done with inputs grounded at the grid of the driver section.
Am I chasing a ghost here or should I be able to get this amp dead silent ?
Any thoughts ?
Thanks,
Joe
Just finished a new stereo amp build using a 6F5P (ECL85) triode strapped with a beefed up split rail power supply and some oversized Musical Power Supplies output transformers. The amp sounds amazing. Despite taking great care in power supply design and layout there is still a minute amount of 120 cycle hum ? The left channel has a bit more hum than the right. The right is nearly dead silent. Speakers are 95db efficient.
The heaters are connected to a virtual ground using two 100R resistors that are connected to the cathode of one of the output tubes. Roughly 30 volts.
I am using a ground bus that only connects to the chassis in one place. The safety ground also connects to this point.
I'm getting about 100uv of ripple on all the B+ lines.
Swapping tubes does not change the hum.
The strange thing is that with both tubes removed I get equal audible 120 cycle hum in each channel. Still have to be very close to the speaker to hear it.
Replacing the tubes the hum nearly disappears as the tubes warm up.
All testing has been done with inputs grounded at the grid of the driver section.
Am I chasing a ghost here or should I be able to get this amp dead silent ?
Any thoughts ?
Thanks,
Joe
Attachments
Possible (likely) magnetic coupling from the PT to the OPT's. Do you have the OPT's secondary (-) terminal grounded to the chassis? Your diagram shows it floating.
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With the tubes pulled you cannot be getting any current through the OPT's so it has to be magnetic coupling or possibly even mechanical vibration induced from the PT into the OPT's. It sounds like a normal level of hum. You have no FB to kill the small stuff, either. The way you characterize it, it sounds par.
The main filter choke is very close to the left output transformer, magnetic coupling is a possibility. Remove the tubes and short the choke, check if the hum issue improves. If you hear an improvement, replace the choke with a resistor (or move it).
I just placed a 3x5x1/2 thick piece of aluminum plate that I had laying around between the PT and the left OPT. Silence ! Both channels are now dead silent. I guess I should have placed the PT towards the front left side of the chassis ? Too late now. The BIG aluminum shield might look cool though ! 🙂
Thanks all for chiming in on this !
Thanks all for chiming in on this !
Hashimoto transformers must know something Hammond engineers don't know. Their PT is always mounted horizontally through the top panel, and the core is fully encased with an iron strip + the bell cover. The botton side that end up inside the case is left exposed for wiring .
I was about to say I am not a fan of running the AC heater lines so close to the volume pot.
I was about to say I am not a fan of running the AC heater lines so close to the volume pot.