R.G. wrote: ↑Sun Aug 17, 2025 1:25 pm
Hmm. I believe that you already made sure that the PT CT went only to the first filter cap negative. Otherwise that would be the next place to look.
I will move it again and double check.
In your schematic, I see that the PT CT is where the HV fuse lives. That means that there is a wire from the PT to the fuse, then a wire to the first filter cap negative. That could implicate the wire routing on this wire path, and you might have dirty fuse contacts. Maybe.
Yes, in Hiwatts the PT HT CT is fused. But it does not get grounded to the first filter cap. It is grounded locally at the chassis under the fuse holder. I will try moving it to the filter cap negative. However the "first" filter cap isn't grounded. It is linked to the other caps and then grounded at a node by the NFB/output, along with the post-presence presence cap.
Rectifier and transformer snubbing pop to mind next. the 5408 is a standard rectifier, so it's prone to generating RF spikes when the diodes turn off, There's a section on rectifier noise in Guitar Amp Wiring Notes on this. You could try putting 0.01/1kV ceramic caps across each 5408, and also a 0.01 ceramic in series with a 1K resistor directly across the two HV leads from the PT. These dodges help prevent RF ringing from the sudden cutoff of the main diodes. Fast, soft cutoff diodes like FREDs are another way to avoid slam-off diode noise.
Yeah I've read about this. But never experienced it because isn't this sort of switching noise well beyond what we can hear? Certainly not in the realm of 120Hz? The diodes in the amp are currently UF type.
FourT6and2 wrote: ↑Sun Aug 17, 2025 6:45 pm
Yes, in Hiwatts the PT HT CT is fused. But it does not get grounded to the first filter cap. It is grounded locally at the chassis under the fuse holder. I will try moving it to the filter cap negative. However the "first" filter cap isn't grounded. It is linked to the other caps and then grounded at a node by the NFB/output, along with the post-presence presence cap.
Maybe they had good luck with that, but it would not be my first choice.
I think it's worth breaking the connection of F1 to the ground/chassis/whatever; then, running a wire from it to the negative end of C13. Let C13 be connected to ground in whatever way it wants. There should be no other connections to the wire from the PT CT to F1, or the wire from F1 to C13 negative. This forces the rectifier pulses to flow only in that wire, not on the chassis and not on any other wires.
Yeah I've read about this. But never experienced it because isn't this sort of switching noise well beyond what we can hear? Certainly not in the realm of 120Hz? The diodes in the amp are currently UF type.
Well, OK, that's all taken care of then.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Heard back from Hiwatt after sending the amp in under warranty. They say they found the problem—it was a bad AC/DC SMPS. Hopefully when the amp returns to me all will be well.
FourT6and2 wrote: ↑Thu Sep 04, 2025 6:48 am
Heard back from Hiwatt after sending the amp in under warranty. They say they found the problem—it was a bad AC/DC SMPS. Hopefully when the amp returns to me all will be well.
Congrats!
And for the electronically challenged, like me:
Switch Mode Power Supply (SMPS): A power supply that uses a high frequency switching regulator to convert electrical power efficiently, typically from AC to DC, DC to DC, or DC to AC and AC to AC. The main components are a rectifier, filter, inverter, transformer, and feedback control.