YBA-1 with confusing 60/120hz at output

[waspclothes]

Hey everyone,
I have a Traynor YBA-1 (earlier version with the choke) that I've replaced all of the electrolytic capacitors on. At the output I'm getting a 110mVpp 60/120hz hum. I say 60/120hz because it isn't a clean wave and has both 60 and 120hz sort of peaks to it.

When I ground the input of the PI, the hum disappears, but when I lift the input of the PI the hum gets louder. I've tried:

- replacing all preamp tubes
- measuring for ripple at the power supply and at the preamp tube plates
- measuring all DC voltages - they match within 5% of schematic.
- swapped the heater wires on one power tube so they stay consistent (noise cancelling)

I noticed that the later YBA-1 schematic seems to add a grid stopper resistor before the PI, so I wired one of those in at a few different values and that hasn't seemed to do anything. I'm at a loss here.. A few things I did notice, but have no idea what they mean:

- the amplitude of the hum is affected by the "bass" tone knob a decent amount.

Anyone know what might be happening?

Schematic on page 6: YBA1Schems.pdf

[52 Bill]

When I ground the input of the PI, the hum disappears, but when I lift the input of the PI the hum gets louder.

- the amplitude of the hum is affected by the "bass" tone knob a decent amount.

This should lead you to believe that the hum is being generated in the stages before the PI and tone controls. Was the hum present before the cap replacements? Are all of the grounds good and tight? Does the volume control have any effect on the hum level?

[tubeswell]

Bad rectifier diode? (with transient switching spikes getting through the PT into the heater winding?)

[Mick Bailey]

Whene you lifted the input to the PI was the NFB still connected?

[waspclothes]

Bad rectifier diode? (with transient switching spikes getting through the PT into the heater winding?)

How do I check for a bad rectifier diode, just sub in a known good one? I did scope the power supply output and nothing looked "odd" in the ripple to me. I believe there was a 5 or 10Vpp sawtooth wave at the first stage of filtering, and after the choke that ripple was down to 10s of mV.

[waspclothes]

Whene you lifted the input to the PI was the NFB still connected?

Hi Mick, I lifted the grid of V3A. So I guess yeah, the NFB was still connected around to V3B.

[waspclothes]

This should lead you to believe that the hum is being generated in the stages before the PI and tone controls. Was the hum present before the cap replacements? Are all of the grounds good and tight? Does the volume control have any effect on the hum level?

Hi Bill, volume control doesn't affect the hum.. neither does pulling V1. tone controls affect it a bit, but never remove it. I can't remember if the same hum was present before the recapping, it was just "noisy" in general.

[waspclothes]

The best I could come up with on this amp is some bad grounding. The old can caps were soldered directly to the chassis, and when I replaced them with F&T caps and clamps, I ran the ground to a transformer lug.
So I moved that ground and soldered it onto the chassis to the same place as where the power tubes are soldered to the chassis and the hum halved (60mVpp at the output). It's not the quietest amp I've played, but at least it's tolerable.

Interestingly, the hum is present even on standby. I clipped in an extra 22uF to the B+ and that made pretty much no difference in the hum, so I'm pretty much out of ideas.

[g1]

The best place to ground the caps would be where the center-tap of the HT winding of the power transformer is grounded.