‘67 Dual Showman Buzzzz

[fdesalvo]

Sadly I’ve posted this elsewhere and am out of ideas, so here we go.

I received a costumers showman which was setup with a drake PT and choke running KT88s. I converted it back to run 6L6’s using hammond iron. Also did some maintenance and repairs:

- rebuilt input jacks (broken resistors)
- full electrolytic refresh
- test coupling and tone caps (no leaking dc)
- check board for stray dc
- replace severely drifted resistors
- clean and retension sockets
- clean and lube pots, clean brass plate ensuring good ground plane
- test grounds
- rebuild bias/rect board
- install new tube set
- bias
- install new trem roach
- install shielded cables to v1/2
- install replacement grounded power cord

Amp is plagued with ~700hz buzz that’s low level present with volumes off and increases in db rapidly with volumes up. Pulling PI stops buzz. Pulling v1/2 has no impact. Not heater related. Jumped in filter caps in parallel no change. No dc on grids. Lead dress per factory. All voltages are with in spec. Agh!! I’m out of ideas and can use some guidance.

[nosaj]

Do you have a scope? it would help to see where in the circuit it is originating. No scope? An audio probe may be just as good.
Not clear but did you put in a hammond PT and OT?
nosaj

[fdesalvo]

No scope :/. Original ot. Just hammond pt and choke.

[nosaj]

Do you have another OT to test with? I think it might be a wise test to try.
nosaj

[Helmholtz]

Does the amp work otherwise? If so I wouldn't suspect the OT.

How do you know it's 700Hz?

Why not heater related?

[fdesalvo]

Does the amp work otherwise? If so I wouldn't suspect the OT.

How do you know it's 700Hz?

Why not heater related?

works otherwise, yes. I have a frequency analyzer that telling me it’s 700’ish. I could’ve spoken to you soon about the heater thing but I highly doubt it.

[nosaj]

Any chance you could use the frequency analyer to probe the audio circuit to find where the 700hz is or isn't?
nosaj

[loudthud]

Is this just the output stage oscillating because the OT was installed with phase reversed ? The oscillation stopped when the PI was pulled

[Helmholtz]

I have a frequency analyzer that telling me it’s 700’ish.

Please post a readable pic of the spectrum.

[fdesalvo]

OT wasn’t touched. Only pt and choke. I’ll post a snapshot. The analyzer is just an iPhone app that I use for troubleshooting.

[nosaj]

OT wasn’t touched. Only pt and choke. I’ll post a snapshot. The analyzer is just an iPhone app that I use for troubleshooting.

So was this noise there previously to all the maintenance and repair?
nosaj

[nosaj]

Audio probe and how to use

[g1]

Is this just the output stage oscillating because the OT was installed with phase reversed ? The oscillation stopped when the PI was pulled

Good point. It's worth disconnecting the NFB as a test even if OT is stock.
OT wires at power tube plates could have got reversed, or even connections from PI plates to power tube grids can get swapped along the way. Especially when conversions have been done.

[fdesalvo]

No change when disconnecting NFB. Amp was inoperable prior due to condition of electrolytics. Screenshot attached. Sometimes reads as 1.5khz.

[fdesalvo]

Good point. It's worth disconnecting the NFB as a test even if OT is stock.
OT wires at power tube plates could have got reversed, or even connections from PI plates to power tube grids can get swapped along the way. Especially when conversions have been done.

Lol man. The prior tech installed ceramic octal sockets. Your post made me push off the bench and take a step back. The blue and brown ot leads were switched according to the layout, though the output jacks are correct. After reversing the plate leads the amp went into that classic screeching, so I had to put them back.