‘67 Dual Showman Buzzzz

[fdesalvo]

Will def have to get a scope at some point. Good advice.

Lead dress: I’ve made some extreme moves and haven’t heard any changes. I ended up arranging per the layout of some of the quietest amps I’ve had on the bench (I usually photo document for reference). I run off the schematic for component values and hit everything with the hioki to validate.

[fdesalvo]

Alright - after a slew of good repairs, I'm ready to jump back into this Fender. The owner brought me a new OT to tack in, so I'll update after this. I highly doubt it's the deal, but am grasping at straws.

[fdesalvo]

Tomorrow I’ll sub in a know. Good OT, but something strange I’ve noticed. Amp has original OT. When it was running KT88’s, the blue and brown ot primaries were reversed at the sockets according to the layout. After the conversion back to 6L6’s the amp ran without squealing. After reversing the wires to reflect the layout, the amp oscillates like it’s backwards. Secondaries are wired properly at the jacks. Is it possible the buzz I’ve been hunting lies in the OT? I know I’ll have an answer tomorrow, but I’m trying to do some homework before I hit the bench.

ty for reading.

[pdf64]

If the buzz stops when the feedback loop is opened, then yes, it’s just positive feedback. eg lift a leg of the 820R feedback resistor.

[g1]

After reversing the wires to reflect the layout, the amp oscillates like it’s backwards. Secondaries are wired properly at the jacks.

Check that wiring from PI plates to power tube grids matches layout.

[fdesalvo]

Thanks, guys.

PI wiring is per schematic/layout. With ot wired per layout, equaling stops with feedback resistor lifted. Buzz I’m complaining about is much louder with volumes off and exponentially so as they are rolled up.

[Tom Phillips]

...PI wiring is per schematic/layout. With OT wired per layout, [squealing] stops with feedback resistor lifted...

There is always a chance that the color coded wiring of an output transformer could be wrong on either the primary or the secondary. That's why it's always best to do the NFB disconnection test. It sure appears that's the case with your particular OT.
It curious and confusing that your latest test results conflict with your post #14 report.

[Helmholtz]

With correct wiring disconnecting the NFB will inrease gain, so any buzz entering the PI will get louder.

[fdesalvo]

Apologies - haven't been getting notifications, so pardon the hanging replies.



Tacking in new OT doesn't resolve anything. Was just a desperate move on my part.
Grounding each respective channels' grids prevents buzz from increasing when raising the volume.
Grounding the PI input doesnt help
Grounding the .iuf cap on the other PI grid (NFB) stops all buzz, except when the channel volumes are raised, then it increases with volume.

Agh!

Every 5 min I'm hearing a digital tone in the buzz that's about 10 distinct notes increasing in pitch. Could it be that this amp is the only one to pickup line noise of the hundreds that have been on my bench in the last 12 mo? Going nuts!

[Tom Phillips]

...Every 5 min I'm hearing a digital tone in the buzz that's about 10 distinct notes increasing in pitch. Could it be that this amp is the only one to pickup line noise of the hundreds that have been on my bench in the last 12 mo? Going nuts!

Any chance that noise could be picked up from a nearby cell phone? Even powered computer speakers pick up interference from cell phones and the powered speakers are relatively low gain devices. It's easy to test just by moving the cell phone.

[Helmholtz]

Grounding each respective channels' grids prevents buzz from increasing when raising the volume.
Grounding the PI input doesnt help
Grounding the .iuf cap on the other PI grid (NFB) stops all buzz, except when the channel volumes are raised, then it increases with volume.

Generally it is not advisable to directly ground any circuit points that carry a DCV (like PI grids) as this upsets tube operating conditions.

See if the speaker jack is well grounded. Use Ohmmeter to check resistance to chassis.

[fdesalvo]

Any chance that noise could be picked up from a nearby cell phone? Even powered computer speakers pick up interference from cell phones and the powered speakers are relatively low gain devices. It's easy to test just by moving the cell phone.

I wish it were the case. I’ve had my phone on airplane mode with Wi-Fi off and even powered down all together. That digital noise is just part of the buzz I’m hearing. It seems there’s a background noise component coming from the feedback circuit (tested by grounding that .1?) and then from either preamp channel with volumes anywhere off of zero.

[fdesalvo]

Generally it is not advisable to directly ground any circuit points that carry a DCV (like PI grids) as this upsets tube operating conditions.

See if the speaker jack is well grounded. Use Ohmmeter to check resistance to chassis.

Only grounded it for a quick moment. Not sure that was even a relevant test. I’m getting .4 ohms measuring jack to chassis. I get .2 ohms when I short the probes together. TY

[nosaj]

Apologies - haven't been getting notifications, so pardon the hanging replies.



Tacking in new OT doesn't resolve anything. Was just a desperate move on my part.
Grounding each respective channels' grids prevents buzz from increasing when raising the volume.
Grounding the PI input doesnt help
Grounding the .iuf cap on the other PI grid (NFB) stops all buzz, except when the channel volumes are raised, then it increases with volume.

Agh!

Every 5 min I'm hearing a digital tone in the buzz that's about 10 distinct notes increasing in pitch. Could it be that this amp is the only one to pickup line noise of the hundreds that have been on my bench in the last 12 mo? Going nuts!

Can you record this sound and upload it? Can you also take this amp to another house or location and repeat the issue?
nosaj

[Helmholtz]

Is the safety grounding intact?
Check for (low resistance) continuity from amp chassis to the safety ground contact of the mains plug.

Is there anything else connected to the speaker output (except speaker)?