So I have a Howler now :) : low productivitiy day

[mikepukmel]

Today was what we call in the business a Low Productivity Day. This learning stuff is expensive.

So I got the Deluxe AB763 correct turret board wired up, double checked the connections that I could with an ohmmeter, visual verification that the wires all go the right places (or so I think).

Fired up the amp, and with all volume controls on min, get a nice, loud howl, not a 60hz hum, much higher. Turn on the amp, in standby, quiet, Turn the standby on, few seconds then off, get a little squeal for a few seconds, which turns into a howl (lower frequency, few hundred hz). No reverb sound at all on the reverb channel. Amp does amplify the guitar.

Shut off, drained the caps, pulled both reverb send and recv. Turned amp back on, same thing. Racket not dependent on any pot setting. All pots set on min (1), still get the howl.

Turn off, drain the caps, pull the phase inverter, turn amp back on, quiet. Measured the voltages around the power tubes, look OK. Did get a couple of measurements around the preamp tubes, before i shut it off, all seem OK.

Total only ran a few minutes, with testing. Was running on the bulb limiter, no indication of a short.

I did put in grid stops, will jumper them out and see if it makes a difference. Did NOT put any grid stops on the phase inverter.

The sound is almost like Ive heard described as having the output transformer wired out of phase. But I did not remove that wiring!!


Next step, maybe take the reverb 12AT7 and swap that into the phase inverter spot? Need to re-recheck the wiring around teh phase inverter. Maybe I left a wire out or have a connection that is not soldered.

[nosaj]

Today was what we call in the business a Low Productivity Day. This learning stuff is expensive.

So I got the Deluxe AB763 correct turret board wired up, double checked the connections that I could with an ohmmeter, visual verification that the wires all go the right places (or so I think).

Fired up the amp, and with all volume controls on min, get a nice, loud howl, not a 60hz hum, much higher. Turn on the amp, in standby, quiet, Turn the standby off, get a little squeal, which turns into a howl (lower frequency, few hundred hz). No reverb sound at all on the reverb channel. Amp does amplify the guitar.

Shut off, drained the caps, pulled both reverb send and recv. Turned amp back on, same thing. Racket not dependent on any pot setting. All pots set on min (1), still get the howl.

Turn off, drain the caps, pull the phase inverter, turn amp back on, quiet. Measured the voltages around the power tubes, look OK. Did get a couple of measurements around the preamp tubes, before i shut it off, all seem OK.

Total only ran a few minutes, with testing. Was running on the bulb limiter, no indication of a short.

I did put in grid stops, will jumper them out and see if it makes a difference. Did NOT put any grid stops on the phase inverter.

The sound is almost like Ive heard described as having the output transformer wired out of phase. But I did not remove that wiring!!


Next step, maybe take the reverb 12AT7 and swap that into the phase inverter spot? Need to re-recheck the wiring around teh phase inverter. Maybe I left a wire out or have a connection that is not soldered.

Flip the leads on the OT I'm thinking. But then I shouldn't be.

nosaj

[mikepukmel]

Thanks Nosaj, will do. Any chance a bad solder joint could cause something like this? The signal caps are pretty big, wondring if I left a wire off or not soldered under there. Will try the OT leads first.

[eschertron]

From post #1 I'm thinking that you will swap out the PI tube (just to verify the tube)? Do that first. Is there an easy place to unsolder (break open) the NFB loop? Do that next. That'll help point to/eliminate the NFB being the squeal. Even if the NFB is wired in phase, wrong-value resistors could make the loop unstable. If you haven't found the problem after the first items, double check all the component values. Yes, I realize you've pretty well isolated the problem to the power amp section.

[Enzo]

It sure sounds to me like reversed phase on the OT primary.

[tubeswell]

What the others said already, it will be positive feedback so swap your OT primary winding ends around to the opposite output tube sockets.

Then take CD voltages at each of the HT power supply nodes (at each filter cap) and at each plate and each cathode, and report back.

[nosaj]

What the others said already, it will be positive feedback so swap your OT primary winding ends around to the opposite output tube sockets.

Then take CD voltages at each of the HT power supply nodes (at each filter cap) and at each plate and each cathode, and report back.

What is CD?

nosaj

[mikepukmel]

Thanks everyone! Over breakfast coffee I went through all of the old photos I have of the previous wiring attempts, and the only thing I can not verify is the two wires under the board that go (from the 220k resistors on the board) to the grid stops of the output tubes. Those may be reversed compared to the previous wiring. Didn't take photos of the bottom of the board before I installed it (for shame, since I got a bunch of photos of everything else )

I will first get voltages at the caps (I think Tubeswell meant DC, right?), and plate and cathodes. I started voltage probing last night but shut it off after a few seconds since the noise was unbearable at near midnight.

back in a few . . .

[nosaj]

Thanks everyone! Over breakfast coffee I went through all of the old photos I have of the previous wiring attempts, and the only thing I can not verify is the two wires under the board that go (from the 220k resistors on the board) to the grid stops of the output tubes. Those may be reversed compared to the previous wiring. Didn't take photos of the bottom of the board before I installed it (for shame, since I got a bunch of photos of everything else )

I will first get voltages at the caps (I think Tubeswell meant DC, right?), and plate and cathodes. I started voltage probing last night but shut it off after a few seconds since the noise was unbearable at near midnight.

back in a few . . .



Then try swapping the two inputs to

I'd reverse the OT leads first. Had this happen the other day on a ristaucrat jukebox. Seems to me the sounds vary from amp to amp on how positive feedback sounds.

nosaj

[mikepukmel]

May sound like a stupid question, but:

If the inputs to the output tubes are swapped, does this have the same effect as swapping the OT primary connections with regard to the phase of the feedback ckt?

[Jazz P Bass]

I guess that the answer would depend ON the feedback circuit.

[tubeswell]

What is CD?

nosaj

LoL. It’s DC when you’re typing fast.

[mikepukmel]

That's what it was: the wiring from the phase inverter to the output tubes was reversed.

In my original wiring, it was wrong compared to the layouts from Fender, but the color coding on the Hammond transformers is reversed as well, so it worked correctly. When I put the new board in, I wired the PI output as per the fender layout, which caused the problem. Heh. <giving myself a pat on the back for not documenting this in my notebook).

Could just barely lift the board up enough to unsolder the two connections underneath. Unfortunately, the new connections are above the board, so I have a bunch more wires up in the air, compared to the nice looking 1963 chassis photos.

Racket gone! Tone sounds nice, I think its a keeper.

With the chassis open, reverb tank sitting next to the chassis on the bench, still have a little hum in the speaker, with volume controls at minimum. Sounds like 60 or 120 (can't tell which). Not noticeable when im planing, but enough to be annoying when not playing, and this is a tiny room.

Have to find a way to track down this hum.

***

I got a bunch of these cool double turret chassis mount standoff terminals, Enzo suggested, put all of the grid stops on these, also put the 1 ohm bias measurement resistors on, so I could route the grounds to the 'star'. The star is getting pretty busy.


Have one turret lug in near the pilot light, will move one side of the 100 ohm heater ground ref. resistors to the lug and route another wire to the star from there.

The hum is about the same character as the first time I fired up the amp, 3 wiring changes ago. Did something wrong or maybe its just the same ground loop I haven't found yet. Could be something I don't know to look for staring me in the face.

[nosaj]

That's what it was: the wiring from the phase inverter to the output tubes was reversed.

In my original wiring, it was wrong compared to the layouts from Fender, but the color coding on the Hammond transformers is reversed as well, so it worked correctly. When I put the new board in, I wired the PI output as per the fender layout, which caused the problem. Heh. <giving myself a pat on the back for not documenting this in my notebook).

Could just barely lift the board up enough to unsolder the two connections underneath. Unfortunately, the new connections are above the board, so I have a bunch more wires up in the air, compared to the nice looking 1963 chassis photos.

Racket gone! Tone sounds nice, I think its a keeper.

With the chassis open, reverb tank sitting next to the chassis on the bench, still have a little hum in the speaker, with volume controls at minimum. Sounds like 60 or 120 (can't tell which). Not noticeable when im planing, but enough to be annoying when not playing, and this is a tiny room.

Have to find a way to track down this hum.

***

I got a bunch of these cool double turret chassis mount standoff terminals, Enzo suggested, put all of the grid stops on these, also put the 1 ohm bias measurement resistors on, so I could route the grounds to the 'star'. The star is getting pretty busy.


Have one turret lug in near the pilot light, will move one side of the 100 ohm heater ground ref. resistors to the lug and route another wire to the star from there.

The hum is about the same character as the first time I fired up the amp, 3 wiring changes ago. Did something wrong or maybe its just the same ground loop I haven't found yet. Could be something I don't know to look for staring me in the face.

Sometimes it pays not to get everything super nice and neat before a sound check. like not cutting OT leads until your sure of phase. And as you've noticed color code will not always match up.

nosaj

[Steve A.]

The hum is about the same character as the first time I fired up the amp, 3 wiring changes ago. Did something wrong or maybe its just the same ground loop I haven't found yet. Could be something I don't know to look for staring me in the face.

So are you using a Fender chassis or an exact clone? I ask because Leo & Co. usually oriented the transformers to minimize the hum.

The hum could be coming from the AC filament supply. One way to check that is with a 6VDC power supply which could be from 4 AA batteries, or even try the 5VDC output from a USB portable power pack. I believe that if you disconnect the AC filament supply with the tubes hot you can hear if the hum disappears as the signal fades out.

If you decide to go with a DC filament supply you can leave the power tubes on the AC supply... the Boogie Mark IIIC schematic as I recall shows a nifty way to get DC for the preamp tubes.

I did not look up earlier threads with you discussing the hum issues so I have no idea what you already tried.

Steve A.

P.S. We usually encounter the initial problem in this thread after replacing an OT so our first thought is to reverse the leads going to it from the power tubes...