Gibson GA50 Clone Noise Issue

[Phil_S]

Keep in mind that only the output trans and chokes have 120 hz in them, so 1) I can’t see how 120 hz could get impressed into the Pt core.
2) he has no issue with 60 hz from the PT getting into the Ot, or at least not to any amount of it being worth messing with spinning it.

No argument from me. There is another post suggesting rotation of the PT is the only way. I only want to point out that I think rotating either the PT or the OT will have similar results. I don't think it matters which one is rotated.
I'm quite interested in seeing the solution.

[didit]

Power transformer magnetic energy field varies around its "sphere", so in this situation rotating the output transformer 90 degrees is likely to be more effective. There are primary and secondary sources for noise emanating from a power transformer. And the secondary as a source may be 120Hz, albeit at lower intensity than the 60Hz that is the primary. That L1 choke however is most probable source.

Best .. Ian

[Stevem]

Sorry, but I totally fail to see how the secondary side of a PTs ac voltage output can be modulated to 100 or 120 hz.

Show me a scope trace of this taking place.

[bepone]

one test:
lets pull the PI tube out, what is scope showing on the speaker?

second test, return PI and pull first tube, what is on the speaker?

[Duck Dodgers]

Went ahead and rotated the OT. Pic below.
No change to scope trace or audible noise. Still has very, very faint 60Hz hum and annoying 120Hz buzz.
While my component placement isn't optimum, I think we can take it off the table as the cause of the buzz.

Included a pic if the bottom as well so you can admire my lead dress.

Unless someone has a reason not to, I will put the iron back to the original placement. That way we don't have changes piled up on changes as we proceed.

Bepone: Not sure what you mean. Check speaker output w/wo V1? (Earlier on I believe I mentioned that pulling the preamp, V1, has no effect.

Question: I first see the 120Hz signal at the inverter grids. I assumed (we all know what that means) that the 120Hz was being introduced there. Is it being inserted further up the system and "trickling" back to the inverter grids?

Off topic: How do I paste a quote from a post with the attribution. High-lighting and using the "quotes" button doesn't seem to do it.

Thanks again.

[Phil_S]

Egad! That isn't lead dress. That's lead pants. No, no, no. All leads stay inside the chassis. Make them shorter, not longer. Remove the whole length of wire, end to end, and use new wire. When I say under the board, I mean between the board and the chassis. You'll need to fish the wire underneath the turret board.

One way to fish underneath the board is to use solid core with a loop on the end. Twist your new lead onto the loop and pull. Snip off the end of the lead that you fixed to the loop. Adjust placement of the new lead. Solder both ends.

[Duck Dodgers]

Egad! That isn't lead dress. That's lead pants. No, no, no.

I can't decide if you're pranking me or not. It was a "funny" hence the .

I did this to turn the OT 90deg. I had to splice the wires and drag them over the edge to hook them back up. It was a temporary expediency to see if there was any difference in the 120Hz noise. The OT position and the, admittedly, ridiculous wiring made no difference.

The plan is to put things back and keep looking for the cause of the 120Hz.

[Phil_S]

I was joking a little (pants), but not pranking. Aside from being dangerous, running those wires close to the power tubes can be a source of induced hum. It made me cringe. I reacted.

[bepone]

Bepone: Not sure what you mean. Check speaker output w/wo V1? (Earlier on I believe I mentioned that pulling the preamp, V1, has no effect.

pull out PI and check for the problem, if dissapears then is in PI or preamp..
also with the scope you can check where is the wire generating 120Hz problem, and put it against the chassis, far away from g1 wires..

[Stevem]

Would you try this one out.
I hook the each output of the PI one at a time.
Unhook it at the input to the coupling cap.

Does that change anything?

Also ground that unhooked input to each cap, does that relieve the issue?

If I hooking either output clears up the issue then on that output leg install that coupling cap 180 degrees the other way and see what you have then.

[bepone]

I have checked little bit better the layout..layout is generating the issue, also schematic how it is drawn.

0 .First critical error in wiring of R11! R11 grounding cannot be wired with output tubes gnds! V2b GND reference must go to the C3 capacitor.

Layout 4-11.jpg

Then in smaller account, next things>
1. power transformer is shooting the residual field into the all magnetic components.. L1, L2, OPT. Because of that PT must be rotated 90 degrees
2. there is wrong placement of anode resistors of PI..they are below the power tubes.. all PI components must be placed below PI with wires going to PI not over the power section elements, so transfer of anode 100k's is neccessary to below the PI
3. power transformer and output transformer wires must be pressed against the chassis, not in the air and close to PI input resistors

[R.G.]

On the 120Hz buzz:
These can come from "slam-off" in solid state diodes (which you don't have) or from spikes on the AC mains wiring riding through the PTs interwinding capacitance, among other places.

[Duck Dodgers]

First off, the easy one.

On the 120Hz buzz:
These can come from "slam-off" in solid state diodes (which you don't have) or from spikes on the AC mains wiring riding through the PTs interwinding capacitance, among other places.

I have checked the 120ac line. Looks clean. I also moved everything from the garage to inside the house. Different 120 leg and away from the solar inverter. No change.

Now, I need some clarification with nomenclature. What is (the?) Pl ? Only anode resistors I see are on V2 P2&5 and V1 P8. Are the coupling caps referred to C12 and C14? (I'm guessing here; turning the cap is to get the "outside foil" on the low impedance side.) From this I would infer Pl is the inverter, V2.

Anyway, it would be a big help for the newbie if you could also reference the component designation.

Would you try this one out.
I hook the each output of the PI one at a time.
Unhook it at the input to the coupling cap.

Does that change anything?

Also ground that unhooked input to each cap, does that relieve the issue?

If I hooking either output clears up the issue then on that output leg install that coupling cap 180 degrees the other way and see what you have then.

As I understand what you want is: Unhook the Inverter output, one at a time, at C12 and C14.
"Also ground that unhooked input to each cap, does that relieve the issue?" not sure what you mean. Ground the Plate?
Would turning the caps one at a time accomplish the same thing?

Sorry this is so rambling. Thanks!

[Duck Dodgers]

Back again..

Disconnected C12 & C14
C14 only connected; no buzz
C12 only connected; about 1/2 to 2/3 buzz. Reversed cap, no change.
Both connected; the usual buzz.

So...since the grid of V2b, is fed from the anode of V2a it, V2b, would would amplify the buzz coming from V2a. This would account for the lower amplitude of the buzz with only V2a connected. Looks like there is a issue in the area of V2a. Sound OK?

Suggestions for further testing gratefully accepted.

[didit]

Hello --

Yes, phase inverter abbreviated is "PI".

I may have missed it -- long shot certainly, but did you confirm the rectifier tube is known good?

Looking over the recent photos and building up a 3D image relating all the components. Suggest you try unfastening the two tone pots and temporarily moving them away from the leads that connect L1 and L2? That V2a grid has that whole nest including the pots, which is close to both the L1 core, its leads, and your B+ string.

Best ..