Help needed to debug 6G2. Very high pitch squeal. - Telecaster Guitar Forum
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Old January 3rd, 2014, 07:25 PM   #1 (permalink)
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Help needed to debug 6G2. Very high pitch squeal.

I got through Paul Ruby's start up guide, to the point where you install the tubes and hook up a speaker. When the tubes warm up, I get a high pitched noise, it's a very high pitch. I quickly plugged in a guitar and turned the volume up to see if any signal passed. I could hear guitar, and as I turned up the volume, the pitch of the noise changed. I installed the bias pot kit from Torres Engineering, and when I turned the pot, it made no difference in the bias voltage. You can see where my meter was hooked in the picture below. The reading I was getting was around 47 mv. As I turned the pot, it didn't change at all. I suspect I have messed up this circuit somehow. Would this cause the high pitch noise? Should I be looking for something else? Any advise on the bias pot hook up?


This is the pot I added


This one shows the bias check points Torres says to use with their kit


I snipped out the resistor here, per their instructions



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Old January 3rd, 2014, 07:36 PM   #2 (permalink)
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have you tried switching the output tube plate connections to the output transformer? Simply reversing these can get rid of the squeal.
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Old January 3rd, 2014, 08:06 PM   #3 (permalink)
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You've got an oscillation caused by one component being in phase with another. First point of inspection would be to confirm your output trafo wiring is correct? Brown and blue wires may need swapping. This is a very common error. I'd try this before looking too hard into the rest of the circuit.


Otherwise, do you have any hi-res photos of the entire circuit?

Here's my DIY 6G2. You need to copy this circuit as my bias trimmer works perfectly fine.




Frist try the trafo wires and if that fails we'll look further into your biasing arrangement.
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Old January 3rd, 2014, 11:02 PM   #4 (permalink)
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Quote:
Originally Posted by Che_Guitarra View Post
You've got an oscillation caused by one component being in phase with another. First point of inspection would be to confirm your output trafo wiring is correct? Brown and blue wires may need swapping. This is a very common error. I'd try this before looking too hard into the rest of the circuit.


Otherwise, do you have any hi-res photos of the entire circuit?

Here's my DIY 6G2. You need to copy this circuit as my bias trimmer works perfectly fine.




Frist try the trafo wires and if that fails we'll look further into your biasing arrangement.
I flipped the blue and brown, and it works now! I was really disappointed in the hum I was getting, I was regretting my decision to run the ground like the original layout. Then I clicked off my fluorescent magnifying glass 6" above the amp, and the hum vanished. I'll get some good shots of the whole thing and close ups of the bias areas. I need to look into the tremolo circuit, it doesn't seem to be working either. On another note, I ran it for about 10 minutes, and I noticed the rectifier tube was glowing purple, and flashing intermittently. It was a used tube that I thought I tested. I need to pull it and swap it out. I'm going to recheck my secondary voltages again too. Thanks for the help.

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Old January 4th, 2014, 06:45 PM   #5 (permalink)
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I swapped out the 5Y3GT and it isn't glowing purple or flashing anymore. I put the other one in the trash. Played the amp for about 20 minutes, and it seems to be working fine. The open issues I have, are the none functioning pot to set the bias on the power tubes, and the tremolo doesn't work. I'm guessing these problems are related because they share some circuitry. Here are some full circuit shots, and a couple half circuit shots. Let me know if you see something.









Thanks for the help.

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Old January 4th, 2014, 08:06 PM   #6 (permalink)
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Before you turn it on again, wire your bias world thusly. See the difference there? You've essentially bypassed your adjustment pot the way you have it (I think!!!).

Also, try that rectifier tube again :). A bluish glow in a tube isn't necessarily bad (For power tubes at least).
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Old January 4th, 2014, 08:12 PM   #7 (permalink)
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Plus is that a 10k pot?
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Old January 4th, 2014, 08:26 PM   #8 (permalink)
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Also, I'm not sure what the Torres kit contains. You may have it right per their instructions. Just saw that part of your post.
Can you post their hook up scheme? Or contact them for support?
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Old January 4th, 2014, 10:46 PM   #9 (permalink)
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Quote:
Originally Posted by andyfromdenver View Post
Before you turn it on again, wire your bias world thusly. See the difference there? You've essentially bypassed your adjustment pot the way you have it (I think!!!).

Also, try that rectifier tube again :). A bluish glow in a tube isn't necessarily bad (For power tubes at least).
I'll check this to my layout and post the Torres layout. I'm pretty sure that tube was bad. It was flashing like a camera, and glowing a pinkish purple. I've seen my 2A3 tubes for my home stereo glow, but never to this extreme. I swapped in a different 5Y3 today, and no purple glow or flashing. I attempted to check the secondaries on the PT to make sure everything was still where it needed to be, but I couldn't get readings that made any sense (they were spot on when I worked through the Paul Ruby start up guide). I thought later that maybe I need to pull all the tubes to measure the PT output. Do you think the miswire on the pot would cause the Tremolo to not work? I don't have a foot pedal, so I made a jumper plug that completes the connection between the tip and ring. Is that right?

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Old January 4th, 2014, 11:40 PM   #10 (permalink)
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The tremolo works with nothing plugged in.
Also, you'll need to jumper (or plug in and turn off) the trem to do the transformer shunt bias method (if that's what you do, I guess not needed with the 1ohm cathode resistor).

Maybe it did go bad then (tube).

Yeah post their hook up real quick so I can see it :)

Is that a 10k pot? What is that resistor off the pot 220 ohm?
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Old January 5th, 2014, 12:08 AM   #11 (permalink)
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The tremolo works with nothing plugged in.
Also, you'll need to jumper (or plug in and turn off) the trem to do the transformer shunt bias method (if that's what you do, I guess not needed with the 1ohm cathode resistor).

Maybe it did go bad then (tube).

Yeah post their hook up real quick so I can see it :)

Is that a 10k pot? What is that resistor off the pot 220 ohm?
The pot measures 191 ohms. the resistor from the pot to ground measures 23.2K ohms. there is also a 1.4 ohm resistor from pin 8 of the 6V6 to ground, and a jumper wire from pin 8 to pin 8 of the other 6V6. Here's what Torres sent me.



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Old January 5th, 2014, 01:04 AM   #12 (permalink)
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Oh ok. I guess it's wired right then.
Hm. I'm gonna look more closely!
So with no plug or jumper, the trem doesn't work?

I'll get back to ya!!
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Old January 5th, 2014, 01:14 AM   #13 (permalink)
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Man. Everything looks right (as far as I can tell).
Do you feel %100 about the underboard connections?
What is your negative DC voltage on the negative side of the 25/50 on the bias board? Is it in the ballpark of the fender scheme?
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Old January 5th, 2014, 01:21 AM   #14 (permalink)
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Oh wait. Your trem wire is going to jack ground. Send it to the tip.
Now the trem shooooould work. But why no change when adjusting bias......
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Old January 5th, 2014, 01:25 AM   #15 (permalink)
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If the tremolo jack is the one with the lone red wire on it, it looks like the red wire is connected to the barrel terminal rather than the tip terminal. Is that the correct way to hook it up?

I guess I typed my response just as Andy posted his.
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Old January 5th, 2014, 10:56 AM   #16 (permalink)
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If the tremolo jack is the one with the lone red wire on it, it looks like the red wire is connected to the barrel terminal rather than the tip terminal. Is that the correct way to hook it up?

I guess I typed my response just as Andy posted his.
Yea, after Andy said running it to ground shuts it off, I noticed I had it soldered to the barrel. I'm going to recheck my bias circuit shortly, and then try the Tremolo out.

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Old January 5th, 2014, 12:54 PM   #17 (permalink)
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Quote:
Originally Posted by andyfromdenver View Post
Before you turn it on again, wire your bias world thusly. See the difference there? You've essentially bypassed your adjustment pot the way you have it (I think!!!).

Also, try that rectifier tube again :). A bluish glow in a tube isn't necessarily bad (For power tubes at least).
Can I use the components I have and wire it like your diagram? I can't understand how the signal flows through the Torres design. Do I need the resistor on pin 8 of the 6V6, I don't understand it's function either? I checked my under board connections and I have continuity from the pot to the diode, diode to resistor, resistor to pin 4, and pot to capacitor, and cap to ground.
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Old January 5th, 2014, 01:13 PM   #18 (permalink)
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If that's a 10k pot, yeah absolutely!
The 1ohm let's you make a simple math conversion to check the current for the amp.
You'll need to know plate voltage (pin three to ground, or maybe to cathode??? I've never used that set up) then plug that into the weber bias calculator and it'll tell you the milliamp for 70% dissipation.
Your millivolt measurement is supposed to equal your milliamp (which is why the resistor should be exactly 1ohm). So you don't have to do the life ending transformer shunt method.
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Old January 5th, 2014, 01:15 PM   #19 (permalink)
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Ps. I should disclaim, I'm no expert :)
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Old January 5th, 2014, 01:33 PM   #20 (permalink)
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Quote:
Originally Posted by andyfromdenver View Post
Before you turn it on again, wire your bias world thusly. See the difference there? You've essentially bypassed your adjustment pot the way you have it (I think!!!).

Also, try that rectifier tube again :). A bluish glow in a tube isn't necessarily bad (For power tubes at least).
Ok, the tremolo works great now that I moved the wire to the tip. I wired the bias pot per your diagram and still no adjustment. the amp is currently running at 44 mv dc after a couple minute warm up. Changing the way the pot is wired, and getting the same result means the problem lies somewhere else in the amp, right?
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