Fender Champ 5f1 clone squeal - diyAudio
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Old 21st February 2013, 01:49 AM   #1
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Default Fender Champ 5f1 clone squeal

Hey! I just joined the forums a couple minutes ago because I have some questions on an amp I am building for the science fair. It's a Champ 5f1 clone, build to the schematics by triode electronics. Got everything hooked up and plugged it in for the first time a couple of hours ago.

All three of the tubes 12AX7, 6V6GT, and the 5Y3GT glow, but about 15-30 seconds of heating up, I get an ear piercing screech through my speaker.

I've looked around and found a couple of things ranging from tubes going microphonic to the output transformer (TF103-ULH from triode) secondaries needing to be switched. I have pictures, but I'm not sure how to put them in here yet. Ill put them up as soon as I can.

Anyway, I tried switching all the tubes out with new ones, and started it up again. Same thing happened. The output transformer had a 8ohm and a 4 ohm secondary, so i tried switching them. nothing happened besides the squeal being a higher pitch. It happens no matter what the volume is at too, by the way. any help?

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Old 21st February 2013, 01:56 AM   #2
kevinkr is offline kevinkr  United States
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Moved. All musical instrument amp posts and threads should be to this forum..

Thanks..

Note that your link does not appear to work.
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Old 21st February 2013, 02:25 AM   #3
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I looked at the layout on http://site.triodestore.com/5F140-18027120VREV2.pdf

I'm sure many of these have been built with no problems. I would suggest that you check the wiring from the 68Ks to pin 2 of the 12AX7. For that matter double and triple check all of the connections. It could be something as simple as a cold solder joint.

It most probably is a compromised wire connection.

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Originally Posted by Exla13 View Post
Hey! I just joined the forums a couple minutes ago because I have some questions on an amp I am building for the science fair. It's a Champ 5f1 clone, build to the schematics by triode electronics. Got everything hooked up and plugged it in for the first time a couple of hours ago.

All three of the tubes 12AX7, 6V6GT, and the 5Y3GT glow, but about 15-30 seconds of heating up, I get an ear piercing screech through my speaker.

I've looked around and found a couple of things ranging from tubes going microphonic to the output transformer (TF103-ULH from triode) secondaries needing to be switched. I have pictures, but I'm not sure how to put them in here yet. Ill put them up as soon as I can.

Anyway, I tried switching all the tubes out with new ones, and started it up again. Same thing happened. The output transformer had a 8ohm and a 4 ohm secondary, so i tried switching them. nothing happened besides the squeal being a higher pitch. It happens no matter what the volume is at too, by the way. any help?

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Old 21st February 2013, 03:55 AM   #4
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It may well be the output transformer being connected in reverse phase. If you lift the 22k feedback connection and the squeal goes away then that is most likely the problem. Reverse the output transformer primary wires.

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Originally Posted by scott17 View Post
I looked at the layout on http://site.triodestore.com/5F140-18027120VREV2.pdf

I'm sure many of these have been built with no problems. I would suggest that you check the wiring from the 68Ks to pin 2 of the 12AX7. For that matter double and triple check all of the connections. It could be something as simple as a cold solder joint.

It most probably is a compromised wire connection.
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Old 21st February 2013, 06:10 AM   #5
Enzo is offline Enzo  United States
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Output transformer reversed is my vote.
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Old 21st February 2013, 07:10 AM   #6
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Output transformer reversed is my vote.
+1. You'd be surprised how often stupid little things like that pop up during initial assembly.
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Old 30th July 2013, 07:21 PM   #7
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I just built a 5F1 and am having squeal problems also. Goes away at full volume, which is appreciable. Tried lifting 22K feedback, reversing OT primary leads to no avail..... Plan to adjust preamp B+ voltages (are a bit low), and looking for bad connections. Also will put a dummy load on it and look at it on my 'scope. Any other ideas? Thanks!
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Old 30th July 2013, 10:59 PM   #8
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I just looked up the schematic and was appalled to find no grid stop on the 6V6. Every single ended 6V6 circuit I've built required one to supress parasitic oscillation.
Lift the wire from pin 5 on the 6V6 and fit a 15K resistor between the wire and its old pin 5 connection. The resistor body should be as close to the tube socket pin as you can make it.
Cheers,
Ian
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Old 31st July 2013, 01:55 AM   #9
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Default ADDED THE GRID STOPPER - STILL SQUEALS

Really cut the treble as well. I have bigger problems I'm finding out. Thought an old OT from a Hammond organ, for a p-p 6V6 amp would work.... Primary DC resistance only 535 ohms... Need 5-8K right? and it has no air gap. Will buy the right OT and go from there. Thanks for the tip! Had heard about grid stoppers (have notes on it somewhere - lots of great information out there, and not enough organizing on my part!). I'm a newbie. Need to build an amp with the proper parts, and THEN start substituting.....
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Old 31st July 2013, 02:26 AM   #10
Enzo is offline Enzo  United States
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You do want a transformer with a gap for the single ended amp like this.

BUT

Don't confuse DC resistance of the transformer wire with the impedance of the circuit. They are unrelated. 5k-8k refers to impedance. a 5k transformer wound with #28 wire will have a higher resistance than a 5k transformer wound with #24 wire. Same turns ratio means same impedance, but thicker wire will have lower resistance.
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