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I built a Fender 5f1 Champ imitation.

I've taken care to start it up gradually (first with no tubes, then just the 5Y3, then the 12AX7, then the 6V6 with the speaker connected and finally connected a guitar to it.) So far I've seen no shorts or blown fuses. The internal capacitor voltages check out.

After connecting the guitar to either of the inputs, I noticed that the speaker doesn't produce any sound, not even white noise (which is also true, when the guitar isn't connected - this I didn't remember to check before plugging in.)

Instead, the output transformer produces a wheezing, buzzing, cackling sound in response to the volume being manipulated (not just responding to the magnitude, but changing the pitch and sound being produced) and interestingly alters the sound in response to strings being struck on the guitar (most notably, the low E string, with higher frequency notes the effect is not that much noticeable.) Again, all the time, there is nothing coming from the speaker.

I've done a basic check of the output transformer (as detailed in this video) with nearly identical results, but without desoldering the transformer from the circuit.

I didn't check the speaker and I honestly wouldn't know how. However, it is brand new and upon visual inspection, I don't see anything wrong with it.

I've uploaded a short video here.

I've used the standard schematic, with a slight modification (the feedback 22k resistor is split into a potentiometer and a 10k resistor in order to control the feedback):

enter image description here

Where could this behavior come from? What should I check?

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  • \$\begingroup\$ All you’ve shown is the effect. Where is the schematic and the video should show the whole unit in detail so hopefully we can pick something obvious. You can use your PC’s sound card as an oscilloscope to see if the amp is oscillating. \$\endgroup\$
    – Kartman
    Sep 10 at 12:07
  • \$\begingroup\$ @Kartman I'll try to get a better recording, How can I use my sound card as an oscilloscope here? \$\endgroup\$ Sep 10 at 15:35
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    \$\begingroup\$ @user3209815: I wouldn't connect a guitar tube amplifier to a PC soundcard. It can be done. The problem is doing it safely without killing yourself or the PC. \$\endgroup\$
    – JRE
    Sep 10 at 15:49
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  • Desolder the speaker and check it using an ohmmeter. It should read around 4-8 ohms.
  • Check that secondary winding and speaker are connected properly (either not opened or shorted.)
  • Most likely, your amplifier is oscillating. Swap the secondary wires.

Just for the case: currently, color1 wire goes to contact A of the speaker and the feedback resistor (22k), color2 goes to contact B of the speaker and ground. After swapping, color1 should go to B and ground, color2 to the A and feedback resistor.

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I listened to your video. It sounds very much to me like it is oscillating.

Oscillating or not, though, the output transformer shouldn't make noise. I think you have a short circuit on the output side. That causes the transformer to pass more current than it ought to. That could make the transformer produce noise.

Disconnect everything from the transformer output and check for short circuits. Fix anything you find.

Try it again after you check and fix the output.

If it still squeals, you'll have to find out how to deal with an oscillating tube amplfier - I don't know anything about them.

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Assuming you have not found a short circuit, and the speaker checks I would suggest changing the capacitors, they have probably failed or lost enough capacatance not to be effective. Caution that circuit has several hundred volts on the B+ whic is coming off the filament of the 5Y3GT rectifier tube. the x-450 is the (x)capacity in microfarads. Those low values are correct at those voltages. You have a total of 4 to replace. Several could be combined in one can, I cannot see what you have. If you have not checked the speaker connect a flashlight battery momentary across the two leads from the speaker and it should click. One way the cone will pull in a bit and turning the battery the other way it will push out a little.

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