squeal when at high volumes

[Andrew]

I recently built the 2 stroke, my first tube amp, from the kit that Andy pulled together! It sounds great but i’m getting an issue with what sounds like capacitors being overloaded and a ton of squeal when i turn the volume to around 10 (out of 12). Unplayable and I immediately have to turn back down. The only modification I made was to use a 22uF cap instead of the 40uF shown on the TAN wiring diagram as that’s what I could find (and the DH showed this so I hoped it should be ok). I saw some notes about using a 100ohm resistor on the green wires on the PT but it wasn’t on the diagram so I was hesitant to put them in. Could that be the issue? Otherwise it sounds fantastic but I just can’t help turn it up to get that crunch! Also, the problem is made worse when I’m in either of the boost modes. There is also a lot more hum when in these modes. Thanks!

[Andy]

The currently available power transformers don’t supply what is called a “center tap”. That was a green/yellow wire in the instructions. The 100Ω resistors are there to create an artificial center tap. This is done by attaching a 100Ω resistor from each green filament line to ground. I usually attach it at the 2 pilot light terminals and ground it at the 3 pin ground terminal.

That is the first thing I would do as it is an important part of the circuit. This link has some more information: http://www.valvewizard.co.uk/heater.html

[Andrew]

Resistors added. I can now get my volume up to 12, with its wonderful distortion, but at higher volumes the tone knob is now unusable. Anything above 0 on the tone with the volume even at 8 or so causes the sound to drop out and a slightly different squeal to happen. If I turn the volume to 5 or so, I can get more out of the tone but still not the full dial for both. Any thoughts?

[Andy]

The next step is to super check all of your grounds. In particular, the panel mount pots. Make sure all your grounding is solid.

After that, I would take some voltage readings and let us know what you get. Pins 1 and 6 of the tube should be around 140-160V. Pin 3 on the power tube should be around 350V, Pin 4 around 300V.

[Andrew]

I checked and rechecked all the grounds and I’m pretty sure they’re all sound.

Voltages are looking off though. My voltage coming in from the wall is 123.6. Having measured all the voltages, I think I’m a bit high across the board too, possibly because of this input voltage? PT is putting out 6.8V (vs 6.3). 12AX7 pin1=153.5, pin6=153.3. Output/6V6 pin 3=391, 4=336, 8=22.5. Rectifier 5Y3S pin8=394vdc. Do you think that’s the cause? If so, is there a way to drop my input voltage? Maybe a high wattage resistor across the input to drop it a few volts?

[Andrew]

One odd thing that is the impedance switch doesn’t seem to do anything. I’m getting continuity across all the leads to each other, regardless of the switch location. I also can’t notice any difference when it’s plugged in on my 8 ohm speakers except a tiny bit less hum at high volumes

[Andy]

You won’t really hear any difference with the impedance switch. But maybe you have that wired incorrectly. The round eyelets should be what you are switching (green, yellow or white) and the odd shaped one (on the same side as one of the round eyelets) should go to the output jack. I think thats how it goes if I recall correctly.

Feel free to post a picture and we can see if it looks right. Also, make sure that the coax cables that you use to connect across the amp have the shielding ground at only one side.

[Andrew]

I’ve uploaded a video of what’s going on as well as a couple shots of the wiring. I have more if that’s helpful. As far as I can tell, I’ve wired this up right. I’m a bit concerned about the voltage but my PT doesn’t have any other wires to drop the voltage as the instructions allude to. I also do not have a different 5Y3 to try out. Any thoughts? I can keep playing it on lower volume/tone but where’s the fun in that?

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[Andrew]

I guess my video was too big so I posted it to youtube. Any ideas??
https://youtu.be/KgACmyqb7vI

[Andy]

Well, that can be a number of things. It could be a cold solder joint somewhere. It could also be the extra gain pushing the tube into self oscillation. I also can’t quite tell from the picture where you have the twisted black and white wires that cut across the amp. Those could be creating an impedance or cross-talk issue.

Often, when I have an issue like this. I use a set of chop-sticks with one hand behind my back and probe all the solder joints and slightly move wires to see if that quiets it a bit.

But we know that voltages are a bit high, so my guess is that it’s going into self-oscillation when dimed.

Also suspect might be the wire that goes from the tube to the tone stack. How do you have that wired?

[Robin]

I agree with Andy, it’sounds like it might be parasitic oscillation. Can you post a photo of the whole chassis? Pushing the leads around with a chopstick might reveal a problem.

[Author]

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