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Registered Member #1412
Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278
This tube power amp is a kit.After I finish it,I the amp is self oscillating.Then I found I forgot to ground the secondary of the output transformer.After I connect them to ground,the self oscillation stop.Why?
Then I test it by connect it to my signal generator and scope the output.When I put square wave into it,I get something like triangular wave on the output when frequency is higer(10KHz).someone tell me this is normal because vacuum tubes are slow.But I have seen other people test their tube amp with square wave and it didin't deform that much even at 100KHz.I also scope pin 6 of the 12AX7, and it already deformed there.How can I fix it?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
hsieh wrote ...
When I put square wave into it,I get something like triangular wave on the output when frequency is higer(10KHz).someone tell me this is normal because vacuum tubes are slow.But I have seen other people test their tube amp with square wave and it didin't deform that much even at 100KHz.I also scope pin 6 of the 12AX7, and it already deformed there.How can I fix it?
This amp probably isn't designed to have a wide frequency response. Your EL34 is running in ultralinear mode, so it will have a lot of input capacitance because of Miller effect. The 12AX7 has a high output impedance, and won't be able to drive that.
It's very rare to see a tube audio amp that can pass a 100kHz square wave. Even if the tube stages could, the output transformer couldn't.
And it's not really necessary. I'd be happy if it could pass the 3rd harmonic of a 7kHz one.
Why did it self-oscillate: Do the wires from the OPT secondary to the speaker terminals run near the input part of the circuit? Capacitive coupling between the OPT primary and secondary could put a high voltage on the secondary if it wasn't grounded. This could then couple back to the input wiring.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Ground loops, make sure that everything i grounded in such a way that it does not form an antenna. My own EL34 amp had oscillation from a circular grounding that was very good at picking up noise, I solved 95% of that problem by twisting all groundwires and have it go down stream from opt to amp to psu to chassis. Another oscillation problem is floating input or ground potential differential between amp and input device, so for now i only use my laptop as input to avoid grounding the amp through the source.
Registered Member #1412
Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278
Mads Barnkob wrote ...
Ground loops, make sure that everything i grounded in such a way that it does not form an antenna. My own EL34 amp had oscillation from a circular grounding that was very good at picking up noise, I solved 95% of that problem by twisting all groundwires and have it go down stream from opt to amp to psu to chassis. Another oscillation problem is floating input or ground potential differential between amp and input device, so for now i only use my laptop as input to avoid grounding the amp through the source.
Mains earth is not avaliable here.So I only connect everything that need to be grounded to negative rail of the circuit.Is this OK?
The price of this amp is approximately 200USD.Is this price reasonable?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It looks like a reasonable kit for US$200, yes. The negative rail of the circuit is "ground" as far as the audio signals are concerned. It should be connected to the metal casing.
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EL34 tube power amp problem
