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Old 18th August 2008, 07:44 PM   #1
Svein_B is online now Svein_B  Norway
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Default Parasitic oscillation? (picture)

Hi!

The subject is a KT88 PP amp. Slight buzzing sound with ear to speaker, especially from the tweeter. On the ouput I see some spikes every 10ms, 20mV p-p. B+ looks clean, sawtoth 2V p-p.

At the anodes I get the scope image below. A spike in a fixed position every 10ms. Zooming in reveals a damped ringing of around 65KHz. Position of pentode/triode switch makes no difference. The same for all 4 tubes.

Is this how parasitic oscillation is supposed to look?

Do I need to put screen stoppers right on tube sockets?

SB.
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Old 18th August 2008, 08:10 PM   #2
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Offhand, I'd think it would be switching spikes from the diodes. You can probe around a bit to confirm. Parasitics are usually higher in frequency.

Yes, stoppers should be right at the socket, as close to the pin as possible.
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Old 18th August 2008, 08:22 PM   #3
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Try a 5 nF 1KV ceramic cap across each diode - looks like diode switching noise to me.
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Old 18th August 2008, 10:01 PM   #4
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Default Re: Parasitic oscillation? (picture)

Quote:
Originally posted by Svein_B
Hi!

At the anodes I get the scope image below. A spike in a fixed position every 10ms. Zooming in reveals a damped ringing of around 65KHz. Position of pentode/triode switch makes no difference. The same for all 4 tubes.

Is this how parasitic oscillation is supposed to look?
No. Those pulses are occurring at a 100Hz rate, so they're coming from the PS. Parasitics either occur as a continuous oscillation that would smear the trace at that horizontal rate, or they'd occur as damped oscillations at the point where the finals cutoff.

Quote:
Do I need to put screen stoppers right on tube sockets?

SB.
Yup.
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Old 19th August 2008, 06:37 AM   #5
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SB,
As the guys above have said - definitely High Voltage power supply diode switching noise. You note it happens just after the voltage peak when the power supply diodes switch OFF.

Given that its provbably a +450 to +500V supply and the diodes see 2 x Vpk of the AC voltage I would actually use 2KV ceramics rather than the 1kV suggested by Tom and my preferred value is 10nF but thats minor detail.

In an effort to treat the disease and not just the symptom I would first fit Ultrafast Soft Recovery power diodes. Again guessing from your amp description you are probably using 1N5408 or similar - try UF5408.

Cheers,
Ian
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Old 19th August 2008, 06:57 AM   #6
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My instinct is to look over the power supply circuit earthing, is it long and where is the input stage wiring in relation to this ? The global negative feedback (if you are using it) wiring from the o/p tranny can be often the most closest to the AC power side and picking the hash up. I always use screened cable for this.
Can't rule out pickup via the heater wiring to input stage, is this earthed ?
In contradition to other users regarding diode types, I've never had this problem.

richj
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Old 19th August 2008, 07:04 AM   #7
Svein_B is online now Svein_B  Norway
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Thanks guys!
I kind of suspected power supply noise, but got a little uncertain since I could not see any spikes on the 420V B+

Output is silent without driver tubes inserted, but comes back with 12AU7 driver tubes (without input tubes).

I will first try some ceramic 5nF/3KV caps across the 1N5408 diodes, and later replace with UF diodes if this proves to be the source.

SB.
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Old 19th August 2008, 07:25 AM   #8
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I have not had that much experience to date with tubes, but i've seen this before on the last amp i built. It turned out to be noise coming in from the heater feed circuit. It was also present on the prototype i constructed before the main amp. In both cases i cured it by filtering the heater supply. It may not be the cause of your noise but it's worth considering. The sawtooth part is deffo your PSU regulation and that will be audiable as a 100Hz buzz.

Leigh
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Old 19th August 2008, 07:43 AM   #9
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Don't waste your time snubbing PSU rectifiers - this is just a nice plain, clean power supply ripple.

The AC waveform component on your B+ should look identical (just higer p-p amplitude).

Pity it is ending up on the output - the problem is a poor PSRR.
Could be down to numerous issues - schematic?


Cheers,
Glen
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Old 19th August 2008, 12:11 PM   #10
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Quote:
It turned out to be noise coming in from the heater feed circuit.
Was the heater circuit powered by a winding on the same transformer as the HV supply? That's a common source of switching noise coupling.
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