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Old 22nd February 2012, 08:00 PM   #11
DF96 is offline DF96  England
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You would normally have switching before the volume control, so any clicks are attenuated. Pulling plugs can create problems because phono (RCA) usually connects/disconnects the signal and ground in the wrong order.

An amp should run OK with no input and just a little buzzing.
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Old 22nd February 2012, 08:56 PM   #12
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by kingneb View Post
A tube amp has the potential to oscillate if there is no input connected or a load on its output. I am aware of this but many people are not.
You shouldn't have this problem. If you do, then you need to fix your open loop design before you add the NFB. (One project I did showed a weak oscillation at ~100KHz with the input unloaded. A grid stopper fixed that problem.)

The only project that oscillated when running with an unloaded input was the audio deck for a TRF longwave receiver that had a couple of op-amps and 120dbv of audio gain. Without the CW BPF switched in, it oscillated at 2500Hz. That stopped with an input load. Given the extreme gain, that (and microphonics) were unavoidable. (And one reason why superhets replaced TRF's: gain distribution between IF and AF.)
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Old 22nd February 2012, 09:05 PM   #13
BRSHiFi is offline BRSHiFi  United States
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My KT88 PPP amplifier resonates when I connect a 4 ohm speaker to the output. This was with the input shorted or a source connected. The frequency was about 310kHz. Its way up there. The 8 ohm dummy load presented no oscillation.

I wonder if that was coming from the speaker winding resonating?

As soon as I can I will try it with other speakers and see what happens.

Last edited by BRSHiFi; 22nd February 2012 at 09:27 PM.
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Old 22nd February 2012, 09:50 PM   #14
DF96 is offline DF96  England
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Either output stage instability, or global feedback loop instability. You need to do some reading.
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Old 22nd February 2012, 09:58 PM   #15
Chris Hornbeck is offline Chris Hornbeck  United States
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It's becoming popular here on DIYAudio to install quenching diodes from the output valves' anodes to ground. This seems to be enough to protect the output transformer from misadventure, and has no important ill effects. Especially with pentode outputs, DIY amps have been known to oscillate without load, and some commercial ones too.

All good fortune,
Chris
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Old 22nd February 2012, 10:01 PM   #16
Wavebourn is offline Wavebourn  United States
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Amplifier Saftey
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So a well designed amp should allow a user to pull an input jack and/or switch it, while being on, without the amp buzzing during the process?
Right. But plug/socket pair must be made such a way that ground is connected before input, or/and both devices are properly grounded.
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Old 22nd February 2012, 10:03 PM   #17
Wavebourn is offline Wavebourn  United States
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Quote:
Originally Posted by kingneb View Post
My KT88 PPP amplifier resonates when I connect a 4 ohm speaker to the output. This was with the input shorted or a source connected. The frequency was about 310kHz. Its way up there. The 8 ohm dummy load presented no oscillation.

I wonder if that was coming from the speaker winding resonating?
Most probably it is caused by wrong ground connections in the amp.
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Old 22nd February 2012, 10:05 PM   #18
Wavebourn is offline Wavebourn  United States
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Quote:
Originally Posted by Chris Hornbeck View Post
It's becoming popular here on DIYAudio to install quenching diodes from the output valves' anodes to ground. This seems to be enough to protect the output transformer from misadventure, and has no important ill effects. Especially with pentode outputs, DIY amps have been known to oscillate without load, and some commercial ones too.
I did such thing to guitar amps that I repaired. No more complains on damaged output transformers after that.
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Old 22nd February 2012, 10:11 PM   #19
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by kingneb View Post
My KT88 PPP amplifier resonates when I connect a 4 ohm speaker to the output. This was with the input shorted or a source connected. The frequency was about 310kHz. Its way up there. The 8 ohm dummy load presented no oscillation.

I wonder if that was coming from the speaker winding resonating?
How many secondary tap-offs? It's highly doubtful that it's speaker self resonance that's doing it. One common way to connect gNFB to the OPT secondary is to make the connection to the highest secondary tap-off, not necessarily the same one to which you connect the speaker. It's something I avoid with my own projects when using stock OPTs that include a variety of secondary tap-offs. It's a convenience for the end user, not having to move the gNFB pick-off point when connecting speeks.

Nyquist Criterion: "The number of unstable closed loop poles is equal to the number of open loop unstable poles plus the number of encirclements of the point: -1 + j0". Of course, you avoid open loop unstable poles unless it's your intention to make an oscillator.

That makes for stability problems since the unused portion of the secondary can have parasitic reactances that are unloaded. Also, connecting speeks with different impedances can change the magnitude and/or the phase of the feedback signal. That can lead to hitting that nasty -1 + j0 point, or coming too close to it (which gives what can be problematic increases in gain over a narrow range of frequencies).
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Old 22nd February 2012, 10:12 PM   #20
Rundmaus is offline Rundmaus  Germany
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Quote:
Originally Posted by Chris Hornbeck View Post
It's becoming popular here on DIYAudio to install quenching diodes from the output valves' anodes to ground. This seems to be enough to protect the output transformer from misadventure, and has no important ill effects.
Do spark gaps of appropriate trigger voltage do the job equally well? I would also like to protect my OPTs, but if possible without giving up the 'no semis' design idea...

Greetings,
Andreas
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