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Old 22nd February 2012, 04:49 PM   #1
BRSHiFi is offline BRSHiFi  United States
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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.

What type of protection circuit should I add to prevent the potential of oscillation with...

Case A. No speaker connected.

Case B. No input connected.

The amplifier is a KT88 push pull parallel unit. Case B would be easy if RCA shorting jacks existed (if they do, feel free to point me to a source).
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Old 22nd February 2012, 05:04 PM   #2
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You can put a sparkgap between the two anodes of 2,4 x Va.
You can put controled avalance diodes from the anode to ground (also 2,4 x Va)
You can put a high power low ohmic resistor at the secondary side of the outputtransformer, switchable when the speakers are connected.
etc. etc.etc.
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Old 22nd February 2012, 05:07 PM   #3
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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.
I am not.

But I am aware that output transformers without load can be damaged.
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Old 22nd February 2012, 06:20 PM   #4
BRSHiFi is offline BRSHiFi  United States
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I have always been under the impression that the oscillation would take out the transformer, not the lack of a load itself.
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Old 22nd February 2012, 06:52 PM   #5
DF96 is offline DF96  England
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The reason many people are not aware of this is that it is not generally true. Valve amps can be damaged by being run with no output load but it is not oscillation but hard limiting which creates the problem. If you can't guarantee a load will always be present you will have to add a Zobel network.

Oscillation with no input connected is probably a sign of poor layout, and possibly poor design with marginal stability. Don't add a 'protection circuit'; correct the design.
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Old 22nd February 2012, 07:00 PM   #6
BRSHiFi is offline BRSHiFi  United States
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My power amp buzzes badly with no input, but no oscillations. Don't pull the plug while it is in operation!
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Old 22nd February 2012, 07:07 PM   #7
GloBug is offline GloBug  Canada
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Pretty much all amplifier stages "buzz" without an input, SS or tube to some degree.

The exception may be in consumer products with integrated procedures to stop this, or in lower gain amplifiers it may be more or less inaudible.
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Old 22nd February 2012, 07:19 PM   #8
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Quote:
Originally Posted by kingneb View Post
My power amp buzzes badly with no input, but no oscillations. Don't pull the plug while it is in operation!
I bet it is, but you're not looking in the right place ! If it buzzes, check unscreened input leads and make sure ones lab bench light doesn't have a flourcesent tube. A well designed amp should be able to accept hot input switching between an o/c and a dummy input load should be reflected in a slight noise level difference. The exception is when those nasty RCA input plugs are used.
I'm just finalising my powerful 250+250W amp; no way would I allow such problems !

richy
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Old 22nd February 2012, 07:24 PM   #9
DF96 is offline DF96  England
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One way to get buzzing when no input is connected is to have a huge input capacitor (too high in value, possibly audiophile boutique component) which acts as one plate of a capacitor. The other plate is every nearby conductor, including mains supply, rectifiers etc.

Try a smaller capacitor. Mount it nearer the chassis. Put a few pieces of grounded wire over the top of the cap to act as a crude electrostatic screen.
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Old 22nd February 2012, 07:42 PM   #10
BRSHiFi is offline BRSHiFi  United States
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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?
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