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Old 17th May 2011, 01:29 PM   #1
M Gregg is offline M Gregg  United Kingdom
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Default Switchable SE.

Just a thought,

Has any one ever used switchable tubes in a SE or PP power amp.
Talking HIFI here.

For example, 3 X EL34 or EL84 SE and being able to switch off 1 or 2 of the EL34's etc to reduce power when not needed.

My question is the primary impedance of the O/P Tx and the change in load conditions / feed back. If this was cathode feedback and the EL34's were connected to the O/P Tx feedback winding as the common Gnd connection.

I guess the tubes would have to be fixed bias.

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Old 17th May 2011, 02:26 PM   #2
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The biggest problem would be the DC current in the OPT, which will cancel out within the primary winding when in PP mode, but not so in SE mode, causing core saturation.

On a more abstract level, it doesn't sound like a particularly good idea to me. An amplifier, like any artifact of engineering, is a compromise that intends to achieve the best possible performance on a set of predetermined parameters. Starting out with two sets of potentially conflicting parameters sounds like a sure-fire way to get a crappy compromise for either set of parameters, or a good compromise for one set and a really bad one for the other.

If you need less power, then just install a volume pot.
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Old 17th May 2011, 02:32 PM   #3
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If you're doing parallel single ended then the primary of the OPT would have to change -- unless you use one transformer per power valve and then series the secondary windings (4R + 4R for an 8 ohm tap when in PSE mode). Then one whole section of the circuit could shut off for night-time listening, etc.
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Old 17th May 2011, 02:40 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Thoughts are to shut the tubes down with Bias.

I know the other way is to triode connect to cut power.

I know that the the primary impedance is normally set for the tube, however how close has it got to be EL34 2.5K - 5k.

The other issue is cathode poisoning. (heater power off).
Its just a talking point at the moment.

Thoughts are 3XEL84 per side. Just for a fun test.

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Old 17th May 2011, 02:57 PM   #5
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Quote:
Originally Posted by M Gregg View Post
Thoughts are 3XEL84 per side. Just for a fun test.
How would 3 tubes work in a PP setting?
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Old 17th May 2011, 03:00 PM   #6
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by mastodon View Post
How would 3 tubes work in a PP setting?
I am thinking SE.


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Old 17th May 2011, 03:04 PM   #7
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Oh right, now I see I read your post wrong all the time. I thought you were trying to develop a concept that would switch between PP and SE.

What would the purpose be of switching off power tubes? Increase THD? Create impedance mismatch problems? I can see why this could be desirable in a guitar amp, but for hifi, I don't really see the application.
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Old 17th May 2011, 03:20 PM   #8
M Gregg is offline M Gregg  United Kingdom
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Its just an interest,

As you say guitar is an application.
However I was interested in powering off tubes and the problems linked to it.


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Old 17th May 2011, 04:32 PM   #9
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The easiest way to switch off an output valve is disconnect the heater. Unless you have silly HT voltages, no harm in it sitting there cold. Cathode poisoning occurs when the valve is conducting, but with temperature-limited emission i.e. cathode warm but not hot enough for normal space-charge-limited emission.
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Old 17th May 2011, 04:45 PM   #10
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by DF96 View Post
The easiest way to switch off an output valve is disconnect the heater. Unless you have silly HT voltages, no harm in it sitting there cold. Cathode poisoning occurs when the valve is conducting, but with temperature-limited emission i.e. cathode warm but not hot enough for normal space-charge-limited emission.
Thank's,

Do you see any issues with heater power up or down while another parallel tube is conducting. i.e. no output just idle current?

So in theory you could have a number of tubes sitting idle and select lets say an el34 or a 6550 and power up the heater to a tube.(As long as the bias was correct and OP Tx matched). So an "A" or "B" select. Or in a multi channel amp you could select just the channels required.


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