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Valve questions from a total numpty

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Drewan77 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Drewan77 Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2016 at 2:35pm
Originally posted by Graham Slee Graham Slee wrote:

Originally posted by morris_minor morris_minor wrote:

Will this work in my amp?


...The device is for artificial insemination of cattle!...

It won't work in Bobs' amp but it might still produce moo-sick, ouch Shocked
Older than I once was, younger than I'll be
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2016 at 4:15pm
Originally posted by BackinBlack BackinBlack wrote:

And there was me thinking it was a Roentgen tube....


Possibly the right answer but x-rays aren't as much fun! Wink
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2016 at 6:04pm
Not got a clue about AI, but each to his own....
As for X-rays not being fun, remember those machines in shoe shops where you stood with your toes in an aperture and looked down into a screen to see your feet (bones) within your shoes?


Edited by BackinBlack - 24 Jan 2016 at 6:07pm
Just listen, if it sounds good to you, enjoy it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Martin1 Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2016 at 7:03pm
Yes I do ,which shows both our ages . They still got my size wrong so was not exactly fool proof
still wondering why the nurse is smiling ,must be my natural innocence .






Martin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2016 at 9:10pm
When I last logged in we were talking about anodes and cathodes. I come back and it's bovine gynaecology and proctology . Yes it's good to share experiences but I thought we'd be onto cathode bypass caps or something similar!

Jon

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2016 at 7:04am
Originally posted by Fatmangolf Fatmangolf wrote:

When I last logged in we were talking about anodes and cathodes. I come back and it's bovine gynaecology and proctology . Yes it's good to share experiences but I thought we'd be onto cathode bypass caps or something similar!



Yes, it has gotten out of hand a little and I must admit my sense of humour got the better of me, but the question was asked LOL

Now, cathode bypass caps are an interesting subject too: They're there to couple the cathode to ground for ac signals. At DC the resistor is required to lift the cathode voltage which makes the grid voltage negative with respect to the cathode. This biases the anode to somewhere between HT and ground. Just the right amount of voltage cathode to grid will bias the anode at half HT voltage allowing it to swing the full ac signal between HT and ground.

If the bypass capacitor weren't there the gain would be reduced and would be the ratio of anode and cathode resistors (given a proper HT rail which many "new" designs don't), so for maximum gain from the valve, the cathode bypass capacitor is required.

It needs to be large enough to do the lowest bass frequency expected and the formula F=1/2pi.RC is used for that. Rearrange to give C = 1/2pi.FR

This particular bypass capacitor is where a lot of tweakers spend a lot of effort on. They seem to try lots of different types of capacitors and often bypass the bypass cap with all sorts of wonderful schemes.

Instead, why not just put a high voltage GP 85 degree electrolytic there? 63V to 100V should work really well. Just ensure it's given sufficient ventilation to keep it cool. Beats Blackgates IMO!


Edited by Graham Slee - 25 Jan 2016 at 7:07am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2016 at 8:22am
The "story" of the valve (tube) is related quite well here:

http://www.allaboutcircuits.com/textbook/semiconductors/chpt-13/the-triode/
http://www.allaboutcircuits.com/textbook/semiconductors/chpt-13/the-tetrode/
http://www.allaboutcircuits.com/textbook/semiconductors/chpt-13/beam-power-tubes/
http://www.allaboutcircuits.com/textbook/semiconductors/chpt-13/the-pentode/
keep clicking next page for more in-depth stuff.

The very first page shows that valves have the same problems as transistors... capacitance.

However, the Tetrode and Pentodes came into being to "solve" the problem whereas different techniques are used with transistors.

Those techniques do not work for high powered high frequency circuits (like some radio transmitter power amps) where valves hold the monopoly.

A good page for determining bias and gain of valves is: http://sound.westhost.com/valves/bias-gain.html
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