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Does Valve Noise Reduce if you Reduce Plate Current?
Does Valve Noise Reduce if you Reduce Plate Current?
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Old 23rd April 2019, 05:58 PM   #1
primalsea is offline primalsea  United Kingdom
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Default Does Valve Noise Reduce if you Reduce Plate Current?

Just as the title says. If you reduce the current through a triode would this reduce the shot/flicker noise?
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Old 23rd April 2019, 06:51 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Triode or pentode?

I read that a guy called Robert several decades ago discovered that noise was reduced reducing the heater/cathode temperature and then the illumination from it. Noise in tubes came from various sources, mechanical, electrical and perhaps, chemical.
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Old 23rd April 2019, 06:54 PM   #3
primalsea is offline primalsea  United Kingdom
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Triode or pentode?

I read that a guy called Robert several decades ago discovered that noise was reduced reducing the heater/cathode temperature and then the illumination from it. Noise in tubes came from various sources, mechanical, electrical and perhaps, chemical.
Thanks. Triode as mentioned in the post .
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Old 23rd April 2019, 07:09 PM   #4
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Noise in tubes has entire books on the topic. It is a very complex theme. If you have time to read, I can suggest the Radiotron, from Langford Smith and is easily downloadable for free in the web. Mathematics of it is well outside of the forum, and from my brain :-) .

Cascodes are by its own nature, high gain and low noise, in DC, audio and UHF.
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Old 23rd April 2019, 07:16 PM   #5
tschrama is offline tschrama  Netherlands
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shot noise increases only with sqrt (current). So if signal scales with current, SNR increase with sqrt(current) as far as shotnoise is concerned.

Triode noise is dominated by the Equivalent Input VoltageNoise, with scales (?) with 1/transductance (IIRC) .. ans since transductance increases with current, triode noise should be lower with higher current.
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Old 23rd April 2019, 08:25 PM   #6
MarcelvdG is offline MarcelvdG  Netherlands
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tschrama is correct about a triode's white noise: higher current means higher noise current at the anode, but lower equivalent noise voltage at the input and higher signal to noise ratio.

However, most valves have substantial 1/f noise at audio frequencies. The equivalent input noise voltage due to 1/f noise increases with increasing anode current.

So all in all, there is an optimum anode current that depends on the triode type and frequency range and frequency weighting you are interested in.

Merlin Blencowe, Merlinb on this forum, wrote an interesting AES paper about it. There is a thread about it:

Flicker Noise dominates triode noise in audio (AES)

Last edited by MarcelvdG; 23rd April 2019 at 08:31 PM.
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Old 24th April 2019, 06:27 AM   #7
primalsea is offline primalsea  United Kingdom
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Thanks. Heavy reading but I’ll fight through it.

The other thing that confuses me is that noise reduces as you reduce heater voltage. This document suggests otherwise, although it is aimed more at higher frequencies going up into RF. My concern is with audio frequencies.

http://www.keith-snook.info/Stuff-yo...20Circuits.pdf
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Old 24th April 2019, 08:08 AM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by MarcelvdG View Post
Merlin Blencowe, Merlinb on this forum, wrote an interesting AES paper about it. There is a thread about it:
Flicker Noise dominates triode noise in audio (AES)
The quick answer:
For each valve type there is an optimum anode current at which SNR will be maximised. This optimum becomes broader for high gm types, so the operating current is less critical for these. Below the optimum anode current, gm falls and EIN will rise. Above the optimum, gm does not increase fast enough to overcome the increasing flicker noise. From my paper I suggest these optimums (optima?):
ECC81 = 1 to 2mA
ECC82 = 1 to 2mA
ECC83 = 1 to 2mA
ECC88 = 3 to 4mA
6J52P = 4 to 6mA
(These are for a 20Hz-20k noise bandwidth. There is a lot of sample variation, of course)

If a valve is operating below 1mA, a small reduction of heater voltage may improve the SNR. Above 1mA the opposite is true; flicker noise dominates and increased heater voltage has a beneficial effect on SNR, but at the inevitable expense of reduced heater/cathode lifetime.

This is covered in my book, too. See page 203:
Designing High-Fidelity Valve Preamps - Merlin Blencowe - Google Books
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Last edited by Merlinb; 24th April 2019 at 08:14 AM.
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Old 24th April 2019, 09:44 AM   #9
DF96 is offline DF96  England
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Increasing heater voltage means the cathode is hotter, hence the cathode space charge is hotter, so you get more noise.

Reducing heater voltage means the opposite, except that it also weakens the cathode space charge which reduces space charge smoothing of shot noise. There is an optimum heater voltage for noise, which may depend on cathode current. It is likely that in many cases the optimum heater voltage for low noise is just below the usual heater voltage.
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Old 24th April 2019, 01:47 PM   #10
primalsea is offline primalsea  United Kingdom
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Super, thank you for that. Summed it al up nicely for me.
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