Fighting thermal noise in tubes - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: October 01, 2022, 09:36:24 pm
content: While delving into more and more sensitive circuits I've started to have big problems with thermal noise from tubes, what are some ways to try to reduce it?Reducing the heater voltage didn't helped a bit in my experience but still was pretty noisy, maybe adding negative feedback? elevating the heater supply? does the anode impedance matter at all? or does the grid impedance matter?I really have no clue how exactly the noise gets produced, is it from the cathode or the grid being heated by the cathode?are there tube types that have inherently lower noise? i.e. are pentodes less noisy when wired up as triodes?the noise I'm talking about is a static noise, apparent in every tube I have tested, some are noisier, some quieter, but they all have that static which I'm pretty sure is thermal noise.also, I've read that frame grid tubes are noisier than normal grid tubes, but in my testing it was the opposite, a pair of Tesla E83CC tubes I have are the quietest tubes I have, but I have 6N23P which are also frame grid that are noisy so maybe it's just a higher quality tube and that's why...

author: TimFox
author_rank: Super Contributor
timestamp: October 01, 2022, 10:17:27 pm
content: Two sources for actual noise (not hum or microphonics) in tubes.A long discussion:https://static1.squarespace.com/static/5f0983034219c6086e8100ac/t/5f457a92bb0b1537ebdead39/1598388884791/Chapter+13+NOISE.pdf(1) There is inherent random noise from the flow (modulated by space charge) of discrete electrons.The textbooks tell us that the equivalent noise resistance (a fictional resistor in series with the grid) decreases with transconductance:RN= 2.5/gm, approximately, for a triode, in the audio range.(2) There is also "excess noise" due to strange effects on the cathode surface, which are not so predictable, but the spectral density goes as 1/f.It is also called "flicker noise" or "pink noise".At higher frequencies, there is also induced grid current noise, due to impulsive effects from the electrons speeding past the grid wires.The reports of higher noise for frame-grid tubes may relate to microphonics, due to the closer grid-cathode spacing (compared to conventional grid construction).Otherwise, frame-grid tubes usually have higher transconductance than others.Pentodes also suffer from "partition noise", due to the random fluctuation of the cathode current into its two sinks: plate and screen.Connecting screen and plate gives almost the same behavior as a true triode.There are possible defects in individual tubes, due to construction or deposition of unwanted material in annoying locations, over and above the inherent noise.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 09, 2022, 12:04:08 pm
content: Heat is noise.Use transistors....

author: jonpaul
author_rank: Super Contributor
timestamp: October 09, 2022, 05:00:26 pm
content: Bonjour à tousVery well researched in 1950s..1960s.Especially by BTL for undersea cables and high end hi-fi eg the highly prized 1965 Marantz 7C preamp.the tube choice, circuits can be optimized for low noise.See book, Noise Reduction by Henry Ott, and other classic textes, also work by Syn Aud Con, AES etc.Some high end hi-fi afficanados believe in cryogenic process of the tubes, like Pearl Audio.Finally noise measurement is dependent on the instrument noise floor and bandwidth, also the weighting curve, eg A weighting C weighting, flat.See the excellent books and app notes on noise measurement of General RadioBon chanceJon

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2022, 07:14:15 pm
content: Quote from: TimFox on October 01, 2022, 10:17:27 pmThat immediately suggests selecting a tube type for high transconductance.Other than that, some tubes were deliberately designed to be low noise.Secondary emission should be a noise source, and can be reduced with lower plate voltage.

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2022, 07:22:06 pm
content: Some of the tubes designed for low noise, e.g. 7025 variation of 12AX7, were probably optimized for low hum pickup from the heater structure.To reduce secondary emission from the control grid, the 6AK5 used gold-plated grid wire.The frame-grid tubes, including Nuvistors, achieved high transconductance with close cathode-grid spacing.Nuvistors had high transconductance at relatively low plate voltage and heater power

author: jonpaul
author_rank: Super Contributor
timestamp: October 09, 2022, 08:59:28 pm
content: Fighting noise implies a struggle, against a new or unknown threat.Generation and surpression of noise has been well charicterized back to 1920s eg Nyquist, Blackman, Claude SannonFine examples of low noise preamps with tubes are in dynamic microphone preamps, and moving coil phono preamps.Great audio designers, like our old friends, Sid Smith at Marantz, and Rupert Neve.they used a shielded input matching transformer, selected tubes in low noise configurations, DC fillament power, to achhieve professional low noise audio.The schematics should be easily availableJon

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2022, 11:49:37 pm
content: Two more recent monographs on low-noise audio amplifiers with tubes:Merlin Blencowe, "Designing High-Fidelity Tube Preamps" (self-published) 2016. Chapter 5 deals with hum, noise, and microphony. This book (listed as 'Valve Preamps' in some listings) is invaluable and should be in every tube person's library.Burkhard Vogel, "The Sound of Silence", Springer 2011 is a very heavy (literally) book, somewhat overpriced, with lots of detailed calculations in Mathcad. It tends to use the E88CC/6922 in his examples, and concentrates on RIAA stages. Chapter 7 discusses noise in tubes, and includes a table of selected tubes (with calculated noise parameters and some datasheet noise values). In his table 7.1, the best tube is the E810F/7788 pentode, which is slightly quieter when triode-connected (1.13 vs. 0.97 nV/rtHz). The E88CC or PCC88 is roughly twice that noise density, so he likes to use two triode sections in parallel to pick up a factor of 2-1/2.Unfortunately, both books do not discuss or list any Nuvistors.

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2022, 06:30:46 am
content: Rebonjour,The fine German mic firms like Gefell/Neumann, Senheisser, etc make mic preamps and condenser mics using tube and solid state preamps.Some of the classics are known for low noise, sure schematics and specs are avail.The old HP analog AC millivolt meters like 3400A, could measure very low noise inputs, the pre 1970s were tube (nuvistor?) based.Finally the classic MMC phono preamps like Marantz 7CJust a few tips!Jon