Low noise, low microphony, low hum valves
The article "A new low-noise circuit approach for pentodes" by Frank Bloehbaum in Linear Audio volume 0 shows that classic audio input stage valves such as EF86 and ECC808 have rather high levels of 1/f noise. This applies especially to the Telefunken new old stock EF86's he measured, these were even far out of spec. High transconductance RF valves are much better, but they are not designed for low microphony and low hum with an AC heater supply.
Do you know of any valves that combine low noise at audio frequencies with low hum and low microphony?
Low-noise? Use triodes. But I am sure u have a good reason to use pentodes so that doesn't help much.
Sorry I'm not much help, I don't know any pentodes that fill your requirements.
I guess everything is relative, and compared to the most silent SS device, even a triode is considered 'noisy' by some people. But I've never ever had any problems with noise due to the tubes themselves, and have come to believe the notion of noisy tubes is another bad rap authored by those who don't use tubes. (Or have only used pentodes).
The article I referred to is about a trick to get rid of the partition noise in pentodes, but my question is more general: do you know any valve, triode, tetrode, pentode, whatever, that combines low noise with low hum and low microphony?
By the way, the article shows that at least for audio frequencies the difference in noise between pentodes and triode-connected pentodes is not that big. For some obscure reason the author sometimes even measures a higher equivalent input noise for a triode-connected pentode that for a normally connected pentode.
Low noise pentodes - take a look at D3A, 6688, 7788/EF810F, C3G, C3M. You should probably plan on heating with DC if low hum is an issue, and these are all variably microphonic - some samples hardly at all, whilst others can be quite microphonic.
Note that the C3G and C3M have fragile filaments and should probably be heated via a CCS and not disturbed in any way until cool.
Practically speaking the difference might be a dB or so..
Direct experience with both the D3A and 7788 indicate that these types are quiet enough whether pentode or triode connected for signal levels down to a mV or so.
The EF86 in my experience is a much noisier pentode than any of the devices I mentioned in my previous post.
I think you will have to select for microphony or design to avoid exciting microphonic tubes. (It can be done)
Are we talking about 1/f noise (from the cathode) or partition noise (white, from the partition of current flow between g2 and anode)? Triode connection of a pentode will not affect 1/f noise, to reduce that you need a better cathode.
Looking for a low noise pentode, try the mullard EF37a, not very used out there so you may get it at a reasonable price...
The only small signal pentode I\'ve used is the E180F, nice NOS version with gold pins and all. Still too microphonic and got swapped with my fav triode.
I\'ve seen some microphone preamps which use a 5879 Pentode.
In the 1960\'s, this tube was used in some very good preamps.
Don\'t know about the availability of the tube today.
valves low noise low microphony
I have built a pile of mike preamps and phono stages and here are a couple of "secrets"
Let\'s talk 60 DB gain, which is where most mike preamp gain stages are when the gain is all the way up, and the phono stage for MM cartridges is the same in the bass region
A. Pentodes are much less noise than triodes at high gain as the screen grid gives pentodes like 5879, ef86, #78, 39/44 and the lost EF40, ef804 and a few other unknowns, and the great 6sj7, 6j7, and 6sk7 pentodes gain levels up to 800, while a realy good 12ax7, can get a voltage gain of 100.
So a 12ax7, running at 60 DB gain, is still much more noise than a pentode at same as the pentode is loafing at that gain level, and designed to have low noise at much higher levels.
I have built several tube mike preamps, on the right side of page one of my old photo album,
Pictures by SalesBoy - Photobucket
you will see a four tube preamp. I built several of these using a 39/44 early pentode, grid cap, and found this tube to be super quiet.
I built two models that used a 39/44 or a EF40 as the first stage, switchable by just turning on the filament of the tube you wanted to run. The sonic and noise difference between a 1939 39/44 and the EF40, a European version of the "EF86 with the right Telefunken style insides" is almost zero.
B. The "lost EF40" and the "EF86 with the right Telefunken style insides" bit is this prroblem with finding low noise tubes.
There have been various versions of any tube made over the years, and by now, most of the better, lower noise, better sonics versions have been collected and used.
I built a series of preamps for a super piezo pickup rig, for testing tube types and preamp circuits to see what version preamp they liked for sonics and noise.
We made one with a 12ay7, and my boss had a friend that had scarfed a pile of 12ay7s that were very special versions and much lower noise than any other.
They might have been made for oh-silly-scopes, HP I think, and that happens that special versions are made for test gear or special equipment, and limited runs.
The EF40 deal is this. I have very super critical studio owners I fix gear for, and some of them have old Telefunken and other German mikes and preamps.
There is a certain version of the E804s and the EF86 tubes that are much less noise and have a "Magic Tone" that no other version has. For a while, these special versions were sold for up to $300 and now are I think, all gone.
The EF40 gag is that you can buy these all day for $5 from ebay, and other than the Euro 8-pin socket, they are usually the best version, plate and grid version of the best EF86.
I have 40, Amperex, Bugle Boy, made in France, NOS in box, EF40s, and I recently tried to sell some, and no interest at all. I just looked and there does not seem to be any out there now, very odd stuff.
But the point is that I have been building gear with 1935 to 1950 tubes, and EF 40s, 6sj7s etc, and building low noise preamps for very little Moo while others fight over a few special rare tubes.
Also, maybe a "C" is that you can also make very low noise preamps by going three or even for stages, running at lower gain levels, where the tubes produce low noise. I know of one great phono stage that used like three different tubes, and ran them at low gain levels, where the tubes had thier best noise region.
A 12ax7 for example, has low noise at about a voltage gain of 30
A EF86, about 100
EF86, E804s, 6sj7, and I have also used 6eh7, 6ej7, have special designed plates and grid structure, with outside shields that give low noise and low microphony
Lots of study, and you need a pile of tubes, different versions to play
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