why do many tube microphones use triode connected pentodes instead of triodes? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: February 17, 2022, 07:42:48 pm
content: I see a lot of tube mics that use triode connected pentodes instead of just triodes, for example the legendary Neumann U47,the capacitance between the control grid and screen + plate is around the same as that of triodes..., so is there a reason for this?

author: TimFox
author_rank: Super Contributor
timestamp: February 17, 2022, 08:43:22 pm
content: Often, the choice of a triode-connected pentode instead of a regular triode is based on availability of a particular set of parameters.Triodes are usually preferred for low-noise applications because they do not have "partition noise" that occurs in tetrodes and pentodes due to the cathode current randomly dividing between screen and plate.Usually, the only downside of a triode-connected pentode is that the screen-cathode voltage rating is typically lower than the plate-cathode voltage rating of the pentode, and thus possibly lower than that of a true triode of similar dimensions.With a pentode, in either connection, thegmreferred to cathode current depends almost entirely only on the screen voltage, where the dependence is that of a triode.The original Neumann U47 selected individual Telefunken VF-14s for noise, grid current, and its good microphonic behavior (only one microphone in the package, please!), but that tube went out of production. Neumann operated it at lower than normal plate and heater voltages to extend its lifetime.

author: TimFox
author_rank: Super Contributor
timestamp: February 17, 2022, 10:52:12 pm
content: Neumann and Telefunken were different companies when the U47 was introduced. The VF14 predated the microphone, and was used for "normal" things during WWII. There are interesting websites dedicated to it that point out the strange voltage environment back then for radio uses.The U47 was distributed in the US by Telefunken early on.A good reason to use the VF14 instead of a nominal 12.6 V tube such as the 12AV7 was the high-voltage, low-current heater, a good match for 48 V DC "phantom power" (reduced voltage to increase life). With a triode mu of about 30, the tube could also operate with a plate supply of 48 V. (The cut-off grid voltage of an ideal triode is minus the plate voltage divided by mu, and grid voltages less negative than, say, -1 V result in grid current.)Another important reason is microphonics: look at the construction of the VF14. 9-pin miniature tubes such as the 12xx7 series have a more microphonic construction.They went to a 13CW4 Nuvistor (13.5 V at 60 mA heater) later when the VF14s went to Valhalla to play with the other obsolete tubes, but with controversial results.Linearity may not have been of primary importance due compared with the non-linearity and relatively low AC output voltage of the capsule.

author: langwadt
author_rank: Super Contributor
timestamp: February 17, 2022, 11:40:27 pm
content: Quote from: ELS122 on February 17, 2022, 11:14:53 pmgoogle to the rescue,https://www.telefunken-elektroakustik.com/product/m-940h-tube-microphone-power-supply-u47-u48wiki claims 48V phantom power came with the transistorized KM8448V through two 6k8 resistors wouldn't power much of a tube ..

author: TimFox
author_rank: Super Contributor
timestamp: February 17, 2022, 11:55:34 pm
content: Maybe not phantom power, but still the same DC voltage for plate and heater supply.There is no reason why a triode and a triode-connected pentode (all other things being equal) would be different in microphonics.Microphonics in tubes depends on the construction, and physically smaller devices may be more prone to vibration due to less space for bracing, etc.There is anecdotal evidence (which I have no reason to doubt, but no evidence to cite) that octal tubes, with their heavy plastic bases wired to a glass stem (such as the 6SL7) are less microphonic (again, all other things being equal) than 9-pin miniature tubes (such as the 12AX7) which have rigid pins through the glass base going directly inside to support the guts.This webpage, from a manufacturer of a replacement module for the VF14, has an interesting essay on the history of the original:http://www.phaedrus-audio.com/VF14M_story.htm

author: TimFox
author_rank: Super Contributor
timestamp: February 18, 2022, 02:32:57 am
content: There is no E field between the screen and plate (in triode connection) so the extra moving parts are not so important.The frame grid tubes (including the Nuvistors) come much later.An excellent frame-grid pentode (in 9-pin miniature) was the E180F, which was used in triode connection for lower-noise applications as well.Back in the WWII American radar development, triode-connected 6AK5s were popular for low-noise.There were a lot of tube models over time, but I don't know of any 50 V heater signal-level triodes. Higher voltage heaters (with correspondingly lower current) were easier to power with DC (for low hum) before modern semiconductor voltage regulators.If you need a high grid-leak resistor, obviously you want low grid current, which is a very interesting topic that I have looked at intermittently over the years. It's easier to obtain with a lower (or medium) mu triode, since that uses a more negative grid bias for a given plate voltage and current. The thing not to do is use a high-mu triode (like half a 12AX7) at low plate voltage, since that requires a less-negative grid with correspondingly higher grid current.Anyway, the article I referenced gives some interesting history about the VF14 tube before the Neumann microphone use.

author: CaptDon
author_rank: Super Contributor
timestamp: February 18, 2022, 04:24:12 am
content: I haven't looked at any specific characteristic curves lately but I thought the deal on triode connected tetrodes or pentodes had the downside of less gain but the upside of more linear transfer across a wider range of plate and grid operating voltages and the reduced stage gain showing a better signal to microphonic noise ratio and possibly less thermal noise contribution?

author: S57UUU
author_rank: Regular Contributor
timestamp: February 18, 2022, 03:04:49 pm
content: If I remember right, ECC88 was a high frequency double triode designed for TV tuners (which of course doesn't mean it didn't work at audio frequencies). EF180 (a succesor to EF80) was mainly for TV IF amplifiers. Because of their TV roots, these were very common tubes 50 years ago.The dedicated audio pentode was EF86, in preamp stages often mounted on rubber vibration isolation.E88CC was just a long life variant of ECC88, etc. Just my 2 cents...P.S.I read the Phaedrus article, can really recommend it. The guys know their stuff.

author: TimFox
author_rank: Super Contributor
timestamp: February 18, 2022, 05:26:30 pm
content: Many RF signal tubes of that era are "remote-cutoff" or "variable mu", designed for good AGC characteristics, and are not as linear as one would like for audio purposes. Allegedly, in the post-vacuum-tube era, some 6ES8/ECC189 dual triodes, pin-compatible with the 6DJ8/E88CC, were re-labeled 6DJ8, but the 6ES8 is a "semi-remote-cutoff" construction designed for use in RF cascode input amplifiers.