Ah, the 27 isn't that bad. Are you willing to pair the #27 with another tube, or do you want to only use the #27?
I've built a preamp or two with the 27, and I've used it as a driver tube in some single ended amps. The real pain is the heater requirement for the #27, but there's a nice custom Hammond transformer made to deal with this:
Transformer - Hammond, Power, 350-0-350V, 70-90 mA | Antique Electronic Supply
I've built a preamp or two with the 27, and I've used it as a driver tube in some single ended amps. The real pain is the heater requirement for the #27, but there's a nice custom Hammond transformer made to deal with this:
Transformer - Hammond, Power, 350-0-350V, 70-90 mA | Antique Electronic Supply
Some of them indeed are, but not all. 27 type is probably the most diverse in internal construction of all antique low mu triodes. The version with mesh plate supported by two rods, and later ST are generally not microphonic, although there could be exceptions.
My favorite circuit for low mu triodes is transformer-coupled differential stage biased by common cathode resistor, with common choke between primary CT and B+. There is a number of suitable high quality transformers made by UTC, Triad, Microtran, etc. No semiconductors, no capacitors.
I designed a few line stages with 27s and a power amplifier for clients about 17 years ago, and got so many complaints about microphonic 27s that I vowed never to use that tube again. I have had problems with them as well. In my case we switched to 56s and in some cases I made additional changes to the design to optimize them for 56s. (Got some complaints that the 56 didn't sound as good.. lol)
I am sure there are good ones out there, but my then clients had trouble locating ones that sounded good to them and that weren't excessively microphonic.
People around here all wanted to use globes which contributed to the problems.
I am sure there are good ones out there, but my then clients had trouble locating ones that sounded good to them and that weren't excessively microphonic.
People around here all wanted to use globes which contributed to the problems.
I've always been under the impression that the microphonics problem is mostly limited to the globe versions and seems to be a common problem with other types of globe tubes too. I'm sure someone will correct me if I'm off base here.
I've never built a preamp with the 27 but I've also been thinking about it for a while. I've also considered using the 37 as an alternative, but only because it uses a 6.3v heater. I use the 27 in this amp I built and I've never had any issues with microphonics but I only use ST shape tubes. I use mesh plate versions which are harder to find in the ST shape but the solid plates sound very nice too.
I've never built a preamp with the 27 but I've also been thinking about it for a while. I've also considered using the 37 as an alternative, but only because it uses a 6.3v heater. I use the 27 in this amp I built and I've never had any issues with microphonics but I only use ST shape tubes. I use mesh plate versions which are harder to find in the ST shape but the solid plates sound very nice too.
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Any progress yet, Biwado? I've got a pair of RCA uy227's. Both very good microphones 
. Despite their con's, I think they look great, so I want to build a nice all globe pre or headphone amp (combined with 31's maybe be, and an AZ1 mesh globe...why not) with them.
One thing I did notice: the filaments seem to flicker. One worse than the other. Has anyone experienced that before? Bad heater insulation? Bad connection? I am using a regulated DC supply, by the way.
Cheers
. Despite their con's, I think they look great, so I want to build a nice all globe pre or headphone amp (combined with 31's maybe be, and an AZ1 mesh globe...why not) with them.One thing I did notice: the filaments seem to flicker. One worse than the other. Has anyone experienced that before? Bad heater insulation? Bad connection? I am using a regulated DC supply, by the way.
Cheers
I'll jump in and revive this thread. Just finished my 27 preamp. 6x4 rec, and dual 15H LCLC PS. The heaters are fed with in series from a 5V switching power supply. Bias is via 2 blue Vishay LEDs in series for -7.4V, and I cobbled together a C4S CCS to load the plates at 5ma. B+ is 150V, plates are at 120ish. Anyway, this amp has been the most challenging and maddening one to date. Microphony isn't awful. Sure it's there, but they don't ring forever, and music doesn't seem to bother them. The main issue is finding good tubes. I've been thru a dozen at least of the globe meshplates. Some are just noisy, prone to EMI like crazy, or just don't conduct right, introducing lots of artifacts. For the LIFE of me I can't get this amp quiet enough. I'm gonna try a 5VCT transformer for the heaters with the center tap elevated to 30VDC. Still, if you can ignore how noisy they are (kinda of pleasant white noise), I do love the sound. Last thing, this is a basic resistive coupled amp, no iron. Anyway, they're worth a try, but be forewarned. If anyone has gotten these bottle quiet, or knows how to find quiet ones, post away!
I've got a preamp which uses 76 tubes. The first pair of 76 tubes were very microphonic. Fortunately, they are relatively inexpensive and easy to find. I bought about 12 of them and have two that are quiet, (an engraved base RCA and a Raytheon), but they are somewhat noisey. It's white noise as described by natzarich. Since I don't listen with my ear 2" from the speakers, it doesn't bother me. I got into tube amps because I wanted to roll some tubes and that's what I have to do with 76 tubes.
27 Tube PreAmp Update
Well, after a couple months of intense work with 27 tubes, I think I must admit they are just too problematic and troublesome to continue on. As I mentioned, I went thru a few pairs of meshplate globes with horrific noise and EMI issues. I just got a pair of later ST shaped bottles with a solid black plate, and tho MUCH quieter and non-microphonic, they're still simply noisy. Lots of hiss and for sure some kind of oscillations/intermodulations, and these noises absolutely follow the tube, as swapping positions proved. I LOVE these archaic tubes, but the truth seems to be they are just that, archaic. Maybe for some these noises are character, but I couldn't get past them. Anyway, to everyone working with or interested in 27 tubes, I wish you luck and keep reporting if you find any resolutions for the issues I had. Cheers!
Well, after a couple months of intense work with 27 tubes, I think I must admit they are just too problematic and troublesome to continue on. As I mentioned, I went thru a few pairs of meshplate globes with horrific noise and EMI issues. I just got a pair of later ST shaped bottles with a solid black plate, and tho MUCH quieter and non-microphonic, they're still simply noisy. Lots of hiss and for sure some kind of oscillations/intermodulations, and these noises absolutely follow the tube, as swapping positions proved. I LOVE these archaic tubes, but the truth seems to be they are just that, archaic. Maybe for some these noises are character, but I couldn't get past them. Anyway, to everyone working with or interested in 27 tubes, I wish you luck and keep reporting if you find any resolutions for the issues I had. Cheers!
I did run them from a LT1084 without too much trouble but used a shunt regulator. Perhaps 1 at 10 was a reject from electrical point of view, not that problematic at all.
For linestage applications the noise level of a 27 is directly related to the sensitivity of the power amp. I ran into problems with a 27 preamp (Emotive Erato) when I used it with a power amp that has unusually high gain, but the preamp was very quiet when I used it with a low-sensitivity 2-stage power amp. Of course the preamp volume control was turned up higher with the low-sensitivity amp. Speaker efficiency is also a factor.
About some years ago I made a 27 preamp. For heater supply I used a 5V smps and serie connected 27 filaments, the middle point between 2 filaments connects to ground. Bias supplied by a 9v alkaline battery. The tube loaded by Tango NP126 line out transformer.
I have around 10 tubes, 1-2 tubes are microphonic, a pair of Majestic tubes hum loudly due to sensitivity to the power transformer electro magnetic field. Other pairs are fine, the sound is very nice.
I have around 10 tubes, 1-2 tubes are microphonic, a pair of Majestic tubes hum loudly due to sensitivity to the power transformer electro magnetic field. Other pairs are fine, the sound is very nice.
Andromeda 27/E83F mu-Stage preamp
Over in AudioAsylum an inmate named Mike C. developed a preamp based on type 27 tubes which he found very satisfying; best he had heard. I have the tubes and will build it when time allows. Of coarse I have not heard it, but I thought Biwado (OP) and some of you on this thread might like to know about it if you don’t, comment on it and even try it.
He started the journey many years earlier, early 2000s, with an E80CC tube in a Kimmel Mu-Stage configuration with an E83F pentode at the top. After all refinements at the end of his journey it was type 27 he liked best, again with the same Mu-Stage. Strangely, Mike C. never mentioned any issues with 27 tube regarding noise or microphonics that I recall. He did employ a constant current heater supply for the 27, so perhaps that dealt with some of it.
Mike C’s hand-drawn schematic is here:
Over in AudioAsylum an inmate named Mike C. developed a preamp based on type 27 tubes which he found very satisfying; best he had heard. I have the tubes and will build it when time allows. Of coarse I have not heard it, but I thought Biwado (OP) and some of you on this thread might like to know about it if you don’t, comment on it and even try it.
He started the journey many years earlier, early 2000s, with an E80CC tube in a Kimmel Mu-Stage configuration with an E83F pentode at the top. After all refinements at the end of his journey it was type 27 he liked best, again with the same Mu-Stage. Strangely, Mike C. never mentioned any issues with 27 tube regarding noise or microphonics that I recall. He did employ a constant current heater supply for the 27, so perhaps that dealt with some of it.
Mike C’s hand-drawn schematic is here:
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