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In Reply to: RE: Poor Design for a Preamp posted by mwhouston on February 26, 2025 at 17:55:07
"Tried this ides with a 6N7 preamp I also had noise issues with and it worked well.Even with my DIY shieldless lead (175pF) into 100K upper 3db was 37KhZ. 6N7 are power tubes and Im not sure of they have a very high frequency range or not. Regardless I'm happy with 37K."
The 6N7 was primarily intended as a Class B output tube. But the 6N7 data sheets (GE, TungSol) suggests multiple uses for the tube including use as a Class A driver and gives suggested operating points for that application. Since nobody in the DIY community seems interested in building Class B amps all the forum discussion on the 6N7 seems to be about using it as an input / preamp tube.
Apparently, there are some glass (G, GT) versions. I have some of the 6A6s which are glass. The 6E6, 79 and 53 are supposedly equivalents or near equivalents. There's also the 5694 which is equivalent or nearly but has separate cathodes.
At one point I was considering building a Japanese SE 2A3 design that uses them on the input but I was going to use the 6A6 version. I like using less common tubes but I don't really have a need for a preamp with that much gain (mu 35).
I have priced on eBay some glass 6N7s and though not cheap well within my budget. They do look good though. But still having three boxes of tubes im trying not to buy more but use up what I have in new DIY gear and move on. I have a number of other metal tubes. All sound excellent in their respective amps.
| retro-thermionic |
Yeah, I don't remember if the 6A6s I got came from eBay or if I found them locally.
I'm less than 30 minutes from Radio Electric Supply (vacuumtubes.net) which is probably the biggest tube dealer in the world. They have warehouses full of tubes, many millions of them, though not all audio related, of course. They ship worldwide and have reasonable prices.
They're generally much cheaper than the "boutique" sellers who give flowery descriptions of how a particular tube allegedly sounds. As if a particular tube sounds the same regardless of which circuit it's used in or any other differences in the rest of the system. Of course, the "holy grail" tube$ are always what they just happen to have in $tock at any particular time.
The guys at RES (brothers, Roy and Dale) are not audiophiles, they just sell tubes. Hence the total lack of BS.
Another very large dealer is about 100 miles south of me in Orlando. That's vacuumtubesinc.com which also ships worldwide.
The only metal tube I've experimented with is the 6AG7. It caught my interest because it's incredibly easy to drive. I have a breadboarded PSE design that uses them with each channel driven by half of a 3A5, which is a dual DHT with a mu of 15.
It's another "inverted SET" (iSET) design which is a term coined by Andy Evans. I built another iSET amp which I call the Nuance. Details are in a thread on Audio Karma.
The "inverted" aspect refers to fact that instead of using DH output tubes driven either by higher mu indirectly heated tubes or multiple stages of somewhat lower mu tubes, the amp is only two stages with input tubes that are DH. The outputs are indirectly heated and to easy to drive, unlike the output tubes that are commonly used in SET amps.
The PSE 6AG7 is on the sidelines for now, though. I'm back to playing with low mu (10 or less) preamp tubes at the moment which is why your thread caught my eye.
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