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In Reply to: RE: Poor Design for a Preamp posted by Tre' on February 16, 2025 at 12:57:28
Tre made the suggestion to use 68k+22k resistors to give me a dropping output level but not to limit upper frequency. 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.
Applying the same 68k+20k idea to the 01a preamp gave me noise level outputs of 1.9mV and 0.9mV a big improvement over the ~4.0mV with the 300k+100k setup. At good volume only by putting my ear to my 93db sensitive 12" woofers do I hear the slightest hum then only in the left channel.
Good result for two preamps so far and two to go. Lucky I have a good collection of Rs to mod the others.
| retro-thermionic |
"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.
"Tre made the suggestion to use 68k+22k resistors to give me a dropping output level but not to limit upper frequency."
Actually, I was the one who suggested using the combination of 68k in series and 22k to ground and said it should improve the -3db point of the upper frequencies.
My earlier post: "OK, so in the case of this 01A preamp, he could use a voltage divider with 68k in series and 22k to ground. This would reduce the voltage about the same as the 330k/100k combo he's using and it would present a load of 90k to the tube and be no problem.
The value of 68k || 22k is 16.62k. If we plug that into the low pass calculator then the -3db point would be 54.75k. That should also not be an issue, right?"
Tre quoted me and commented about the corresponding -1db point, which he said would be 27.375kHz. He suggested that, from a technical standpoint, this would still not be enough to "get the filter completely out of the audio band".
I'm glad to hear that, from a practical standpoint, it seems like it improved things subjectively. I try to understand the technical end of things - hence my questions in this thread - but I'm not convinced that striving for technical perfection is always audible beyond a certain point. Sometimes "better than it was" is good enough and going to more extreme lengths is just a waste of time and money if it doesn't yield audible improvement. IMO, but that's just me.
I guess for an engineer technical perfection IS the goal so it's a valid pursuit regardless of whether it's audible or not.
I understand how it's easy to get confused though. AA's software is ridiculously out of date and totally lame. Unlike other sites like DIY Audio, Audio Karma, and others AA doesn't allow you to easily quote a previous post without rewriting it and adding quotation marks. As I have done above.
Unless someone makes the effort and adds a quote manually it's often impossible to tell which previous post someone is responding to.
I suspect that AA will never modernize their software. I don't know about the site as a whole but the forums I used to visit regularly years ago are pretty much dead in the water now. Some of them now go weeks without a post where, in the past, there were a large number of new posts and threads every day. Sad.
I still find some useful and interesting posts on AA but I rarely post here anymore because of the lack of activity. Fortunately, the search function yields a wealth of information from the time when AA was a vibrant forum.
Well thanks for the suggestion. Probably like you Im on a number of forums mainly an Australian one which is where I live. Id like to think we are all here to listen and to help where we can fostering friendship and help in a common field of interest.
Two preamps to re-mod. Ill post results.
| retro-thermionic |
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