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In Reply to: RE: Its value is much lower posted by Leo loves music on November 14, 2024 at 12:25:19
Its a single 6SN7 section in an octal base. Its got bandwidth well past 100KHz without breaking a sweat. So it shouldn't be slow.
The 'warmth' comes from the operating point, the fact that the circuit you propose is single-ended (so has a bit of 2nd harmonic) and no feedback (I'm guessing at the latter).
It could work fine as you propose. One issue you will want to really think about is what happens when you power up the circuit and the tube isn't warmed up. If its wired as a plate follower there will be a Voltage spike at the output of the coupling cap that could damage a solid state buffer!
A cathode follower won't have this problem of course but then you wouldn't need the solid state buffer either.
Follow Ups:
How about two protection diodes. The output impedance of 6J5 is about 10K - 20K. So it shouldn't be a problem.
You don't want the diode to conduct with audio. Most diodes conduct around 0.7V.
So I imagine 10 of them in series should do the job. You only have to worry about a positive pulse. That would limit the output swing of the tube to 14V total before the diodes conduct. That's plenty of overhead for driving any amplifier.
Alternatively you could put a diode between the input of the buffer and its Vcc+ so that it only conducts if the +input Voltage is higher. Some semiconductors don't like that sort of treatment though.
"Alternatively you could put a diode between the input of the buffer and its Vcc+ so that it only conducts if the +input Voltage is higher. Some semiconductors don't like that sort of treatment though."
This is what I meant.
Do you know where can I get a big aluminium chassis like this one?
https://www.ebay.com/itm/256080818362
This issue with this one is too expensive. Also the front panel is too thick that I can't put the dact attenuator on.
E-Stat's suggestion is a good one. But I'd be surprised if Par-Metal has anything as inexpensive as you found on eBay. But worth a look!
Buckeye is also a good source but again I think you'll find their stuff to be more expensive.
I got some nice national union 6J5s and some nice supreme mcaps. Don't know when I can start the project as there is no chassis yet.
Power Supply duty, these are very nice caps. I have started use these in my Tube preamp power supply upgrades. Get the ones that are for Professional Power supplies, you will see in the Datasheet applications what they are used for.
For power supply, I will use my own designed SMPS. Some cheap Panasonic capacitors.
Maybe I can go with game plan 1 first so I can use this cheap, blue, ugly chassis. Plan 1 is easier and only needs a mid-size chassis. Plan 2 is much more complicated and needs a bigger chassis to fit more stuff. When I have a chance to get a better bigger chassis then I can do plan 2.
You might consider breadboarding your circuit ideas and listening first instead of choosing one just because it will fit in the chassis you have on hand.Some other thoughts . . .
Most all amps can be driven to full power output from a line level source without any additional gain at all. So try a "passive preamp" if you haven't already. Yet many people find that an active preamp stage offers an improvement over a "passive preamp" (aka volume control) and enhances dynamics. If you're driving a SS or Class D amp you also need low output impedance.
For some reason most preamp designs use higher gain tubes which also have higher plate resistance. The higher the gain the more gain that gets dumped to ground. And, because of the high plate resistance, a more complex circuit must be used (cathode follower / some type of buffer) to lower output impedance. While the 6J5 is certainly not high gain (mu 20) it does require a buffer for use with SS / Class D.
I'm a fan of simplicity and not an engineer. So I prefer to use tubes that fit my needs with less engineering complexity and expense. So I would suggest that if you don't really need gain, start with a low gain tube, mu of 10 or less. And, if you need low output impedance, use a tube whose plate resistance is also very low.
I realize that simplicity doesn't appeal to everyone. Some people just want an engineering / design challenge. It takes skill and creativity to take a tube that isn't ideally suited for a particular task and make it work.
If you breadboard then you can try a variety of circuits and eventually commit to building one that you know you like because you've heard it and compared it to other alternatives.
Edits: 11/16/24
at Par-Metal where I purchased one for a DIY attenuator box.
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