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In Reply to: RE: 6SL7 posted by GeorgePope on March 20, 2017 at 01:36:42
The simple answer is : there are NO pin alternatives to the 2A3 pin arrangement.
The two-stage DC circuit's primary gain is from the driver, where driver choice and implementation is most urgent when seeking perfection.
My entire response addresses, and is entitled " 6SL7" and all pin comments were referring to an octal socket's pins. That would be the 6SL7, where alternative better choices exist.
As examples of three "non-octals", with linear graphs and mu of 100 : 1/2 12AX7 as a 9 pin mini, 6AV6 with symmetrical plate as a 7 pin mini and diodes tied to plate, and a 7B4 seated in a loctal socket.
There are compromises in any and ALL choices of high mu driver tubes for a direct coupled JJ 2A3-40 amp.
For example, the latter two tubes I mentioned are more microphonic than the first tube mentioned, but when employed just one time on the input, its OK in real-world use. The latter two tubes have symmetrical plate-filament-grid structures. All three tubes given as examples, are mu of 100, thus are preferable, normal listening-wise, to a 6SL7 octal-based tube IMHO.
Edits: 03/20/17Follow Ups:
Ok , whatever you say , although a 2A3-40 is a 300B with folded filament and the driver tubes you suggest do not make sense for use with this tube . It will work but I suspect significant HF loss and major issues when driving the 2A3-40 grid close to 0V . A 12AX7 (and I'm sure this applies to 7B4) is not really a good driver unless it is driving a pentode . In fact it has higher Rp and is much less linear than 6SL7
I think you're barking up the wrong tree regarding this mu of 100 thing and also your idea about 6AV6 . If you tie the diode plates to the triode plate , the cathode will start pulling quite a substantial current . The triode section in terms of signal will be 'shorted out' .
Usually redundant diode sections are tied to the cathode .
A normal E.E. will tie the diodes to ground. LOL, one good EE I trust has already told me to do so. Another tells me, last night, the two diode sections, tied to ground, will lose high end, more-so than when tied to the plate. You suggest to tie diodes to the top of the cathode. I don't know who to listen to for sure.
Separate point: you fail to look at the driver requirement in any realistic way George IMHO. I hope you realize, I am talking about a two stage amp that is directly coupled, Driver's plate to Finals control grid. This makes for an ease of driving the Finals, as a direct couple. If it were a cap couple, ( the worst way to couple two stages ) you would be correct, your analysis, view. If it were to be a transformer couple, ( far better than a cap couple ), you may still be a correct. But I am talking of NO devices at all to " drive through " between the two stages , except a few inches of high quality wire, silver, silver coated, etc, etc.
Direct coupling greatly eases the burden on the initial driver stage. A huge percentage of DIY amps in the Orient are Loftin White variants, over several decades now, mainly....because it sounds good. Isamu Asano's amps, Nobu Shishido's, all used mu of 100, low current triodes.
An EE type will typically look at tube choice from their textbook basis only - in term of maximum swing as it approaches 0 VDC of bias. That is hardly how the circuit operates IMHO!!!
I totally agree, you are correct about when "approaching zero" , but you are considering the wrong things !! Its like a MD operating on Cancer, when all the patient really needed is to eat fresh fruits and lots more organic vegetables.
In the real world, 99.999 percent of the time, the mu of 100 tube is centered at about 1.5 VDC of self bais at idle, running maybe -0.5 to -2.5 VDC of bias, playing music at normal ( even loud ) levels in the home. I design and optimize for that parameter, normal use, on a 98 dB ALTEC VOTT or ALTEC 604.
I doubt you have listened to such a circuit, particularly an optimized one, as I describe, much, if any at all.
From your comments, and understanding,you certainly wouldn't ever design it. It would require thinking outside of the box. Most trained E.E.s typically can not mentally do that, due to their schooling, training and thought processing, .... poor fellas and gals !!
A mu of 100 driver is clearly more fun, listening-wise, executed as good as possible, then is a mu of 70 driver!! Herb Reichert "got it", when he used the term " presence " of the performers " being in the room ", at normal listening levels - during RMAF 2016 last October. I happened to be there, listening with him, when he visited. He " got it" upon listening. A mu of 70, or 50 driver, can't do that.
You likely have little or no experience with this circuit, well executed. I can understand that. I have substantial listening experience of this George.
When looking at a 12AX7, 6AV6, or a 7B4, as a DCed 2A3 driver - also include your 6SL7, I would suggest all four are very similarly and nicely linear, in the ranges they will operate in for this circuit, 99 percent of the time. I am excluding " zero bias " conditions, because it is simply backwards to what occurs in the real world use George.
So...get with it, think out of the box, not textbook and formula rigid . The truth will set you free. Results is what counts, not formulas and theory that does not totally explain all that is happening !!!
As for the question of what to do with the 6AV6 diodes, tie them to plate or ground, or top of the cathode as you suggest, I am entirely "open" to see what others, who think they have an answer, say about this.
I have not yet used the tube. I do plan to, driving a Type 46, next month for two friends' DC amps, and I would be open to LEARNING from experienced technicians, on what to do with the two diodes, how and why.
So far, two people I respect, have given me opposite answers. You suggest a third approach, the cathode. What do I know ??? I was a business major.
Let us see what smarter than I, more experienced people, can suggest, and WHY they suggest it. Fun !
Thanks for posting,
You do not want the diodes to be conducting or have any electrodes floating . Connecting the diodes to plate will not work . Connecting to cathode or as you suggest 0V in fine
Apart from that , I fail to see any relevance within your post .
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