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In Reply to: RE: Small IDHT tube cathode bypassing posted by 91derlust on July 06, 2015 at 00:44:48
Here's another option for you to consider which should work quite well with the 6C6: Try grounding the cathode and use a 10 meg grid-leak to create the bias (this necessitates AC coupling to the source)! I use this method almost exclusively when using high-mu triodes in the front end (and contrary to popular belief, this does not result in hum issues); some pentodes, like the 6C6, also can be used that way. GE published some recommended values for the plate/screen load as a function of supply voltage which you can look up (if you can't find it, look up the 6J7- same tube electrically). The advantage of this method is that there are no frequency response issues, elimination of 2 components from the signal path and, as the tube ages, it will automatically adjust its bias based on the available space charge.
Follow Ups:
Hi,
But isn't there a huge disadvantage, that you have to add a series coupling cap in the signal path. Not only that, its placed at the lowest signal level of the entire amplifier.
I build two stage DC amps because I want to eliminate all caps that are in series with the signal path. Sure sounds better to me than ANY cap couple SET amplifier !!
If you have to bypass an Rk with film caps, that is not any disadvantage at all. Its an advantage.
You can select the best caps, plural, and get the amp to play exactly the way you want it to, in EVERY range, at the speaker voice coil, and to your ear, where it counts the most.
Of course, you have to experiment and listen, and TEACH YOURSELF how to bypass properly, what values and brands work best.
Quality - wise, Dennis tells me in 7-2015, Mundorf Silver and Oil has an entire line up of values, that will do the job. Usually, I experiment, even snuck in some Russian surplus caps here and there. Dealer's choice !!
No, my fellow hobbiests, having an Rk to bypass can be a BIG advantage, if you teach yourself how to do it well ! Yes, it costs, but it costs more to fumble on an amp, and never get it to perform at a certain level. I'm getting too old to be listening to mediocre amps, its gotta please me, its the WEAK link in the High Efficiency audio chain, at least 98% of the SETs out there are mediocre in my opinion.....
Jeff Medwin
'I build two stage DC amps because I want to eliminate all caps that are in series with the signal path'
Perhaps you should remove all cathode bypass capacitors ? Any signal through the grid also flows through these
Al
Yes sir, great idea. I'll just put in an inexpensive solid state POS, a LED, etc etc, and have no frequency compensation, sound shaping at all.Jeff Medwin
Edits: 07/07/15
These terms are very revealing of your goals. That's perfectly OK, but many do not share those goals. And those goals are not attainable without introduction of "colorations" that most of us would like to avoid. (Coloration seems to be one of your goals, on the other hand.)
Anyway, you like to deride those who criticize your approach without having tried it. In this case, try using a Schottky or red LED and NO bypass capacitor, on the cathode to ground connection to bias a small signal triode. You might change your tune, or maybe changing your tune would constitute "sound shaping".
These terms are very revealing of your goals. That's perfectly OK, but many do not share those goals. And those goals are not attainable without introduction of "colorations" that most of us would like to avoid. (Coloration seems to be one of your goals, on the other hand.)
That is the conclusion I have arrived at as well. Although ultimate neutrality is not my primary goal, I'd still like to avoid gross colouration and maintain acceptable transparency, while achieving other goals.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Both of you speak from NO listening experience with this technique, especially properly executed. Your lack of experience allows me to write off your "guesses and comments" as useless, which IMHO, they are.Sincerely,
Jeff Medwin
Edits: 07/09/15
Jeff, I deleted my previous response. I do not want to get caught in negative exchanges with you - they are miserable experiences that achieve nothing that justifies the effort.
As you know, I sought the type of experience you apparently demand. The reality is that you repeatedly mandated that experience be limited only to your Rx. Any expression of creative or independent thought - especially experimentation with other methods - has been vehemently discouraged. This behaviour has been repeated continually in our personal correspondence. Because of this, I requested that personal communication between us cease. I will endeavour to extended this agreement to public exchanges as well.
All the best to you Jeff,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Jeff,
you like the sound of a dynamic volume expander and distortion. That is okay, and there is absolutely nothing wrong with creating a unique sound generating machine.
dynamic volume expander and high distortion I disagree with.
bandwidth limited yes.
There was a post by tube wrangler that claimed -1/2dB at 19KHz or something to that effect.
Perhaps the EML tubes and all his other special techniques allow for such performance from a SE 7B4 DC 2A3 circuit.
my point was that claims of high distortion are exxagerated.
Based on your ARTA graphs, the distortion of the SE 12AX7 DC 2A3 was NOT unusually high from what I remembered seeing.IMO, it looked OK.
Not sure how much your SE 2A3 circuit deviated from what DF or JM have done.
Edits: 07/13/15
since jeff's circuit is a moving target I'm probably off by a mile. :)
I'll take the high road on this, because there's no such thing as "winning" this argument. But for the record, I meant no criticism of your preferred approach; I merely recommended that you might try another that is diametrically different, just to hear the result for comparison.
With a Schottky biasing a wimpy triode such as a 12AX7 , it may work but the impedence will be far higher due to the low current through the diode . Y ou would also need far more diodes stacked to get the required bias . I seem to remember Naz proposed a solution using a star power LED for around 1mA or so (archives?) .
Al
But it does not negate the basic idea. I've never done it with a 12AX7 or with currents as low as 1mA. I got the idea originally from Naz.
91,
May I ask what output tube you plan to directly couple the 6C6 driver tube to? I to very much like the sound of the 6c6 pentode.
Thanks jet
My pleasure! IIRC, you had once posted some of your experiences with resistors on a similar amp on A'gon... Are you still running without a cathode bypass on the 6C6?I will be directly coupling the 6C6 to a 2A3 run at 42mA and 250V. Now, before people think that I am blindly following the Jeff and Dennis Rx:
> The operating point has been reported as sounding pretty decent by a few people.
> I think Emission Labs ~mu 4 tubes might sound really nice with a ~1:6 mA:V ratio. This is something that Thorsten recommends with 300Bs, but it might apply to 2A3s and maybe even EML 45s.
> I am loading the 2A3 with 4K2, not 2k5. I have two beautiful 5.9Kg custom nanocrystalline beasts sitting on my shelf...
> The cathode bypass will be sized for a ~8Hz -3dB point, not 20 - 25Hz (which could work well in the right amp, but I am not experimenting with that for this build).I will likely post the final build here, but would be happy to keep you updates as to my failures and successes as the build progresses.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 07/10/15
I've got a fixed bias 2A3 on the bench at present . I have three pairs of 2A3 : RCA/Shuguang2A3C/Sovtek2A3 . All need more than 42mA to start singing , I use 48mA at present with 260V A-K with a 5k OPTX . May be worth bearing in mind once you've got it up and running to play with the op points a little
Al
.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
91, I am still running my 6C6 driver without the cathode bypass cap and I like the way it sounds but it provides a little less gain to the output tube. Please do keep me posted on your amplifier.
.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I think you should look at how the signal flows in your amp as you don't seem to understand about signal paths and impedences . What is wrong with SS in the cathode ? You don't need a bypass cap on a diode/LED as the impedence is low , look at the impedence of a bypass cap at 20Hz and compare this with the couple of ohms a schottky diode offers . Obviously you have never tried or even considered this...
Al
Thanks to you both!
I have thought about using Mundorf SiO... should be nice in an amp that has a gentle roll-off in the highs (as does the 6C6-2A3... unless I use positive feedback to equalise it). Our local distributor usually has pretty competitive pricing on them.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
How about using a REAL triode for the 2A3 driver tube, only THREE parts inside it ???
2A3 is easy to drive in a DC configuration.
The WE91 is narrow band and nothing special at all. Dennis used to throw them in the dumpsters when re-doing theaters, and replace them with his modded Dyna ST-70s, 'till he figured out he could resell them abroad.
Jeff Medwin
The subject title of the original post was misleading... I should have said IDH tube not IDHT (old habits die hard!). The 6C6 pentode meets my practical requirements and I also like it sonically.When it comes time to try something different I may go Willie style and try an all DHT amp.
BTW, did you make a conscious decision to troll, or does it simply come naturally:
"The WE91 is narrow band and nothing special at all. Dennis used to throw them in the dumpsters when re-doing theaters, and replace them with his modded Dyna ST-70s, 'till he figured out he could resell them abroad."
You know full well that what I am doing is rather different to the WE91. Please, share your experience of the circuit I will be building. Based on your own demands for experience, you must have considerable experience, else you would be, by definition, a hypocrite.
Regards,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 07/07/15 07/10/15 07/10/15
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