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In Reply to: RE: Not an Irritation posted by Garg0yle on December 05, 2014 at 08:57:16
A well designed DC amp will require NO trim pots of any sort. Trim pots are baloney, junk design. Amps should really be engineered to have NO pots in them, good amps !!The op points should be thoughtfully centered ( and conservative ) such that changes in AC line, and tube aging, are FULLY taken into account at the onset. Trim pots are band aides for incorrect designing.
Me personally, I have been working with DC SE amps since 2007, and I have never had a failure of any sort.
GarGoyle, you have mail. Punching a hole for an Octal high mu driver tube is a mistake of sorts. A 9 pin miniature will sound better, it does not have all the soldered connections to the octal's socket pins, and is "less hazy" more distinct sounding since the wires come directly out of the glass envelope, and do NOT terminate into solder connections (eight) on an Octal tube socket. Maybe its not much of a factor on a 6SN7 with a mu of 20, but it certainly IS audible ( as a degrade ) on a mu of 70 to 100 tube.
LMAO, it will be a cold day in Hell when anyone beats half a Telefunken etc. 12AX7, under one milliamp, DCed into a 2A3, IF you know how to do it. Proof .... is in the pudding !!! I am working on a stereo 1/2 12AX7 - JJ 2A3 amp for myself as of this very moment, applying new things I've learned. Installed L1/L2 for each channel - this afternoon. No pots in my amps !!
Gotta go now, tonight is the Kansas City Symphony's performance of G.F. Handel's "Messiah" , and I can't wait to hear / experience it !!
Jeff Medwin
Edits: 12/05/14 12/05/14 12/05/14 12/05/14Follow Ups:
"A well designed DC amp will require NO trim pots of any sort. Trim pots are baloney, junk design. Amps should really be engineered to have NO pots in them, good amps !!"
To be fair trim-pots vs soldered in components is really a matter of semantics, but I get your point. If the drift stays within ones design parameters then no harm done.
"LMAO, it will be a cold day in Hell when anyone beats half a Telefunken etc. 12AX7, under one milliamp, DCed into a 2A3, IF you know how to do it. Proof .... is in the pudding !!! I am working on a stereo 1/2 12AX7 - JJ 2A3 amp for myself as of this very moment, applying new things I've learned. Installed L1/L2 for each channel - this afternoon. No pots in my amps !!"
Does the Rp of the 12AX7 limit the bandwidth in your application?
"Punching a hole for an Octal high mu driver tube is a mistake of sorts."
To quote Bart Simpson, "you're damned if you do, you're damned if you don't."
I will have a look at your paper, enjoy the show.
"A well designed DC amp will require NO trim pots of any sort. Trim pots are baloney, junk design. Amps should really be engineered to have NO pots in them, good amps !!"
Well, in a cathode circuit, maybe unnecessary. But, there are some nice, low wattage pots available - probably better than the metal film resistors some folks like to use.
As for trim pots in general, I use them sparingly, but they do have their place. Every amp I coceptualise, I ask: in a DHT cathode, does a hum-nulling circuit using a high quality, high-turns, low value bulk-foil trim-pot provide better sonic and practical results than the alternatives. To my mind, and how I use it, with my current level of knowledge, in my system, yes. I don't want hum pushing through the quiet music I enjoy in the wee hours (in a quiet neighbourhood).
Now, I know, the counter-argument is that am not as enlightened as those who might take a different POV. I'd be curious as to what noise levels folks are achieving when trying different, commercially available DHTs and running a CT filament transformer with no hum pot. There is a fair bit of variation amongst even the highest quality tubes.
Cheers.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
Interesting discussion, and one we all have to consider.
Variations in DHT filament balance are all over the place. In the case of the 2A3's, I once had an AVVT mesh-plate that delivered .8 MV at the amp output. That was with A Hammond filament trans, and A.C. direct-heating.
That .8 mv was rare, it's mate delivered 2.3 mv. The usual was about 4-6 mv, and I'd try to arrange these into nearly alike pairs, preferring to leave the cathode with nothing but a resistor and cap on it.
I'm still pretty hung up on this, and am trying to talk a trans-builder into making me a truly balanced filament trans. It will not, of course, balance-out wayward tubes, but at least the transformer as a hum source can be minimized!
---Dennis---
The experience here is consistent with mine as well.
In general, I think one can count on 2mV from an AC-heated 2A3, allowing for swapping in a variety of tubes. I know Gordon Rankin does a lot better, with careful attention to the details and I presume some willingness to cull the noisy tubes. Most of the time that's just fine, you don't hear it.
I did experiment more with 300Bs, which have twice the filament voltage and generally produce around 5mV hum with AC filaments. Using the winding centertap, I found anywhere from 4mV to 25mV depending on the tube (and culling one with visibly warped filaments). Too many speakers will make 5mV audible.
I prefer DC for 300Bs, and hum pots for both 2A3s and 300Bs even with DC. One of these days I'd love to play with one of those EML tubes with the filament centertap pulled out for connection!
Hi Paul,
My EML V4 centre tapped tubes are being made right now for Feb. Shipping.
I'll compare them to the standard SPs using CT hammond 266m2 and report my findings re. sound differences and hum performance.
Only problem is as these are specially made I will have to accept the matching of the pair 'as is'.
Also re. The Ultrapath cap, I found more important is the C2 bypass in the power supply to improve the sound.
Cheers
Smart
I'd be curious to know how accurate the filaments can be centre-tapped... maybe enough to get the noise down to acceptable levels and not need a pot in the cathode circuit (and able to keep the filament transformers out of the local current loop).
I'm temped to try it for my next build, but for comparative reasons, I'd like to be able to use the tubes I have invested in. Would be easy enough to convert over at some later stage though.
As for expected noise levels, I don't really know. Some folks say 3-4mA is acceptable when using a pot with AC fils on a 2A3, but I am with you, 2mA is an achievable target. Whether there are real sonic benefits with the lower hum, beyond hearing the noise, I don't know. Tradeoffs: to pot or not to pot?
Cheers.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
Can't wait to get my hands on those!
Will try to report what happens....
---Dennis---
Not knowing how your filament transformers are constructed, is it at all possible to partially unwind the long half to achieve a closer balance?
> > > I'm still pretty hung up on this, and am trying to talk a trans-builder into making me a truly balanced filament trans. < < <Sounds interesting Dennis; if you don't mind, keep us informed of your progress.
I'd like my amps to be as simple and needing as few adjustments as possible... and I'd like them to be quiet, stable and reliable... oh, and sound just as I like them! Seems like we have similar priorities, even if our methods differ. Of course, my implementation is not as evolved as yours.For my first amp build I used a 27-turn, 10 Ohm Vishay, bulk foil pot and even with less than ideal grounding (to my mind) I have been able to get noise down to ~1mV on one channel, and somewhere between 1mV - 2mV on the other using the EML solid plates (noise with the mesh plates was a touch higher). This is okay I think; I can't hear hum more than about a foot away from each of the 99dB+ efficient speakers. The difference between 2mV and 6mV hum is noticeable, I think, but it might just be "expectation" effect. As an aside, I find it interesting that my amp seems quieter than the two solid state amps it has replaced, despite using ac fils... maybe that slight hum is less offensive than whatever noise some solid state amps produce?
I am looking forward to my next build, but still have not settled on how I will manage hum...
Cheers.
"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki
Edits: 12/06/14 12/06/14
Not all hum is alike-- like everything else, it has its own internal components.
A "little bit" seems to aid both interconnects and speaker cables, keeping a bias signal on them so that they really respond quickly to musical transients-- capture the "leading edge" as musicians say.
So, how much is too much, then, and will cause musical capture to start going "South"?
How much is too much depends on what components are in the hum, and what your speakers will tolerate.
In GENERAL, most of us like to see less than 4 mv-- if you're going to measure things. That's just a rough clue-- not an absolute.
Solid State amps may use center-tap (2 diode) circuits for H.V., and others will use a Full-Wave Bridge-- (4 diodes). Not all Solid-State amps use chokes-- some will just load-up the caps & rectifiers--only-- supply and then try to "regulate" it into submission!
Choke-Input supplies are used in the BEST Solid-State amps, just as in the best tube types.
In any case, many S/S amp's hum artifacts will often be very different sounding than the same "hum" levels from tube amps.
Interestingly, when the best possible tube amps are compared to the best possible Solid-State amps-- the two not only sound identical, but so do their hum components.
Small world!
---Dennis---
" Does the Rp of the 12AX7 limit the bandwidth in your application? "Only if you are able to think conventionally !! But in REALITY :
Not really, if you consider that the 12AX7 is just ONE part of a totally optimized amp we execute in modern times.
The 2A3 is very EASY to drive in a Direct Couple !! And of course, we have an actual, not theoretical, historical record of this : 1947 used a mu of 100 high Rp tube ( 6SF5 - Lipman and Robin, later "FI" amps ) as did the DC amps of Isamu Asano ( 2A6 at 0.43mA.) and Nobukazu Shishido ( 12AX7 at 1 mA. ). About 25% of the DIY Japanese amps were L-W variants, according to Jean Hiraga if I recall it properly.
If you wanna do it on a "proof by listening" , we have on A.A., Smart845's SupaNova Type 45 DC amp review, which I helped him build. I can refer you to various amplifier reviews found on Dennis Fraker's "Serious Stereo" web site.
In late 2014, we are able to do it even better, new parts and technologies are applied - positively.
Jeff Medwin
Edits: 12/06/14
"Gotta go now, tonight is the Kansas City Symphony's performance of G.F. Handel's "Messiah" , and I can't wait to hear / experience it !!"
Hear / experience it huh?
What kind of power amps are they using in the PA system?
What about crossovers? I'll bet DSP based feeding bi/tri-amped arrays!
What kind of wire are they using, particularly on the low level microphone feeds?
How many high feedback OPAMPS are in that audio board the signals have to transverse through?
Hi Gusser,It was a live unamplified concert, four soloists, a powerful 250 member mixed choir, and a Harvard ( summa cum laude ) music conductor. Not a single microphone, op amp, speaker, or electronic device was needed.
So you have struck-out on every account Gusser. Way to go.
Its the new multi-zillion dollar Helzberg Hall, about a three story pair of clamshell like buildings, with seating for several hundreds, and no microphone was needed. No prosentium arch either, Mr. Fulton would approve. Almost like a Greek ampi-theatre, but in 2014.
See the photos of the hall, especially the side-view shots down the page. Drool. It was fabulous. Everything is "up to date" in Kansas City.
Jeff Medwin
Edits: 12/06/14
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