Re: 2A3 - Really a beautiful power tube!

Absolutely agree. I've gone through the whole gambit of PP tube amps of all
power sizes, moved on to SET's and settled on the 2A3 as having the most
truly beautiful and natural sound. I wrote a piece on a few forums asking
for other opinions, and most agreed that the 2A3 won out over 300B and 45.
I felt that the 45 gave a percieved "cleaner" sound, but it was almost
clinical--and not natural. I like a sound that is as close to a classical
live concert in a good hall, as possible. There is a fair amount of
reflection which colors the sound in a live concert--the 2A3's seem to do a
very good job of mimicking this. I also found that the supposed improvement
from cobalt OPT's was not to my liking either.

I've gone through a whole set of 2A3's NOS to find the best. My favorite is
a pair of no-name RadTel's from the 30's. Next is RCA silver, and then
National Union, then RCA black. The SovTeks and ValveArt's just plain
stink.

I use Lowthers which gives plently of volume in my listening room. Even a
71A amp has enough power for me.

"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message
news:43183CA0.EDCEDB20@xxxxxxxxxxxxxxxxxxxxx
>I finally have hooked up the stereo amp which uses 2A3
> to my speakers which are about 90dB sensitivity.
> I find I could listen all day and night to this underpowered
> amplifier.
> Whatever its doing it is doing it RIGHT.
>
> The conditions are as follows.
>
> Power tranny is Hammond type 370FX, and with
> a 240V: 2 x 2.5V filament tranny.
> Hammond 10H choke used with a quad of Sonic Frontier
> PS caps of 1,100 uF each, so that with seriesed caps
> I have 550uF, 10H, 550uF, as the CLC to supply power to the
> pair of amp channels.
> Rectifiers are IN5408 SS diodes. I don't strictly believe in tube
> rectifiers;
> I like them, I even use them sometimes; I just don't think they are
> indispensible.
> But in this case the use of a tube rectifier would have reduced the
> available B+
> and forced me to use fixed bias, which I don't like for any SE amps.
>
> No need to dual mono power supplies.
> 180uF used to further filter the input and driver stages.
>
>
> Output tubes are 2A3, RCA, NOS?, maybe used, not sure.
> Ea = +270V, Ia = 57mA, cathode bias with 820ohms
> bypassed with good quality 100V x 470uF.
> heaters are AC 2.5V with a pot for hum nulling.
>
> OPT is a 2.5k : 4/8/16 Hammond 25 watt rated SET gapped OPT
> type 1627SE.
>
> OPT is set up to match 2.5k to 4ohms, but I find 5 ohms is the most
> correct load.
>
> At 4Watts I get excellent open loop BW with this OPT
> and freedom from saturation, of which there is only a slight trace at
> about
> 7 Hz.
>
> The OPT has queer resonances above 30kHz, makinf the use of
> global NFB difficult for ppl with no clue about how to overcome such
> connundrums.
> It must be due to primary being wound close to the secondary,
> and the absence of enough interleavings of windings, and the absence of
> sufficient
> insulation thickness.
> Nevertheless, the 1627SE did turn out to be very usable, and sound is
> glorious.
>
> Driver tube is an ancient Radiotron 'made in australia' 6SN7,
> used but OK. 1k Rk bypassed with 470uF, RL = 34k, 0.47 uF Wima coupling
> cap,
> 150k grid bias R for 2A3, 1.8k series grid stopper.
>
> The 6SN7 is operating fairly linearly and comfortably.
> Without Ck bypass, the driver thd reduces, but then overall thd rises
> a lot since less even order Dn cancelation takes place.
>
> The fully bypassed Rk on the 6SN7 gives lowish Ra of about 5k for the
> paralleled tube
> and is thus a sufficiently low enough drive resistance to enhance the
> micro detail behaviour of the
> 2A3, ( very necessary imho to get the best out of not just a triode
> output tube,
> but ALL output tubes.)
>
> The input tube is 1/2 a 12AU7 of unknown lineage; it measured nicely,
> and I like the 12AU7 for inputs; its warm, accurate, and sounds
> glorious.
>
> Its Rk is 1.8k, and also bypassed with 470uF, and taken then to 100 ohms
> for
> the global NFB to be applied.
> RL is 50k, 0.47uF to a network for LF gain stepping, 1M+0.033uF
> plus 220k, typical in many of my amps to vastly
> improve the bass stability when the 12dB of global FB is applied.
>
> The FB resistor divider network is 750 ohms to 100 ohms
> with a phase tweaker bypass cap across the 750 ohms = 1,000 pF.
> There is a 470pF plus 2.7k zobel network connected from the
> driver grid to 0V to control the HF gain and ultimate phase shift of the
>
> 12AU7 to 6SN7 interface.
> A further 5.6ohms+0.47uF Zobel network is across the OPT secondary,
> and finally I got the amp to be fairly ring free when connected to
> any value of cap without a pure R load, and when using a square wave.
> Open loop BW with all compensation zobels in place was 20Hz to 20kHz,
> -3dB,
> and with 12 dB of FB the full power BW is 5Hz to 55kHz, 5 ohms.
> It remains the same at lower levels of power.
>
> A 5 kHz square wave is a bit ragged due to the mentioned OPT design
> simplicity,
> but the sine wave response between 30Hz and 20hHz right up to clipping
> is dead flat.
>
> A 50k Alps Black pot is used for the gain control.
>
> Input sensitivity with 12dB applied global NFB is 0.7Vrms.
> The amp's use with an CD player is fine.
> Just no need for any preamp.
>
> The sound is that of a far more powerful amplifier,
> with tremendous sense of rightness, instrument detail, warmth without
> muddle, not the slightest hint of any lack of bass, the usual beautiful
> mids, and detailed top end.
>
> The amp measures well at less than 1% thd at 4 watts into 5 ohms,
> mainly all 2H.
> But at normal levels of a watt the 3H and other H are present below the
> 2H
> which can be seen on the CRO when monitoring the THD at all levels.
> Ro is 0.47 ohm, and although my speakers vary between
> 4 and 20ohms, the sound appears firm, well timbred, and there isn't any
> hint
> of the sound being that from an inadequate amplifier.
>
> I played Handel's water music, the sound track of 'The Pianist' for my
> tests plus
> a not so wonderful Naxos CD, 'The Best of Bach'.
>
> Who says SET amps suck?
>
> Certainly not I!.
>
> Methinks that if the owner wanted more power, he could simply swap the
> 2A3 for a KT90 since the PT has a 6.3V heater winding, and then
> remove the series R I have placed in the B+ lines to trim the B+
> to where I want it, and with 270V, and with the UL tap on the
> OPT that is provided, maybe 9+ watts of SEUL would be available.
>
> The now unused 1670 OPT UL tap is at 40%.
> But it could be at 60%, if you use the plate winding reversed,
> thus further drifting a tetrode towards triode, but still not
> suffering a huge loss of power due to grid current limitations.
>
> 300B could also be used, and with slightly more Ia,
> but with the same low Ea, thus suiting moderate power and good matching
> to
> low value loads, which is a major failing in many SET amps;
> they try to match so max power is at 8ohms, but that is
> nearly always wrong with modern speakers, and
> max power should occur at 3 or 4 ohms.
>
>
> Patrick Turner.
>
>
>
>


.

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