Re: 2A3 - Really a beautiful power tube!
- From: "Fabio Berutti" <fabio.berutti@xxxxxxxxxxxxx>
- Date: Mon, 05 Sep 2005 09:46:51 GMT
Just a brief note concerning the cost of a PP arrangement: there are still
some good NOS Svetlana 6S4S for sale (or 6C4C as it looks in Cyrillic), a
6.3v-octal base version of the original dual-plate 2A3, which cost some 25$
per tube, if You spend some time "googling" around (http://rvd.gstube.com/
or http://www.tubes.ru/GlassTubes.html). 100$ for the whole power tube
complement is not that much, I'd say. Sure, if You want four RCA single
plate NOS the figure goes up quite a bit.
"Andre Jute" <fiultra@xxxxxxxxx> ha scritto nel messaggio
> One of the finest amps I ever heard was a PP 2A3 from the late lamented
> Audio Innovations called IIRC The First.
> In Patrick's application SE 2A3 will be more than adequately powerful
> to drive the HF, which really doesn't require even a whole watt even on
> orchestral fortes. (Experiment: make a filter to lose everything below
> 8K5Hz. Listen to the remainder. It is a sort of buzz, not music. Its
> actual power content is fractional. Most people a lot younger than the
> average age of RATs cannot hear that there is a difference between one
> of my Impresarion with the tweeter and one without, the one without
> tapering off about 10kHz.)
> But I have my doubts about SE 2A3 even with a sensitive fullrange horn.
> A DHT is usually decently silent but I like further to silence it with
> a high load on the plate. When you do that the power falls alarmingly,
> soon to under one watt, which is not too hot on even a Lowther horn if
> you want to play orchestral music. With a Lowther horn (c100dB/w/m) I
> like about 4W of very silent power, which is why I use 300B instead of
> 2A3. To address Fabio's remarks, I don't think a 3.5W 2A3 SE with a
> 97dB speaker is a rock'n'roll combination. Far too much powere required
> in those bass "transients" which is rock'n'roll can go on forever.
> Also, while theoretically I like the *concept* of PP DHT, I am a
> Calvinist who shrinks from such expense when a triode-linked PP EL34
> amp will give you the same power and 99 per cent of the sound for half
> the price. There used to be a salesman called Zip on RAO (he was
> disgraced when he couldn't distinguish one cable from another); Zip
> used to tell people who complained about the price of the amps he sold
> that they lacked commitment to their music, as if how much you spend is
> a mark of merit. Zip was full of shit. I have passion for my music but
> there is no reason to let it interfere with common sense.
> Andre Jute
> Gotta sleep. E&OE.
> Fabio Berutti wrote:
>> A well-known manufacturer of 400W amps like Mr. Turner is someone to
>> when he says that as low as 4W can still be enough, if all the "sound"
>> is properly set. I treasure my own 2A3, particularly for its
>> sound, which is not "weak" or "tubey" indeed. The only limit of the 2A3
>> is "loud" music: my LS are about 97 dB, but when listening the Stones,
>> Mick simply isn't shouting and Keith's guitar isn't as mean and nasty as
>> Nevertheless, this is not a real limit: a HiFi set is a sort of a musical
>> instrument. Just like a Stratocaster is not considered "bad" 'cause it's
>> not suited for playing Bach, a 2A3 SE is not "bad" 'cause it's not good
>> AC/DC (the Oz group, not the power supply); mr Turner correctly quoted
>> the amp was very good for playing chamber music, not trash metal. In
>> I agree with Mr. DeWaal: a 2A3 PP (or a 6B4G) is probably the closest
>> to the "perfect" sound: DHT, relatively low driving requirements,
>> low output impedance (3K5 typ), reasonable B+, and an output power of
>> 10-12W which is enough even for the 9th symphony's chorus or the
>> of "Back in black"... if You have efficient boxes and your neighbors are
>> There are thousands of "viable" circuits, but I'm still persuaded that
>> an engineering point it's better to avoid problems (or distortion) than
>> create some and then try to get rid of them. The 2A3 provides a nearly
>> perfect transfer function of its own, therefore the "tinkering" required
>> make it "20 to 20k" can be kept to a minimum, which ultimately results in
>> unpredictable "electronic traps", fixing some bug here just to make a
>> one somewhere else.
>> The only problem with a 2A3SE with good LSs is that suddenly You really
>> how badly many CDs are recorded. Some jazz LPs from the Fifties are lots
>> "Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> ha scritto nel messaggio
>> >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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