Simple 300B SE Amp
I have recently assembled the 'Simple 300B SE amp' from the circuit drawing shown in the FCUPS manual.
Many thanks to Thorsten for his help and advice during the build process.
The music making ability of Ergo/Ella chasis with DIY Hi Fi 5K amorphous Nickel output trannies, dual FCUPS PSUs, 300B heater modules and fixed bias module is everything I'd hoped it would be.
I have now changed the 6SL7 driver stage for the D3A driver stage,
suggested by Thorsten.
Each D3A has a CCS on the anode set
to 10mA and a 220 ohm cathode resistor bypassed with a 100 uF cap.
I have a spare DIY Hi Fi bias module and wonder if it is feasible/desirable to change the D3A to fixed bias? Would using CCS and fixed bias on the same stage work? I assume i would
need coupling caps on the inputs to block the -dc on the grids of each valve. What value should they be?
Many Thanks Kevin M.
It is not recommended to exceed 50K...100K DCR for these tubes (D3a et al) grid return, so that forces our capacitor value. With 100K we need at least 0.22uF, with 50K at least 0.47uF. Given that bias module puts out a lot of negative grid voltage I think a 10V or so set of Zenner clamp's should be used to avoid getting the grid too negative, this can be done safely with the Active Bias Module...
So, 10V zener clamp across the output of the active bias module, 100K resistor for gridleak, 0.22uF input coupling cap (PTFE & Tinfoil recommended) and 39R cathode resistor for D3a for 10mA.
That's it. Fixed bias input stage.
HOWEVER, there is an even easier and more fun way.
Use battery bias. It is described here by Troels Gravensen:
TRAM WOT Preamp with Battery Grid Bias
This is all near the end of the page, together with some comments from a certain "Peter Pan" (my Guild name in Harvey "Gizmo" Rosenberg's Triode Guild)...
I've pondered the merits of both LED and battery bias and wondered how they might compare to fixed bias in the D3A driver stage.
I guess you've pre-empted my next question by pointing the way to battery bias.
I looked at the very interesting Troels Gravesen article and will see what i can do to implement It. 10mA through a 220 ohm cathode resistor gives me 2.2 Vdc bias.
A single lead/acid cell would give be about 2.2 V so maybe that would be the answer. Thanks, Kevin M.
The "Battery in series with the grid" method is to me preferable to any other, including cap coupled fixed bias.
B+ = 247 Vdc. 3V lithium 'coin' battery fitted into a small battery holder with - conncted to the 1 K grid stopper and + to the input. 10M resistor and 10.4 nF silver
mica cap fitted across battery terminals.
The circuit you pointed me to had the wiper of the (volume control?) connected to
the bias battery but no grid leak resisitor.
I assumed i needed one and tentatively put a 100K between the battery + and earth. Do i need one and if so what value would be most suitable?
No hum or noise even close up to my PM6As!
Initial Impressions are that the bias batteries are there to stay.
Oooooh I like It!
Thanks yet again for some great advice, Kevin.
Most people would throw a major wobbler at putting a battery in series with the "holy signal", after all the battery is right in the signal path, right...
Except, as in theory the grid current is zero (and in reality is next to zero) ther eis no current flowing, hence the battery cannot distort...
Yet people think that the Nicad's they put into the cathodes are not "in the signal path", even though any change in signal will change the current going through them appreciably and any distortion that appears across the battery as result appears in series with the signal...
Do keep us posted. May be worthwhile showing your build (and the unusual tricks) in the Tube Forum, also with pictures... It should pretty good on the Ergo chassis... ;-)
I've been experimenting and replaced the (3.3v) lithium batteries with 2.4V (non-rechargable) NiMH 'coin' batteries.
These batteries seem to work/sound? as good as the lithium ones and 2.4V is alot nearer to the original 2.2V used with cathode bias.
I'm loath to play with the CCS as I set them using a mA meter so I cheated and
added resistors between the FCUPS and CCS. 3K (10W) brought the anode volts
down to 180Vdc. I could try 2K and raise the anode to approx 210 Vdc.
With AVC volume control at 0, a faint hiss can be heard with
an ear very close to my PM6As!
With 3.3 V battery bias and 250V on anodes I thought that decay on, say, piano and recorded ambiance/acoustic were much improved. Front-to-back depth seemed very slightly reduced and a very, very slight 'glare' was noticible. The 2.4V bias gives similar Improvments but front-to-back-depth is better. The overall feel is slightly
'darker' and more 'see through'/ glare free/relaxed. May be it was the anode volts being too high? I'll think about some photos but that may have to wait till later as
I'm away with work soon.
imprved 'timbre' and front-to-back 'depth' is
A friend is very impressed with my amp and has asked me to build him one.
He has a pair of Audio Note ANEs (the newer 97db/1 watt variety).
He'd prefer an amp set up to give more power than the nominal
six watts mine puts out.
If I used more conventional 2.7K ohm output Transformers could i
put the B+ to something higher, the 300B bias to 80mA and still use the
same 'simple 300B SE amp' circuit with D3A driver stage?
He's after best possible sound with more watts...
Would this be the best approach or would one of the other DIY HIFI kits
be more suitable?
He's owned a couple of very famous 300B amps but is looking for something
a bit special sound wise.
Any Ideas. Kevin.
You can read on that Amp here:
6moons audio reviews: DIY Hifi Supply Lux 91 Max Monos
I build a close version of the "simple SE" (okay, not so close, choke load for D3a not CCS, self biasing 300B, tradistional CLC PSU) for a friend in england who lend him to the reviewer, who liked them, but would have liked more power...
So the Lux 91 was born, specifically to drive above average, but below really high efficiency speakers...
If your speakers allow, a Lady Day 91 or a "Simple SE" are IMHO slightly better choices, but these trade power for finesse. The Lux 91 has a little less finesse, but much more brawn, which with speakers like the AN-E is what is needed IMNSHO.
We had a similar discussion when i was looking for an amp to drive my 100db
speakers and you recomended the simple SE amp in a Ergo chasis.
This was an excellent choice so I guess I should go with your
recommendation of the Lux mono max for the ANEs.
These would be within my friend's budget and i would't even have to build them!
He sort of fancied something along the lines of what's offered by the company
who makes his speakers. I built and heavily upgraded their KIT amps
before moving on to something more to my taste.
I'll see what he says. Kevin.
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