My 6SL7, 2A3 SE design

[mig-ru]

ok this is the final version of my 6SL7, 2A3/6A3/6B4G Single Ended design. feel free to critique. also was wondering if this will drive 91dB speakers to sufficient volume. and seeing as to what happened to my last post, if we could stick to the matter at hand... thx.

[dhaen]

Dmitri,

"Keeping to the matter at hand": I don't know.

What is your anticipated output power?

Also, have you considered what the unbypassed cathode resistor is doing to the anode resistance of the 2A3, and it's already low value of anode load (the transformer).

My thoughts,
Cheers,

[Brett]

Hi Dmitry,

First things first.

<b>wondering if this will drive 91dB speakers to sufficient volume</b>

No. About 10dB more sensitivity in the speakers, or 10dB more amp power would be my <i>minimum</i>

<b>feel free to critique.</b>

OK, but unless you listen at low volume to simple bass light music, the amp won't produce enough power, and will sound disappointingly wimpy.

Output stage.

- biassed incorrectly. An Rk of about 750 ohms will give approx 60mA and Vg = -44V, right smack bang in the middle of the 2A3 sweet spot.

- bypass the cathode resistor with about 100uF of quality capacitor.

- 330V for the output stage is a bit high. In line with my bias recommendation above, reduce it to 295 - 300V

- grid leak is a bit high in value. I'd try 1/2 to 1/4 of this and adjust C1 up accordingly. An IT or a grid choke would be <i>much</i> better.

Driver
- Why 6SL7? It will have a gain if about 70 in this application (so amp sensitivity of 600mV) which is a bit high for modern systems, and may present some noise issues, especially with an active preamp. With a pre, you'll also be throwing away tons of gain.
The output Z of the driver will be about 60K. Yuk!. It'll sound like an old fashioned clock radio. A 6SN7, 6H30, ECC99, C3m (trioded) or even one of the 6C45/417/5842 would be better, with the ECC99 being my first choice (ECC99 ; B+ = 300V, Va = 100V, Ia = 20mA, Rk ~300ohm probably bypassed, Ra = 10k, at a guess)

DHTs sound a lot better driven by a tube with some grunt, or all you hear are the driver colourations very clearly. I know there are some people here who will disagree with this, but this is my own direct experience, as well as people like Paul Joppa.

- unless the pot at the input is there as your only volume control, get rid of it.

PSU
- First C is OK, choke should be about 10H, second C about 100uF. If you drop the output stage voltage to 300V as suggested, you can probably run the driver off the same supply. Note, I haven't run the numbers on the simulator yet, but that should be close. Of course another LC stage wouldn't hurt.

- Add some common mode chokes to the filament supplies for the 2A3's, as well as some smallish caps right across the socket pins, to remove any HF noise that comes through from the mains trans. The filament on a DHT is as sensitive to noise as the grid.

- Mains trans should have an electrostatic sheild (EI core), or be of dual bobbin constuction. A seperate ES sheilded filament trans would be nice.

HTH

[analog_sa]

Dmitry

6SL7 is a very pleasant sounding tube but it is not a driver for a 2A3. I have tried this combination and the sound is pale and anemic although very clear. If you like dynamics and bass get a juicier tube.


cheers

peter

[James D.]

Hi Dimitry,

Please listen to Brett's comments on your design. He is giving very, very good advice. The same advice was in your last post before the thread discussion widened.

I recommend you build the design your way and see how it sounds and then revamp it along the lines Brett suggests and listen to that. See if you think we are giving good advice.

Your power supply design is not going to remove enough mains ripple to prevent hum on the output. According to psu11 sim. you have over 700mV of ripple for the 2A3 and 235mV of ripple for the 6SL7 - as this is running at a gain of 70 you will have lots of hum on your output. If you do not have access to bigger chokes and capacitors then I recommend a regulated power supply of any type...You should be aiming for less than 5mV ripple for the 2A3 and at least an order of magnitude less for the 6SL7 - IMO.

ciao

James

[mig-ru]

forgot to mention that the 6SL7 is biased at -1.5V and the 2A3 at -40V. also the 6SL7 is setup to produce a gain of around 47 as far as i can remember from my calculations, not the maximum 70. also, for the lack of cathode bypass capacitors is to minimize distortion. and to answer dhaen in what my anticipated output is, well somewhere near a 12AX7 sections in series with EL84 output amp runnin near 2-3watts. also, Brett i dont see how u can suggest a 6SN7 instead of a 6SL7, seeing as how the 6SN7 has a max gain of only 20. ive seen a few amp designs using a 6SN7 put in series then driving a 2A3, so that would b about the same as using a single 6SL7, unless there is some current difference between the two. please reply.

[analog_sa]

Hi Dmitry

A gain of 20 is probably acceptable. The main advantage of the 6SN7 is the lower Rp - 7.7k which in fact is not quite low enough.


peter

[dhaen]

Hi Dmitri,

I think your LF distortion may be worse with no cathode bypass, of course this depends on the inductance of your transformer, but probably.

By leaving the cathode unbypassed, you are providing some local NFB, but the Ra of the 2A3 is then higher, and so it has to work harder to get any current into the output transformer.

Also, the parasitic capacitance of the transformer will become more significant, so HF may drop slightly.


That's how I see it.


Cheers,

[fdegrove]

Hi,

Also, the parasitic capacitance of the transformer will become more significant, so HF may drop slightly.

Audibly and measurably so.

Setting a side PSU filtering and input stage,this what I would do:

B+ 310 VDC will give you about 300 VDC on the 2A3 anode.

R6 lower to 750 R + add Ck 100 mF 160 VDC.

R5 lower to 220K.

And with a bit more work you loose C1 too and go Loftin White style.

Cheers,

[Brett]

Originally posted by mig-ru
forgot to mention that the 6SL7 is biased at -1.5V and the 2A3 at -40V.

I ran your numbers in Livecurves, and I get Vg = -59V, Ia = 48mA, with a loadline that dips well down into the highly pinched section of the curves. I estimate about 2.7W (2A3 cathode bypassed), with a heap of distortion, even by SET standards.

also the 6SL7 is setup to produce a gain of around 47 as far as i can remember from my calculations, not the maximum 70.

I don't have a very good set of 6SL7 curves in front of me, so I'll take your word for it. However with such a high Ra, and a low B+, the current through it will be very low, raising the Rp even higher than I calculated. Again, you'll be running it down into the highly non linear parts of the plate curves. Therefore even more gutless than I thought, and whilst possible suitable for a guitar amp gain stage, it will sound like crap as a driver.

also, for the lack of cathode bypass capacitors is to minimize distortion. and to answer dhaen in what my anticipated output is, well somewhere near a 12AX7 sections in series with EL84 output amp runnin near 2-3watts.

Lack of cathode bypasses in both these stages will increase distortion for the reasons outlined previously in this post, and the one before. It will also produce almost no power, as the output impedance of the 2A3 stage wil be greater than the load.

An EL84 is not going to give 3W SE; 1.7W is greater than 8% HD in triode, let alone what it would have in pentode or UL

also, Brett i dont see how u can suggest a 6SN7 instead of a 6SL7, seeing as how the 6SN7 has a max gain of only 20. ive seen a few amp designs using a 6SN7 put in series then driving a 2A3, so that would b about the same as using a single 6SL7, unless there is some current difference between the two. please reply.

Interesting that you choose to pick the 6SN7 out of the list I mentioned. Whilst I generally like the 6SN7, I don't think it makes a good DHT driver. ANY of the others would be far better. Even with less gain it (6SN7) would still be a better choice, because, bypassed a 6SN7 will actually drive it decently (not really well). What is the point of having a linear tube (2A3) and then putting an athsmatic driver in front of it to colour it all to hell? And this with inefficient speakers.
BTW, two cascaded 6SN7 sections would give more gain than a single 6SL7.