audio-talk

I've been planning something a bit more exotic for my current C3m>300B amp, been toying with the idea of a DHT driver. I have bought in some 45's and some 71A's but was out of action in all respects for a while last year . Now I'm getting back into some experiments, it feels like this 'upgrade' ( hopefully ) project is nearer . I 'm discussing anode chokes with Mr. Purvine of O-Netics right now, so seems like a good time to post a circuit .
Have a look at this, see what you think and if you have any ideas for variants or improvements . I think it looks quite tasty. I know Paul has something with 3 DHTs currently , and Nick built an amp with a 45 ( + aikido ) and a 45 driver for the 300B , so it's nothing really new .
Mark

Cor blimey - we'll all be doing it soon.

The only bit I'd quibble with is the front end simply because I'm determined to use a 26 on the front end.

I've been doing sleepless nights over this sort of design (three stage all DHT sort of thing - Thorsten's 300b, 300b, 300b stuck in my mind a while back)

I've got all the bits and even drilled out aluminium plates to take all the bits and allow chopping and changing. I still have a couple of cheap Hongkers plate chokes left over and Hammonds to come from Philip but I also have a pair of Mr Purvine's excellent 3k outputs (even though I wasn't supposed to get them!) waiting patiently in the wings.

onwards!

I know what you mean about the 26, but it just wouldn't have enough gain , and the ecc40 is a very nice sounding valve.

I would go for 1st stage 26 gain 8 and has a full sound (body) to add to 10y-801a gain 8 driver has lots of low end drive and very good mids and would give a 300b hf lift.

I can see the appeal of the 26, but as Nick says, not enough gain . This circuit matches my current amp exactly for gain, which works well with the phono and CD player.
10Y/801 is another option Will , they are lovely valves, and the typical higher anode voltage would put my grid a little lower for direct-coupling , giving the ECC40 a bit more room ( ie. anode around 160-180, and so a higher anode load possible ) but ..

...Choke would need a lot more henries , and I want to run over 20mA for grunt into the 300B, so those chokes could end up being monsters, and I DO want a choke-loaded driver.

Future upgrades might also add a grid-choke instead of the resistor .

Mark

This is the circuit that's had me thinking for a while ; Jono Noble mentioned his design to me :

http://www.audiodesignguide.com/se/katelelo.html

Fixed bias on those of course - takes a cap or two away but adds another bit of complexity .
I eventually found out there are curves for the MRO PT8 , there's an equivalent with info - 3A/141A .

One prob for my circuit - does anyone know of any small toroidal or EI transformers for about 4V at 1.8 to 2A to feed the DC supply for 45's ? I reckon I'll need 4V to throw some away in smoothing , correct me if I'm wrong .

Mark

One prob for my circuit - does anyone know of any small toroidal or EI transformers for about 4V at 1.8 to 2A to feed the DC supply for 45's ? I reckon I'll need 4V to throw some away in smoothing , correct me if I'm wrong .

I used a 6.3 torroide and then a current reg, seemed ok with a bit of heatsinking.

Other option is wind some turns around a toroid. Should be easy enough to get 4v or so,

I'm watching with interest, though I'm some way behind Mark.

Looking through the audiodesignguide site I came across a 6EM7-300BXLS circuit: http://www.audiodesignguide.com/my/new3 ... 0bxls.html

Looks interesting, anyone got any thoughts?

I've used the TS9 but it was ages ago
the base is a short pin ux4.
Sound is very good, top end is very very good.
Va 130v Vg -1.5v Ia 0.7ma Ri 55k gain 28
Vf 2.0v @ 0.25amp

Looking through the audiodesignguide site I came across a 6EM7-300BXLS circuit:

Ah, yes, the one with the back to back resistors,

I guess this will drive a 300b as well

http://home.lurcher.org/nick/audio/set/gm70ckt.gif

Actually, the last 300b I had here before it got taken to bits was usinga 6em7 like that, but with a 1:1 interstage instead of the choke.

How do you rate the 6EM7 as a driver for 300Bs etc. Nick? On the face of it, the combination could be quite good, but is it fair to say it hasn't stood the test of time in your amps? I should try feeding the Barker/Kaufman in to the 300Bs sometime...

It was the best combination I found. It didn't stand the test of time, as the entire 300b amp was taken to bits.

I will see if I can knock one together for this sunday at Simons if you are interested, I think I have the bits.

I am still waiting on some parts for the 211, so its something to do

Sounds like a plan Nick. I don't have anything new to bring but I could bring the WE91s and the KRs for comparison perhaps?

Sounds like a plan Nick. I don't have anything new to bring but I could bring the WE91s and the KRs for comparison perhaps?

Sounds good, If I build it with AC heating for the 300bs, then we can swap to the KR's without worrying about the heater supply having kittens.

Or the KRs! (I've put them through enough already...) Sounds good to me. I still want to hear what differnce your buffer stage makes in place of the passive I use too.