Future plan for 300B XLS amp

[IslandPink]

Thanks for the tip on 'Rapid' , Nick .
There's a little toroid that's 7VA with 2x 7V that will be ideal to feed one ECC40 .

Mark

[IslandPink]

So anyway
A day or two back I read some comment by Mark Kelly that if you were direct-coupling, you could stand the output valve on VR tubes to avoid a big resistor and cap . So I had a think and came up with the following option for the driver stage . I wonder is there any big problem with this ? I suppose it classifies as fixed bias . With the choke load, the current through the OD/OC will be pretty stable too . Should brighten up the top-plate of the amp if nothing else .
MJ

[Nick]

The only thing that would worry me is the impedance of VR tubes is not linear over audio frequencys.

[IslandPink]

OK, so is it any different to the situation where the VR tube is a shunt element on a power supply taking the return path ? ( which works )

[JamesD]

If you use the appropriate Zobel network across the tube you can cancel the tube inductance. This works well from about 100Hz up but I'm not sure its so good below that...

I've tried it and it sounds nice but I always end up converting to a decoupled resistor - their is some magic in the decoupled resistor inspite of the pain of tweaking the cap type to get the sound 'right'.

James

[Nick]

I will see if I can find the reference to the problem, but it was described as being due to the conduction being down to ions instead of electrons, and the resultant much larger transit time.

Though I agree that the same could be the case for shunt use.

[IslandPink]

Ok, thanks for the advice chaps . It's not really a hardship to stick with a good NI resistor and a BG VK cap , should sound fine . 11k and 22u gives around 0.65Hz as the -3db point if I'm not mistaken .

If Bud comes up with a wild price for the 50H chokes, what other options could I consider ? ( 45's at around 22 or 23mA quiescent is my target ) .

Mark

[Nick]

When they start taking orders again, you could get a pair of AE 1:1 interstages, they are about 45H @ 50ma. use them either as a choke or interstage.

[IslandPink]

Thanks for the tip Nick...but how soon will it be till they start taking orders again ?
Let's see what Bud comes up with first , because he can usually supply in a small number of weeks .

Meanwhile, the circuit Dave Slagle showed is this one ( it came from the 'forum that cannot speak its name' ! ) Hope he doesn't mind me posting it here . Pretty much state of the art :

[Nick]

Blimy, I see what you mean.#

AE Are taking orders again on March 24th

[david C]

as an aside, does anyone have a view on the AE interstages?

[Nick]

You can ask folk tomorrow, as the 300b that I am builiding to take to Simons tomorow has a pair of them in it, and also Colins gm70s have an amorphous version in them.

One thing to bear in mind is they are bifilar (at least mine are, which I suspect are a early version, as the way they are wired haschanged), so if the windings are not in phase you will loose a lot of the higher frequences.

[david C]

Ta Nick,

[IslandPink]

Maybe I should just leave the amp as it is ?
It's pretty fashionable :

http://www.6moons.com/audioreviews/yamamoto6/a09s.html



MJ

[IslandPink]

Just instructed Bud Purvine to build me a pair of anode chokes.
55H , 265DCR , M3 core. Will take max 85mA DC , which is way more than I need but hopefully very linear over typical operating range.

10 weeks delivery .

Mark