300B SE amp: One more one

[Yvesm]

Word less . . .







Yves.

[Gluca]

ives ... stop making clever amps.

[Rod Coleman]

Yves,

I like pentode driver for 300B too! 6BW7 at 170V/10mA sounds nice.

But if we are to be KISS, do we really need the fixed bias windings and network? Why not be more KISS with a cathode resistor?

And then you can use MKP capacitors in the Western Electric network for better sound!

With ANSAR CPA series you can get good sound for a low enough price -

http://www.cricklewoodelectronics.co...me.php?cat=155

[kevinkr]

Originally Posted by Rod Coleman

Yves,

<snip>

But if we are to be KISS, do we really need the fixed bias windings and network? Why not be more KISS with a cathode resistor?

And then you can use MKP capacitors in the Western Electric network for better sound!

<snip>

Should be mu + 1 since the noise injection point is the cathode, in practice I've found the value is not that critical.. I have used this with considerable success in a couple of simpler designs where the improved psrr was worthwhile, that said there is nothing like no cathode bypass capacitor and for this reason I still prefer fixed bias. My best sounding designs IMHO and by general consensus all use fixed bias - you are swapping slightly more power supply complexity for a simpler signal circuit path IMHO..

YMMV and just my

[Yvesm]

Hi Rod,

It's true that cathode bias could made things more KISS, but, as kevinkr, I really dislike any kind of bypass capacitor at so critical location.

"Fixed bias forever"

Yves.

[planet10]

Yves,

Thanx for another interesting amp.

dave

[Rod Coleman]

Salut Yves,

The cathode capacitor must be best quality MKP, for sure. Certainly no electrolytic, however fancy.

With fixed bias, you still have capacitors in the signal path - C2 in this case. This is the 300B input (grid) network current, too - I would say this is even more critical than cathode current! Noise from the fixed bias components, like D3 will also be injected here.

[Rod Coleman]

The B+ regulation looks interesting -
How does the 350V regulator sound, compared to using a choke-input power supply?

[Yvesm]

Originally Posted by planet10

Thanx for another interesting amp.

You're welcome

Originally Posted by Rod Coleman

Salut Yves,

The cathode capacitor must be best quality MKP, for sure. Certainly no electrolytic, however fancy.

With fixed bias, you still have capacitors in the signal path - C2 in this case. This is the 300B input (grid) network current, too - I would say this is even more critical than cathode current! Noise from the fixed bias components, like D3 will also be injected here.

Mmmh, yes, zeners are noisy and it's why C2 is here !
However, at this point (bottom of the grid leak resistor), noise should not be a real problem.
The shunt regulator (D3 + Q1) is supposed having the capacity to absorb possible 300B grid current while maintaining stable voltage accross it.

How does the 350V regulator sound, compared to using a choke-input power supply?

Can't tell ! ! I never used choke filtered supplies since rugged silicon diodes and decent litycs capacitors were indroduced (thanks to SMPS requirements). . . some decades ago
But, should a power suply "sound" ?
Probably it does cos in SE design signal has to pass thru it and the tube before reaching the OPT primary.
My goal was to keep PSU internal impedance low (says 10% of the Rp of the 300B) across the whole audio range.

I've tried to do "something else" and it seems to my ears that this thingy does not sound like a typical 300B SE.
This was the callenge I wanted to fullfill, sorry if this lent me to some heretical approach

Must I add that I'm not a pure SE designs lover ?
Anyways, I had pleasure building as well as listening that.

Yves.

[Rod Coleman]

Salut Yves,

Tell us about the output transformer - was it the quality you used at ETF2009? With success!

The design of that transformer would be interesting to see.