Request for design guidance with Fixed Bias Output Tube

[RhythMick]

Did anyone read the article on the Andy Grove parallel 300b? First stage; I can’t figure why he’s shunting the cathode resistor and cap instead of bypassing.

Do you have the link to the article ? I saw the diagram Nick pasted but can't find the link in Google

[izzy wizzy]

Is he doing that to reduce the impedance and therefore increase the gain of the stage? As is, the 680R is the DC cathode resistor to set the bias and the shunted small value R is for AC to control the gain.

[izzy wizzy]

So - here's my current thinking. As always very happy to be guided ...

I have an unused secondary winding on the mains transformer which is 150-0-150 @ 20mA which seems to me ripe for providing bias +/- rails. Solid state rectification and filtered not regulated (as the valve B+ supplies aren't regulated). L&R Channels are fully independent. With LCRC filtering and a steady draw current I'm expecting to have say +100 / -100 available on the bias rails.

The amp is SE with balanced inputs : Input transformer to 26, LC coupled to 45, LC coupled to 300B, OPT to speakers (Lowther DX4 in parallel with powered 15" subs).

Tentative plan is to switch to fixed (adjustable) bias : 300B and 45, possibly the 26 too. Cathodes would be grounded. Something like 4 to 8 mA per valve would stay within reason for the transformer (builder confirmed higher mA not a problem but the voltage will drop a little). 5687 doesn't look comfortable at this low current level - either choose a different valve or use a different mains TX.

Along with choice of valve I need to choose cathode follower load to a) be within the max grid resistor for the following valve and b) allow the bias voltage to be varied within the range desired by adjusting the bias on the cathode follower. As the CF bias is varied, the CF current varies and so does the 300B bias.

Just about choosing the valve and load now, right ?

I'd be concerned about injecting noise into the more sensitive parts of the amp by going fixed bias on the first 2 stages. Seems like more hassle than it could be worth.

Nothaving done any sums, I think I'd want more HT + and - for a driver of that type. Not sure of the swing you need to drive the 300B but I think you'd want quite a bit of HT in reserve and not be up against any limits hence non-linerarity.

[Nick]

Just about choosing the valve and load now, right

You could just use current sources as the CF load and not worry about it.

[Nick]

Is he doing that to reduce the impedance and therefore increase the gain of the stage? As is, the 680R is the DC cathode resistor to set the bias and the shunted small value R is for AC to control the gain.

Ahh, I see, you mean the first stage. Without the 33R resistor the cathode cap will remove any AC feedback if its switched to use feedback.

[RhythMick]

I'd be concerned about injecting noise into the more sensitive parts of the amp by going fixed bias on the first 2 stages. Seems like more hassle than it could be worth.

Nothaving done any sums, I think I'd want more HT + and - for a driver of that type. Not sure of the swing you need to drive the 300B but I think you'd want quite a bit of HT in reserve and not be up against any limits hence non-linerarity.

Ah - see my thinking is very flawed. Applied to the grid of the 300B is of course a signal which could swing at full volume and if the 26 and 45 are at full gain, maybe 180v pk. That's not going to be driven by a Cathode Follower with such a small HT is it.

Back to reading.

[Nick]

You might be able to get away with a source follower, but no. I use +- 300v rails in my 211 which uses a el84 to drive the grid. You should get +- 200v from that transformer with SS rectifiers and CLC filtering though.

[Paul Barker]

Ahh, I see, you mean the first stage. Without the 33R resistor the cathode cap will remove any AC feedback if its switched to use feedback.

Thanks

[Paul Barker]

Rythm Mick I don’t know of the article, but Andy Grove designs usually published in a mag with a text explanation.

Nick, doesn’t help understanding the nodes above the 33r, but looks to me the 6au6 never passes any current. So circuit printed above doesn’t work.

[Nick]

Nick, doesn’t help understanding the nodes above the 33r, but looks to me the 6au6 never passes any current. So circuit printed above doesn’t work.

Again, not sure what you mean. I see 2ma through the anode and 1ma through g2, which gives 3,2ma through the cathode, with a 680R cathode resistor matches the 2v on the cathode.

[Paul Barker]

Thanks again Nick; I’d not followed the feedback path. Since global feedback isn’t part of my vocabulary I was blind to it and as part of the image I see on the phone it was out of site underneath. Never gave it a thought, it was bleached from my conscious mind.

[RhythMick]

Nick has inspired me to go learn about FET-based source followers. My brain hurts !

I'll start a separate thread for it as I have LOTS of (probably dumb) questions about that !
http://www.audio-talk.co.uk/phpBB3/view ... =11&t=7863