801a amplifier

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Paul Barker
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#16 Re: 801a amplifier

Post by Paul Barker »

I realise after something @Nick said I’d gone overboard on driver. Max grid current of 801a at chosen operating point is 6mA for a miniscule part of the signwave when grid drive hits +30v at 50v end of load line.

That in mind I’ve penned a driver circuit. The 5687 runs at 23mA.

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Nick
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#17 Re: 801a amplifier

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I would have just driven the 5687 grid off a pot connected between 0v and -ve (with resistors either side), and used a current source (SS or the other half of the 5687) as a load and driven the grid of the 801a from the 5687 cathode. Far simpler (and reliable) than trying to make the 5687 auto bias itself to the required position. Plus its both adjustable to suit the 801a and also will tend to track the supply rails varying if not regulated.

You could still inject into it for hum suppression if its seen to be a problem. Personally I find getting quiet rails and supplies is simpler and more effective than such methods as they make unsupportable assumptions about linearity.
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#18 Re: 801a amplifier

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I realise after something @Nick said I’d gone overboard on driver. Max grid current of 801a at chosen operating point is 6mA
It wouldn't matter if it was 30ma, the 5687 is able to pass that much current if needed.
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#19 Re: 801a amplifier

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In case someone doesn’t know the hum cancellation is more complexed but all I’ve shown is where it’s injected back in. At 801a filament supply it’s divided down with a pot and fixed resistors, adjust on test.
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#20 Re: 801a amplifier

Post by Paul Barker »

Nick wrote: Wed Jul 07, 2021 4:36 pm
I realise after something @Nick said I’d gone overboard on driver. Max grid current of 801a at chosen operating point is 6mA
It wouldn't matter if it was 30ma, the 5687 is able to pass that much current if needed.
Thanks for waking me up to this Nick. Having struggled on the drawing board balancing both 6080 and 6bx7 to allow use of a balanced power supply while also ensuring more voltage swing than necessary which we should observe or were limiting the output valve, worst thing we could do. That’s when you’re listening to the driver more as it’ll clip prematurely and not do it as nicely as an 801a.

The mental gymnastics I had to traverse not forgetting anything were blocking my path in such a way I nearly gave up and went with two supplies of different voltages. But the 5687 middle operating point of the 3 suggestions of RCA sat the output potential bang on. So power supply much simpler. Now the project is awakening.
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#21 Re: 801a amplifier

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Nick wrote: Wed Jul 07, 2021 4:36 pm
I realise after something @Nick said I’d gone overboard on driver. Max grid current of 801a at chosen operating point is 6mA
It wouldn't matter if it was 30ma, the 5687 is able to pass that much current if needed.
I take you’re point and agree.

I went auto for the anti-ossillation benefit and 5687 feedback benefit. If this costs too much impedance with no actual better sound I’ll try fixed bias off voltage devider as you say. Indeed the hum cancellation and fixed bias resistor network would combine with the bias network taking over elements of the hum cancel network serving two purposes.

I’ve used Stephies circuit before and found it simple to build totally reliable 22db quieter and no strange effects.
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#22 Re: 801a amplifier

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The hum cancel is at 100hz a lot better than the 30khz AC filament supply we used to do. Which you pointed out to me the problem it caused, which you tried warning Alex of but he wasn’t interested in you’re opinion at the time.
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#23 Re: 801a amplifier

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Obviously circuit not yet trialed and as Nick mentioned means of correcting output valve bias is with driver bias. I’ll put a higher value pot alongside the cathode resistor which would require recalculation so I can bias everything up. I plan to sense current and display it so while listening I can correct anything that shows up fast, and always have an eye on conditions. The adjusting pots will be placed quite visible so easy for me. Ill put a metal screen over them so non valve amp builders are not given a pot to play with.
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#24 Re: 801a amplifier

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just had to buy a 5687, cant believe it.
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#25 Re: 801a amplifier

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The output valve B+ is in the plan to have valve regulated power supply, voltage on display and tickle pot as for other pots, protecting the ignorant.
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#26 Re: 801a amplifier

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I went auto for the anti-ossillation benefit
But you aren’t driving long lines or a capacitive load.
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#27 Re: 801a amplifier

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just had to buy a 5687, cant believe it.
I refer the reader to my use of el84 as a cathode follower :-)
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#28 Re: 801a amplifier

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Not missed.
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#29 Re: 801a amplifier

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Taken youre recommendation compared the two valves. EL84 triode looks considerably more linear with more transconductance, and not much rethink to operate it at similar parameters. So I’m off to eBay see if I can find those Russian military equivalents.

Thank you Nick
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#30 Re: 801a amplifier

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