801a amplifier

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Paul Barker
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#121 Re: 801a amplifier

Post by Paul Barker »

Meanwhile

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Paul Barker
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#122 Re: 801a amplifier

Post by Paul Barker »

Roxy giving body language to say “I own this” or is she saying “ooh that’s nice heat”

So gm70 no good in gk71 base. Either need 813 base (which I have) : carefully drill out one ceramic hole.

Or scout eBay for gm70 bases, These look good.

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I presume you solder to the silver sockets’ anyone?

Had to buy with graphite gm70. £108 total delivered and taxed take off £68 for cheapest pair of gm70 elsewhere = £20/base.

Yes, worth it.
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#123 Re: 801a amplifier

Post by Paul Barker »

You may recall the concern I had at wearing out my few 801a valves. Down to 4! At same time I mentioned this I bemoaned the fact no suitable alternative valves.

So I was idling and looking once more at 1619. It fits the bill.

Using 6v6 triode curves. As Ive said before there is a misdirection about this valve on audio forums. It is not a directly heated 6L6, it is a directly heated valve closest to 6v6. Not intended as an equivalent, but should operate closely to 6v6, whereas its not on the same page as the 6L6. I think that whole thing was a mistype by one person once and that syndrome of repeating the error by people who dont verify things for themselves.

That corrected, the 6V6 triode connected into 9k operated at 14 watts dissipation, 350v quiescent in A1 provides 2.22 watts, @+10v A2 3.47 watts, @+20v A2 5 watts. (All calculated from measuring the characteristic curves) ((ve - va) * ( ve - va)) / 8*9000.

I cant calculate 2nd harmonic distortion graphically because the characteristic curves Ive found dont go beyond 500v and anyway the curves 2nd distortion pinch as expected. If A2 is encroached into as pointed out with 801a analysis above odd order distortion would sky rocket. But anyway, with this less power valve I cant ignore the A2 likelyhood. Which means fixed output valve bias, and in my case cf drive, since Im not interested in mosfet power drive. ACFII screams at me, but I may not add that task, rather pop a cf single stage in. Or probably nothing built.

Valve costs 1619 (which I dont need to buy, but replacement cost £15 each) 801a ( £70 to £150 each).

I need to drop the b+ from 630 volts to 350 volts. For breadboarding simplest option variac after seperating filament supply for rectifiers. Long term, alternate transformer.

The figures given are for one valve into 9k, but as with the 801a vesrion the 9k os what you drawer into the characteristic curves. The two output valves per channel are pse connected and the opt is 4k5. So teh actual power output doubles to 4.44 watts, 7 watts 10 watts.

What am I expecting ? a sweet sound rich in second harmonic, enough to play my music at home, much more distortion than the 801a version. But I may like that! Not a waste of effort anyway: plentiful, affordable replacement valves.
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#124 Re: 801a amplifier

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Couldn't agree more! recently put together a PSE 1619 SE amp (295V ht - 3.5k load - driven by 12gn7 a'la Pete Millett) very sweet sound + 1619's £8.00 from Langrex!) and ac htrs are fine!
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#125 Re: 801a amplifier

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😎 👍 I null the hum anyway connecting ac filaments each if a pair out of phase. With push pull you connect the pair in phase. Seperate windings, same core. A different core for other channel.

The 801a connected so has no filament hum..

The last 1619 amp I built was push pull. It was mint. I lost the phase splitting transformers! Still smarting from that loss🤧😒
Last edited by Paul Barker on Tue Dec 27, 2022 12:21 pm, edited 1 time in total.
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#126 Re: 801a amplifier

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Me too 😊
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#127 Re: 801a amplifier

Post by Paul Barker »

OTOH, I have 11 10y’s so as they use same base and heater as 801a one less thing to change. At 350v 25mA into +30v A2 7.4 watts x 2 per channel into full distortion. I was going to add a cf dc driver and negative supply anyway. Worth trying if only as phase one. But might surprise, one way or the other.
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#128 Re: 801a amplifier

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I think I’ll aim at a new goal, distortion curves at lower B+ voltage, but with +30v max grid voltage rather than zero positive grid, seems to look better.

So how to implement? Drive it with direct coupled el84 CF and a +/- power supply

Remember when looking at the curves power output is double. Distortion curves reveal no more than 9 watts, so double that 18 watt from pse 801a’s under 1% second, 3rd and 4th stay low to 9 watts. Just before clipping it should sound great.

10 mA grid current for parallel output valves doesnt seem difficult for el84. Characteristic curves look pretty linear left to right extremes at this operating point.

When I can do it is the big question. Not yet!
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#129 Re: 801a amplifier

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#130 Re: 801a amplifier

Post by Nick »

I would put the cap on the bias for the CF between -200v and the center. That way on power up the bias for the output stage starts -ve.
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#131 Re: 801a amplifier

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Nick wrote: Sun Jun 11, 2023 12:08 pm I would put the cap on the bias for the CF between -200v and the center. That way on power up the bias for the output stage starts -ve.
Thanks Nick.
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#132 Re: 801a amplifier

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Ive got more 10Y’s than 801a’s.

So here are 10Y adjustments, looking like 8 watts looks ok.

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#133 Re: 801a amplifier

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Goes without saying the distortion spectrum of the 801a and the 10y isn’t that of a lush triode like 2a3. Hence the Pentode Voltage Amplifier, but rather than seen as adding odd order harmonics to the blend to bring better timbre to the output valve s contribution, the aim is to oppose the odd order harmonics of the output valves…
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#134 Re: 801a amplifier

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Adjusted for Nick’s helpful suggestion.

I could make the two resistors which set the EL84 bias and hence the output valve bias various arrived biases for other output valves, aswell as a linear potentiometer to arrive at values. So the first two valves could make a complete preamp with different biases for different output stages.
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