801a maximised class A2 excursion.

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JamesD
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#106 Re: 801a maximised class A2 excursion.

Post by JamesD »

That looks really nice!

This is a great project :-)
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Paul Barker
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#107 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Yes, It will be interesting, I can imagine the 801a would be great up to 7 watts, and most likely would beat the 811-10 in the first watt quality. But just as the 801a comes up to 2nd 3rd and 4th taking off together skyward the 811-10 has headroom.

And the way these things generally work out some will like A over B and some will like others way.

But it’ll be fun.
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Paul Barker
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#108 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

I’ll probably adjust for the output Valve biases at the first stage, so I’ll have to make sure first stage is linear at both output valve biases which is most likely -32 and -10 which is a fair difference. If I can’t achieve that I’ll do what I can at first stage and finish adjusting with regulation adjustment of the negative power supply for the CF direct driving the output valve.

If I can get it all done in stage 1 it would make life easier.
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Paul Barker
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#109 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

The main b+ is easy, my bread and butter, choke input LR smoothed R being shunt valves.

The B+ for driver and can get picked off from a tap off the vr string. With great pulling 50mA max for the vr’s used so as to provide grid current.

The negative supply same method but back to front so vr’s backwards.

Quite an enterprise. But I know that type of power supply is on the money. But big heavy and costly. Oh so worth it though.
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Paul Barker
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#110 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

In my heart I’d love to expand to 211 but 811-10 is enough and much easier as it’s a plug-in. I’d starve 801a with 6.3v so filament supply same. Just the bias of output valve to alter.

Reading forums on 10y as input valve not everyone likes it some report microphony. I might use MH41 conventionally for va and E280F for CF driver at its max 40 mA 4 watts. Would still give tiny bit of headroom and unlikely to draw above 40mA maybe 811-10, we don’t have grid current curves for it.

811-10 biased more positive than 801A to provide max power of 14 watts. But should kill the 801a after 8 watts Svetlana claim 1% thd at max power for 811-10.
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Mike H
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#111 Re: 801a maximised class A2 excursion.

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Image
 
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#112 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

I am very pleased to have found my smaller transformers, which I think are 4k5. As they rae a lot more manageable sized than my Tribute’s, and the best sounding transformers I have on a par with Tribute at the lower level stuff; I thought I’d check the distortions.
ImageImageImage

Don’t be frightened by the 4th harmonic, the point is even ta 10k load 3rd and 4th rise rapidly and all three distortions rise rapidly before 8 watts..

From the power/distortion graph, up to 6.5 watts it’s very linear. I can see this working as a 6.5 watt amp. But if you allow Stephies data to go to +50 grid volts the distortion goes worse sooner.

The lower load impedance has a silver lining; the grid current, max grid current < 10mA with 10k you’re > 30mA.
Last edited by Paul Barker on Sun May 24, 2020 2:20 pm, edited 1 time in total.
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Nick
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#113 Re: 801a maximised class A2 excursion.

Post by Nick »

How does the anode load affect grid current?
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#114 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

I’ve changed the quiescent point which looked better after fettling the options on the spreadsheet and deciding on this one. The actual quiescent point is in the data. You enter anode volt and cathode current, software predicts bias voltage.

IGNORE THIS POST I ASNWERED A QUETSION THTA WASNT ASKED.
Last edited by Paul Barker on Sun May 24, 2020 2:32 pm, edited 1 time in total.
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#115 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Nick wrote: Sun May 24, 2020 2:13 pm How does the anode load affect grid current?
Sorry, I answered that wrong, now I see you asked why grid current much lower.

That is because the 4k5 load intercects the most positive grid voltage at a higher anode voltage than a 10k load. Grid current is heavy influenced by the anode voltage at the time. So it stays in a much more low level all the time. With the 10k load it very swiftly rises to 20 mA as anode volts drop towards 50v.
Last edited by Paul Barker on Sun May 24, 2020 2:31 pm, edited 1 time in total.
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#116 Re: 801a maximised class A2 excursion.

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Nick
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#117 Re: 801a maximised class A2 excursion.

Post by Nick »

Ahh, I see, you mean at the extreme end of the transfer curve. But for most (more than 100v on the anode) of the time the grid current looks constant(ish).
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#118 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Yes that’s it, which means that in reality less is more.

Over the years some impressive claims of power output from supposedly impressive people are swallowed by the ignorant. But the reality is there is a cliff edge well short of fanatical power output claims.
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#119 Re: 801a maximised class A2 excursion.

Post by pre65 »

Paul, I wondered if you knew about Pete Milletts A2 buffer (source follower) circuit and the PCB he does ?

http://www.pmillett.com/A2_buffer.html

https://www.ebay.co.uk/itm/DIY-PCB-MOSF ... 1438.l2649
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Paul Barker
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#120 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Thanks buddy noted.

I’ve crawled back to my tube only comfort zone for the now.
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