801a maximised class A2 excursion.

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

Post by Nick »

I would make sure the 2sk170's you have are real, they haven't been made for years. I use LSK170's which are a re manufacture of a equivalent part.
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Paul Barker
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#92 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Thank’s Nick I’ll order you’re suggestion.
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#93 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Drifting away from transistors right now, I’ve ordered some 6J4’s, I have 6bx7’s I’ll take refuge back with valves. To me it’s more a comfortable place to be.

I kept thinking how nice the ACF2 Nick built for sounded that day at Owsten. That circuit proved itself. But I always end up back at Stephies version of things.

So gathering parts for Stephies 841 circuit.Image

Except exchange 841 for the Output valves aforementioned. Bias I’ll require is more negative so I’ll have to juggle with the amplitude of the negative rail for greater headroom.
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#94 Re: 801a maximised class A2 excursion.

Post by Mike H »

Ah well, if you're going to have a negative rail – now imagine that the 6H30 is a high Voltage MOSFET – probably won't have to change anything else :D
 
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#95 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Ideally it needs to run at 5mA that should work then. But I’d have to buy some, I’ve got some smaller high transconductance Russian valves, not as well rated and little known, but second best; should be good enough.I’ll give that a go. And I’ll have 600v B+ so load resistor can be mahoosive. When talking to Morgan (not lately, that time in Chris Vryonides flat[perfect man pad]) he said he didn’t know why I didn’t do that: hinting it might sound better than the ccs.
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#96 Re: 801a maximised class A2 excursion.

Post by cv »

What a blast from the past! Just to point out "pad" is superfluous.

Ps JamesD - great to see you back!
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#97 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Hi Chris, we had fun!
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#98 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Just saw a 801a design I doodled about the time I brought the push pull amp to Owsten, when we decided to have an idht friendly context. Which I think Nick won but everybody brought something really good.

The 330v B+ was the power supply I had for the PP amp, the 801a thirsts for more. So that’ll alter.

Image

The concepts are about right bud if I were to build this I’d dc out the coupling cap.

Might sound better with buffer into 10y, and once you’ve done that, might aswell ground the 10y grid.

Anyway, not a million miles from a sensible voltages sweet sounding little amp. That interstate is the Hammond bifilar.
Last edited by Paul Barker on Sun May 17, 2020 9:59 am, edited 1 time in total.
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#99 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

This has cathode bypass caps, but depending on the ‘A’ (The figure for (A)ctual gain), I may play with thermionic diode bias with the objective: no caps.

There won’t be any capacitors in the power supplies either.
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#100 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Mike H wrote: Sat May 16, 2020 5:12 pm Ah well, if you're going to have a negative rail – now imagine that the 6H30 is a high Voltage MOSFET – probably won't have to change anything else :D
Couldn’t find my Russian ones. But while looking found E810F and with a triode connected impedance 1k2 I can use the interstage here.
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#101 Re: 801a maximised class A2 excursion.

Post by IslandPink »

Classy valve !
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#102 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Turns out I have 6 E180F (one of which GEC CV3998 rest Mullard) F and 2 E280F for which the data sheet provides triode curves at 200v 40mA the curves show healthy (for CF [note that the other elements mu and rP not relevant, though nice at that operating point]) 30 ma/v transconductance at 40 mA Quiescent operating point it should rise to the grid current of output valves whichever I go for 801a won’t demand more than 30mA.

So at this stage at least we have headroom. So as long as I get the rest right the output valve will be where it clips. Nastily at that as it will go at voltage drop off and current drop off but work right up to those cliff edge limits.
Last edited by Paul Barker on Mon May 18, 2020 8:06 pm, edited 4 times in total.
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#103 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

I can’t beat Stephie’s original operating point. The 9 watts is theoretical but from the distortion graph it’s clear we have to see it as an 8 watt power output. Up to 8 watts though the valve should show worthy.ImageImageImage
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#104 Re: 801a maximised class A2 excursion.

Post by Paul Barker »

Nice to see very low distortion from “first watt” right upto 5 watt close to zero distortion. 6 watts no qualms 7to 8 nearing the end 9.1 watts don’t happen drops off the cliff edge! On paper. Same base as 811_10 so thatwould plug straigh in, with minor bias adjustmet.

I could try program in 811_10 data, in time; lots of work with the vernier dial gauge. Now that I’m back working, less time for this project.

But the project is on!
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#105 Re: 801a maximised class A2 excursion.

Post by steve s »

Good stuff Paul.. nice to see got a good plan.
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