801a amplifier

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Paul Barker
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#1 801a amplifier

Post by Paul Barker »

I intend to drive the 801a with a direct coupled 1/2 6BX7 cathode follower. 6bx7 on a +/- power supply to set bias of 801a.

So I have a question? would 8ma in the high mu (80) voltage amplifier mh41 drive the 6bx7 CF ?

8mA most I can get for two good reasons. Max anode volts is 200 at which max cathode current I can achieve is 8mA at -1v. Bearing in mind the 8mA is also the maximum dc current of the Hammond 156C;150 henry anode choke load: mh41. Ive gone for 150 henry for overkill to ensure good bass.

what i’m trying to avoid in this design, is multiple LF early roll off and early phase shift at bass.The achilees heel of 801a making it sound thin. If that werent a problem I could use two stages IT. but three transformers in circuit, unless you can afford no expense spared, expect bass roll off and phase shift multiplied by each stage adding up to pants bass performance.

The 1/2 6BX7 CF as a driver for the class A2 801a can send 40mA into the grid of 801a headroom since max anticipated grid current 30mA. Each section can do 150v @ 40mA for 6 watts to stay within 12 watt max both sides. This seems ideal.
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Nick
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#2 Re: 801a amplifier

Post by Nick »

I used el84 to do the same thing to drive the 211, but yep sound like a good plan. I would have thought the cathode follower will be a easy drive, there is no miller to worry about.
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#3 Re: 801a amplifier

Post by Paul Barker »

Thanks Nick?
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#4 Re: 801a amplifier

Post by Paul Barker »

As for the operating point of 801a ignore the theoretical 8.45 watts, in Stephies worksheet, Up to 7 watts no distortions passed 1% but up to 5 watts no distortion to speak of.

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#5 Re: 801a amplifier

Post by IslandPink »

Nice project. Which outputs do you have for 801's ?
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Paul Barker
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#6 Re: 801a amplifier

Post by Paul Barker »

They’re off an 845 Chinese amp. 4.5 kw primary, but with 4 ohm or 8 ohm output options so connecting 8 ohm speakers to 4 ohm becomes 9k. I know primar inductance will be a compromise.

I’ve used them before and they sounded really good.

I have also some smaller EI laminations 50% nickel so I plan to assemble them interleaved (no gap) for parafeed, no dc. So I can taste a nickel option at low power but should sound great which may compensate for the capacitor.

But then I might wind the 50% nickel as a phase splitter interstage transformer make output transformers from same material reminiscent of Sukumas use of 801a. No cap on PP opt. but phase splitter will be parafeed as primary sees dc otherwise.
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#7 Re: 801a amplifier

Post by Paul Barker »

There are still plenty of 10y and 801a used working or nos tested.

At least thats one remaining tube type we arent beholden to the Chinese for.

Would I buy a pair of PSV 845 WE pretender for £750? NO!

Would I buy a nos 801a for £150? Yes Yes Yes and Yes. theres plenty out there.

Ive got 5 801a and 7 10y from when I spotted them still offered for £80. Like I bought all px4s Jim Fish brought to Milton Kenes vintage fare. only 6, but £60 each.

I just dont get why people dont do this its a better investment than anything else ive known.

Just dont invest in crap. theres plenty of new crap on sale today. Dont expect that to become anything more than a turd.
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#8 Re: 801a amplifier

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Can’t get the required pp voltage swing from 1/2 6bx7 at the dissipation allowed with 30mA current.

So 1/2 6080 curves checked; provides the voltage swing with excess and can easily operate at 50mA and it’s got bags more if required. So I’ll pmunb for 50mA drive because it lifts the quiescent point higher to enter more linear part of curves. Any current beyond 30mA not required, but like voltage swing many years ago I think it was Gordon Rankin who pointed out not to limit the output stage in the VA or driver, design for 1/3rd more. I think he said a 1/3rd.

At 50mA current the first two stages will drive 833a etc down to a mofo.

But I’ll be happy just driving the 801a with more than enough volts Peak to peak at more than enough well provided current.
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#9 Re: 801a amplifier

Post by steve s »

Paul Barker wrote: Mon Jul 05, 2021 10:21 am

I just dont get why people dont do this its a better investment than anything else ive known.

I can't answer that one Paul, but It's my view too
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#10 Re: 801a amplifier

Post by Nick »

steve s wrote: Tue Jul 06, 2021 10:21 am
Paul Barker wrote: Mon Jul 05, 2021 10:21 am

I just dont get why people dont do this its a better investment than anything else ive known.

I can't answer that one Paul, but It's my view too
I would have said this is only true if you can sell them while you are around. If you don't, then the estate is left to sell them and may not know the value, or have someone to sell them to, or thank you for the room full of valves with no idea what to do with them.

It also assumes that there will be a market for them in n years time.
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#11 Re: 801a amplifier

Post by steve s »

The investment is also in sound quality .. but you make some valid points nick

I would say for myself is that I've already got back what I've spent on valves over the years, if they all go into the skip when I've gone well so be it, I've got the pleasure of being able to build what I want, and in my view, being able to use the best there is.
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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#12 Re: 801a amplifier

Post by Nick »

The investment is also in sound quality
Yes, I agree, but I don't think that was the way Paul was referring to them. But again, in your sense, the investment only has value if you use them, in which case the other meaning of the value is reduced maybe to zero.
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#13 Re: 801a amplifier

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I’ll be selling valves at best price when my only income is paid by the pension element of my and my employers National insurance contributions of a lifetimes work.

keeping anything I may want to listen to for the next 37 years .

They’ll be the first assets to go into the spending money stash.

Property I don’t want to steel the children’s opportunity to get a fillip after the sad news of my watchful eye over them is extinguished. But if things get too hard I reserve the right to remortgage.
I would go forward in the mindset that I don’t want to spend the children’s inheritance but not if my quality of life at an unknown future date needs support the inheritance would provide in part or in full.
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#14 Re: 801a amplifier

Post by Paul Barker »

Diana will be equal partner in such decisions, if she’s still breathing.
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#15 Re: 801a amplifier

Post by Paul Barker »

I’ll get my coat
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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