801a amplifier

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Paul Barker
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#31 Re: 801a amplifier

Post by Paul Barker »

Good price. I’ve ordered 5 Russian types so they’ll do for a start. I’ll raise the +/- to 160v and it’ll balance up to -44 v naturally also at -7 bias. Power supply choke input 6em7 series pass regs.
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#32 Re: 801a amplifier

Post by Paul Barker »

Suddenly on my history you tube suggests a parallel SE 801a amp.

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Why didn’t I think of that already. Much better plan. It makes better use of my 5k OPT,s. Can use the same driver and the drivers power supply has more current than required so if driver needs beefing up it’s got loads in reserve as have the el84’s.

So I’ll get better matched design with double output into 5k 8 ohm or 4k5 4 ohm and they were made for 845 so can take the current. The inductance will be better for bass result. I darent take an 801a to Nick with great highs but thin bass. I’d be laughed out the place.

The project is looking exciting already.
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#33 Re: 801a amplifier

Post by Paul Barker »

So into true 5k transformers parallel 807’s will deliver 14 watts with better looking distortion profile.

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Notice at 7 (14) watts 2nd is dominant at about 0.3 % 3rd and 4th are right down at 0.1 % best not expect >8 watts but even at 8(16) watts still 4th doesnt rise much but at 8.5 watts 2nd and 3rd about to converge below 1%, but the ratio of 2nd to 3rd doesnt look great at that max power, Probably an effect of A2. Nice though at 16 wats 2nd dominant and still 0.8% 3rd limited to 0.2% and 4th 0.1%. So it should be a shoddy 16 Amp PSE quite a handy amp.

The Nickel cores will get shelved as they cant do that. 45 se at 1.6 to 2 watts parafeed Nickel OPT,s : a future project, maybe after state pension kicks in 2 years and 3 months, um I wasnt just cought counting down? But it has to be experienced.
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#34 Re: 801a amplifier

Post by Paul Barker »

Ill be a far time building the amp, so for recreation Ive been playing with operating points.

When deciding where to operate a class A valve you have to position it within the grid lines equidistan to the Peak to Peak swing before cutoff at both ends and if that means last grid line at either end might give you crashing clipping with more 2nd harmonic to the ight hand side, so if you want to avoid too much 2nd you operate valve to the left cutoff not the right. For lowest 2nd you shift left, for max power you sit middle for diddle. All relevant to SE amps.

But I first found from driving the sv 810 into A2 in the sense that if you bias youreself leftish to clear the pinched up are right side. It was an absolutley dlightfull sounding amp and definately a 300b killer, in the sense it was more open. The 300b with the 6sn 7 (which added to 300b spoils it further. [but there is hope the 27 sets the 300b free of its over middle tendencies]). 300b is actually a great valve handled differently.

But back on this project. Ive found slight changes in bias B+ and amount of A2 by positioning leftish. Id thought Id clear pinching right and make a better distortion profile. But results were dramatically going into higher distortions of higher order in A2.

Not necessarly the final resting place. But I like distortion profile of 550v 34 mA bias -61 grid volta into 10k p to 6 watts.

So if the music signal isnt greater than what settles at 6 watts (12watts two valves) it would sound undistorted. As Im using parallel pair Id see 12 watts of quality. This only requires 5 of positive grid volts.

So the graphs tell the story, but the amp wont know the graphs, and it will produce sound out to the beyond the perfect operating point because if the signal and drive is there itll go as far as 30 positive grid volts neither will it cut off to the right, but there will be an almost vertical distortion chase. i think the best of it will be in A1 not A2, but A2 is there for the extremes.

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#35 Re: 801a amplifier

Post by IslandPink »

Do you remember what were the guidelines regarding how to get the best out of two parallel valves ?
I'm thinking about how you best match them as a pair, to get low distortion. It's a long time since I thought about this, it must have been around the time I built the first 2A3 PSE amp.
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#36 Re: 801a amplifier

Post by Paul Barker »

No, there was a Glass audio article on parallel valves which was inconclusive with a bias parallel valves were fine. That’s all I recall. I subscribed to Glass Audio but can only place a few of the mags and that article I haven’t found.

I’ll try match the valves best endeavour and have a go. We’ll have to wait and see.

Apart from doubling the power the main benefit I’m chasing is the transformers I’m using are a true 5k, so there contribution to the whole will be at their best, rather than a 5k transformer pretending it’s 9k. Primary inductance isn’t set for 10k even if it doesn’t matter about that elsewhere, we must hold in our thoughts the tendency of the 10 family to come over light on bass.

Seems confusing but the 8ohm output is 5k into 8ohm and the 4 ohm output is 4k5. So using youre 8 ohm speakers through the 4 ohm connection youre getting 9k in all but primary inductance.

So all other things equal parallel looks better use of what good components I know these transformers to sound. I’ve had my best ever sound on these transformers at home, corroborated by a silent forum member who also heard them and was blown away.

So this is a combination of factors I’m balancing.

If I had a pair of Westrrn made 845’s I’d build that amp, but I don’t have a single 845.
Last edited by Paul Barker on Sat Jul 10, 2021 7:07 pm, edited 1 time in total.
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#37 Re: 801a amplifier

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With historic cost to me of 801a’s £80 a pop I’d prefer to use those true Western made valves than a pair of Chinese made We845 copies for £750.

Added to that I actually have 7 of the 801as which were at that price point so don’t have to spend money on 801a’s.

If
I live long enough I’ll wind Nickel parafeed opts to a true 10k then go single output valves.

I don’t think I’ll get egg on my face building this amp with parallel valves. But we’ll find out one way or the other. The bass will be better though than tricking the output transformer into 9k, also the distortion of 9k rather than 10k is not insignificant. The 801a is a valve which well placed on the curves is sweet as but a little away from that sweetness is nasty especially thin bass.
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#38 Re: 801a amplifier

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Of course when it’s built you can pull out two valves change speaker outlets and you’re point will be made for good or for bad. Maybe at home it will spend it’s time single ended until I’m playing bass heavy sound when I’ll turn off, plug two more valves in and shift outputs. In years to come it will be that easy to demonstrate, as I have a feeling it’ll be a keeper.
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#39 Re: 801a amplifier

Post by IslandPink »

I'm completely in agreement with all of this, there seems to be something, maybe with the amounts of higher-order distortion in NOS valves that seems better, I've heard it a few times. With Russian valves it very much a mixed bag, a lot sound poor like cheap Chinese remakes, but then you have exceptions like 6N6P, GK-71, GM70, 4P1L.
Yes, and bass - too often it's lost due to implementation, rather than the output valve being intrinsically bad.
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#40 Re: 801a amplifier

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I’m undecided how to power rate the 10y (not the oxide 10y a as I have non)

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I read somewhere but can’t find it to quote accurately , the 10watt rating of the 10 was improved to 13 watts in the 10y. But the extra rating didn’t land on the RCA data which leaves it at 10 watts.

This is the amp in building upside down showing 10y on left and the ceramic base 801a on right with its partner discarded to demonstrate 10y.

since both lit up valves are at 7.5v only you can say each have identical plate both have thick ceramic top supports 10y has 1/2 thickness at bottom. 801a plate is no different at all but there is more space before the ceramic rails and that’s it. So I suspect they held back on giving the 10y an up rating on paper from the 10 but it can hack it.

But anyway I’m building with 801a. The nickel transformers of the future will find a friend in the 10y.
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#41 Re: 801a amplifier

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Heater voltage.

Blast, I’ve found the longish complex heater voltage discourse at Jacmusic.

I’ve taken days to get my heaters bang on 7.5 but it appears in flight they were running at 7 Volt with engine on, 6.3 engine off. And the oxide filament versions (10 and VT25A) are 7 volt on the data sheet.

I think I’ll change my heater supplies more conservative.
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#42 Re: 801a amplifier

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Little progress but to give me a lift I popped valves in. 10y’s ans MH41.

Plans changed to need more bases for el84 and D3a

But it’s nice just handling the valves.

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#43 Re: 801a amplifier

Post by IslandPink »

Nice !
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#44 Re: 801a amplifier

Post by Cressy Snr »

Looking good. :)
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#45 Re: 801a amplifier

Post by Paul Barker »

Well the NightinGale Audio project box wasn’t intended for so many chops and changes so it’s beginning to look like a Swiss cheese. But after the eventual build is complete :

first iteration relatively simple

VA D3a triode LCL coupled, because the grid chokes are high ac load with relatively low dc load 0.1 uF Russian Teflon caps to begin.

EL84 triode CF direct coupled driver.

Because I’m limiting for simplicity direct coupling to the driver, I can’t control the 10Y/801a bias with the driver b+ alone.

I also had to do mental summersaults figuring out how to regulate the minus rail of a balanced +/- driver supply.

So for HT I’ll device a regulated output stage B+ with headroom in order to provide for various B+ options for example when using 10y or 801a or even by the basing single 300b 2a3 45 and also Svetlana 811/10. The two British bases in front no longer required could house output valves px25. The 5k load would maybe struggle with 811/10 so in their case 4 ohm tap for 8 ohm speakers in their case as I only have two.

Instead of balanced driver supply I’ll make two seperates another b+ and a stand alone b- both series pass regulated for tweaking operating point and achieving correctly biased output valves.
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