45W SE amplifier

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JamesD
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#1 45W SE amplifier

Post by JamesD »

Picking up on Nick's comment that for 20W one would probably want to go PP ( I sort of agree using a Lynn Olsen style PP or maybe better yet a Ralph style PP), I remember seeing a 45W GK-71 triode SE amp from a Russian manufacturer (I think) and I wondered what it would look like from an operation point and OPT transformer spec... so I ran the figures graphically and plugged into an SE design spreadsheet from Voltsecond and I got 45.5 Watts output at Va=1500V, Ia=100mA, Vg=-260V and a 15K:8R output transformer with 240H inductance... which pretty much makes the argument for not doing SE but playing around with the spreadsheet does produce a possible 30W design with Va=1000V, Ia=120mA and Vg=-165V and a 6K OPT at 95H and that is just about do-able. Lots of op points for 20W SE and under 5K loads... but again - not trivial designs to build. These are all Class A1 designs.

SV572-10 can do 40W at 1000V B+ but that is Class A2...

So for 20W I have a really nice design for a PT15 PP Ralph style yielding 15Watts in Class A and 22W in Class AB1 before it poops out. Va=700V...

ALso looking at a couple of Sakuma style SE and PSE designs that use some nice 2nd harmonic distortion cancellation for some very impressive performance - but using lots of expensive iron!

I raise seperate threads to talk about the last two beasties

J.
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pre65
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#2 Re: 45W SE amplifier

Post by pre65 »

[quote=JamesD post_id=175446 time=1589388350 user_id=113

So for 20W I have a really nice design for a PT15 PP Ralph style yielding 15Watts in Class A and 22W in Class AB1 before it poops out. Va=700V...

J.
[/quote]

I've still got 4 of PT15 in War department boxes. :wink:
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IslandPink
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#3 Re: 45W SE amplifier

Post by IslandPink »

Interesting stuff James. I must check the spec on the 6k/8R transformers I have from Bud. I think they are about 30H , and maybe I asked for 15W, not sure. I wasn't intending to run that much voltage, maybe 700 to 800Max, you can get some useful power from a GK-71 at those levels.
It's worth noting that even at 460V B+ , making about 3W , the one I tried, with Andrew's fil supply, sounded somewhat better in the bass than a KR300B. It was a really nice, rich sound.
It may be that this is because the anode impedance of the bigger valve is not swinging around as much as a 300B for the same power output. It was a lightbulb moment reading that article from years ago, by Steve Bench, pointing out that an SE valve like a 300B varies its anode impedance by about 3:1 when it's running near full power output - one less-considered source of distortion, especially with reactive loads like a speaker.

'course my intention is to keep voltage at a level where I can still run a shunt-reg supply...
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JamesD
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#4 Re: 45W SE amplifier

Post by JamesD »

I am having such a debate with myself over what main amplifier to build - the confusion started with my having a WAD2A3PSE chassis and valves in store so first thought was to build that but with an upgraded design and new iron as 8W was close enough to my target requirement of 10W output as to not hear any difference.. My loudspeaker assumption was a minimum sensitivity of 90dB/1W/1M so 10W gave me 100dBA peak... more than enough at home and normal listening within the first watt - The first watt is the deepest and all that... It has a sensible B+ voltage to but its PSE...

And the PT15 sounds better than the Sovtek 2A3 and gets to 8W at 600V... so maybe thats the way to go....

But at 600V the GK-71 is coming on stream hard and sounds great... but then it needs that huge heater supplying but that gives it prodigious bass... etc. etc...

And maybe 20W would be better after all then I can get to 103dB and thats about where Mahler Symphony No. 8 peaks in an concert hall - my favourite Symphony... now that is PT15 PP territory or a really big GK-71 SE amp...

too many choices... so build a small amp and speakers to get going...

The 2A3 PSE is out - PT15 and GK-71 remain in play. Ralph style PP or sakuma SE is part of the debate too... I'll start a new thread tomorrow on the this.

ciao

James
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pre65
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#5 Re: 45W SE amplifier

Post by pre65 »

Have you got some GK-71 James ?
The only thing necessary for the triumph of evil is for good men to do nothing.

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Nick
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#6 Re: 45W SE amplifier

Post by Nick »

Picking up on Nick's comment that for 20W one would probably want to go PP
Yep, though I don't follow my own advice. The old 211 that I got out the garage and am using again at the moment uses a 950v HT, 211 cathode to ground and grid driven via a el84 CF from a split rail power supply. Gets somewhere in the region of 30W at a push.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
JamesD
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#7 Re: 45W SE amplifier

Post by JamesD »

Hi Phil,

I have four of the beasties so far if I go the Sakuma route I'll need more...

Hi Nick,

Yeah that is a lovely beastie :-) but I find I'm a little wary of voltages over about 700V now... Don't know why I used to work with 25KV all day - i guess its getting older and lack of practise. Well I could sort out the latter :-)
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#8 Re: 45W SE amplifier

Post by steve s »

I made a se pt15 amp years ago. It sounded like my px 4 amp, lovely valve to listen too, I was told it sounded like a Px25 when trioded, and it did.
I may have started the trend, I had about 30 valves at one time, you could buy them for 10-15£ each.. Bargain!
But you need a little more inductance, mine was light in the bass, which suited the back horns I was using at the time.
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
JamesD
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#9 Re: 45W SE amplifier

Post by JamesD »

The PT15 has an Ra about 2000R or so at Op points around 600V so it needs a chunk of inductance for good bass but its very encouraging to hear that SE sounds similar to the PX4 & PX25 :-)

I did a PP version driven by an Aikido front end back in the day and it was really nice before I blew up the power supply...

My PT15 cost me £20 each from Langrex... those were they days :-) Got my N43 from them too at £7:00 each... wish I had bought more now...
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#10 Re: 45W SE amplifier

Post by steve s »

I used mine with a 5. 5k load in se, a little low but i could not tell..
Push pull is another world to me. There's an amount of signal cancellation, valve and component matching needs to be perfect to get close to what se sounds like, or run higher classes to stop the signal overlap, disadvantages both ways to get watts.. But get it right and a nice amp can be the result.
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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