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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Sep 2022, 9:51 am   #41
Peter.N.
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Default Re: What makes a good valve amplifier?

I had a couple of friends at school in the '50s who like me went into the TV trade and we used to build our own amps and compete on who could make the best bass boost circuit. We both had a chassis, mains transformer, output transformer and a few octal valve holders.

The chassis had many unused holes due to the number of times we had rebuilt them. Initially we had a couple of 6V6's and probably and ECC33, I have forgotten now, then we graduated to 6L6's or even KT66s and I reckoned all our amps sounded excellent feeding a 10" speaker in a sand filled baffle, mono of course, stereo hadn't started yet.

We had loads of fun with those (the neighbors probably didn't) and I can draw you a circuit of those basic amps even now from memory.

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Old 23rd Sep 2022, 7:19 am   #42
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Default Re: What makes a good valve amplifier?

My school mate [he worked all his life in the TV service sector] did exactly the same thing and managed to get a surprising amount of volume and bass from our efforts! I have built several amps over the last few years without the aid of a diagram. It sticks in your brain. Sad I know but I didn't have to look up the valve base connections.
A teenage life spend buried in valve data books can probably be blamed.. John.
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Old 23rd Sep 2022, 9:20 am   #43
kalee20
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Default Re: What makes a good valve amplifier?

I played around making amplifiers out of old TV's... Frame output transformer makes a fair heater transformer; Audio output transformer for its intended purpose; HT straight off mains; etc.

Even with a 6V6 overrunning the poor little OP transformer, bass response can be pleasing. What the figures looked like, I don't know - I only measured the hours of pleasure and the number of smiles.

But, what makes a good valve amplifier? Low distortion (sub-1%); adequate frequency response (30Hz - 18kHz +/- 1db); low output impedance to give good loudspeaker damping, are generally accepted. To that I'd add smooth overload characteristics, reliability, long-life.

How to achieve that? Lots of ways... I'd suggest running output valves at no more than 75% of rated maximum anode dissipation. Using good-quality components (paying attention to voltage rating of metal-film resistors). Plenty of iron in the output transformer. Good regulation in the power supply, unless using pure Class A. Plenty of decoupling between stages - using common HT impedance to hold-up bass response by positive feedback is flaky! Aiming for good frequency response and low distortion open-loop, and using NFB as the icing on the cake (and to keep control of OP impedance, if using pentodes or BT's), are all 'musts' for me.
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Old 23rd Sep 2022, 9:53 am   #44
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Default Re: What makes a good valve amplifier?

"What makes a good valve amplifier?"

Isn't answerable without some clarification of "For what purpose?"

Guitarists sometimes want to 'play' the overload characteristics or non-flat frequency responses, rectifier droop and even stuff akin to crossover artefacts as part of the performance. An ideal amplifier lacking all imperfections would not float their boat. It's no good for their purpose.

Some hifi people make claims to produce the truest possible replica of what was recorded on the record/tape/CD/download they bought. Choosing an amplifier other than the most linear/flat/etc one is therefore hypocrisy. Can't have that!

Some hifi people choose imperfect amplifiers because they like what they do to the sound. This is a free and honest choice. Honest so long as they're prepared to admit it.

Some people choose an amplifier because of the appearance. To look techno, to look blinged up to hell, or to look stripped to absolute minimalism. Ears are relegated.

Some people choose an amplifier because of what's said about it, maybe they want to have something seen as elite, for status reasons, maybe they want something that's accepted as worth getting by a lot of people. Some follow the pronouncements of gurus, some follow the masses. Buy a PL12D turntable because so many people have them, there can't be much wrong or we'd all know, your gregarious instinct will thank you. Buy an Unattainable Pinnacle (tm) turntable because other people will look up to you.

Some people choose an amplifier on its paper specs, maybe supported by independent reviewer measurements.

Some people choose an amplifier because they like the details of the circuit design.

For a manufacturer, the goodness of an amplifier mostly shows up in the overall profit.

Some people just decide to design/build their own for no better reason than they can. Maybe they can't afford any of the commercial ones they would have liked? Maybe it's just arrogance?

I suppose I fit into the arrogant bracket, but I enjoy it so the amp I've been using for a lot of years is good for me. It's not necessarily good for anyone else, but a small run got built.

David
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Old 23rd Sep 2022, 1:48 pm   #45
kalee20
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Default Re: What makes a good valve amplifier?

Good points - fitness for purpose is key!

The above list is fairly comprehensive, one thing missing could be weight and portability, if it's a guitar amplifier.

So it's for the OP to define that, now.
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Old 24th Sep 2022, 5:33 pm   #46
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Default Re: What makes a good valve amplifier?

Thank you for the replies. A very interesting discussion and quite a few pointers. I'm building a single ended stereo amp (my first one) at the moment so will compare this one to my more 'hifi' push pull amps. I'm still working things out and finding out what I like.

David, I enjoy building them too, because I can, because it's fun and because it's a challenge. I do however follow established circuit designs (Mullard, Olson, Armstrong). My original question was because all these sound the same to me - but then they would because they all set out (I assume) to achieve the same goals which were listed above.

I wonder if a modern 'hifi' valve amp with all the bells and whistles costing five digit figures will deliver anything more in audio terms compared to the classics we're all familiar with.
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Old 24th Sep 2022, 9:52 pm   #47
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Default Re: What makes a good valve amplifier?

Of course it will Gabe. The lectrons are expensive ones, not the crappy old second hand ones you and I use.

Joe
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Old 25th Sep 2022, 6:50 am   #48
Gabe001
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Default Re: What makes a good valve amplifier?

In all seriousness, the only valve amps I've heard are the ones I built myself, with Chinese iron, which is surprisingly chunky for its price. I don't want this to end up an audiophoolery thread. I'd expect that if I pay paying 5-10 grand for a Macintosh valve amp (for example) the OPTs would be top notch, which should in theory make a difference.
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Old 25th Sep 2022, 7:43 am   #49
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Default Re: What makes a good valve amplifier?

Hi.
My everyday amp is a home built Mullard stereo 3-3. It has top quality output transformers and a mains transformer that would easily run four of these amplifiers. The HT is also stabilised. The amp has had many great comments over the years regarding sound quality even from a high end audio dealer who said that those expensive amps with a 300B triode don't come near it, remember all comments were subjective but we know how good the spec is on the 3-3 don't we.
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Old 25th Sep 2022, 9:17 am   #50
Robert Gribnau
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Default Re: What makes a good valve amplifier?

Since the output transformer is so critical, I like the path Philips followed for some time, namely OTL valve amplifiers in combination with 800 Ohm loudspeakers. I more or less copied two of those amplifiers (but without the preamplifier and tone controls), using them with an active valve crossover. The screen grids of the EL86's are being fed by a double choke. There is positive as well as (of course much more!) negative feedback going on in them. See the first attachment for the schematic of one of them.

Commercially it was probably not the wisest of choices because of the 'forced marriage' between these amplifiers and the 800 Ohm loudspeakers.

Besides selling a range of full-range 800 Ohm loudspeakers, Philips also sold an 800 Ohm woofer, the AD5201A. It has a rather unique design, making the front of the styrofoam conus look a bit like a piston moving inside a cylinder. I was fortunate to get my hands on a pair and made sealed enclosures for them. See:

http://otl800.blogspot.com/2014/02/g...s-ad5201a.html

Philips never sold 800 Ohm tweeters though. Smaller full-range loudspeakers were mostly used for that role. I combined my pair of AD5201A woofers with a pair of 9710AM-01 full-range loudspeakers in seperate open baffles, with a pillow stuffed at the back. See the second attachment.
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Old 25th Sep 2022, 10:14 am   #51
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Default Re: What makes a good valve amplifier?

Quote:
Originally Posted by Gabe001 View Post
... I wonder if a modern 'hifi' valve amp with all the bells and whistles costing five digit figures will deliver anything more in audio terms compared to the classics we're all familiar with.
In the end a lot of the shortcomings in hi-fi sound (by which I mean the air vibrations, not the subjective experience inside our heads) come from the speakers. A speaker is a bunch of compromises. One of them is between physical size, distortion and sensitivity. Through the 1970's and 80's, as silicon made watts cheap, speaker designers were able to make smaller, lower distortion units and those are often what amps have to drive today. So what modern valve amps can deliver that the old favourites (Quad II, Leak Stereo 20, Radford STA25 etc) didn't is watts.

You mention McIntosh in another post. Despite having been in the valve amp business since 1949, their lowest powered valve amp in current production is a 75W/ch unit and the rest are all 150W/ch or more. Of course this is 'only' 10dB more than the Quad II gives, but if you're driving modern 84dB/W@1m speakers like the Bowers and Wilkins 607's rather than, say, my 45 year old 91.5dB/W@1m Yamaha NS1000's then you'll find most of that extra power is eaten up by the loss in speaker sensitivity.

Cheers,

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