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

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Old 18th Mar 2023, 10:47 am   #61
GrimJosef
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Default Re: Parallel single ended valve amps, any experience?

I confess I only mentioned the 304TL because, as per the thread title, it really is four separate triodes in parallel in the same envelope (Eimac would sell you just one, as the 75TL, or two - yes, you guessed it - as the 152TL, but somehow the 304TL seems to have been the most popular). There was a higher-mu version too, called, unimaginatively, the 304TH.

Cheers,

GJ
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Old 19th Mar 2023, 1:21 pm   #62
knobtwiddler
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Default Re: Parallel single ended valve amps, any experience?

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Originally Posted by Radio Wrangler View Post
4CX250Bs? Eimac again.

The $CX250B is well known in RF power amplifier circles. They were designed for non-linear Class-C operation, and intermodulation characteristics are not wonderful. People have used them in 'linear amplifiers' for SSB, but the linearity is only marginally acceptable. They are a high gain device (High perveance, too) and prone to UHF self-oscillation. The main defence is to use special valve sockets with integrated ceramic decoupling capacitors on to fingerstock connecting with the g2 ring. They can give peculiar behaviour like negative current at g2. So having g2 off to a transformer tapping is asking for trouble.

If asked to nominate a valv least likely to be used at audio and not allowed to name obvious non linear and switching wierdos, I'd have suggested 4CX250s.

Impressive work, if they've tamed those little devils at audio.

David
I count a few mastering engineers amongst my friends and I've never heard of anyone using the 641 in the field. It's incredibly rare and has long been a mythical beast to me (I just want to know about the O/P transformer!). I found out about it through my AES compendium of papers, so I'd imagine it to be stable, as the paper would have ben strictly peer-reviewed and none other than Rudy Van Gelder co-authored the article (he is probably the most famous recording engineer in Jazz - recorded Coltrane and many other icons).

You can see the AES aricle here: https://www.technicalaudio.com/pdf/F...rchild_641.pdf

Quote:
these tubes, even though half the size of 6L6s, are capable of approximately 1000 watts in Class B
As you will see in the audiophoolery thread Fairchild valve equipment is at the absolute top end of prices for vintage gear. There is a PSU for sale if you put 'Fairchild 641' into Google. It's a bit of a beast!

edit - lots of selenium in that PSU by the looks of it!

Last edited by knobtwiddler; 19th Mar 2023 at 1:27 pm.
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Old 19th Mar 2023, 1:42 pm   #63
Radio Wrangler
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Default Re: Parallel single ended valve amps, any experience?

I wasn't saying that 4CX250b cannot be made stable, just that it's an epic task and whoever did it with audio sized values around it probably won a medal. Full power needs 2kV so that output transformer is going to be wild!

David
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Old 19th Mar 2023, 2:44 pm   #64
knobtwiddler
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Default Re: Parallel single ended valve amps, any experience?

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Originally Posted by Radio Wrangler View Post
I wasn't saying that 4CX250b cannot be made stable, just that it's an epic task and whoever did it with audio sized values around it probably won a medal. Full power needs 2kV so that output transformer is going to be wild!

David
I didn't interpret it that way. All I know is that the 641 was made in very small numbers and I've never spoken to anyone who's used one in the wild. The AES paper was likely published before many facilities adopted the system, and for all we know, there could have been behaviour in the field that wasn't envisaged by its designers (or not). I've never heard of such a powerful cutting amp being used. It was common to see Crown DC-300 amps feeding heads, but 1KW is on another level, particularly of the valve variety.

Maybe all the 641 amps have been snapped up by wealthy audiophiles and used as home amps...This has occurred with the Technics SP-02 lathe motor. In Japan, you see whole Neumann lathes as home playback decks...

NB - the head Fairchild designer, Mr Narma, is held in high regard in academic circles. He also designed the 670 limiter. https://www.aes.org/aeshc/jaes.obit/...7_11_PG931.pdf
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