ECC91 Common cathode double triode as stereo amplifier

[Steve_Bell]

I would like to use an ECC91 common cathode double triode as a stereo amplifier. Would bypassing the cathode resistor at a low audio frequency achieve this, rather than acting as a differential amplifier?

[kalee20]

Yes. But why not use triodes with separate cathodes? A common cathode will give crosstalk, especially at lower frequencies where the bypassing is correspondingly less than effective.

[Steve_Bell]

I will probably use a separate cathode valve such as an ECC83 for the reasons you suggest but just wanted to explore the possibility of using a cheaper alternative ready to hand.

[turretslug]

No harm in trying something like this that is both plentiful and under-employed- there's not a great deal of call for these valves that were once churned out in swathes for professional VHF use.

As the actual need for stereo separation decreases at lower frequencies, you might not notice much image degradation anyway- we're used to seeing things like CD players claiming huge separation over the whole audio range but the acoustics of real-world recording mean that far less is achieved in practice and it diminishes further at the LF end. No suffering will ensue from experimentation....

[Sideband]

Don't know how good ECC91's are at audio. I think they were designed for RF. Certainly less than ideal with the common cathode.

[Radio Wrangler]

Odd choice of valve. It's optimised as a small RF power amplifier. Works nicely as a VHF oscillator and mixer. I built 2-metre converters around them.

As a stereo amp, you'll have to decouple the cathode heavily to minimise crosstalk and this will mean there is no degeneration in the amplifier, it's set up for max gain and max distortion.

What do you need the amplifier to do?

David

Edit: started the message then left the editor open too long!

[Synchrodyne]

I understand that the ECC91 (6J6) is somewhat microphonic. It was an RCA WWII valve intended for military VHF applications, then adopted post-WWII for TV and FM front end use. GE introduced the 12AT7 in 1947 as an improvement for this purpose, less microphonic as that had been a potential problem in FM and split-sound TV applications. Although that said, RCA continued using the 6J6 until the 6X8 became available in 1951.

I guess that whether microphony is an actual problem in AF service depends upon the application and the signal levels. With domestic receiving valves, it was a potential concern with the first AF amplifier (maybe dealing with half a volt or so of input AF) in respect of acoustic feedback from the speaker in the same cabinet. I think that Philips had something to say about microphony in its Book IIIA.


Cheers,

[Argus25]

Originally Posted by Synchrodyne

It was an RCA WWII valve intended for military VHF applications, then adopted post-WWII for TV and FM front end use.

RCA deployed them in a unique TV tuner design for the 621TS that was designed during the war & released in 1946. I'm pretty sure RCA developed the 6J6 for this set. It was part of their technology drive to improve all aspects of TV design at that time which culminated in their 621TS masterpiece, which set the standard for all other TV designers in the post war period.

The 621TS tuner used three of them, one for a differential neutralized RF amplifier, one for a mixer and the other for a marvelous Kalitron local oscillator. They also had an amazing gigantic mixer coil, the tuner circuit is here in one of my articles at the ETF:

http://www.earlytelevision.org/holden_621.html

I can't see why the 6J6, like many RF valves couldn't used for audio aside from the microphony issue.(notice one in the tuner had a thick lead shield) I have even seen the EF50 used as an audio valve by wrapping it up in sponge rubber and placing it in a rigid tube. I guess as audio valves get more expensive, this may be the inevitable fate for RF valves too. It might drive up their prices, Currently the 6J6 is plentiful and as "cheap as chips".

[Synchrodyne]

Originally Posted by Argus25

RCA deployed them in a unique TV tuner design for the 621TS that was designed during the war & released in 1946. I'm pretty sure RCA developed the 6J6 for this set. It was part of their technology drive to improve all aspects of TV design at that time which culminated in their 621TS masterpiece, which set the standard for all other TV designers in the post war period.".

Well, perhaps not according to RCA’s own writings. A good source here is the article “Miniature Tubes in War and Peace”, in RCA Review for 1947 June, p.331ff. It is available here: https://www.americanradiohistory.com..._Issue_Key.htm.

I have pulled out a couple of pages. The first covers the WWII military application origins of the 6J6:



The second shows the immediate post-WWII RCA miniature valve range, for consumer as well as industrial products, and which included the 6J6 and other WWII valves such as the 6AG5, 6AL5, etc., as well as the new issues, 6BA6, 6BE6, 6AU6, etc.



Cheers,

[Argus25]

Really interesting documents. Aside from the military applications, in radar and coms gear, the early consumer equipment using 6J6's would have been Televisions mainly but there probably was amateur gear too. The 6J6 must have been a very attractive option when RCA first released them and I see them along with the 6AG5 for TV use.

The 6AG5 mentioned in the documents was an amazingly popular pentode, used in many early post war turret TV tuners and sometimes in the IF's as well. Probably they would be suited to audio work too, aside from any microphony issues.

[Diabolical Artificer]

It looks a likely candidate for a LTP PS or differential IP though. Couldn't find it's u/gain, though it looks low from the curves. If not the 91 how about the 86 or even the 85 at a pinch, pends how much gain you need, you could use them as cascodes.

Andy.

[Radio Wrangler]

The 6J6 has a large, flat cathode. On each side of it is a separate, flat, grid. Then beyond those are the separate, flat, anodes. The anodes have a stiffening/cooling fold, but the flat face collects the electrons.

It's like a miniature QQV06-40.

Without an all-encompassing anode or shield, they're quite showy when the heater's on.

I can confirm the microphony, I found VHF oscillators readily converted vibration to FM. At first I thought it was the tank circuit and tried reinforcing it, but in the end I had to accept that a lot of it came from the valve. The sensitivity is also somewhat directional.

The construction tends to show up the effects of mechanical tolerances, the two sides are not well matched.

David

[Synchrodyne]

Originally Posted by Diabolical Artificer

If not the 91 how about the 86 or even the 85 at a pinch, pends how much gain you need, you could use them as cascodes.

The ECC85 was certainly used as an AF valve, despite its origins as an FM front end valve. Armstrong used it as such in some of its late 1950s chassis, such as the Stereo Twelve, which I think had three of the type. I imagine that Armstrong preferred to standardize on one double triode type, and with the ECC85 required for the FM front end, found it also suitable for use at AF.

How the ECC86 would work as an AF valve I don’t know. It was intended to be an FM front end valve for 12-volt HT car radio receivers.

Originally Posted by Argus25

The 6AG5 mentioned in the documents was an amazingly popular pentode, used in many early post war turret TV tuners and sometimes in the IF's as well. Probably they would be suited to audio work too, aside from any microphony issues.

The 6AG5 begat the RCA 6CB6 in 1950, followed by a whole raft of American VHF pentodes in part endeavouring to solve TV AGC problems. But that I think is another topic. The 6AU6 sharp cutoff HF pentode was used as an AF valve, though, and some data sheets show that was part of its mission. Amongst VHF pentodes, the Mazda 6F13 found some AF applications, and its predecessor, the SP61 was the basis for Baxandall’s tone control circuit in 1952.

Returning to AF double triodes, what about the 6L19? Are any still around?


Cheers,

[Herald1360]

Why not just use two? Cheap as c
chips is the attraction anyway. Put both halves in parallel and treat as simple triodes or use as LTP diff amps.

[kalee20]

With the OP running at 8.5mA each section, I'd guess he's using at a reasonably high level, driving a low-resistance (or capacitive) load anyway - power output triode maybe? Steve, please share!

[Steve_Bell]

The DC operating conditions were obtained from the Tung-Sol data sheet for the 6J6 from 1956. There is a section showing the operating conditions for a "Class A1 Amplifier".

I think it draws too much HT current compared to, say, and ECC83. I don't have a definitive design in mind but just wanted to explore.

[Refugee]

Would they work as phase splitters in push pull amplifiers?

[Radio Wrangler]

SSSssshhhhh !

It only needs one to try it and report on air, authority, soundstaging and granularity, and before we know it an economical RF resource will be gone.

David

[Cobaltblue]

Love it !

I got hundreds of 6AM6's with the blue screen

How do I promote those at £10 a pop so I can retire

Cheers

Mike T

[Radio Wrangler]

£10 a pop would never do. They couldn't be any good at that price. Just not esoteric enough, not exclusive enough. They need to be well over a hundred quid. You need to pick out some visible feature of the electrode structure, then you need to learn the in-vogue adjectives and pick a selection of them to write into a review. For safety's sake write your review in terms of audible superiority when playing a highly esoteric direct-to-vinyl disc of an artist who has vanished without trace such that no-one else will have a copy. You might have to bump off the artist just to be sure. Build up a history for yourself (and several aliases) on a few hifi groups. Postings must be of 10-30 words, followed by three pages listing every hifi item you have fictitiously owned. Don't forget to post a few arguments amongst your aliases for added verisimilitude. Above all make sure each alias' equipment list is different or you'll be spotted. Now create sworn affidavits from a few of your aliases that the magic 6AM6s made you have to listen to your whole record collection again! Provide a list of the collection, all five of them! All unique special cuts no-one would want to listen to the music of and no-one has ever heard of.

And you're made!

David