6J5 or one half of 6SN7 as an output stage?

[G4XWDJim]

Neil,

Here's the audio circuit of the R107 and a page of component values.

If you'd like a couple of 6J5s to play with just say and I'll send you two metal ones maybe a couple of ECC82s and EL91s as well.

Jim

[ColinTheAmpMan1]

Fender used a paralleled double-triode for driving a reverb unit a good few years ago. The AA1069 circuit for the "Pro Reverb Amp" uses a 12AT7 and has 400V on the anodes via a transformer driving the reverb unit. A 220R cathode resistor bypassed with a 25uF/25V capacitor gives +8.0v on the cathode and it is all finished off with a 1MO grid resistor. It is quite feasible to use it to drive a small loudspeaker for "bedroom" practice. The circuit (and similar ones) can be found from a number of sources.
I hope this is of use.
Colin.

[Heatercathodeshort]

You will get more volume than needed. Most radio receivers with high wattage output stages were only used in a domestic situation at little more than 1 watt if that. One half of a 6SN7 was used for frame output in a couple of TV chassis. J.

[FrankB]

Per Ludlow Sibley's book: Tube Lore, the 6J5 is 1/2 of a 6SN7.
P.29
This book is, IMHO, the essential "Bible" for tubes, or anyone who uses them.
At least 3 addendums have also been published. If you buy a copy, be sure it has all of them, or write Lud to obtain them.
ISBN # 0-9654683-0-5. My copy is a 1st ed. Pub in 1966.

(Joining the Tube Collectors Assn., {Lud is the President},and buying ALL the back issues is to be highly desired.
It provided weeks of reading for me and so much knowledge that was unobtainable elsewhere.) Plus Lud is a great guy to correspond with.
Feel free to PM me for any info on TCA.

[Neil Purling]

How complicated would an output transformer be to run both parts of the 6SN7 in parallel?

[dr_nick]

I have somewhere in my mind that if run in parallel, and reasonably matched, then an appropriate transformer would be one that present 1/2 the impedance that would be optimal for one valve on its own.

No need for multiple windings - just a single primary circuit.

Nick.

But note that the anode current will be twice as big - so needs accounting for in the specifications.

[Neil Purling]

So, both anodes connected together, through the same o/p transformer. Separate cathode bias components or just one resistor & capacitor?

[ms660]

http://www.r-type.org/articles/art-123b.htm

Lawrence.

[dr_nick]

Lawrence,

thanks for that link - helped to refresh my memory on the ra issue for valves in parallel.

Neil,

anode-anode, grid-grid and cathode-cathode; all electrodes wired in parallel. Single set of cathode bias components (one resistor, one capacitor); and both anodes to the cold end of the o/p transformer.

It's all in Lawrence's excellent link in the section 'Parallel and Push-Pull Output Valves'.

Nick.

[Herald1360]

There may be some benefit from individual cathode bias resistors and bypass caps- if one valve goes low emission, that will prevent the other from taking excess current and it will also improve the dc balance between the valves somewhat.

Unless you're using fairly well matched NOS valves it's worth the extra few pence- it's not as if you'll be churning out thousands and counting the beans.

[boxdoctor]

I must agree with Herald1360 (above). Its certainly worth the small extra effort and expense to fit individual cathode bias networks.
Some people seem to have the misconception that using a common cathode bias network in some way "balances" the two valves.
In fact it does just the opposite. Its just penny-pinching!
I always modify ( with the owner's permission) the bias circuit of any Hifi amp that I get for repair if a "common cathode" bias network is originally fitted. Tony

[turretslug]

Agree re. common cathode resistor- it might have a certain apparent elegance on paper, but it's an unstable situation just waiting to tip one way and snowball. Remember that we're discussing paralleling signal-type valves, so even combined anode current won't be more than 20mA or so, i.e. well within the expectation of common-or-garden single-ended output transformers. For matching, the RS universal replacement type that often crops up at vintage fairs, junk boxes etc. has a lot of ratio options- some folk get awfully doctrinaire about precise matching forgetting that anode loading is a multi-way compromise/optimisation of several factors and that the load impedance, i.e. loudspeaker is almost inevitably far from precisely defined or constant over its frequency range anyway. It's valves we're discussing after all- they won't blow up and die in a nano-second if you don't get everything spot-on straight away!

[kalee20]

Yes, definitely agree with separate cathode resistors (and bypass capacitors).

If you have two 6J5's in parallel, you can even pull one of them out and run with the remaining one, at slightly less available volume, without underbiasing it - something you couldn't do with a common resistor. (Not that it would complain, unless you left it running for many hours).

[Neil Purling]

Sideband: Did you make any note of the level of the signal driving the grid of the ECC82?

[Sideband]

No I didn't I'm afraid and after my experiments I took the set apart again. I think the anode bend detector gave quite a high output...probably a couple of volts.