10-18W JCM800

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Littlewyan
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10-18W JCM800

Post by Littlewyan »

I'm going to rebuild my Pignose G40V (converted JCM800) into a proper JCM800 chassis but slightly smaller head cabinet. At the moment I only get roughly 10W Clean out of it and the transformers are an 18W 8K OT and a 340V HT PT. I'm using 6V6s in the output stage.

Now, I have a 6.6K 50W OT spare that I'm thinking about using instead. On one hand I would get a bit extra power I think and it may sound better. On the other hand it might make the PT look a bit silly size wise and it may not sound as good because its not getting pushed as hard as it normally would.

What are your guy's thoughts?
strelok
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Re: 10-18W JCM800

Post by strelok »

http://www.radiostation.ru/tubes/6V6GT-GE.pdf

According to this datasheet for the 6v6, power output starts to go down ever so slightly at loads above around 7k or so. Additionally THD starts to rise rather dramatically. Granted this is for 250V on the plate, so it might be a bit different for your case. If you went to a ~6k load you might see the clean output jump to maybe 12-15 watts. However if you want to see a lot more power you're probably going to have look into higher B+ voltages.

That said what's your screen voltage and plate dissipation set to?

You could try the 6k 50 watt transformer, if you like the sound you could keep it, but I'm inclined to say that you'd want to get a smaller one (they're not very expensive, shouldn't be more than about $50, tweed deluxe type would be fine.) My intuition says that having a transformer that's capable of being pushed close to its limits would sound better though I can't say I've tried one that's way over sized for the application. Personally I'd go with the smaller one just to keep the weight down.
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Littlewyan
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Re: 10-18W JCM800

Post by Littlewyan »

Screen Voltage is going to be roughly 335V once I install a choke instead of a resistor. Plate dissipation is 70%. I think i'll stick to the current OT as thats for 18W. Can't raise the plate voltage much more because the EH6V6s can only take 350V on the screens and I don't like the sound of the JJ 6V6s. Ill leave it as it is. Be interesting to see if the power goes up much when I up the screen voltage.

Interestingly the EH6V6s I have in there draw very little screen current at idle, below 1mA. If you look at the datasheet for the Tung Sols they draw 4mA at idle.
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martin manning
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Re: 10-18W JCM800

Post by martin manning »

At 335V on the anode and screen 8k is a better match.
pdf64
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Re: 10-18W JCM800

Post by pdf64 »

strelok wrote:http://www.radiostation.ru/tubes/6V6GT-GE.pdf
According to this datasheet for the 6v6, power output starts to go down ever so slightly at loads above around 7k or so. ...
Just to note that 'operation characteristics' chart on p6 is (almost certainly) in regard of SE operation.
strelok
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Re: 10-18W JCM800

Post by strelok »

Very true. Also when you're calculating bias are you doing it with 14 watts as the plate dissipation rating to get to 70%? The reason I ask is a traditional 6v6 is rated for 12W while the new JJ's and EH's are rated for 14. Probably won't make much difference but I figured it was worth mentioning.
sluckey
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Re: 10-18W JCM800

Post by sluckey »

That 14 watt rating has been around a long time.

In 1959 RC-19 listed the max dissipation as 12 watts, but in 1960 RC-20 was revised to show the max as 14 watts.
strelok
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Re: 10-18W JCM800

Post by strelok »

Ah, I didn't know that. Every old datasheet I've ever seen for a 6v6 had it listed as 12. Good to know.
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