Hi!
I'm looking at the N78 pentode, got some for next to nothing. Interesting tube, but I'm confused at the suggested operation voltages for push-pull in both pentode and triode modes. The values are much higher than the ratings, and this is in a datasheet. Can someone please explain?
Thank you!
Jose
I'm looking at the N78 pentode, got some for next to nothing. Interesting tube, but I'm confused at the suggested operation voltages for push-pull in both pentode and triode modes. The values are much higher than the ratings, and this is in a datasheet. Can someone please explain?
Jose
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That would be related to average power dissipation. But usually the maximum plate voltage rate applies at idle, and that means up to 350V quiescent plate voltage, while the maximum rating is 250V.
Let's compare with an EL34. Maximum anode voltage is 800V, and none of the suggested operating points use 1000V.
For Push Pull vs Single Ended both in Pentode mode, wouldn't the peak potential seen between G2 and the Anode be less for a given DC anode voltage ?
I have run the n78 at 320v 35ma push pull in triode mode. The spec sheet says it can be run at 350v 22ma in triode mode but I haven’t been that brave. I have seen some discussion about how optimistic GEC was with some of their ratings. Maybe they wanted you to replace tubes more often lol.
That's exactly my opinion. Just surprised to find that optimism in a formal datasheet.
It is about the quiescent point. No signal applied, both tubes conducting. 350V in both plate and G2 (triode strapped), 100V above the maximum rate.
It does not provide an answer to your question, but I found the discussion in this thread relevant and quite interesting. Apologies if you have seen it already. I suppose the key to understanding this contradiction is in the testing assumptions underlying the “ratings”, whether they are design-centre, maximum or absolute maximum ratings. On some data sheets this distinction is noted, but your N78 seems to be silent on it. https://www.diyaudio.com/community/threads/a2134-vs-n78-tube.412008/
Notice the reference in post #11 to the EL84 sheet where a similar pattern occurs w.r.t. the initial characteristics “rating” and later pentode and triode PP use cases.
http://www.r-type.org/pdfs/el84.pdf
As an aside, the ratings for EL84 in 20% vs 43% distributed loading in AB1 PP shows higher distortion and higher output for the 23% condition. I thought 23% presented the lower distortion.
Notice the reference in post #11 to the EL84 sheet where a similar pattern occurs w.r.t. the initial characteristics “rating” and later pentode and triode PP use cases.
http://www.r-type.org/pdfs/el84.pdf
As an aside, the ratings for EL84 in 20% vs 43% distributed loading in AB1 PP shows higher distortion and higher output for the 23% condition. I thought 23% presented the lower distortion.
I'd say that the EL84 datasheet makes sense. First, the voltage/bias etc values used to measure gm, u, ra. That is not related to maximum ratings. Then the operating modes for different use cases, then the limiting values. In no case the suggested operating voltages go above the limiting value.
If a manufacturer only specified maximum design ratings, like in the N78 datasheet, then the use cases should not suggest operation above those, IMHO.
@jcalvarez,
Yes, you are right about the EL84 data and I stand corrected.
Looking further into N78 I found 6BJ5 listed as an equivalent. The linked data sheet for 6BJ5 from Amalgamated Wireless Valves in Australia listed the Design Center maximum values for Plate and Screen voltage as 350 V and 275 V respectively, as is used in your Marconi sheet for the high voltage use cases.
http://frank.yueksel.org/sheets/201/6/6BJ5.pdf
I get the sense they didn't see the numbers as an absolute uncrossable line*. Not like with sand devices. You could find other tubes that are shown in the data sheet doing things outside the specified max ratings - for whatever reasons. Maybe a sort of unspoken disclaimer "You can do it but if it fails you won't get your money back." sort of thing? I've seen it quite a few times re: Max Plate Dissipation. (Here's one)
*I remember a Bendix sheet giving a list of max figures with a note about not exceeding more than one at a time.
*I remember a Bendix sheet giving a list of max figures with a note about not exceeding more than one at a time.
I dunno, that's three and a half watts hotter than the spec sheet op point. ( nearly 50% more ! ) Doesn't sound like you're terribly lacking in bravery. 🙂
TV sweep tubes often have operating use case voltage and current ratings that are greater than the max dissipation rating. In this case the tube is NOT subject to over dissipation. A TV sweep tube never idles. It must run at full power output all of the time. There is no volume control on the sweep circuit. If there was the picture would disappear when the knob was turned down.
My favorite example for this is the 6W6GT tube. This tube is specified for both TV vertical sweep use where it has a 7.0 watt dissipation rating, but the operating conditions show 300 plate volts and 60 mA of average plate current which would generate 18 watts of dissipation. I can verify by personal experience on several tubes that they will NOT survive in an SE audio amp under those conditions. Some will red plate, some may not, but they will eventually go into a runaway condition at 50 mA and 300 volts. They do sound nice until they die. The vertical sweep circuit puts out 5 to 10 watts of a sawtooth wave continuously, forever. The actual power level is determined by the picture tube size and deflection angle. In some older TV's there WAS a volume control on the vertical sweep. It's called the vertical height or just height control. It's on the back of the chassis and intended for tech use to adjust for tube differences. It has a very limited range. The 6W6 also has audio output specs which show a higher 10 watt dissipation rating, but much lower operating conditions. Here it idles at 9.4 watts.
The 6W6GT was given several different heater voltages and also sold as the 12L6, 25L6 and 50L6 which is found in audio output use only. The data sheet is exactly the same only the higher sweep duty ratings are absent.