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Why doesn't this 6SJ7 work?
Why doesn't this 6SJ7 work?
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Old 30th June 2019, 02:54 PM   #31
mandu is offline mandu  Singapore
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[QUOTE=avtech23;5838843]Ok so the culprit was a bad connection downstream of the heater chain.



You put all efforts to test. Best of luck.
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Old 1st July 2019, 01:40 AM   #32
PRR is offline PRR  United States
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Quote:
Originally Posted by avtech23 View Post
... dissipation at 12.1%....
So? This is NOT a Power Amp. Oh, all amplifiers have to make Power, but the signal power here is so incredibly small (microWatts) that there is no reason to run the tube HOT.

Anyway you know the current is less than 1mA--- 270V supply and 270V plate resistor. If the tube were a dead short, it would pass 1mA of DC (and zero signal). More likely the plate should sit 1/3rd-2/3rd of supply so 0.3ma to 0.7mA. Sure, less than 1/10th Watt Pdiss. They don't make tenth-Watt tubes so we use a Whole Watt tube and work it easy.

Why low current? Cost of B+ filtering in this very low-level stage. And Gain: voltage gain of tubes and FET rises as current falls. Gm does not fall as fast as current.
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Old 1st July 2019, 02:45 AM   #33
avtech23 is offline avtech23  Australia
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Why doesn't this 6SJ7 work?
Thanks for taking the time to explain.

I wasn't thinking that every tube needs to be 'cranked up to 11', but I was concerned because the current and voltages were way down the bottom of the datasheet graphs and did not look right. I was expecting dissipation to be something like 50-60%, in the more linear area of the graphs.

But it makes sense, as you explain, with such small signals.

For this version, D is 330V so top current is 1.2mA, and 0.33mA at the 1 Meg.

I'm just waiting on some 6V6S tubes now, as the plates are set at 425v which is way too high for the 6P6S that I have on hand. Then I'll see if I'm getting a nice sine wave..
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Old 3rd July 2019, 03:40 AM   #34
PRR is offline PRR  United States
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> the more linear area of the graphs.

Linearity is all relative.

Plotted to 10mA max, yes a half-mA looks scrunched.

Plot just the range to 1mA. Now the linearity is the same around 0.5mA as it was around 5mA on the 10mA graph.

Large changes should be plotted on log graph. (Somehow this was rarely done, though the MIT Radar books do.) Now the low end looks like the high end, maybe better. Plot the 3/2 power of voltage and the graphs get straight. It's all relative.

Anyway how much linearity does a film projector need? (The nonlinearity of optic film is horrible.) Especially at this part-Volt level?
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Old 3rd July 2019, 04:16 AM   #35
avtech23 is offline avtech23  Australia
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Why doesn't this 6SJ7 work?
Quote:
Originally Posted by PRR View Post
Anyway how much linearity does a film projector need? (The nonlinearity of optic film is horrible.) Especially at this part-Volt level?
This one is a conversion of a 1940's projector amp into a guitar amp. The optical tube is gone as are the old defective bits that would likely catch fire if I were to power up.

I guess the same question applies... How much linearity is required for a guitar amp.....

Thanks for the pointers with the data sheets. I'm not used to deciphering the tube data sheets, so this is also a learning project for me.

Looks to be working pretty nicely now :-)
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Old 6th July 2019, 09:20 PM   #36
jjman is offline jjman  United States
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Quote:
Originally Posted by maxhifi View Post
That's the old fashioned way to do it... on more modern schematics, lines crossing at right angles = no connection. Lines crossing at right angles, with a dot placed on top of the junction = connection. It's clear that's the convention this schematic follows.
Yes but there are missing dots in many places, so confusion is possible.
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Old 6th July 2019, 10:10 PM   #37
kodabmx is offline kodabmx  Canada
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Why doesn't this 6SJ7 work?
Quote:
Originally Posted by avtech23 View Post
I'm just waiting on some 6V6S tubes now, as the plates are set at 425v which is way too high for the 6P6S that I have on hand.
I've run them that high...Hell, I've put them in place of 6P3S "just to see" and while they got HOT, they worked fine. I didn't run them that hot (20W Pd) for long though.
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