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Old 3rd April 2003, 11:54 PM   #1
JoeBob is offline JoeBob  Canada
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Default Effects of high heater voltage?

When I was trying to find out why a 6922 in a common cathode circuit was oscillating I also noticed that I had the heater voltage too high (6.5V instead of 6.3V), so I proceeded to adjust it to 6.3V (when fully heated) and added grid stoppers to try to fix my original problem. My question is, what are the consequences of a higher heater voltage, could that have been originally responsible for the oscillations? If it's possible (or maybe I'm way way off, I'm new to tubes) and that could have been the cause I'll take out the grid stoppers and pobe with my scope again (as it is, it's rock solid, but I'd like to be able to eliminate the grid stoppers), but if there's no way that was causing it, I won't waste me time. Again, thanks for all the input from you guys.
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Old 4th April 2003, 12:47 AM   #2
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Most data sheets say +/-10%. 6.5V is only 3% high, and for all either of us know could well be meter error (..depends on the meter).

The only way heat could be related to oscillations is by heat varying the transconductance of the tube - and this is N/A because of the emission curve of oxide-coated type cathodes. Grid stoppers are a seperate deal, and the need depends on the circuit.

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Old 4th April 2003, 12:54 AM   #3
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Default The grid stoppers...saga...

Quote:
Grid stoppers are a seperate deal, and the need depends on the circuit.
Finally!!...I agreed with someone!!

Cheers,Tim!!
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Old 4th April 2003, 06:01 AM   #4
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Tim
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Old 5th April 2003, 11:33 AM   #5
EC8010 is offline EC8010  United Kingdom
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Default Re: Effects of high heater voltage?

Quote:
Originally posted by JoeBob
I also noticed that I had the heater voltage too high (6.5V instead of 6.3V), so I proceeded to adjust it to 6.3V...
Where did you measure? You'd be surprised by how much voltage can be dropped down a bit of twisted heater wire. It's vital to measure at the valve base.

Mullard quote 5% tolerance on heater voltages, but mains voltage varies, so AC heaters need to be set as closely as possible (and ideally measured with a true RMS meter). Regulated DC supplies can be set to precisely the correct voltage, so it seems foolish not to do so...
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Old 6th April 2003, 05:21 AM   #6
JoeBob is offline JoeBob  Canada
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Actually I did measure on the tube socket pins, and it is from a regulated DC power suply, silly me was absent minded when I set the voltage, that's why. I was just wondering if this was the cause of the problems I had been seeing, I guess not, so I'll have to look elsewhere to eliminate the grid stoppers.
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Old 6th April 2003, 05:33 AM   #7
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Why would you want to eliminate the stoppers?

Tim
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Old 6th April 2003, 05:54 AM   #8
JoeBob is offline JoeBob  Canada
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Because without them the circuit oscillates, which leads me to believe that there is something wrong with the circuit. Seems to me that if everything was alright, they wouldn't be needed.
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Old 6th April 2003, 06:34 AM   #9
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Likely you'll have to do more than change components around. With fidgety tubes you'll probably have to change the layout altogether, be it longer or shorter wire runs between parts.
You might try neutralization, check out ham-radio books for how to do that. (Not like it's worth the price of a grid stopper. )

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Old 6th April 2003, 02:34 PM   #10
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Default STOPPERS...AGAIN.

Hi,

Quote:
Seems to me that if everything was alright, they wouldn't be needed.
Since you have a scope you could try to lower the value of the gridstoppers untill oscillation shows up again.
Make sure the body of the resistor is soldered as close as possible to the grid, otherwise the resistor is not quite as effective.

Don't forget the 6DJ8 is an RF tube and oscillates quite easily.

Another way is to use a small ferrite rod to kill off parasitic oscillation but they can be quite tricky to implement correctly.

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