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7JP4 Heater protection in series string sets
Of course, installing a 6.3 volt transformer to power the 7JP4 heater would provide the most constant source of voltage, but other schemes have been mentioned here.
One scheme places a surge suppressor diode accross the heater source to limit the high voltage surge during turn on of the set (sometimes as much as 12 volts). The part number mentioned is P6KE8.2CA... is this correct and still thought to provide good protection? I can order some on the 'bay. Other schemes?? For grins, last night I experimented with two banks of ten series-connected 1n4005 diodes, which limit the heater voltage to about 6.4 volts at 600 ma heater current, but at the high turn on surge current allow a momentary heater voltage of nearly 10 volts! jr |
#2
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I'm not sure. The filament runs on 6.3 VAC RMS. Doesn't that mean 8.9 volts peak to peak ?
So I would think that using a P6KE8.2CA would result in a clipped sinewave across the CRT filament during normal operation. So it would be getting less power than normal. Curious if anyone has put a scope across the filament leads with the TVS diodes installed. |
#3
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Good question!
The Tektronix DMM 850 is a true RMS meter, plus I used the ac+dc position, just in case there was a net dc offset due to diode differences between the two strings of ten diodes. I think the measurment should be valid? I wonder if the PKE8.2 will be substantially better at limiting the true RMS surge voltage. jr Bonus points... id the chassis... Last edited by jr_tech; 05-08-2017 at 12:54 PM. |
#4
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I'm thinking you should be using TVS diodes rated for 9 volts rather than 6.8 since the CRT filament should have about 8.9 volts peak to peak during normal operation.
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#5
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Here's a lengthy discussion of the topic: http://antiqueradios.com/forums/viewtopic.php?t=132513
I find the specs on the P6KE8.2CA kinda confusing. I was expecting one of those values to be 8.2 Breakdown voltage - 7.79 Clamping voltage - 12.2 Working voltage - 7.02 |
Audiokarma |
#6
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Thanks for the link... lots of good info there. I went ahead and ordered some 8.2 devices, will give them a try to see what they do to the RMS voltage at the 7JP4 heater pins during turn on surge and normal operating conditions.
Still scratching my head over the specs that you posted, will try to determine the slope of the voltage vs current curve around the 8.2 voltage point. Possibly it could have considerable slope. jr |
#7
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Quote:
I got my 8.2 diodes yesterday and experimented last night... true RMS turn on surge is indeed limited to less than 7 Volts, but normal CRT heater voltage is only about 5.9 Volts RMS... cathode is a little cold, causing some loss of brightness. See scope trace. I ended up sticking a back to back pair of 1N4005 diodes in series with the 8.2 Volt TVS diode, yielding a near perfect heater voltage of about 6.25 Volts RMS during normal operation, and limits the turn on surge to just slightly over 7 Volts. An added benefit, warm up time of the 25L6 tubes is decreased, since the CRT surge (12-13Volts) being shunted by the TVS diode to the rest of the string. thanks for the help, jr Update: attached second pix from better 'scope... with tv on an isolation transformer I don't need to use the battery powered "floating" 211 'scope. . Last edited by jr_tech; 05-18-2017 at 10:10 AM. |
#8
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Quote:
1 mA current is also the specified current for most MOV's "varistor voltage" and the same for some silicon carbide (old school) varistors. There are exceptions, but most modern datasheets adhere to the "voltage at 1 mA" standard. I have a Fluke 332D voltage standard - handy for checking higher voltage varistors (VDRs) - my NOS and removed stock all checks within about 5% of the varistor voltage. It's kinda cool to "dial-up" to the 1 mA current, and read the voltage directly from the front panel knobs. $2,000 tester for 50 cent parts... Also handy for checking meters and ammeters.
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Brian USN RET (Avionics / Cal) CET- Consumer Repair and Avionics ('88) "Capacitor Cosmetologist since '79" When fuses go to work, they quit! Last edited by Findm-Keepm; 09-29-2017 at 06:39 PM. |
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