7JP4 Heater protection in series string sets

[jr_tech]

Originally Posted by bandersen

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.

Correct!

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.

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