Starting to tear into my Pilot TV-37U (with ballast tube). Noticed that the SAMS (62-16) matches the set rather than the Rider. The 3KP4 CRT (V21) does not appear to be first in the filament string and my set has a ballast tube listed as M4.

I have heard that the CRT filament needs to be protected b/c during startup it overheats b/c of too much voltage draw until the other tubes in the string heat up. Is this the case in my set where there is a ballast and the pic tube filament does not appear to come first in the string? Should I still take the measures to protect it? I was thinking of installing a CL-90 thermistor at the place indicated by my pointer in the below photo. On the AC line before the 33 ohm 4 watt resistor. Any advice would be appreciated.

Thx.

here is what I was told to do. Works great Its from tubesrule


A lower cost, less intrusive, and better regulated solution is to use a simple bidirectional TVS diode (Transient Voltage Suppressor). This is basically two back to back zener diodes. By carefully selecting the breakdown voltage, not only do you eliminate the cold start surge, but you are now clamping the voltage on the filament to a set maximum, so regardless of what the line voltage is, or transients on it, the filament will always be protected. No modification of the chassis is required, and it can be installed in seconds!

Since we want 6.3Vrms on the filament, we need to clamp at about 8.9V maximum. In practice we actually want to clamp a bit less than this to limit the maximum rms voltage, so a standard 8.2V TVS is used. The diode is simply added across the crt filament leads at the 25Z6 socket

you also might want to check this out.
http://videokarma.org/showthread.php?t=264074

Ha! I totally forgot about that thread. Thx for reminding me about it. That's easier and should work great.

Hey, I cannot seem to find a TVS with the correct specs on Mouser. Can you help me out? Looks as though I should be looking for one with a clamping voltage from 8.2-8.9v. They all seem to have clamping voltage specs higher than this.

I am thinking this one would be fine (standard 1.5KE8.2CA diode), except the clamping voltage is listed at 12.1 V.

http://www.mouser.com/ProductDetail/Littelfuse/15KE82CA/?qs=sGAEpiMZZMuNo3spt1BaV%2fHTyk9HbrguHrLgImFOKf8% 3d

Hey, I cannot seem to find a TVS with the correct specs on Mouser. Can you help me out? Looks as though I should be looking for one with a clamping voltage from 8.2-8.9v. They all seem to have clamping voltage specs higher than this.

I am thinking this one would be fine (standard 1.5KE8.2CA diode), except the clamping voltage is listed at 12.1 V.

http://www.mouser.com/ProductDetail/Littelfuse/15KE82CA/?qs=sGAEpiMZZMuNo3spt1BaV%2fHTyk9HbrguHrLgImFOKf8% 3d

This diode is an industry standard 1.5KE8.2CA diode available for about 35 cents. With this diode in place, the voltage on the filament never got above 7.0Vrms regardless of the line voltage or during a cold start. Here is the cold start filament and plot:

This device will tend to clip the tops of the waveform even in normal operation (at 6.3 VAC RMS, the peak voltage is 8.9 V). This could be a bit of an anti-brightener if the tube is weak at all. Probably no harm in going this route, but if your tube is weak and you're starved for brightness, a different solution might be better.

Tom
You are correct that during normal operation an 8.2V TVS will clip the sine wave, however it will clip at about 8.5V while a 6.3Vrms signal is about 8.9V peak, so the clipping is minimal. I don't have my notes right now, but the measured RMS difference was negligible, which means the change in heating effect of the filament will be negligible. Now if you do have a weak crt, and the they typically usually run higher than 7Vrms in these sets, then yes the emission will be noticably less.

Darryl

Think I will restore and power up on my variac very slowly. Then see how my picture looks. If my CRT is strong and I am not starved for brightness I will then go with the TVS link I posted above.

If I do have a weak CRT can I then put in a CL-90 thermistor at the AC line before the filament string for a solution to the filament voltage spiking on power up?

If I do have a weak CRT can I then put in a CL-90 thermistor at the AC line before the filament string for a solution to the filament voltage spiking on power up?

Not a good idea! The voltage on any given series heater at any given instant is highly dependent on the warmup state of the other tubes, and unless they all warmup in exact synch* there will be over and or under voltage transients on the heater (which is why TVS diodes and or filament transformers are added to these sets).
*The odds of that, if not slowly variaced, are comparable to those of turning off your clothes dryer and finding all the clothes inside neatly folded and stacked!

Ok so either I have to power up very slowly on a variac each time, or install the TVS diode. Got ya.

Thanks everyone.