I am working on bringing back a Pilot TV-37. I have taken the crt filament out of the circuit and during troubleshooting have been running it from a bench power supply. I want to run a filament transformer. I've read that a 600 ma is appropriate. I have a NOS transformer but rated at about 1.5a. My concern is that space is tight, and hey, I might need that heavier unit for some future project! Question: do modern black box monitors & tv sets use seperate filament transformers? If so, would they be up to this task? Regulated close enough? (converted wall warts occured to me, but I know some can run away sometimes) I guess this shows how familiar I am with this modern junk! I had a 9" solid state Philco-Ford with a busted crt so I dug it out but found it to use a 12v filament & a larger transformer powering other things. If modern stuff does have the right transformer I shouldn't have much trouble finding one for free.

Modern TV sets either run the CRT heater from its low voltage DC supply (that also runs the tuner, IF, and such) or powers it with a few turns of wire wrapped around the flyback transforemr core, like they used to do to get filament power for the high voltage rectifer tube (like a 1B3). So a modern TV set won't likely have anything usable for this. Other parts like capacitors in solid state TVs and computer monitors I found can be used for recapping various high voltage tube circuits.

Be careful when picking a place for the heater transformer that the transformer's magnetic field doesn't leak into the CRT causing beat patterns of deflection.

I think I'd just go for a new filament transformer unless you can find a new old stock one...what is the filament voltage...6.3 volt? If it's worth the special care of converting to a dedicated filament circuit from the series circuit it's worth the real right part.
Hardly any modern device except say boomboxes use regular 110-volt to low voltage transformers anymore, it's all switching power supplies.

If it's a 6.3 volt filimant, a transformer shouldn't be at all hard to find. Try taking apart some little alarm clock/clock radios, those typically have little transformers that will be 12 volt, you might be able to find a center tapped one. I've scavenged transformers out of all sorts of stuff, it just takes some looking sometimes... Start picking up small electronics found in the trash, things like cassette players, clock radios, cd decks and older VCR's typically use power transformers.

Or, you could just go to Rat Shack, believe it or not, they still have a couple useful things, and they do still have small 12.6 volt transformers center tapped at 6.3 volts.

What was wrong with the filament power source in the television? I wouldn't see why only the CRT would lose it's filament source - normally they get it from the same place that the rest of the tubes do - or is this one of those series string sets without a filament transformer? If so, then won't all the rest of the tubes be running a little hot with the CRT removed from the string?

-Ian

I think this is a series string filament set which does not use controlled heater warm up time tubes so there is a surge of current in some tubes (such as the CRT) when the set is turned on and is hard on the CRT filament...replacing the CRT filament in the string with a resistor and providing a seperate trans for the CRT filament removes the surge problem from it.

Yes, Chad is right-these Pilots are notorious for blowing the crt filament. That already happened to the orginal 3KP4 so I have a green phosphor 3KP1 in there now. With a resistor in place of the filament the voltages look good. I will do a little more scavenging around here before buying a new one, which isn't that big a deal. There are ways to add surge suppresion of some sort to save the crt but I kinda like this way better.

Yes, Rat Shack does still carry those small transfomers. That amazes me... considering you can't even buy a plain speaker in a box... or even a replacement fuse for some of their older digital meters! I don't even know why they call theirselves Radio Shack anymore... considering they hardly carry the basics!

I think this is a series string filament set which does not use controlled heater warm up time tubes so there is a surge of current in some tubes (such as the CRT) when the set is turned on and is hard on the CRT filament...replacing the CRT filament in the string with a resistor and providing a seperate trans for the CRT filament removes the surge problem from it.

Another possibility would be to use a pair of high powered zener diodes (in series, cathode to cathode) rated for around 8V (this value selected to pass the peak voltage of 6.3VRMS = 8v +0.7V diode drop of the other zener in forward conduction mode). So when the set is turned on, the diodes will clamp the heater voltage to around 8V equivalent RMS, which a heater is designed to take for short periods of time. It is a *heater*, not a filament, yes?

You can hide the diodes below the chassis.

wa1ise's idea is great - but I still like the idea of the seperate filament transformer best from a tube safety point of view, since it totally takes the CRT out of the possibility of ever getting more than 6.3 volts. You could hide the new transformer under the chassis too. And, to take up the 'slack' in the filament string, you could simply replace a 6SN7 with a 12SN7, or a 6SQ7 with a 12SQ7, etc. and not have to worry about a resistor added into the circuit.

-Ian

you could simply replace a 6SN7 with a 12SN7, or a 6SQ7 with a 12SQ7

-Ian


Plug 'n' play?? Uh, no, you could not. Tubes with different filament currents are staggered into different sections of the string. Maybe replacing a 25Z6GT with a 35Z6G (both 300ma) would make enough difference in the string, but adding a resistor is probably the best idea.

Hi Bryan,
I think I have a spare small Stancor 600ma transformer if you pm me with your address I'll drop it in the mail to you.
Terry

Plug 'n' play?? Uh, no, you could not. Tubes with different filament currents are staggered into different sections of the string. Maybe replacing a 25Z6GT with a 35Z6G (both 300ma)

Ah, so since a 6SQ7 is a 300ma tube at 6v, and the 12SQ7 is a 150ma tube at 12v, it wouldn't work because then you'd have one tube in the string that needs a different amount of current... and in series current is distributed evenly and voltage drops can differ - vs. in parallel where voltage is a constant and current differs.

Duh.

Well, so much for a convenient, elegant way to do it :). Yeah, resistor...

-Ian