I heard that some people add a thermistor to their tube electronics so that when the power switch is turned on, the initial rush of current is dampened as the thermistor heats up. The idea is to prolong the life of the components in the TV, especially the heater filaments. Does anybody here have any experience with this?

I know that "instant on" tube TVs can accomplish a simplified version of this, by using an external power switch to determine when the "stand by" power is turned on to pre-warm the tube filaments for a minute. Then the TV's power switch is used to fully power on the TV. The thermistor mod sounds much more convenient though.

The instant on feature was in no way intended to prolong the iife of anything, and in fact did quite the opposite.

There was no minute before power on standby setting for instant on sets. Most of them never powered off the heater, instead supplying half voltage the entire time the set was off. Some of the higher end consoles had a vacation switch on the back to turn off instant on.

Instant on is really only beneficial if a set is going to be on 8 hours a day and get turned off and on a LOT in that time.

Most color sets that had a thermistor and early Dumonts (that had a RC time delay feeding a relay) had the cushioning in the B+ circuit only and whacked the heaters with direct on-off-on action.

Zeniths at one time had a thermistor to limit filament inrush. IIRC, they were sued by other makers saying the claimed benefits were untrue.

Instant on. A fun topic.
First the B+ already has inrush covered by the usual 5.6 ohm
fusable.
As far as filaments go you could add a thermistor thats 0 ohms
hot & maybe 120 ohms cold. Needs to carry a few amps also.
I bet it will fail faster than a filament so why bother. Just leave the
set unplugged & wait 30 secs to use it.

Instant on was sold on the fact that tube life will be better
due to less shock to the filaments, maybe some truth in that.
Here's the catch. Most sets with instant on were all tube PCB based
sets & the cheap brands. Basically the sets cooked themself to death.
I had even pull a hoz out / damper tube & the socked came with it !

Early all solid state sets used it also. The Zenith Chromacolor & CC2
sets had it ( D-F line). They are considered the longest lasting CRT's
ever by most here. So on a 100% solid state set there may very well
be a good argument for instant on.

enuf fer now
73 Zeno:smoke:
LFOD !

In parallel heater sets I'd reckon over %95 of all tube failures were NOT heater failure (usually worn out cathode, or shorts). On series string heater sets the rate of heater failure was higher than parallel sets, but it was typically only 1-3 tubes in the string and usually the ones with higher heater voltage. If the set isn't color and a series string there's fair odds that a standard color thermistor would work... could also install a series heater resistor (or a diode) and a time delay relay to short it out.

The instant on feature was in no way intended to prolong the iife of anything, and in fact did quite the opposite.

I understand that instant on was intended to make a tube TV turn on faster, at the expense of lowering tube life. However, if an external switch is used to remove all power from the TV when not in use, and the external switch is turned on a couple minutes before the TV is turned on, then the instant on feature is used as a “slow start” for the filaments to prevent a current rush.

The DIY tube audio enthusiasts use various slow start circuit designs for their filaments and their B+ to slowly ramp up current into the filaments. A thermistor in the filament circuit seems to be a simple and inexpensive mod to prolong filament life.

I've noticed that in tube TV sets, the tubes tend to fail either because the cathode becomes eroded with time and emission drops off, or, because they develop heater/cathode leakage and fail to operate properly. Either that or because it's an RCA and is populated with 6GH8As. Is filament burn out really a factor ever? That's got to be an uncommon mode of failure - I'm not a repair man but I've been using and maintaining tube TVs for 30 years, and have never seen a filament burn out.

A lot of audiophile stuff is kind of overkill for general purpose electronic applications like a TV. Have a look at how marginal power supplies are in an average TV set compared with an audio amp for example. Also, a lot of big audio amps are really hard on great big power tubes, running them very hot for low distortion, where a TV is much kinder.

The idea is to prolong the life of the components in the TV, especially the heater filaments. Does anybody here have any experience with this?

On the list of television tube failure modes, open heater/filament is extremely rare. Most common are low emission, shorts, interelectrode leakage, grid emission, gas, intermittents (shown by tapping on the tube), and heater-cathode arcing in damper tubes (also shown by tapping). By contrast, in series-string radios, open heater in hot-running tubes like 35W4, 50C5 etc. is very common; back in the day it was the most common failure in radios, followed by filter capacitors.

Ditto
On TV's rare. If it happens its usually the Hoz out / damper or the RF amp.
True also on AA5's. We had a quick filament checker just for that. Plug it
in & if the test light lit tube OK.
Had 7, 9, & 12 pin sockets.

73 Zeno:smoke:


On the list of television tube failure modes, open heater/filament is extremely rare. Most common are low emission, shorts, interelectrode leakage, grid emission, gas, intermittents (shown by tapping on the tube), and heater-cathode arcing in damper tubes (also shown by tapping). By contrast, in series-string radios, open heater in hot-running tubes like 35W4, 50C5 etc. is very common; back in the day it was the most common failure in radios, followed by filter capacitors.

I also agree that most tube failures do NOT involve open filaments. Some times a thermistor (G.E. CL60-5 amps or CL90-2 amps) is used to mitigate an upgrade that makes the power supply more efficient and robust.

RCA's CTC11 color chassis used a thermistor on AC line, later RCAs with automatic degaussing used a thermistor ahead of the rectifiers only.

Motorola also used a line thermistor "tube sentry" and may also have been accused of unsubstantiated claims of extending tube life.

On some HiFi consoles, a 3DG4 or 5U4 is used as full-wave rectifier but its heater consumes an additional 15 watts - 100% waste heat. The fact this tube needs to warm up along with others mitigates over-voltage during warm-up. If the tube were replaced by two silicon rectifiers (1N4007), B+ voltage may top 400 volts before settling down under load, stressing electrolytics unnecessarily. This is a good argument for using a thermistor, not so much for the tubes, which are tougher and replaceable.

On many of these console stereos, power transformers can be a delicate device capable of failure due to transient overloads or being inadvertently left on. To address that, add [email protected] ohm 10 watt resistors in series with AC line to drop 123 VAC to 117.

I also agree that most tube failures do NOT involve open filaments. Some times a thermistor (G.E. CL60-5 amps or CL90-2 amps) is used to mitigate an upgrade that makes the power supply more efficient and robust.

RCA's CTC11 color chassis used a thermistor on AC line, later RCAs with automatic degaussing used a thermistor ahead of the rectifiers only.

Motorola also used a line thermistor "tube sentry" and may also have been accused of unsubstantiated claims of extending tube life.

On some HiFi consoles, a 3DG4 or 5U4 is used as full-wave rectifier but its heater consumes an additional 15 watts - 100% waste heat. The fact this tube needs to warm up along with others mitigates over-voltage during warm-up. If the tube were replaced by two silicon rectifiers (1N4007), B+ voltage may top 400 volts before settling down under load, stressing electrolytics unnecessarily. This is a good argument for using a thermistor, not so much for the tubes, which are tougher and replaceable.

Zenith in particular has power transformer fails from shorted rectifier tubes, overfused at 4 amps line and being left on inadvertently. To address that in a restoration, add [email protected] ohm 10 watt resistors in series with AC line to drop 123 VAC to 117.

Motorola also used a line thermistor "tube sentry" and may also have been accused of unsubstantiated claims of extending tube life.


I may have confused Motorola and Zenith on this point. I do remember the "tube sentry" claim.

I just use these either on the filament string or off the AC line in.

https://www.mouser.com/ProductDetail/995-SL15-22102