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Thermistors to prevent heater current inrush?

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Soothsayerman

Soothsayerman

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Joined 2011
#1
I know in solid state design thermistors are used but, could thermistors be used to prevent heater current inrush on tubes?

or maybe the interior temperature differentials of tube amps wouldn't really lend itself to using thermistors?

I am sure there is a very good reason they aren't used that I am missing....

Thanks

-David
 
#3
Wavebourn said:
Yes, I used thermistor in series with primary until I found a better way to reduce all inrush currents at once.
Click to expand...

Gotcha, I hadn't considered that! Solving the problem before it becomes a problem! I knew it was something big that I was just missing because I never see them in circuit designs really.

Of course that begs the question, how did you solve it? It may be too complex to go into or proprietary and I completely understand if so! 😀

Thanks for the quick reply!

-David
 
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#13
needtubes said:
And that way is?
Click to expand...

That way is, all inrush currents sum up in the primary side of the power transformer, according to their transformation ratios. If you add a resistor in series with the primary all voltages on each secondary will go up with the same speed, no matter how many cold filaments, rectifiers and capacitors you use.
So, my solution was, instead of guessing and calculating when all inrush currents go down to certain safe level I decided to let them to define the time constain. One relay after one of rectifiers that shunts a resistor in series with the primary solves the problem very well. Simply, but effective.
 
#15
In a very basic sense for the generic valve amplifier - is there any 'problem' to start with?

The only tangible 'problem' I see is that any primary or secondary side fusing can then be selected at the lowest current rating if the initial current surge can be suppressed - which then benefits the amp from better fuse protection.

Perhaps the effective source resistance increase could keep a valve rectifier below its peak current rating if the design was pushing beyond such limits.

I'd have thought heater current reduction to near steady state was not really a one-one correspondence with output tube loading of B+ (and hence any problem related to short term over-voltage of B+).

So this topic may be just an ss amp related concern.
 
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#16
Some of todays current production 5AR4's have less than stellar quality control with respect to electrode alignment and cathode coating uniformity. There have been batches from all the major manufacturers that "spark out" on initial power up. Designs like the SSE that come close to the rectifier peak current rating have been known to eat brand new 5AR4's.

Adding a CL70 or CL90 depending on the amps size in series with the power transformer's primary, and a CL140 in series with the HV secondary CT lead will increase the warm up time a few seconds, but has been proven to save rectifier tubes. Adding a 1N4007 in series with each plate of the 5AR4 helps too.
 
#18
tubelab.com said:
Some of todays current production 5AR4's have less than stellar quality control with respect to electrode alignment and cathode coating uniformity. There have been batches from all the major manufacturers that "spark out" on initial power up. Designs like the SSE that come close to the rectifier peak current rating have been known to eat brand new 5AR4's.

Adding a CL70 or CL90 depending on the amps size in series with the power transformer's primary, and a CL140 in series with the HV secondary CT lead will increase the warm up time a few seconds, but has been proven to save rectifier tubes. Adding a 1N4007 in series with each plate of the 5AR4 helps too.
Click to expand...
I like to use damper diodes with controlled warm up 6cl3 and the like. George how are you after last Thursday ?
 
#19
Yes, I observed the destruction of a 5AR4 upon switch off of the standby switch. Investigation revealed that opening of the standby switch causes a high voltage spike across the HV secondary of the power transformer. The magnitude of this voltage spike is dependent on when in the AC cycle the switch is opened and the quality of the transformer.

The spike results in an arc through the 5AR4 and a new arc resulting in destruction of the tube on next use. 5U4's and 5Y3's may arc, but it doesn't seem to destroy the heater like it does the cathode in a 5AR4.

Modern cost reduced transformers like the Hammond 200 series and their Allied equivalents are worse case. I observed over 2.5 KV across the 750 volt secondary in an Allied 6K7VG. Older (50's vintage) power transformers never went over 1200 volts. Normally the load on the other windings (heaters) will provide enough damping to absorb this transient. Modern transformers seem to have less weight for a given VA rating, so I assume that the higher magnetization levels and reduced coupling from winding to winding have contributed to this.

I no longer recommend the use of a standby switch in my HiFi amps, but it seems that an avalanche rated rectifier diode across the standby switch or even a suitably rated capacitor reduces the transient.

All SSE's in the past 3 years incorporate the CL140 and 1N4007's on the PC board. This has resulted in virtually no start up issues with rectifier tubes, but I have not tried a standby switch in any recent HiFi or guitar amp.
 
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