When some tubes may not like full HT while cold and when I drained the remnants of my finances on some rare and extremely expansive tubes – I'd rather take all precautions, even against remote and unproved possibilities.
It looks that the best precaution measure would be to feed the filaments with about 50% rated voltage when the amp is idle and provide some lower value HT, may be about 100V. It may increase the cost by additional power transformer, but that cost increase is minor compared to the cost of the tubes.
It looks that the best precaution measure would be to feed the filaments with about 50% rated voltage when the amp is idle and provide some lower value HT, may be about 100V. It may increase the cost by additional power transformer, but that cost increase is minor compared to the cost of the tubes.
I've owned a few of those. Amazing how long they keep going. But guess what? They all have Tube rectification.
Did rectifiers in that radios survive? Providing "HT delay" they run with anode voltage applied BRFORE they are hot. Also, they are often DIRECTLY heated. Also, they suffer from big current DURING HEATING PERIOD.
Actually the best precaution is to use a voltage that slowly rises as the circuit is turned on. That way you won't have tubes die b/c of cathode poisoning (no plate voltage with hot cathodes), and you won't have molecules fall off and hit the grid b/c sudden B+ on cold cathodes.
I understand that people take all precautions so whatever gives that comfy feeling is a good thing.
But I will no longer bother include a standby switch on my amps.
Whee doggies, Jethro - what a lively discussion!
I couldn't agree more. The only problem is that we can only get the 10-year-old stuff here in NC. Just as well; I can't afford the 40-year-old stuff even if it were available.
Touche. Tubes are - *gasp* - expendable.
Now, where's my soldering iron...
I couldn't agree more. The only problem is that we can only get the 10-year-old stuff here in NC. Just as well; I can't afford the 40-year-old stuff even if it were available.
Touche. Tubes are - *gasp* - expendable.
Now, where's my soldering iron...
SY, your perception of the reliability of commercial products is neither the subject under discussion, nor relevant to the degraded performance claimed by the author of that note. Would anyone on this forum complain about an amp because a valve went noisy after a few years use, and worked fine with a new one installed? But that same user may not have known whether the sound quality degraded during the life of the valve. The amp may not even be good enough to resolve the difference.
But that doesn't mean I'm going to treat my DIY amps like that, because things I make myself should be as good as they possibly can, and be stable in use over years of service.
van de Weijer's note offers a warning about an effect that MAY occur. Until someone offers something else, authored by someone qualified to add to the debate (ie real experience of cathode design) - we have nothing to go on but guesswork. You guess one way, I guess the other. That's fine - this is DIY.
No. We learned to delay B+ for magnetrons only in radar stations, and similar tubes with high anode voltage and high cathode current density. For the reception/amplification tubes we learned that they have higher anode voltage rating when cold. It is a diametrically contrary to the myth discussed here.
In case of defective tubes used.
By the way, I have a solution for you.
It is a power supply I designed for a tube radio.
A first filament voltage goes up slowly, so no filament stress.
Second, B+ waits for tubes to start conducting. It does not work as a switch (off/on), but it's speed of raising of output voltage depends on the current drawn from it, due positive feedback by current that works until it comes to the stable condition and starts regulating voltage. Such a way coupling capacitors have time to be charged gently, and don't provide abrupt grid currents.
Enjoy!
I hope, since you are here, you know from which side to hold a soldering iron!
What is compelling to me is that tubes have a cold anode voltage rating. Vao is the maximum voltage that can be applied to a cold tube. Some datasheets call this the non-conduction i.e. zero current case, others call it the cold case.
As long as this isn't exceeded, I feel safe in applying B+ before warmup as long as all other voltages are also stable. (I do worry about inrush current on startup)
I offer the following suggestions as to why the idea that B+ on a cold tube is harmful:
"Why do tubes fail?":
Back in the day, that was a million dollar question and everyone wanted to have the answer. There may have been a lot of speculation.
Warmup delay:
The standby switch is not necessarily there to save the cathodes. Back in the day, people didn't experience the joy and anticipation we DIYers do during the warmup period. It was more like "c'mon... c'mon... c'mon... warmup you bastard!" With the standby switch, the amp could be ready to go instantly when needed. TV sets were made to keep the filaments warm so users wouldn't experience the warmup delay. I don't recall tubes lasting any longer in these sets.
Early SS rectifier conversions
There might have been some association of tube failure with the conversion to SS rectification, but that's hardly surprising given the impact of this on B+ voltage, both inrush and steady state. SS conversions killed a lot of tubes, but there's no need to blame B+ on a cold cathode.
Transmitting tubes:
Some transmitting tubes data sheets recommend delaying B+ until the cathode is warmed up. For example, Eimac states in the 4CX250 datasheet "it is recommended that rated heater voltage be applied a minimum of 30 seconds before other operating voltages are applied". This is in the section relating to limiting maximum cathode current.
As far as logic goes, it would be pretty hard to prove shakti stones don't make a difference...
As long as this isn't exceeded, I feel safe in applying B+ before warmup as long as all other voltages are also stable. (I do worry about inrush current on startup)
I offer the following suggestions as to why the idea that B+ on a cold tube is harmful:
"Why do tubes fail?":
Back in the day, that was a million dollar question and everyone wanted to have the answer. There may have been a lot of speculation.
Warmup delay:
The standby switch is not necessarily there to save the cathodes. Back in the day, people didn't experience the joy and anticipation we DIYers do during the warmup period. It was more like "c'mon... c'mon... c'mon... warmup you bastard!" With the standby switch, the amp could be ready to go instantly when needed. TV sets were made to keep the filaments warm so users wouldn't experience the warmup delay. I don't recall tubes lasting any longer in these sets.
Early SS rectifier conversions
There might have been some association of tube failure with the conversion to SS rectification, but that's hardly surprising given the impact of this on B+ voltage, both inrush and steady state. SS conversions killed a lot of tubes, but there's no need to blame B+ on a cold cathode.
Transmitting tubes:
Some transmitting tubes data sheets recommend delaying B+ until the cathode is warmed up. For example, Eimac states in the 4CX250 datasheet "it is recommended that rated heater voltage be applied a minimum of 30 seconds before other operating voltages are applied". This is in the section relating to limiting maximum cathode current.
As far as logic goes, it would be pretty hard to prove shakti stones don't make a difference...
I have examined and stripped for parts many pieces of military electronic equipment that were equipped with a time delay relay on the plate transformer.
John
In the Johnson era, however...
I recently used a D-51 with the original tubes in it. Excellent amp. D-76, D-150, these were deservedly classics.
As for military gear, it is certainly the case that HV applications (like 3kV on a 4CX1000) are a different animal.
That's the point delay lovers routinely ignore and-IMHO-the worst tube killer.
Don't forget large cathode by passes that have to be charged too !
Never built an amp with delay switch, at most an NTC in the PSU tranny to reduce inrush current . . . s you don't need to oversize the fuse
Yves.
I heard a theory as if 3 Great Pyramids on Giza Plateau are precise aligned to North Pole, and resemble projection of 3 stars in Orion belt in December 2012, in order to stop Earth's crust shift, caused by precession angle and planet's paradise.
Can somebody prove/disprove this "theory"?
The Earth's crust is more valuable than selected quad of EL156 tubes, so it is better to be cautious and believe in the theory, right?
There were low (relatively) voltage applications that used them, but the military tends to write specs in general terms, so most equipment had a delay whether they really needed it or not. My own opinion is that the best medicine for long term reliability in tube equipment is a thermistor on the primary of the power transformer. Bendix recommended current limiting resistors on the plates of their high-reliability rectifiers and, get this, a 45-second warm-up time for their amplifying tubes but with plate and filament voltages applied simultaneously.
I do know that running a cathode below it's recommended operating temperature long term can shorten tube life.
John
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.