I have a pair of date-matched NOS GE 6BL7 tubes. I ran them in a amp I'm working on, and noticed one illuminated much more quickly than the other. I swapped sockets, thinking I had a voltage mismatch, but the issue followed the tube, not the socket.
Is this normal variation? Does it indicate a problem with the tube?
Is this normal variation? Does it indicate a problem with the tube?
I think some tubes can have differing amounts of the heater exposed at the top or bottom of the cathode, and those exposed parts flare up when the amp is powered up. Older tubes can also flare more than new tubes due to thinning of the coating on the heater where is it exposed. I have an amp with 3 12AX7s in series from a 36v secondary, and one of them really lights up. This one has extra stress on it as the heaters find their equilibrium at startup.
Whether this is an issue or not depends on who you ask. Many of the older guard who grew up with tube technology think it is normal behaviour, whereas as the newcomers look at soft start techniques to look at ways to minimise the jolt on the heater as it heats up. (Like a lightbulb the resistance changes with temperature, so ramping up the voltage slows down the heating stage). A tube with excessive flash may well be a sign that the tube has been used for a long time, so could be a pointer for replacement.
I have fitted an Ametherm Inrush Current Limiter (220R 1A) on the on-off switch of my recent build, which settles down to about 1R when it has heated, and this makes a big difference to the powering up. Heaters come up slowly, and the supply to the tubes comes up like a tube rectifier was being used.
Whether this is an issue or not depends on who you ask. Many of the older guard who grew up with tube technology think it is normal behaviour, whereas as the newcomers look at soft start techniques to look at ways to minimise the jolt on the heater as it heats up. (Like a lightbulb the resistance changes with temperature, so ramping up the voltage slows down the heating stage). A tube with excessive flash may well be a sign that the tube has been used for a long time, so could be a pointer for replacement.
I have fitted an Ametherm Inrush Current Limiter (220R 1A) on the on-off switch of my recent build, which settles down to about 1R when it has heated, and this makes a big difference to the powering up. Heaters come up slowly, and the supply to the tubes comes up like a tube rectifier was being used.
I refurbish valve amplifiers from the 50s and if the valve reads good, it stays with the amplifier.
After over 70 years of being switched on and off, the heaters do not suffer. It is the budding "designer of nowadays" that thinks we need more soft starting.
A total myth but if that is what you want to waste your money on, be my guest ... fill your boots.
After over 70 years of being switched on and off, the heaters do not suffer. It is the budding "designer of nowadays" that thinks we need more soft starting.
A total myth but if that is what you want to waste your money on, be my guest ... fill your boots.
I think it is a matter of bare heater wire (bare from isolation) rather than exposition. Anyway, out of experience it won't harm.
Best regards!
Best regards!
Let me ask a related question to your experience: I have read that it is beneficial to switch on the heater circuit first to let the tubes warm up before turning on B+. Is there any truth to this?
HV power on delay
You can setup whatever time delay you want, connect after C7 (may be lift one leg of R17, then connect this module in between) Get 6.3vAC module
https://www.ebay.com/itm/265565092223?hash=item3dd4e9b97f:g:6owAAOSwcPdiF395
I use that to delay B+ for 30 seconds o my 300B
You can setup whatever time delay you want, connect after C7 (may be lift one leg of R17, then connect this module in between) Get 6.3vAC module
https://www.ebay.com/itm/265565092223?hash=item3dd4e9b97f:g:6owAAOSwcPdiF395
I use that to delay B+ for 30 seconds o my 300B
I got the two-position power switch from Broskie, so I just plan on doing it manually. Wanted to know if there was actually any benefit, or if it just stroked my "fancier is better" Audiophile ego 🙂
I guess benefit for life span of the tubes. The support to turn on filament on for 30 seconds before HV B+
Check this out http://www.valvewizard.co.uk/standby.html
Check this out http://www.valvewizard.co.uk/standby.html
The 12A_7 tubes have several different kinds of filaments.
Example the ECC81/12AT7:
1. Filament wires that go up and down several times in the cathode; they all are straight wires, and they do Not light up brightly at power on.
2. Filament wires that go up once and down once in the cathode; and that Do light up brightly at power on.
They have spiral wires, no straight wires, and no coating at the header, no coating until just before they enter the cathode. Telefunken tubes did this.
3. Filament wires that are spiral wound. The spiral goes up once, and comes down once in the cathode. They do Not light up brightly at power on. They have uncoated straight wires from the header, and then they are formed into spirals that are coated before they enter the cathode.
Who knows how many other versions of filaments there are to these tubes?
Does it make any difference?
Perhaps, in the amount of hum introduced into the cathode?
But essentially, they all work pretty well.
Do not loose any sleep worrying about these differences.
Many tubes last for thousands of hours and many decades.
We are talking about consumer radio tubes and audio tubes.
We are not talking about high power transmitting tubes with no getter on the glass envelope, and filaments that take more power than the plates of an audio tube can take, etc.
Your Mileage (and your tube May Vary).
Example the ECC81/12AT7:
1. Filament wires that go up and down several times in the cathode; they all are straight wires, and they do Not light up brightly at power on.
2. Filament wires that go up once and down once in the cathode; and that Do light up brightly at power on.
They have spiral wires, no straight wires, and no coating at the header, no coating until just before they enter the cathode. Telefunken tubes did this.
3. Filament wires that are spiral wound. The spiral goes up once, and comes down once in the cathode. They do Not light up brightly at power on. They have uncoated straight wires from the header, and then they are formed into spirals that are coated before they enter the cathode.
Who knows how many other versions of filaments there are to these tubes?
Does it make any difference?
Perhaps, in the amount of hum introduced into the cathode?
But essentially, they all work pretty well.
Do not loose any sleep worrying about these differences.
Many tubes last for thousands of hours and many decades.
We are talking about consumer radio tubes and audio tubes.
We are not talking about high power transmitting tubes with no getter on the glass envelope, and filaments that take more power than the plates of an audio tube can take, etc.
Your Mileage (and your tube May Vary).
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JonSnell's comment is pretty much truth.
Maybe today's manufacturing of tubes needs "helper tweaks" due to not adhering to the old standards of tube making, I don't know.
But I've got original tubes in regular service with no problems. (GE-RCA-Tungsol-Raytheon-Realistic "lifetime"-etc)
And none of them need or have to be babied and stroked and patted on the head with delayed B+ stuff.
And at the shop, restoring hundreds of 1929 to 1960's radios, phonos, etc, I rarely came across tubes that needed replacing, or were weak in emission.
That "delayed B+" stuff was generated on the internet and is basically an internet-born obsession.
There are a lot of "new gurus" out there that dream this stuff up, when it wasn't a problem for decades.
Delayed B+ might be warranted for transmitting tubes operating in transmitting service, but for ordinary tubes used in almost any situation, the possibility of cathode stripping due to B+ not being delayed is not a serious consideration that one should worry about. Only situation I can think of danger from the B+ coming on too soon, is where there is direct coupling of a grid to a previous stage, and there not being proper bias and the grid ending up at much too high voltage at turn on. But a diode clamp should easily solve this problem. There might be some other cases where it matters, but most of it is people needlessly worrying about something that they understand incompletely at best.
For those amplifiers that have B+ capacitors that have a voltage rating quite a bit Less than the unloaded B+ voltage . . .
You have to either use very fast warming output tubes, or slow warming rectifiers, or some other form of delayed B+ circuits.
Otherwise, your B+ capacitors may have a very 'short' lifetime (short is both the truth, and a pun).
Just my opinion
You have to either use very fast warming output tubes, or slow warming rectifiers, or some other form of delayed B+ circuits.
Otherwise, your B+ capacitors may have a very 'short' lifetime (short is both the truth, and a pun).
Just my opinion
"Date matched" has little to do with actually being matched. perhaps two made on the same day are more likely to be the same, perhaps not...
Some tubes suffer more than others from surging heater current on startup. as someone else pointed out above, it could be the heater is more visible, and thus you're seeing the effects more. I've seen many more tubes dead from no emissions than from open heaters. I've seen many more tubes with dead shorts in them than tubes where the heater actually died.
If you're using multiple tubes that don't have controlled heater warm up times in series, you're more likely to have heater problems. 12AX7 has no controlled heater warm up time, 12AX7A does. If these are new production tubes, there's no telling whether they were designed to actually meet 12AX7A specs or not.
are you using NOS 12AX7A here? otherwise one tube will bear the brunt of the load more than others.
This is a wise point to mention, but IMHO, a reflection of poor design engineering to not have the capacitors rated for the unloaded B+ voltage.
Per the Original Post:
Get your high power magnifier out, bright diffused lighting, and see if the construction of the two 6BL7 tubes filaments are different.
I have seen production line products that were one way in the morning; and products that had been modified in the afternoon
. . . the date codes were the same.
Get your high power magnifier out, bright diffused lighting, and see if the construction of the two 6BL7 tubes filaments are different.
I have seen production line products that were one way in the morning; and products that had been modified in the afternoon
. . . the date codes were the same.
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anchorman,
You understood my intent.
Yes, I should have made it more clear for newbees . . .
Always use B+ capacitors that have voltage ratings higher than the highest B+ voltage: with no B+ load, and with power mains at maximum voltage conditions.
You understood my intent.
Yes, I should have made it more clear for newbees . . .
Always use B+ capacitors that have voltage ratings higher than the highest B+ voltage: with no B+ load, and with power mains at maximum voltage conditions.
For normal things like home amplifiers at the 300-350V B+ levels, I use filter caps rated at 450V, never had any issues.
I've got an amp that has 500V rated capacitors from the factory. fully loaded B+ is around 480V, unloaded B+ is much higher than that. Also depends on what it's plugged into (115V vs 122+).