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Old 15th December 2012, 08:21 PM   #31
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I mean say 6 years is too much time for burn-in or breakage.
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Old 15th December 2012, 09:46 PM   #32
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Originally Posted by Rod Coleman View Post
I hope DIYers are not scared away from the EH 300B gold grid though - the newest version of these is very well constructed and aligned, and it sounds far better than previous versions.
Yes, that's what I find as well. Really good tubes!!
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Old 16th December 2012, 04:18 AM   #33
cotdt is offline cotdt  United States
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For one, those EH have really thin filaments compared to other 300B's.

Today I saw an old 1940s paper saying that 6SN7s in digital devices (computers) were more reliable when never turned off. Whenever one of the 20,000 6SN7's failed, the system would go down. The failures were almost always during turn-on or cool-down. The reliability of the 6SN7s improved hundred-fold when simply kept on. It took them years to realize this fact.
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Old 17th December 2012, 01:02 AM   #34
tcqanh is offline tcqanh  Viet Nam
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Using them with ac-heating, especially where the transformer is able to supply many amperes at start-up, is likely to be the biggest risk. When the filament winding is on the same transformer core as the HT(B+) supply, a high starting filament current is certain, because the HV secondary is effectively unloaded at switch-ON. The filament winding will give higher than normal voltage, and the cold-filament resistance of the 300B is less than 1 ohm (instead of 4 ohm hot). In this case, starting-current of over 5A is likely.
High current for filament at starting lasts very very fast. I think it is not major cause for filament's breakage. filament material is special for hot temperature.

WE 300B ampli have used also DC filament only with capacitors filter, but these tubes get very long life.

Almost indirectly heater tube use AC or DC (without CCS) for heater but I never see a case of broken heater for those one.
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Old 17th December 2012, 01:35 AM   #35
kevinkr is offline kevinkr  United States
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Originally Posted by tcqanh View Post
High current for filament at starting lasts very very fast. I think it is not major cause for filament's breakage. filament material is special for hot temperature.

WE 300B ampli have used also DC filament only with capacitors filter, but these tubes get very long life.

Almost indirectly heater tube use AC or DC (without CCS) for heater but I never see a case of broken heater for those one.
I've got to agree with Rod on this based on some experience with various brands of 300B and AC, DC and CCS based heating where I've experienced the best long term reliability with CCS heating of dhts.

The original WE amps all used AC heating, and probably filament windings with reasonably high dcr to limit cold filament inrush.. (They did not miss much)

Magnetic field strength during inrush and local hotspots both contribute to filament stress, a CCS substantially reduces both.

Comparing an idht filament with a dht filament is not valid due to many differences in construction, they generally run a bit cooler and are also more physically constrained than a dht filament is.
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Old 17th December 2012, 02:19 AM   #36
tcqanh is offline tcqanh  Viet Nam
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Originally Posted by kevinkr View Post
Comparing an idht filament with a dht filament is not valid due to many differences in construction, they generally run a bit cooler and are also more physically constrained than a dht filament is.
Material for heater of ID or DH tube is metal too, but somes are better or worse depend on the kind of material.

Almost ID tube's heater have DC resistance below 1 ohms, so they get big current in start of burning like DH tube. They is cooler than DH tube... I'm not sure about this

Some tube ID tube get flash at right time of turning on the power. You can see that phenomenon on many 9 pins tube of Mullards . Why ? I guess they maybe have very low DC resistance heater. But these tube is longevity.
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Old 17th December 2012, 02:36 AM   #37
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I think filaments burning out is an even worse problem for IDHTs.
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Old 17th December 2012, 04:33 AM   #38
kevinkr is offline kevinkr  United States
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I think filaments burning out is an even worse problem for IDHTs.
What makes you say that? I've built and sold dozens of amplifiers using all sorts of idhts and have yet to hear of a single filament failure, OTOH nearly every dht based amplifier has experienced one or more filament failures, none used ccs heating of dhts..

In 30yrs I've personally had exactly two idht fail due to filament burn out - both Mullard 12AX7A that fried due to inrush. (The bright flash, both of these tubes were NOS purchased in the 1970s and were barely used when they failed - both in the same amp.)

My current system has 18 idht and 4 dhts, previous iterations had as many 30 idhts and 4 dhts - none have ever lost a filament. My MR-71 tuner (not counted above) has all of its original tubes except in the multiplex and audio stages which I modified 3 decades ago..
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