Hi folks. Quick question about DHT filaments. I designed a build to incorporate two switches, one to fire up the filaments, and the other to pass the B+. To my chagrin, I just discovered after turning b+ off, I'd forgotten to turn the filament switch off last evening. Unfortunately, I'm talking vintage 45 globes. Duh. :-( As a general proposition, how much damage have I done? (The amp sounds fine right now.)
I really do not think you caused any damage this time.
A complete schematic of the amplifier and power supply might be nice to have for this thread.
You might investigate the possibility that after the amplifier is on and warmed up, you or someone else might mistakenly switch off the filaments before you turn off the B+.
You might investigate the possibility that your power company causes a hot start of the amplifier (the amp is powered and warmed up,
then the power goes out briefly, and comes on again quickly) all before you can turn off the power to the amplifier, or turn off either of the filament and/or the B+.
Most carefully designed amplifiers can be designed to handle most situations.
Some questions to ask yourself:
Is the B+ tube rectified; and does the rectifier have a directly heated filament, or is the rectifier one that has both a filament and cathode (indirectly heated)?
Is the B+ solid state rectified.
Is the B+ using a choke input filter
Is the B+ using a cap input filter
Are the 45 filaments AC powered
Are the 45 filaments DC powered
Is the DC power soft starting
And other questions about the topology.
Example, with an indirectly heated driver tube that RC couples to the 45, the 45 filament may be warm before the driver tube is.
That could put B+ from the coupling cap onto the 45 grid when it is already warmed up.
DC coupling from an indirectly heated driver tube plate to the 45 tube grid, could cause similar problems.
My simple single ended 45 stereo amplifier I designed and used at work, ran for years without any trouble.
A complete schematic of the amplifier and power supply might be nice to have for this thread.
You might investigate the possibility that after the amplifier is on and warmed up, you or someone else might mistakenly switch off the filaments before you turn off the B+.
You might investigate the possibility that your power company causes a hot start of the amplifier (the amp is powered and warmed up,
then the power goes out briefly, and comes on again quickly) all before you can turn off the power to the amplifier, or turn off either of the filament and/or the B+.
Most carefully designed amplifiers can be designed to handle most situations.
Some questions to ask yourself:
Is the B+ tube rectified; and does the rectifier have a directly heated filament, or is the rectifier one that has both a filament and cathode (indirectly heated)?
Is the B+ solid state rectified.
Is the B+ using a choke input filter
Is the B+ using a cap input filter
Are the 45 filaments AC powered
Are the 45 filaments DC powered
Is the DC power soft starting
And other questions about the topology.
Example, with an indirectly heated driver tube that RC couples to the 45, the 45 filament may be warm before the driver tube is.
That could put B+ from the coupling cap onto the 45 grid when it is already warmed up.
DC coupling from an indirectly heated driver tube plate to the 45 tube grid, could cause similar problems.
My simple single ended 45 stereo amplifier I designed and used at work, ran for years without any trouble.
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Thanks so much for the thoughtful response! My amp is basically a simple 45 incorporating some elements, primarily the paralleled 6SL7 driver, of the Morrison Micro 2A3, modified to a 45. I wasn't happy with the PS voltages available with the Micro design (though gotta admit it sounded great!), and so the project morphed to more of a Simple 45 design, which to my ears sounds great as well, if perhaps not quite as fast. That said, I'm itching for a Micro redux with some 45a's on the output. But that'll have to wait. :-(
Well-considered or not, the 5AR4-rectified PS is mostly outboard, and slowly ramps up the B+ for drivers and outputs. The outboard also houses an AC filament supply for the drivers. The outboard is, as above, independently switched. The amp chassis contains the AC filament supplies for the 45's, and it too is independently switched. My practice is to turn the amp chassis on first, which warms the 45 filaments, followed a few moments later by flipping on the B+ and 6SL7 filaments. Of course the point is to avoid cathode stripping on the 45's. The other night, however, after flipping off the outboard (B+), I simply neglected to turn off the amp chassis. Ergo, my urgent post.
Your thoughtful post gave me much to think about and I sincerely appreciate it. Your comments on the above scheme always welcome. Thanks again.
Are you switching AC to transformers for B+ and filaments?
If so, wire AC to the filament switch first and have it go to the B+ switch such that any time filament is off, B+ is also off. This will insure that one can only have B+ if the filaments are turned on.
If so, wire AC to the filament switch first and have it go to the B+ switch such that any time filament is off, B+ is also off. This will insure that one can only have B+ if the filaments are turned on.
At work we have a timer switch for our kettle. If just cuts of the supply after a preset number of hours. I hate that feeling of getting on a plane and wondering if I turned off all the switches back home, so looking out for something for the hifi plug.
Thanks for the reply. Much appreciated. B+ (input and output) and 6SL7 input filament (AC) derive from the same power transformer, cannibalized from a vintage Eico ST40. So the lone voltage applied to the signal side at switch-on is the filament supply to the 45's. All considered, I think I'm OK--the amp is running fine and sounds good. Think I escaped this time, but your suggestion is a good one and I'll remember it moving forward. Thanks again.
Use two DPDT switches in this arrangement and it doesn't matter which switch you use first. It comes from an old Glass Audio magazine.
http://audioratbag.blogspot.com/2014/04/hv-delay-using-two-dpdt-switches.html
ray
http://audioratbag.blogspot.com/2014/04/hv-delay-using-two-dpdt-switches.html
ray
Use buck converter to supply 2.5Vdc to the 45 tube. The converter is DC voltage regulated and current limiting. Also, it is important to keep the B+ and idle current within its limit. Use 5AR4 indirect heated rectifier for slow rise of the B+. These should be the good ways to prolong the life of the valuable 45 globe tube.
Johnny
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