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24.27.41.140
In Reply to: RE: Need for Soft-Start / Standby for Solid State Rectification? posted by Chip647 on April 12, 2024 at 11:56:45
It's an urban myth propagated by people who don't know better. The voltages present in receivers and most amplifiers aren't sufficient to damage the tubes. The only real concerns are A) negative bias comes to life quickly if it's a fixed-bias output stage, and B) all the filter and decoupling caps must withstand the surge voltages that occur before the tubes conduct. Every decoupling cap down the line must be rated for full unloaded B+.I'll just add that I've purchased many amps/receivers from the '50s and '60s with SS power supplies that still had the original tubes and were working well.
Edits: 04/12/24Follow Ups:
Regarding soft start?
Cheers
The only thing I've run across that needs some thought is directly heated tubes with indirectly heated drivers that are directly coupled. Some tubes are more sensitive than others, and some circuits are more risky than others. For example, a choke loaded driver with power taken from the output tube cathode/filament (the monkey-on-a-stick" topology) provides some protection from grid-cathode arcing.
MMMMMM monkey on a stick is one of my favorite audio delicacies.
clippy
!
The Mind has No Firewall~ U.S. Army War College.
On my ECC35 DC PX4 amp, instead of using indirect heat rectifier, I use 5R4 type tube with a delay timer, plus tube voltage regulation for B+, delay timer is set to 1 minutes, it allows me to cooks the PX4 and ECC35 filament first for a about a minutes, then roughly another 12 seconds before B+ ramp up to full 420V.
It may be over kill but for DC circuit, I think this is the best way to protect those rare triode power tubes.
My 211 SETs use a choke-loaded IDHT follower direct-coupled to the output stage grid. Despite powering the driver from its own power supplies, there was significant drift of output stage idle current in the prototypes. The current would consistently creep downward more than 10-12% during the first 30 minutes or so from cold. I traced this to gradual conductance changes in the driver tube itself.
Stabilizing this was tricky, because the amplifiers are designed to operate in A2 when required. A2 causes output stage current to change, so cathode current couldn't be used to regulate bias. In the end, I designed a circuit that detects and integrates output stage grid voltage. This doesn't change, because signal voltage is always symmetrical about the zero crossing, and the driver has the headroom necessary to remain linear under all driving conditions.
The control circuit for this ended up being more complex than I originally anticipated, but idle current is rock steady. You've probably seen this before Paul, but I'll post the partial diagram here for the benefit of anyone else interested.
I might need to defer to the opinions of others on that point, because I don't have long-term experience with all the various tube types or different operating voltages. However, my 211 SETs use a supply operating above 1KV, and they see only a relatively short delay from a 5R4GYB. I also have a 811A RF amplifier operating at 1,500V, and its SS supply comes up with no delay at all. The tubes in both amps have been trouble-free.
Hard not to get paranoid sometimes :-)
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