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Do DHTs need HV delay?

As this is my first foray into the DHT world, and I have 2 SET amps in house now, I am wondering if applying HV at the same time as heater voltage is detrimental to tube life of DHTs like it is for indirectly heated cathodes?

I would like to make the mods, delay etc while I have the amps on the bench for tweaking, if it's a requirement. Both these amps have diodes in the HV supply and share the same xfmr as the heaters and are instant on. I'm going to add a thermistor to the AC line to reduce surge on the big 805 amp.



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There are only a small number of types where delayed B+ is required - mercury vapor rectifiers, and a few low mu IDHT high perveance triodes (6AS7G, 6080 etc). There are also a few DHT types with thoriated tungsten filaments where delayed B+ is required/recommended, otherwise I'd say not required based on 4 decades of designing and building tube amplifiers.
Definitely agreed that HV delay is only required for a narrow category of fairly specialized tubes.

And I definitely don't want to start a dispute on this topic that can easily turn severely disputatious.

My opinion, and it is only opinion-

At the same time, there's data that a much wider variety of tubes end up with reduced life if run for extended periods with too low a filament voltage. I view the tube heating up with full B+ from the outset as a something that might be a form of much shorter interval of running with filament undervoltage - shorter but maybe more extreme. Multiply by all of the on-cycles over the hoped-for-life duration of the tube, and it may or may not cumulatively make a big difference. To know if it made a difference that actually mattered, you'd have to do some sort of test based on lengthy side-by-side cycling of two otherwise identical circuits, exactly the same except for presence/absence of the B+ delay, and you'd have to run the test over a quantity of samples of tubes, and maybe across makes of tubes, to really have informative results. As far as I know no one has run, or is likely to run, that test. It's very rational to think it probably doesn't matter at all, but it is not totally irrational to want to remove the potential for something that might shorten the useful life of the tubes.

A lot of the commercial tube equipment that had solid state rectifier supplies was from the later-to-sunset era of tube equipment, where it was assumed that tubes were consumables, and also usually assumed that replacement tubes were readily and cheaply available and/or of no cost concern to end users. On anything built now, there are definite advantages to solid state rectifiers that vastly outweigh the absence of a B+ delay in commercial product in which the inclusion of an additional B+ delay will equal some mix of higher cost to buyers or lower profit to manufacturers and retailers- or a DIY project in which someone concludes (entirely reasonably) that the B+ delay isn't worth their incremental bother or expense.

Myself, given a clean slate, and in anything I'd build or significantly re-work using more expensive/special tubes, I'd generally prefer to have B+ delay, based on the potential for it to increase the lifespan of what are now more scarce/expensive/upscale tubes. But that's just my preference, and I tend on these sorts of things to like to err on the side of preferring the largest possible margin for reliability and longevity.
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2017-11-24 11:21 pm
Madison WI
For what it's worth thoriated tungsten filaments are more tolerant of positive ions than oxide cathodes and can tolerate a depletion of the electron cloud much better than their oxide counterparts.

Care and Feeding of Power Grid Tubes (put out by Varian) has some good information on this. In the section on tube life they even go so far as to suggest running reduced filament voltages in certain applications. They suggest finding the point at which performance starts to decrease, then running the filaments slightly above than that point.

For oxide cathode tubes like a 3CX1500 you would definitely want a delayed turn on for the HV supply. That said, anything using a 3CX1500 would have separate switches for plate and filament voltages so it is less of an issue. Whether this is realistically an issue for tubes like 6L6s or EL34s I do not know.