Timed switch on B+ in power supply, good idea?

[shif]

I’m building a power supply for an OTL headphone amp project. The design uses silicon diodes to rectify B+ rather than a vacuum tube rectifier. I’m curious if this will lead to cathode striping and premature tube death since the heaters are not fully hot prior to applying B+. I’ve got ultra fast recovery diodes (MUR460) and plenty large filter caps on this project.

I am considering installing a timed switch on the leads just before the diodes. This would prevent power to the diodes until it timed out after about thirty seconds. I have an IDEC GT5 miniature on-delay timer with a 120vac coil that would be perfect for the job. It has a DPDT configuration and it's coil is powered directly from my mains switch.

Will this do the job or will it cause a voltage surge on the outputs when the HV is suddenly applied to the diodes? Will running my tranny for a short period of time with no load cause issues with heating and early death of the tranny? It is a Hammond 369EX transformer with 190-0-190 volt secondary, the timer has 250vac contact rating.

Seeking ideas and opinions from those with more experience than myself.

Thanks,
-S

[ray_moth]

A delayed B+ is a good idea; 30 seconds should be more than adequate.

To avoid a charging surge, you could use a thermionic diode in series with the SS rectifiers to achieve both a delay and a soft start. This would be effective but probably inconvenient.

Alternatively, you could connect a high value resistor (say 47k) across delay relay contacts, to allow some light current to flow into the capacitors before the relay switches on.

[Sherman]

Originally posted by shif
... The design uses silicon diodes to rectify B+ rather than a vacuum tube rectifier. I’m curious if this will lead to cathode striping and premature tube death since the heaters are not fully hot prior to applying B+. ...

Thanks,
-S

If there is no input on the grid (ie volume control fully attenuated position) there should be no cathode stripping. Don't apply a signal until the tubes heat up. I'm not an expert but I have done some relatively extensive searching/reading on this topic and have begun to think various schemes to prevent cathode stripping are mainly solutions in search of a problem. (Let the flames fly.)

Alternatively just put in a standby switch. Flip on the main switch which powers the heaters and after the tubes are warm turn on the standby switch to allow the HV to flow.

[jlsem]

If there is no input on the grid (ie volume control fully attenuated position) there should be no cathode stripping. Don't apply a signal until the tubes heat up. I'm not an expert but I have done some relatively extensive searching/reading on this topic and have begun to think various schemes to prevent cathode stripping are mainly solutions in search of a problem. (Let the flames fly.)

What difference does having no signal make? The tube plate is still has a zero signal current. I'm no expert either, but a lot of real experts have spent a lot of real time looking to solve the problem of cathode stripping.

John

[EC8010]

You might like to read the data sheet for the 6528. The valve manufacturer specifically forbids application of HT until 30s after heater power is applied.

[SY]

Shif, another Mick Abrahams fan!

EC, this recommendation is peculiar to the 6528? I've seen other data sheets for other tubes that allow all voltages to be applied simultaneously- I think this is true for the 12B4.

The unease occurs when (as is normally the case) the question of warmup goes unaddressed. I'm more comfortable using a delay (Pascal's wager), but I do wonder how often it's really necessary.

[EC8010]

6528 was the first one I could think of off the top of my head, but there are others. They seem to be the high mutual conductance types and/or the ones with very high current densities at their cathode. My feeling is that it's not worth taking the chance if it can be avoided cheaply.

[jlsem]

6528 was the first one I could think of off the top of my head, but there are others. They seem to be the high mutual conductance types and/or the ones with very high current densities at their cathode. My feeling is that it's not worth taking the chance if it can be avoided cheaply.

All of the Bendix T-series require cathode warm-up times on the order of 45 seconds, including the rectifiers.

John

[ErikdeBest]

Alternatively, you could connect a high value resistor (say 47k) across delay relay contacts, to allow some light current to flow into the capacitors before the relay switches on.

I like this solution a lot, but (well, I admit I have not searched a lot) I have not found a relay that can handle 400VDC. can someone give a model number of one of such relays. Thanks.

Erik

[SY]

You want to put the relay on the primary side, not in series with the DC. It's very easy to find relays rated for power line voltages.