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
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
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.
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.
shif said:
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.
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
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.
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.
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.
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
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
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.
Thank you all for your ideas.
Today I smoke tested my power supply and it works as expected. I ran both secondary leads through each half of the DPDT switch (timer relay). After reading these posts, I've decided to change the layout. I plan to run the center tap through one half of the relay and use the other half to switch in a bleed-off resistor when the relay is off (power-off condition). This will discharge the B+ supply making my project safer to play with. The heater supply will discharge through the filaments and my ubiquitous blue LED mounted in the front panel.
-S
Today I smoke tested my power supply and it works as expected. I ran both secondary leads through each half of the DPDT switch (timer relay). After reading these posts, I've decided to change the layout. I plan to run the center tap through one half of the relay and use the other half to switch in a bleed-off resistor when the relay is off (power-off condition). This will discharge the B+ supply making my project safer to play with. The heater supply will discharge through the filaments and my ubiquitous blue LED mounted in the front panel.
-S
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