Poor man's time delay?

I have lots of time on my hands and am contemplating building another ST-35 with some more bells and whistles. Bob and Ray's time delay board looks kinda' cool but I understand that it's probably overkill for this particular amp. B+ is in the neighbourhood of 380VDC which I guess, isn't too high.

I do like the idea of heating the tubes prior to adding the HV however and wonder if this idea would work: Put an off/on switch on the unit - the original ST-35 never had one - in typical fashion so the power transformer heater wires are delivering the 6.3VAC to the tubes right away, but have another switch on the two red HV wires before the diodes. This way, the tubes would heat up for the amount of time you choose, and then you flick the other switch to allow the HV to flow. You would want to turn off the HV supply before the heater supply when powering down of course.

Does this sound too simple to work?

Captain Coconut wrote:I have lots of time on my hands and am contemplating building another ST-35 with some more bells and whistles. Bob and Ray's time delay board looks kinda' cool but I understand that it's probably overkill for this particular amp. B+ is in the neighbourhood of 380VDC which I guess, isn't too high.

I do like the idea of heating the tubes prior to adding the HV however and wonder if this idea would work: Put an off/on switch on the unit - the original ST-35 never had one - in typical fashion so the power transformer heater wires are delivering the 6.3VAC to the tubes right away, but have another switch on the two red HV wires before the diodes. This way, the tubes would heat up for the amount of time you choose, and then you flick the other switch to allow the HV to flow. You would want to turn off the HV supply before the heater supply when powering down of course.

Does this sound too simple to work?

make sure the additional switch is capable and isolated for 350VAC !

Yeah, that was my thought too. There probably aren't too many DPDT switches out there with that voltage capability - if any. Maybe put a SPST switch on the power transformer center tap instead?

Captain Coconut wrote:Yeah, that was my thought too. There probably aren't too many DPDT switches out there with that voltage capability - if any. Maybe put a SPST switch on the power transformer center tap instead?

http://picclick.co.uk/22mm-Panel-Mount-DPST-Latching-Control-Push-Button-291736993859.html

They make them.

Cheap, too. But a bit bulky.

I suggest a relay - but that leads you back to the board.

Enjoy! It is a worthwhile endeavor you are contemplating.

Yup - common trick on the old ham tubers. Those are notorious for taking forever to warm up, so an extra switch for the heaters is just the ticket for tickling the beast so it stabilizes fast when you flip the main switch.

Here's the mod on my ol' Siltronix ...



Of course, on some of those, we're talking HOURS, not seconds ...

Got that here as well ... nothing wrong with a little redundancy, eh.

Got that here as well ... nothing wrong with a little redundancy, eh.  

I just added a barrier strip to an existing mounting point near to the AC line, then cut the neutral lead, soldered that to the terminals, and bridged those with the thermistor. Most will recommend a CL80 as they have a higher amp rating, but I prefer a CL90 as you have less power loss and a larger surface area for better heat dissipation, and I've never had one fail with some serious loads. You WILL want to provide plenty of clearance around a thermistor, as they do get HOT in operation.

About that power loss ... keep in mind a thermistor never opens completely, so you'll see a couple percentage points less throughput, which can be a problem if your B+ is already borderline. I metered mine after installation, and even with the bucker in line, I'm comfortably on the low side of spec.

Captain Coconut wrote:I have lots of time on my hands and am contemplating building another ST-35 with some more bells and whistles. Bob and Ray's time delay board looks kinda' cool but I understand that it's probably overkill for this particular amp. B+ is in the neighbourhood of 380VDC which I guess, isn't too high.

I do like the idea of heating the tubes prior to adding the HV however and wonder if this idea would work: Put an off/on switch on the unit - the original ST-35 never had one - in typical fashion so the power transformer heater wires are delivering the 6.3VAC to the tubes right away, but have another switch on the two red HV wires before the diodes. This way, the tubes would heat up for the amount of time you choose, and then you flick the other switch to allow the HV to flow. You would want to turn off the HV supply before the heater supply when powering down of course.

Does this sound too simple to work?

Giving this another thought, it could be done differently:
Use a FET to ground the midpoint of the B+ circuit, the function as :
1/ at power on it's in open state.
2/ the negative voltage across the fet will charege a cap that controls the gate
and slowly turns the fet on ( start to conduct)
3/ After some time ( 30s ?) the fet will conduct fully and it should be helt at
fully conductive state during the amps usage

This will create a true "soft-start" where the B+ will ramp up slowly. If done
properly it will turn off the fet when power is lost and always go throug
the same soft-start whenever power is (re)applied, this will save the tubes from
flashing in case of power glitches.

Advantages : no mechanical switches, no human intervention, it will
automatically "do the right thing".

The logic to control the fet is left to the student  as an exercise :-)

A fully functional circuit is presented in :
http://www.diyaudio.com/forums/tubes-valves/298660-st-70-power-supply-questions.html
although it actually breakes the B+ on the plus side, but that will be equally good.

I'm using a CL-90 now with good results. Maybe that's all that is required for this amp. As Bob mentioned in another thread, there are many ST-35s still in service that don't have any "protection". But there's no harm in obsessing. Or is there?

I finally added a power switch to mine. Nice to have. Mine was already using the Gillespie EFB board so I'm not too sure what else I could have done to add bells and whistles I'd really notice. And I'm thinking that the advantage of a standby power arrangement would not matter as much with the EL-84s as it would with a big 6L6 or a KT88. What else are you planning to do to soup it up?

daveshel wrote:I finally added a power switch to mine. Nice to have. Mine was already using the Gillespie EFB board so I'm not too sure what else I could have done to add bells and whistles I'd really notice. And I'm thinking that the advantage of a standby power arrangement would not matter as much with the EL-84s as it would with a big 6L6 or a KT88. What else are you planning to do to soup it up?

My recommendation is to remote-control the on-off, here is one example
of a device that does this at a reasonable cost :
https://www.sparkfun.com/products/13015
( the device shown can control 40A, devices for less power is available. Formfactor
and mounting is perfect for internal mounting, no isolation pads needed)