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How best delayed B+ with SS rectification

cubby01

cubby01

aka Buck
I'm going to do a build with SS diodes for PT rectification. In this case I've got a stand alone filament tranny. Wondering what recommendations are for managing tube warm up before hitting them with B+.

One thought was two toggle switches; one turns on the heater TX and the other turns on the big PT.

Another thought was a 3 position toggle; Off-Heaters-Everything.

I guess There's also some relay type delay that could be built in but I've not looked into that at all.

What else is there? And what works well for you folks that have done this?

Thanks,
 
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If you want to do 2 switches, I recommend you use 2 double-pole switches.
Wire a pole from each in series to feed the B+ power.
Wire a pole from each in parallel to feed the heaters.
No matter which order you hit the switches, one switch on gives you heater power, and the B+ comes on when both switches are on. There is no combination that gives you B+ without heaters on.
 
BrianLW said:
If you want to do 2 switches, I recommend you use 2 double-pole switches.
Wire a pole from each in series to feed the B+ power.
Wire a pole from each in parallel to feed the heaters.
No matter which order you hit the switches, one switch on gives you heater power, and the B+ comes on when both switches are on. There is no combination that gives you B+ without heaters on.
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Thanks Brian, From the Radio Handbook circa '41 I believe. that was kinda my thought with the 3 way switch although I hadn't sourced the switch yet. I'm liking the 2 switches the more I think about it.

@timgr - I'll have to consider that 6BE3 tube for the future. I'm not wanting to add another tube for this one.
 
I don't think most audio tubes in hi-fi run at high enough voltages to worry too much about just turning things on, but sometimes I put a varistor in series with the output of your rectifier, gives the tubes a few seconds to warm up and good if you have a lot of capacitance in the power supply. Similar to having a tube rectifier as far as soft turn-on.
edit: uh, I meant to say thermistor.
 
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drembo said:
I don't think most audio tubes in hi-fi run at high enough voltages to worry too much about just turning things on, but sometimes I put a varistor in series with the output of your rectifier, gives the tubes a few seconds to warm up and good if you have a lot of capacitance in the power supply. Similar to having a tube rectifier as far as soft turn-on.
Click to expand...

Well that does bring up a good point. I've seen it said that "low" voltages its's not a problem. "Low" is a relative term and I'm not sure where the cutoff is. I always assumed it was in reference to like around 200vdc preamps. But is 550vdc still considered low? Might be compared to 900vdc B+ in some amps. I'm sure there are really high voltage amps. Where's the line? :dunno:
 
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Check out Amperite delay relay tubes such as used in the Dynaco MK VI. They made a variety for both 9 pin and octal sockets. You can get them with a variety of heater voltages and delay times. You can source 115 volt heater versions so that you don't add to the existing filament load.
 
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hey-Hey!!!,
Just keep in mind the ratings on the delay relay's contacts; something like 3A of 120VAC...IIRC the folks at Dynaco put the relay in rectified B+ on the Mk.VI. That is WAY over its limits. Put the Amperite relay on the primary of the HV power trans. That way you run the heater trans on power up with the switch( and hopefully the bias trans ) and when the delay relay heats up it then powers the B+.
cheers,
Douglas

ps...the MK.VI only partially separaqtes the delayed B+; the power tubes get it instantly, the input stage gets it after the relay closes.
 
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Bluelobster said:
Have you thought of using a thermistor in the power supply for a soft start?
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Yes, have thought a little about it and have some CL-90 types handy. But, I would only want this on the PT (B+) right? Actually the filament TX I'm thinking of using is old and could probably use the benefit of a slight voltage drop. If I put a Thermistor in front if each of the two transformers then I'd still need two switches and have to stagger the 'ON' times right?
 
PakProtector said:
hey-Hey!!!,
Just keep in mind the ratings on the delay relay's contacts; something like 3A of 120VAC...IIRC the folks at Dynaco put the relay in the HV secondary CT on the Mk.VI. Put the Amperite relay on the primary of the HV power trans. That way you run the heater trans on power up with the switch( and hopefully the bias trans ) and when the delay relay heats up it then powers the B+.
cheers,
Douglas
Click to expand...
Thanks for posting, that just went in my notebook.:thmbsp:
 
Any "normal" switch contacts can cause "unwanted" effects when you put 500 or so volts across them. Since you are doing separate transformers, it becomes a no brainer IMHO to deal with the primary side. I would just use a $1.50 thermistor before the primary of the HV TX. How much current you are dealing with and how much delay you want would be the deciding factor of which device(s) to use.


Edit, that's not to say that they don't make thermistors for high voltages, they do, and they are cheap as well.
 
In older equipment I have seen with a grounded center tap B+ winding, the B+ switch is often in the CT ground leg. Low current at that point compared to switching the primary.
 
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BrianLW said:
In older equipment I have seen with a grounded center tap B+ winding, the B+ switch is often in the CT ground leg. Low current, low voltage.
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Yes, and that's an excellent spot for for a CL whatever too. A normal 6L6 amp can get by with a very small CL 120 right there, or you can even use a 1/4 amp, or 1/2 amp type as a combination inrush limiter / fuse as well.

Here's the 120 that I typically use there. http://parts.digikey.com/1/parts/705811-current-limiter-inrsh-1-7a-10ohm-cl-120.html
 
BrianLW said:
In older equipment I have seen with a grounded center tap B+ winding, the B+ switch is often in the CT ground leg. Low current, low voltage.
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ummmmm...that is not a low voltage position; it is the same voltage as putting it at the other end where the rectifier is.
cheers,
Douglas
 
PakProtector said:
ummmmm...that is not a low voltage position; it is the same voltage as putting it at the other end where the rectifier is.
cheers,
Douglas
Click to expand...

Yeah, now that I think about, I see what you mean. Still was very common, and avoided switching primary current. But not low voltage - I stand corrected.
 
BrianLW said:
Yeah, now that I think about, I see what you mean. Still was very common, and avoided switching primary current. But not low voltage - I stand corrected.
Click to expand...

But very low current which is often the limiting factor when using CLs in the primary. Another good one for that spot is the 140 (1.1 amps 50 ohms).
 
hey-Hey!!!,
You can lower the effective current requirement with a bit of insulation( use neither toxic or flamble stuff; glass wool is good). The rating is set to give enough heating to drop the resistance a set amount, if you're going to pull less, than keep the heat in and get the same hot performance at lower current.
cheers,
Douglas
 
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