Hi,
I'm assembling a tube phono + line preamp. Both stages are fed by the same PS which I plan to build in the same chassis (but separated by a partition and enough space).
I would like to add a switch to disable the phono pre if I'm not using it, and save tube life. The way I thought of it working was to add a 3PST switch that would cut the B+ and filament supply (AC, so both wires would be connected to the switch).
The challenge I came up with is that the B+ is around 250V and the filament supply rating of the transformer is 4A. I noticed that switches are rated for both voltage and current, so I would need a 500V, 4A 3PST switch that would make a real bulky and hard to find piece of iron.
I also saw that some switches are rated for e.g 10A / 250V or 15A / 125V, so I wonder if there is a correlation between the two, and I could run a high-voltage, low-current (B+) and two low-voltage, high-current (filament) wires through the same switch?
Is this a good idea at all to mix these paths, or am I setting myself up for a safety hazard? If it is a good idea, are my assumptions correct?
Thanks
gm
I'm assembling a tube phono + line preamp. Both stages are fed by the same PS which I plan to build in the same chassis (but separated by a partition and enough space).
I would like to add a switch to disable the phono pre if I'm not using it, and save tube life. The way I thought of it working was to add a 3PST switch that would cut the B+ and filament supply (AC, so both wires would be connected to the switch).
The challenge I came up with is that the B+ is around 250V and the filament supply rating of the transformer is 4A. I noticed that switches are rated for both voltage and current, so I would need a 500V, 4A 3PST switch that would make a real bulky and hard to find piece of iron.
I also saw that some switches are rated for e.g 10A / 250V or 15A / 125V, so I wonder if there is a correlation between the two, and I could run a high-voltage, low-current (B+) and two low-voltage, high-current (filament) wires through the same switch?
Is this a good idea at all to mix these paths, or am I setting myself up for a safety hazard? If it is a good idea, are my assumptions correct?
Thanks
gm
Traditionally in radio receivers was tipical to cut the DC mantaining heaters on. In any case, if no plate current, no damage to the tube. Heaters on mantain cathodes "clean". Moreover when heaters was in series string.
I have that built into all of my preamps. Why light up and waste 8 tubes and 50W of power? It's being used to switch 12V for heaters, and 330VDC for B+. Yes, it exceed the voltage ratings but the current is so low it doesn't matter (50mA).
I made a board for it. 5$ plus shipping.
I made a board for it. 5$ plus shipping.
Notice that most HV switch ratings are for AC only. Most of their DC ratings are much lower voltage.
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I wanted to point out this part, because it's easier to find far more current-overrated than voltage-overrated switches. So, if my B+ is 250V/150mA (tx rating), would a 250V/10A give me the missing headroom, or would I still be running the switch right at capacity?
The selector switch for the inputs switches ground to relays, and the ground of this relay is simply connected to the phono position and turns on with the input. You can rig it anyway you like to switch 12V into the relay though.
Sure they will, but it's so slow that almost all tubes wear out from low cathode emission before a heater failure.
Hm, it looks like I'm going into the exotic realm for a simple switch now. After a couple of hours searching on Mouser and Digikey, any relay rated for over 250VDC is in the hundreds $ and mechanical switches are almost inexistent.
At this point the best solution seems to be a reasonably priced 2PST rated for 250VDC / 4A, driving the B+ on one pole and a 12V lamp + a cheap 24VAC / 4A relay for the heaters. More complex than I wished, but I guess I could keep the heaters on if that's not a concern, from what I read in the comments.
At this point the best solution seems to be a reasonably priced 2PST rated for 250VDC / 4A, driving the B+ on one pole and a 12V lamp + a cheap 24VAC / 4A relay for the heaters. More complex than I wished, but I guess I could keep the heaters on if that's not a concern, from what I read in the comments.
If it were me, I'd have one dedicated power supply for the phono, and another dedicated power supply for the rest.
Each with separate AC line power switches. Of course, the entire phono stage could be in an entirely separate chassis also.
Each with separate AC line power switches. Of course, the entire phono stage could be in an entirely separate chassis also.
Good Guitar Amplifier Parts shops will have the Carling switches used in millions of Fenders for killing 500V of DC. Way past their tested ratings but time-proven.
I know this guy is good. Top three switches on this page. Hoffman knows how to abuse an amplifier, sells to boutique builders who let him know if his Carlings give trouble. While we know of each other, he sure is not going to give me kick-back on a few 5-buck switches.
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Switches
The DPDT switch would do both heaters and B+ even though it is not a classic Carling.
I know this guy is good. Top three switches on this page. Hoffman knows how to abuse an amplifier, sells to boutique builders who let him know if his Carlings give trouble. While we know of each other, he sure is not going to give me kick-back on a few 5-buck switches.
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Switches
The DPDT switch would do both heaters and B+ even though it is not a classic Carling.
You actually want a DPST. You only need an off, not two on positions. But the "other throw" has to bang into something, a switch factor has lots of contacts, so they usually offer a DPDT rather than a DPST. (Unless you order a million.)