l am in the process of prototype 300B amp based on JE labs circuit.
l want to use a meanwell adapter 5V 7amp to power both channels 300B output tube.
The heater of 6sn7 will also use a 6v ac dc adapter with ground elevation.
Is this feasible , will circuit work
and safe?
l want to use a meanwell adapter 5V 7amp to power both channels 300B output tube.
The heater of 6sn7 will also use a 6v ac dc adapter with ground elevation.
Is this feasible , will circuit work
If I understand, you want to use one 5V 7A switching supply to power two 300B filaments in parallel (1.25A + 1.25A = 2.5A)
1. The Left and Right channels need to have their own individual 880 Ohm self bias resistor and 100uF capacitor.
Using a single 440 Ohm resistor and single 200uF and connect the filaments in parallel might be a problem.
That is especially true if the 300B tubes are not perfectly balanced, since they will draw different plate currents, perhaps 55mA for one and 65mA for the other, perhaps even more un-balanced than that.
Very well balanced 300B tubes should draw the same plate current, then you can use a common 440 resistor and 200uF cap.
But next, what about the following:
2. 300B filaments are about 1 Ohm when they are cold. That is 5A per filament at start up; or 10A at start up in parallel.
Can the switcher take a 10A transient of cold filaments?
Once the 300B filaments are warm, 2.5A total, can the switching power supply run correctly with only a 2.5A load, instead of a 7A load?
3. The safety of the switcher power supply not only depends on its ability to deliver 10A start up current, but also how well it is designed and built to stringent safety standards.
It does show Line, Neutral, and Ground . . . that seems to be for a 3 wire IEC power cord (and you need a 3 wire power mains outlet to make that work properly).
4. And why does the other picture show a 6V 4A label?
Is that for the two 6SN7 tubes?
0.6A + 0.6A = 1.2A
But the cold filaments of the 6SN7 take lots more than 1.2A at start up.
1. The Left and Right channels need to have their own individual 880 Ohm self bias resistor and 100uF capacitor.
Using a single 440 Ohm resistor and single 200uF and connect the filaments in parallel might be a problem.
That is especially true if the 300B tubes are not perfectly balanced, since they will draw different plate currents, perhaps 55mA for one and 65mA for the other, perhaps even more un-balanced than that.
Very well balanced 300B tubes should draw the same plate current, then you can use a common 440 resistor and 200uF cap.
But next, what about the following:
2. 300B filaments are about 1 Ohm when they are cold. That is 5A per filament at start up; or 10A at start up in parallel.
Can the switcher take a 10A transient of cold filaments?
Once the 300B filaments are warm, 2.5A total, can the switching power supply run correctly with only a 2.5A load, instead of a 7A load?
3. The safety of the switcher power supply not only depends on its ability to deliver 10A start up current, but also how well it is designed and built to stringent safety standards.
It does show Line, Neutral, and Ground . . . that seems to be for a 3 wire IEC power cord (and you need a 3 wire power mains outlet to make that work properly).
4. And why does the other picture show a 6V 4A label?
Is that for the two 6SN7 tubes?
0.6A + 0.6A = 1.2A
But the cold filaments of the 6SN7 take lots more than 1.2A at start up.
Regarding points 1 to 3 raised. Can l solve the problems by using two 5v 7A dc smps?
The 6v smps is for 6sn7 heater
The 6v smps is for 6sn7 heater
In terms of cold heater current you need a supply with 2x or more current rating of the warm heater current. Even so the supply may soft start a couple of times before staying on. Avoid more than 5x rating as the supply can go into cycle skip and generate audio tones. Yes you do need separate isolated outputs or separate supplies for the 300B's.
They are some modules that seem to offer isolated outputs:
https://www.aliexpress.com/item/32921995271.html
They are some modules that seem to offer isolated outputs:
https://www.aliexpress.com/item/32921995271.html
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Meanwell make very reliable and quiet SMPS. I use them all the time for heater supplies and as a rule rate them for twice the nominal heater current. The advantage of these supplies is that their current limit uses a hiccup mode. This repeatedly tries to fire up into the load but never exceeds the thermal limits of the switching transistors. It works well for tube because the pulses of current gradually warm up the heaters enough for the load current to drop and this process repeats until the load drops below the max rating. I have experimentally found that doubling the rating always will fire up heaters. You can look at the output voltage with a simple meter and watch each hiccup as the output voltage steps up. It usually take no more than three hiccups to reach normal operation.
Cheers
Ian
Cheers
Ian
My 300B tubes are 0.7 Ohm when Cold.
When they heat up, they are 4 Ohms.
0.7 : 4
To mis-quote George Orwell's novel "Animal Farm":
"All switcher power supplies are equal; but some are more equal than others".
Your Mileages May Vary.
When they heat up, they are 4 Ohms.
0.7 : 4
To mis-quote George Orwell's novel "Animal Farm":
"All switcher power supplies are equal; but some are more equal than others".
Your Mileages May Vary.
What about using twice the voltage needed, and use a 4R power resistor in series with the filament? That way you solve the cold start issue, at the expense of a hot resistor. Should work, no?
Are they also quiet enough for a filament on a directly heated tube?
If not, then perhaps a higher voltage unit to power the filament through a buffer of some sort, like the Coleman reg for one example ?
Yes but terribly wasteful. If you want to get complicated then wire a resistor in series with the filament and then have a relay short it out after a few seconds.