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Old 10th June 2013, 08:53 AM   #11
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what about boosting the heater voltage by using double-voltage circuit and tune the correct voltage with resistors?

or just use the 6V and tune the correct voltage with resistors?
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Last edited by ranhaber; 10th June 2013 at 08:55 AM.
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Old 10th June 2013, 09:42 AM   #12
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Hi!

voltage doublers work best in low to medium current power supplies. For a heater supply you would need rather large caps. Doable, but keep in mind that the current load on the transformer secondary doubles when it works into a voltage doubler circuit.

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Old 10th June 2013, 10:29 AM   #13
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by Vinylsavor View Post
Hi!

voltage doublers work best in low to medium current power supplies. For a heater supply you would need rather large caps. Doable, but keep in mind that the current load on the transformer secondary doubles when it works into a voltage doubler circuit.

Best regards

Thomas
I second that.
I would not advise that approach, even if it is doable to some degree.
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Old 10th June 2013, 10:32 AM   #14
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What about using a pair of ordinary high-current silicon diodes, that will be added to the heater chain to drop the heater voltage by about 0.7V. Like the picture below.

i mesured 6.1V unloaded. so will it be good for 5V heater rectifier?
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Old 10th June 2013, 10:33 AM   #15
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Hi!

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Originally Posted by FoMoCo View Post
I would not advise that approach, even if it is doable to some degree.
Agreed! The heater winding might heat up a lot due to the current spikes.
Better get a proper power transformer for the task

Thomas
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Old 10th June 2013, 10:34 AM   #16
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Hi!

Quote:
Originally Posted by ranhaber View Post
What about using a pair of ordinary high-current silicon diodes, that will be added to the heater chain to drop the heater voltage by about 0.7V. Like the picture below.
Those diodes will dissipate the same amount of heat as properly sized resistors for the task. No big advantage. With resistors you can better fine tune the voltage to the required level.

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Old 10th June 2013, 10:45 AM   #17
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by ranhaber View Post
What about using a pair of ordinary high-current silicon diodes, that will be added to the heater chain to drop the heater voltage by about 0.7V. Like the picture below.

i mesured 6.1V unloaded. so will it be good for 5V heater rectifier?
In my opinion:
1) Diodes are better when the load varies a lot. This means a (semi)constant voltage drop. But, as already mentioned, you can't easily tune the voltage.
2) Resistors are better when the load is constant. You can more easily tune the voltage drop and the constant drop of a diode isn't needed. In addition, the resistor helps with inrush current.

In both cases roughly the same power is dissipated. (It's actually slightly less in the diode, but I'll skip the math here. The difference isn't relevant.)

For a tube filament, number two applies. I'd use the resistor if given the choice between resistor and diode.
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Old 10th June 2013, 01:36 PM   #18
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Given the local supply voltage and the 230V primary I would expect the secondary voltages to no more than 5% low under load. I'm wondering about the accuracy of the voltmeter used to make these measurements.

I am not a fan of boost/buck winding connection when secondaries are used to do it for safety reasons. A small 9V - 12V filament transformer configured as an auto-transformer on the primary side of the transformer is a lot safer and IMLE works very well. (Also leaves those filament windings available for possible use.)
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Old 10th June 2013, 01:54 PM   #19
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Default after a good night's sleep......

Ranhaber: A comment and an idea......

In your configuration in post #10 since you are using two windings in series with the primary you need to have all of the windings in the proper phase. If one (or both) of the LV windings are bucking instead of boosting, your voltages will be lower, or only slightly raised.

Try it with one of the 6.3V windings to start, and convince yourself that you can boost/buck, then add the second winding.

Here is another idea that may be a lot more practical and eliminates the isolation and saturation issues, and may even offer some improvement for a Tubelab SE:

Don't boost anything; use the transformer as-is. This gets you 350-0-350 or so which is more than enough for 300B operation.

Use a 6.3V winding with a dropping resistor for the rectifier. (Experts: Is this practical?)......or diodes using the forward voltage drop?

Use two 5V windings in series for around 9V into the filament regulator. This provides additional voltage headroom into the filament regulator which may be a good thing. The Tubelab SE regulator has minimal regulation with 6.3V input. This higher voltage will burn more heat; you'll need ample heat-sinking.......We'll have to look at the specs for the regulator to verify.

For those following along, here is the schematic:

Schematic

Last edited by boywonder; 10th June 2013 at 02:02 PM.
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Old 10th June 2013, 01:56 PM   #20
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Quote:
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I'm wondering about the accuracy of the voltmeter used to make these measurements.
I'm wondering that too.........perhaps a CAREFUL measurement of the house voltage at the wall...
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