Hi everyone,
I want to use a transformer for a project that has two 6.3VAC secondaries. I want to elevate the heaters above ground as I have previously on other projects.
I have two questions:
Can I wire the two 6.3VAC heater taps in parallel and then elevate by using the center reference as usual or is this not a good idea with two secondary windings? I know that I would at least have to wire the two start points together and the two finish points together of the two secondary coils to keep the phase the same. Any other considerations?
second:
Can I elevate just one of the secondaries as for the preamp tubes? And leave the heaters for the power tubes referenced to ground? I don't see why I cant but while I am posting I thought I should ask.
I want to use a transformer for a project that has two 6.3VAC secondaries. I want to elevate the heaters above ground as I have previously on other projects.
I have two questions:
Can I wire the two 6.3VAC heater taps in parallel and then elevate by using the center reference as usual or is this not a good idea with two secondary windings? I know that I would at least have to wire the two start points together and the two finish points together of the two secondary coils to keep the phase the same. Any other considerations?
second:
Can I elevate just one of the secondaries as for the preamp tubes? And leave the heaters for the power tubes referenced to ground? I don't see why I cant but while I am posting I thought I should ask.
Tying the two heater windings together shouldn't be a problem as long as you pay attention to the phasing of the windings (as you noted in your original post). This is commonly done on some split-secondary transformers (for example, the Triad VPS12-3400 which is a split-secondary 6.3/12.6 transformer) to increase the current capability of the windings - two windings in parallel is OK, just mind the phasing (which you will probably have to verify with a dual-channel scope).
Elevating the paralleled heater windings will not hurt anything - you're just raising the heater winding potential relative to the amplifier ground, both windings will be at the same potential but no current will be flowing from the bias network.
Similarly, biasing one heater winding while leaving the second winding referenced to ground should not cause problems. No current is drawn from the bias network, you are just elevating one transformer secondary to a higher DC potential with respect to the rest of the amplifier circuitry.
There should be no problem with this as the transformer itself doesn't "see" the higher elevation of the one heater winding and the higher DC potential of the one winding doesn't affect the transformer operation as regards the other windings.
Personally, for a preamp I'd elevate the heater winding to the preamp tubes to reduce the possibility of heater-induced hum. Such hum is not typically a problem on the output (power) tubes, but elevating their heaters as well doesn't cause trouble (as long as you mind the maximum Vhk spec of the tube...)
Good luck with the project!
Sam
Elevating the paralleled heater windings will not hurt anything - you're just raising the heater winding potential relative to the amplifier ground, both windings will be at the same potential but no current will be flowing from the bias network.
Similarly, biasing one heater winding while leaving the second winding referenced to ground should not cause problems. No current is drawn from the bias network, you are just elevating one transformer secondary to a higher DC potential with respect to the rest of the amplifier circuitry.
There should be no problem with this as the transformer itself doesn't "see" the higher elevation of the one heater winding and the higher DC potential of the one winding doesn't affect the transformer operation as regards the other windings.
Personally, for a preamp I'd elevate the heater winding to the preamp tubes to reduce the possibility of heater-induced hum. Such hum is not typically a problem on the output (power) tubes, but elevating their heaters as well doesn't cause trouble (as long as you mind the maximum Vhk spec of the tube...)
Good luck with the project!
Sam
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Duh... How access to modern equipment has made me lazy....
You can determine phasing of the transformers with an AC voltmeter. Just connect the heater windings in series and measure the AC voltage across the two outside wires. If it's zero or nearly zero, you're measuring across the in-phase legs - just connect these two wires together and you're done.
If you measure 2x the heater winding voltages, you're measuring across the opposite phases. Swap the wires to parallel the transformers.
Simple!
You can determine phasing of the transformers with an AC voltmeter. Just connect the heater windings in series and measure the AC voltage across the two outside wires. If it's zero or nearly zero, you're measuring across the in-phase legs - just connect these two wires together and you're done.
If you measure 2x the heater winding voltages, you're measuring across the opposite phases. Swap the wires to parallel the transformers.
Simple!
You're welcome! In further thought, if you elevate one heater winding and leave the other winding referenced to DC ground, the only concern I would have is the insulation breakdown voltage rating between the two heater windings.
It isn't usually specified on transformers, but even a cheapie transformer should be able to take 50-60V bias between the heater windings without breaking down. The enamel insulation used on the wires, plus any inter-winding tape, should get you into the 50-60V range without any worries and without making a lot of assumptions about how that particular transformer was constructed.
If you're biasing the heater for the top tube in an SRPP (or similar) stack, you might use a >100V bias on the top-heater in which case the safest approach IMO would be to use a separate heater transformer for that tube, or bias the windings such that the difference in bias between windings is in the 50-60V range.
I know that transformers we use routinely have a HV winding and one (or more) heater windings on the same bobbin. Certainly, the insulation between the HV windings and the LV windings can take the voltage difference (and more, for margin).
This would lead one to assume that the LV windings have the same voltage rating with respect to each other - but that might be a major assumption with dramatic consequences if not true. Depends on how much influence the bean-counters had in the manufacturing process, and if there was any $$ to be saved using cheaper wire for the LV secondary windings...
I'm starting to understand why fellows like Tony take all that trouble to wind their own transformers - if you know what is in the tranny, you don't have to guess! Unfortunately, we don't all have the time or skill to make our own so we have to take a best (and safest) guess.
Good question you posted - it's made me think a bit, which is always a good thing! Rust never sleeps
Hope this helps, and best luck with your project!
Sam
It isn't usually specified on transformers, but even a cheapie transformer should be able to take 50-60V bias between the heater windings without breaking down. The enamel insulation used on the wires, plus any inter-winding tape, should get you into the 50-60V range without any worries and without making a lot of assumptions about how that particular transformer was constructed.
If you're biasing the heater for the top tube in an SRPP (or similar) stack, you might use a >100V bias on the top-heater in which case the safest approach IMO would be to use a separate heater transformer for that tube, or bias the windings such that the difference in bias between windings is in the 50-60V range.
I know that transformers we use routinely have a HV winding and one (or more) heater windings on the same bobbin. Certainly, the insulation between the HV windings and the LV windings can take the voltage difference (and more, for margin).
This would lead one to assume that the LV windings have the same voltage rating with respect to each other - but that might be a major assumption with dramatic consequences if not true. Depends on how much influence the bean-counters had in the manufacturing process, and if there was any $$ to be saved using cheaper wire for the LV secondary windings...
I'm starting to understand why fellows like Tony take all that trouble to wind their own transformers - if you know what is in the tranny, you don't have to guess! Unfortunately, we don't all have the time or skill to make our own so we have to take a best (and safest) guess.
Good question you posted - it's made me think a bit, which is always a good thing! Rust never sleeps
Hope this helps, and best luck with your project!
Sam
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