Yes, generally you should connect all secondaries (but no need to connect centre-taps too). If the two transformers are identical then you can connect secondaries in corresponding pairs. If they are not identical then you need to wire all secondaries in series on each, then connect to the other. A few low VA secondaries can be omitted.
An isolation transformer is one whose primary role is isolation (but sometimes may transform voltage too). A power transformer is one whose primary role is providing certain required voltages (but almost always will also provide isolation). The case of a transformer which is required to provide an output voltage equal to the input voltage falls between these two stools, so then what you call it depends on the context.
Having now understood what the OP is doing, I can now see how he described it in his first post. I assumed that he had two power transformers with their outputs wired together, rather than two wired back-to-back.
An isolation transformer is one whose primary role is isolation (but sometimes may transform voltage too). A power transformer is one whose primary role is providing certain required voltages (but almost always will also provide isolation). The case of a transformer which is required to provide an output voltage equal to the input voltage falls between these two stools, so then what you call it depends on the context.
Having now understood what the OP is doing, I can now see how he described it in his first post. I assumed that he had two power transformers with their outputs wired together, rather than two wired back-to-back.
In my case, all the secondaries provide at least two amps per.volt. Does this mean that I should connect all the secondaries together to get the best results with respect to regulation and Ohmic losses? I'm looking for a rule of thumb for the future. Is amps per volt the measure to go by, or is it just the total VA? For example, if I had two secondaries available to connect, one being 10 volts at 100VA, and the other 10 volts at 20VA, would connecting the lower VA per volt winding limit the performance of the two transformers together when they are connected as described above? Thanks again to everyone for all the help with this.
Confirming Scottj's "Yes, to 20VA".
The lowest current rating winding sets the maximum continuous current of all the connected windings.
The lowest current rating winding sets the maximum continuous current of all the connected windings.
omitting the 9Vac leaves you with 24+24+10Vac = 58Vac
The lowest current rating of those windings is 2A. That's the 116VA
If you only use one 24Vac winding, you get 48VA
Use two 24Vac windings and you get 96VA.
It's clear that omitting the 9Vac winding, gets to the highest VA even though it wastes some copper.
But that also removes the smallest increment tapping if you want to make adjustments.
If you really only want 80mAdc @ ~111Vac, then you are only using 9VA and this is tiny compared to the 76VA available when using all the copper.
But a warning.
Connect any two windings together, either in series, or in parallel and you run the risk of a potentially disastrous shorted winding.
Use a Mains Bulb Tester to power ON. Check your voltages thoroughly to ensure you have the phases of all the windings correctly connected.
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> The amp is Aren van Waarde's
Which specifies 30VA for the HV supply.
> 48VA, 48VA, 24VA, 18VA
You have in hand 138VA times two or 276VA, to do a 30VA job.
A two-ton truck to carry a sedan's load.
In a rational world, this is poor economics.
However a bird in the warehouse is worth (costs) more than two pigs in your hand.
And I'm not opposed to over-engineering. Especially if it fits. Especially if it saves money. I just re-set a 6x6 post under my house where a 4x4 would have been fine. The 6x6 was on site and the 4x4 would have to be bought and carried.
Seeing as how any one 48V winding is more than the 30VA needed, I'd just connect one 48V and get on with it. The other windings might be heater supply?
If you are really curious, try that then try both 48VA windings. I see these results:
1) voltage goes up very slightly due to marginally lower losses.
2) voltage goes down slightly, heat rises, if the windings are not exactly matched and useless circulating current happens
3) voltage goes to zero and smoke comes out, if the windings are out of phase. (Take Andrew's note about Lamp Limiter seriously!)
Since the up-side is small, and the worse-case stinks, I'd apply K.I.S.S. and just do one winding.
Note that this amplifier is NOT critical. Output is ample with anything "near" the noted 150VDC. THD at a given output will scale inverse of voltage, but it won't vanish at any reasonable supply, and if it gets "colored" at low voltage, well, isn't that why you want a hot tube instead of a clean cool chip?
> a mechanical buzz that is easily audible
Toroids can buzz also. Have you power-up these doughnuts to check that?
Toroids are often supplied 115V/230V. As an extreme measure, you could wire as 230V and apply your 120V. Core flux is much lower, many buzz sources are less. Total VA is half at best, but you are 9V over-kill and 4X over is still generous. I would however now try two "48VA" (now 24VA) windings to jumper the power.
Which specifies 30VA for the HV supply.
> 48VA, 48VA, 24VA, 18VA
You have in hand 138VA times two or 276VA, to do a 30VA job.
A two-ton truck to carry a sedan's load.
In a rational world, this is poor economics.
However a bird in the warehouse is worth (costs) more than two pigs in your hand.
And I'm not opposed to over-engineering. Especially if it fits. Especially if it saves money. I just re-set a 6x6 post under my house where a 4x4 would have been fine. The 6x6 was on site and the 4x4 would have to be bought and carried.
Seeing as how any one 48V winding is more than the 30VA needed, I'd just connect one 48V and get on with it. The other windings might be heater supply?
If you are really curious, try that then try both 48VA windings. I see these results:
1) voltage goes up very slightly due to marginally lower losses.
2) voltage goes down slightly, heat rises, if the windings are not exactly matched and useless circulating current happens
3) voltage goes to zero and smoke comes out, if the windings are out of phase. (Take Andrew's note about Lamp Limiter seriously!)
Since the up-side is small, and the worse-case stinks, I'd apply K.I.S.S. and just do one winding.
Note that this amplifier is NOT critical. Output is ample with anything "near" the noted 150VDC. THD at a given output will scale inverse of voltage, but it won't vanish at any reasonable supply, and if it gets "colored" at low voltage, well, isn't that why you want a hot tube instead of a clean cool chip?
> a mechanical buzz that is easily audible
Toroids can buzz also. Have you power-up these doughnuts to check that?
Toroids are often supplied 115V/230V. As an extreme measure, you could wire as 230V and apply your 120V. Core flux is much lower, many buzz sources are less. Total VA is half at best, but you are 9V over-kill and 4X over is still generous. I would however now try two "48VA" (now 24VA) windings to jumper the power.
Sounds like the nice little transformers they had at Active Surplus. Pity that they closed down 🙁
DarpMalone:
They are from Active Surplus. I bought them at their closing sale for very little, and finally found a use for them. I miss that place. Now it's only A1, and they are much farther from me.
AndrewT:
I have them hooked up with both 24 volt windings now and used a bulb tester on the power up with no issues. The voltage went from a 150 B+ when I was using the previous buzzing EI transformers, to 165 B+ with the toroids. The EI transformers ran hot, while the toroids barely get warm. Overall, a big improvement.
They are from Active Surplus. I bought them at their closing sale for very little, and finally found a use for them. I miss that place. Now it's only A1, and they are much farther from me.
AndrewT:
I have them hooked up with both 24 volt windings now and used a bulb tester on the power up with no issues. The voltage went from a 150 B+ when I was using the previous buzzing EI transformers, to 165 B+ with the toroids. The EI transformers ran hot, while the toroids barely get warm. Overall, a big improvement.
Same here. I'm in Pickering so I only get out to A1 Parts when I really need something. Active Surplus was close enough that I'd go just to browse and leave with a bunch of junk i don't need... I guess that's the silver lining. 🙂
That would be true if you are connecting secondaries in series, then joining the two ends to the other transformer. This is not the best way to do it if the secondaries have different current ratings. Instead, connect in pairs as I described in an earlier post. Then using the 9V winding still adds something. You get the full 138VA.AndrewT said:
I have everything connected in pairs as referred to by DF96, with like colour attached to like colour, of two identical transformers. With the exception of the 9 volt winding which is centre tapped (orange-yellow-orange), the windings have distinctly coloured wires.
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