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03-28-2025, 02:49 PM
Hi Guys
From our writings, the ideal power transformer for a tube power amp has these windings:
PRI-1: BLK (120) - WHT (0)
PRI-2: BRN (120) - ORG (0)
Electrostatic shield - GRN
SEC-1: RED (hi-V) - RED (0), appropriate current, plate
SEC-2: VIO (hi-V x 0.2 to 0.15) - VIO (0), 100mA bias
SEC-3: YLW (6) - WHT (0) - YLW (6), appropriate current
Note that the bias winding is not specified as a set voltage as it is on so many PTs for tube amps; instead it is proportioned to the high-voltage plate winding.
For example, say we need 500Vdc, we would want the raw bias supply to be between -100Vdc (hi-V x 0.2) and -75Vdc (hi-V x 0.15) to accommodate every tube sample from low-gain to high-gain. This would also accommodate using a bias regulator if the amp is to be Power Scaled. In absolute voltage terms, when the bias supply is 20% of the plate supply there should be no tube that is uncontrollable, where at 15% very few tubes remain uncontrollable.
Note that in the 6/12 heater system, the power tube heaters are distributed over each side of the CT and it does not matter if these loads are equal. Maximum current is based on the sum of individual heater loads.
For example, say there are 2x EL-34 and 3x 12AX7.
One EL-34 socket is wired to each side of the CT.
The three 9-pin socket are wired across the full 12V (pin-9 not used).
The load per side is equal to one EL34=1A6, plus 3 x 150mA = 450mA
The entire winding is then rated as 6-0-6V or 12Vcr at 2A05.
Some octals need 1A8 each for their heater, so we might spec the winding at 3A or even 4A. The hidden "magic" of this approach is that the heater winding is only rated for half the power tube complement plus the small-tube complement AND with fan-out wiring only needs small-gauge wire to each octal socket.
If there is a need to power relays or switching circuits, or other things, we can add an auxiliary winding having BLU wires. This may be as follows:
SEC-4: BLU (12) - BLU (0), 100mA, or whatever current is needed for the auxiliaries. 12Vac accommodates the use of a 7812 3-pin voltage regulator to give 12Vdc for relay coils, etc. The AUX winding and related DC output has no "polarity" until you ground one end or the other.
If you need split-rails for the auxiliaries, then a 24Vct winding is needed, as:
BLU(12) - WH (0) - BLU(12) at the needed current.
Of course, the aux windings can be other voltages as appropriate to your needs.
If the PT is for other power tube types, such as 300B, 2A5, etc, then appropriate heater windings are needed, where each winding is given its own colour
From our writings, the ideal power transformer for a tube power amp has these windings:
PRI-1: BLK (120) - WHT (0)
PRI-2: BRN (120) - ORG (0)
Electrostatic shield - GRN
SEC-1: RED (hi-V) - RED (0), appropriate current, plate
SEC-2: VIO (hi-V x 0.2 to 0.15) - VIO (0), 100mA bias
SEC-3: YLW (6) - WHT (0) - YLW (6), appropriate current
Note that the bias winding is not specified as a set voltage as it is on so many PTs for tube amps; instead it is proportioned to the high-voltage plate winding.
For example, say we need 500Vdc, we would want the raw bias supply to be between -100Vdc (hi-V x 0.2) and -75Vdc (hi-V x 0.15) to accommodate every tube sample from low-gain to high-gain. This would also accommodate using a bias regulator if the amp is to be Power Scaled. In absolute voltage terms, when the bias supply is 20% of the plate supply there should be no tube that is uncontrollable, where at 15% very few tubes remain uncontrollable.
Note that in the 6/12 heater system, the power tube heaters are distributed over each side of the CT and it does not matter if these loads are equal. Maximum current is based on the sum of individual heater loads.
For example, say there are 2x EL-34 and 3x 12AX7.
One EL-34 socket is wired to each side of the CT.
The three 9-pin socket are wired across the full 12V (pin-9 not used).
The load per side is equal to one EL34=1A6, plus 3 x 150mA = 450mA
The entire winding is then rated as 6-0-6V or 12Vcr at 2A05.
Some octals need 1A8 each for their heater, so we might spec the winding at 3A or even 4A. The hidden "magic" of this approach is that the heater winding is only rated for half the power tube complement plus the small-tube complement AND with fan-out wiring only needs small-gauge wire to each octal socket.
If there is a need to power relays or switching circuits, or other things, we can add an auxiliary winding having BLU wires. This may be as follows:
SEC-4: BLU (12) - BLU (0), 100mA, or whatever current is needed for the auxiliaries. 12Vac accommodates the use of a 7812 3-pin voltage regulator to give 12Vdc for relay coils, etc. The AUX winding and related DC output has no "polarity" until you ground one end or the other.
If you need split-rails for the auxiliaries, then a 24Vct winding is needed, as:
BLU(12) - WH (0) - BLU(12) at the needed current.
Of course, the aux windings can be other voltages as appropriate to your needs.
If the PT is for other power tube types, such as 300B, 2A5, etc, then appropriate heater windings are needed, where each winding is given its own colour
