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01-06-2025, 09:14 PM
Hi Guys
Hammond 369AX is the correct universal replacement for 269AX. Both have the same secondary voltages at the same currents.
The 50V bias tap is a misnomer here, as it requires that the plate winding CT be grounded. In that situation, the 50V tap would be half-wave rectified to produce about -80V while a half-bridge tied to the winding ends would produce a plate supply of just 175V or so.
Why is the 50V tap on all the 300-series transformers? At the time they were designing these, I was discussing possible improvements and modernisations for the classic designs with one of the engineers, Doug Hutt (RIP). He agreed that there should be bias windings, but instead, all these designs were given taps as Hammond decided to retain the CTed plate winding instead of opting for a non-tapped winding requiring the use of a full bridge rectifier. This was to satisfy the very large body of tube amp enthusiasts who often favored tube rectifiers. (Some of those modernisations were incorporated into the 290-series). The use of a standard-voltage bias tap was also something I tried to dissuade them from, but they did it anyway.
In any case, both the 269AX and 369AX have a CT on the plate winding and the Tube Reverb project does not use it - it is taped and stowed, or soldered to an unused terminal on the terminal strip.
The 300-series got more than the usual dual primaries to accommodate 120-240V mains. There are taps for other voltages and a 40s-style method for matching to 100V for Japan.
You do not think of 50Hz and 60Hz being significantly different in frequency, but that 10Hz - really, the loss of 10Hz - requires a bigger core with higher magnetising currents at 50Hz than at 60Hz. All those primary windings and taps, and the "proper" consideration for 50Hz make the core and the entire PT larger and heavier than the 120V 60Hz 269AX.
The alternative is to use two PTs, one that can provide the heater current and one for the plate supply. For the latter, a 230V to 230V device would work, or dual primaries to 230V out.
London Power's PSU-PRE provides this capability with the option for 6V or 12Vregulated DC for heaters. When using 12A_7 tubes, 12V heater wiring is always preferred. Since the project only has two tubes, you could use PSU-PRE-12, but if you wanted to add a preamp or so, PSU-PRE-24 will support up to four tubes.
You could use the same transformers as in PSU-PRE, and mount them on perf-board. You could also get channel-mount versions or go for full toroids. Depends on how much space and $$ you have.
Hammond 369AX is the correct universal replacement for 269AX. Both have the same secondary voltages at the same currents.
The 50V bias tap is a misnomer here, as it requires that the plate winding CT be grounded. In that situation, the 50V tap would be half-wave rectified to produce about -80V while a half-bridge tied to the winding ends would produce a plate supply of just 175V or so.
Why is the 50V tap on all the 300-series transformers? At the time they were designing these, I was discussing possible improvements and modernisations for the classic designs with one of the engineers, Doug Hutt (RIP). He agreed that there should be bias windings, but instead, all these designs were given taps as Hammond decided to retain the CTed plate winding instead of opting for a non-tapped winding requiring the use of a full bridge rectifier. This was to satisfy the very large body of tube amp enthusiasts who often favored tube rectifiers. (Some of those modernisations were incorporated into the 290-series). The use of a standard-voltage bias tap was also something I tried to dissuade them from, but they did it anyway.
In any case, both the 269AX and 369AX have a CT on the plate winding and the Tube Reverb project does not use it - it is taped and stowed, or soldered to an unused terminal on the terminal strip.
The 300-series got more than the usual dual primaries to accommodate 120-240V mains. There are taps for other voltages and a 40s-style method for matching to 100V for Japan.
You do not think of 50Hz and 60Hz being significantly different in frequency, but that 10Hz - really, the loss of 10Hz - requires a bigger core with higher magnetising currents at 50Hz than at 60Hz. All those primary windings and taps, and the "proper" consideration for 50Hz make the core and the entire PT larger and heavier than the 120V 60Hz 269AX.
The alternative is to use two PTs, one that can provide the heater current and one for the plate supply. For the latter, a 230V to 230V device would work, or dual primaries to 230V out.
London Power's PSU-PRE provides this capability with the option for 6V or 12Vregulated DC for heaters. When using 12A_7 tubes, 12V heater wiring is always preferred. Since the project only has two tubes, you could use PSU-PRE-12, but if you wanted to add a preamp or so, PSU-PRE-24 will support up to four tubes.
You could use the same transformers as in PSU-PRE, and mount them on perf-board. You could also get channel-mount versions or go for full toroids. Depends on how much space and $$ you have.
