Tube Reverb PT from Tonnes of Tone

[luxpaul]

Hello.

I am thinking about building the Tube Reverb from the Tonnes of Tone book.
I am based in Europe so I would need a 230v Primary Power Transformer.
The one suggested in the book is the Hammond 269AX.

Does anyone know of an equivalent (or suitable) 230v PT ?

Many thanks
Paul

[Sherlok Ohms]

Hello Paul

Hammond has their 300-series power transformers with universal primaries, essentially the same as the North American version but able to be used everywhere. The numbers are generally similar just with a 3 instead of a 2 at the beginning.

269AX you would expect to be replaced with 369AX, however, the plate voltage on the 369AX is 250-0-250V instead of 300-0-300V. The 369BX is a direct replacement.

Hammond sells world-wide so you might be able to get them from Farnell. The North American distributors like Digikey and Mouser also ship to Europe.

Cheerio

[luxpaul]

Hi Sherlock.
I just checked and the 369AX is 125-0-125 but then so is the 269AX.
However the 369 is almost double the weight and larger and it also includes a bias tap which I don’t need.
I would imagine that also make it more expensive than the 269AX.

So would like to know if there are any other alternatives.

Thanks again

[K O'Connor]

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.

[luxpaul]

Many thanks for the thorough reply and suggestions Kevin.
I will take some time to think about it but at least I now have some options.

Regards
Paul