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Output and Power Transformer Identification, Testing and Impedances |
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Output and Power Transformer Identification, Testing and Impedances |
The following information is supplied to assist in determining the condition of output and power transformers. The DC resistance measurements can be useful if it is required to assess a transformer prior to purchase of a second hand amplifier. (In the authors experience, the TL/12 "enclosed" output transformers have a reputation for going open circuit on the primary).
The primary impedance and screen tap percentage can be useful knowledge if you need to have a new component wound, or intend to build/replicate your own Leak amplifier. Note that Leak used two different screen tap percentages - 25% and around 50% (the classic recommendation is for 43% as noted by Hafler and Keroes when they first publicised this output stage, but Mullard noted that values in the range 20% to 50% are also applicable).
Later models of the TL/12 apparently included screen-taps, but these were never used in production - all TL/12's were triode connected.
All the information below has been determined by
measurements on actual transformers, and is therefore subject to variation
and experimental error. A rough indication of the expected tolerances on
the measurements are given below.
OutputTransformers:
| Leak Transformer ID Number (stamped on bottom) |
Leak Amplifiers that use this Transformer |
Primary Experimental Error Approx +/-250 Ohms |
Primary Experimental Error Approx +/-10 Ohms |
DC Resistance Experimental Error Approx +/-10 Ohms |
Screen Tap +/- 2% error |
| TL/12/T2/2 Open Frame | TL/12 prior to about '51 / '52 |
5800 | 280 | - | no taps |
| TL/12/T2/2 Enclosed Casing | TL/12 1951 / 52 onwards |
5500 | 280 | not available | 47% |
| 1360 | TL/10 | 9100 | 200 | 105 | 49% |
| TL/25/T2/2 | TL/25A | not available | not available | not available | no taps |
| not available | TL/25 | not available | not available | not available | not available |
| 3925 | TL/25 Plus Stereo 50 |
7000 | 102 | 54 | 49% |
| 8013 | TL/50 Plus | 5200 | 45 | 22 | 49% |
| 3921 | TL/12 Plus Stereo 20 |
8600 | 165 | 91 | 49% |
| 8615 | Stereo 50 late models Stereo 60 |
5600 | 102 | 22.5 | 25% |
| 8778 | TL/12 Plus late models Stereo 20 late models (seen on Sep 1961 model) |
not available possibly 8600 |
not available possibly 140 Ohms |
not available possibly 40 Ohms |
not available possibly 25% |
Replacement Suppliers for Leak Transfomers:
Majestic Transformer Co. Sowter Transformers |
Harbuch Electronics Unit 9 40 Leighton Place Hornsby New South Wales AUSTRALIA 2077 Phone: +61 2 9476 5854 Fax: +61 2 9476 3231 (will wind to spec) |
Power Transformers and Chokes:
| Amplifier Model | Transformer ID Number |
Choke ID Number |
Resistance (Ohms, +/- 10 Ohms) |
| TL/12 Open Frame |
TL/12/T3 | TL/12/T1 |
19 (ht-0-ht) 160 |
| TL/12 Closed Frame |
TL/12/T3 | TL/12/T1 |
19 (ht-0-ht) 160 |
| TL/10 | 1414 | - | 24 (ht-0-ht) |
| TL/12 Plus | not available | - | 20 (ht-0-ht) |
| TL/25 Plus | 3942 (grey chassis, 240Vac) | - | not available |
| TL/50 Plus | 8011 (champagne chassis, 240Vac) | not available | not available |
| Stereo 20 | 8162 (240 Vac) | - | not available |
| Stereo 50 | 8190 (240Vac) | - | 8.5 (ht-0-ht) |
| Stereo 60 | 8932 (240Vac) 8589 (110Vac) |
not available | not available |
Note: "ht-0-ht" means the resistance of the main high tension winding measured across the pins feeding the main rectifier valve/tube.
Notes from Leak Amplifier Owners:
1) Stereo 20 power transformers appear to have been revised during the production run. Early models had 300-0-300 volt HT secondaries, but this was revised to 280-0-280 V, possibly from a propensity for this transformer to overheat. The Leak circuits do not document this change. Comments from owners are welcome.
2) Different HT secondary voltages have been noted on TL/12 Plus amplifiers. One unit with close to 350Vdc HT on the main HT, and the other with around 320Vdc on the main HT. There was also a different output voltage from the power transformer secondary on each unit (similar to the situation described in (1) above.)
Wanted:
Additional data to fill the gaps in the above tabulation.
Acknowledgement:
The technical information above was kindly supplied by various people, mostly via the Internet.