Leak Logo 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.


ID Number
(stamped on
Leak Amplifiers
that use this

Load Impedance 
(anode-anode, Ohms)

Experimental Error Approx +/-250 Ohms

DC Resistance
(anode-andoe, Ohms)

Experimental Error Approx +/-10 Ohms

DC Resistance
(screeen tap to screen tap, Ohms) 

Experimental Error Approx +/-10 Ohms

 Screen Tap
Percentage Turns out
from Center Tap to Anode

+/- 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.
245 Rossmore Rd
Dorset BH12 2HQ
United Kingdom
Phone: +44 (0) 1202 734 463
Fax: +44 (0) 1202 733 793
Web: http://www.transformers.uk.com

Sowter Transformers
The Boatyard
Cullingham Road
Ipswich IP1 2EG
United Kingdom

Harbuch Electronics
Unit 9
40 Leighton Place
New South Wales
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
(Ohms, +/- 10 Ohms)
Open Frame
19 (ht-0-ht)
Closed Frame
19 (ht-0-ht)
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.)


Additional data to fill the gaps in the above tabulation.


The technical information above was kindly supplied by various people, mostly via the Internet.


Designers and Manufacturers of Specialised Electronic Instruments for the Communications Industries