Leak Logo Restoring the Leak
TL/12 Amplifer
 

If you aquire a Leak TL/12 Amplifier it you may need to replace some or all of the resistors and capacitors on the main circuit terminal board, or other components. Shown at the right are a pair of Leak TL/25A amplifiers prior to restoration. These units have been in commercial theatre service for over 30 years and were in need of restoration.

This page covers the following topics:

Resistors
Capacitors
HT Filter Capacitors
Iron-Cored Components
Connectors
Valves / Tubes

Resistors

The original amplifier used carbon resistors, some of which were of the "body-end-dot" variety (that's the way you read the value), and some of which had the more modern coloured bands.

Below is a list of all the resistors and capacitors in the amplifier to assist with purchasing and replacing components.

The choice of resistor type is to an extent personal. It may be desired to keep the amplifier close to original, in which case the replacement resistors should all be "carbon film" variety. However, "metal film" resistors are another possibility although are reputed to give poorer sound quality, despite technically superior performance. Don't forget to keep your TL/12 looking first rate by soldering all resistors in place with the tolerence bands all facing the same direction.

 Quantity  Original Value / Type  Recommended Replacement

3

2

1

1

3

1

1

1

1

3

2

3


1

 

100 Ohm, 1 Watt, Carbon

600 Ohm, 5 Watt, Wire Wound

1 kOhm, 1 Watt

2.2 kOhm

10 kOhm

22 kOhm

57 kOhm

68 kOhm

100 kOhm

470 kOhm

1 MOhm

1.5 MOhm


Feedback Resistor with TL/12/T2/2
(standard output transformer,
for 2, 8, 18 and 32 Ohm loads):
2 Ohm strapping: 3 kOhm
8 Ohm strapping: 6 kOhm
18 Ohm strapping: 10 kOhm
32 Ohm strapping: 12 kOhm

Feedback Resistor with TL/12/T2/1
(optional output transformer
for 1, 4, 9 and 16 Ohm loads):
1 Ohm strapping: 2.2 kOhm
4 Ohm strapping: 4.7 kOhm
9 Ohm strapping: 6.6 kOhm
16 Ohm strapping: 10 kOhm

1 Watt

Wire Wound, 5 Watt minimum

1 Watt

1 Watt

1 Watt

1 Watt

56 kOhm, 1 Watt

1 Watt

1 Watt

1 Watt

1 Watt

1 Watt


1 Watt


3 kOhm
6.2 kOhm
10 kOhm
12 kOhm




2.2 kOhm
4.7 kOhm
6.8 kOhm
10 kOhm

Sources for Resistors:
The Parts Connection
Farnell
RadioSpares
See Also Suppliers Page.

Capacitors

There is every chance that the original TCC Metal pack capcitors on the main board have become leaky, and will require replacement, or that some capacitors have already been replaced with an inferior type in the past. The TCC Metal pack capcitors are a very good paper/metal film capacitor, and should be replaced with the best capacitors available. Unfortunatly it is not possible to purchase new types the same as the old ones (although the author did find some new-old-stock ones, however even these were leaky and un-usable).

A good choice is a capacitor withe a polypropylene dielectric, and metal-foil, or metal-film electrodes. Radio Spares and Farnell both carry suitable capacitors with 1000Vdc rating that have the original "tubular" construction, with "pigtail" leads like the originals. The Farnell units are made by LCR in the UK.

 Quantity  Original Value / Type  Replacement

5

2

3


1

 1 nanoF / 100V

250 nanoF / 600V / paper & metal film

25 microF / 50V / electrolytic

30 microF / 16V / electrolytic


Feedback Capacitor with TL/12/T2/2
(standard output transformer,
for 2, 8, 18 and 32 Ohm loads):
2 Ohm strapping: 200 pF
8 Ohm strapping: 100 pF
18 Ohm strapping: 100 pF
32 Ohm strapping: 100 pF

Feedback Capacitor with TL/12/T2/1 (optional output transformer
for 1, 4, 9 and 16 Ohm loads):
1 Ohm strapping: 500 pF
4 Ohm strapping: 500 pF
9 Ohm strapping: 300 pF
16 Ohm strapping: 300 pF

silvered mica or polystyrene

polypropylene / 630 V min.

electrolytic / 100V min.

electrolytic / 25V min.


Silvered Mica or Polystyrene


2 x 100 pF
100 pF
100 pF
100 pF




270 pF + 330 pF
270 pF + 330 pF
2 x 150 pF
2 x 150 pF

Sources for Capacitors:
The Parts Connection
Farnell
RadioSpares
See Also Suppliers Page.

 

HT Filter Capacitors

A key feature of the LEAK TL/12 is the use of paper-in-oil capacitors for filtering and de-coupling the HT rail. These capacitors have a very long life indeed, and should never need replacing. In fact it is undesirable to replace them at all, unless it cannot be avoided (eg: because they have been removed, or have failed from leaking oil).

Further information on how to replace these capacitors is being prepared and will be added when available. It relies on removing the existing capacitors from the square can and putting new capacitors into the existing can, so that the original appearance is maintained. A polypropylene motor-run capcitor (4microF/44OVac from Farnell) is used for the main rectifier filter, and 4 x 10 microF/630V polypropylene"Solen Fast Caps" for the remaining capacitors (from "The Parts Connection"). A side effect of this replacement is that the HT ripple in the amplifier is vastly reduced as the capacitance has been increased.

Shown at the right are a pair of capacitor cans from some TL/12's. The original oil capacitors had failed, so were removed from the can. The replacement capacitors (as described above) were placed in the box, with medium density fibre board lining the box. This assisted to locate the capacitors physically, and retained the "solid sound" the original box had when tapped with the hand.

Iron-Cored Components

Fortunately the iron-cored components are usually fairly reliable, however failures can occur. If the amplifier is operated long term with leaky coupling capacitors this can induce a power transformer failure from the increased quiescent current in the output stage.

If the power transformer has failed, or the filter choke failed (approx 6 to 7 Henries in value, with a current of 130 mA), these components should be able to be re-wound by a small transformer manufacturer in your area. You will need to inform them of special requirements regarding physical exits of winding taps to suit the terminal layout on the baseplate, and makes sure thay can still fit the original transformer shrouds on the re-wound unit.

If the output transformer has failed, this is a more serious problem, as selecting a suitable manufacturer for a re-wind will be a difficlut task. In the UK, Sowter Transformers of Ipswich may be able to assist, and in Australia, Harbuch Electronics of Sydney may be suitable (see Suppliers Page). The alternative approach is to try and locate an original spare from some of the audio dealers in the UK (also see Suppliers Page).

Connectors

The input connector to the TL/12 is an octal socket. To avoid drilling holes in the chassis it is possible to make up an adaptor using an octal plug and RCA socket, join via a short lead. Also it may be possible to mount an RCA socket in the shroud of the octal plug, so the adaptor is a single-piece unit. Octal plugs are available from Farnell Electronics.

The loudspeaker connector on the TL/12 is a 2-pin Bulgin chassis-mount plug. The mating cord-mount 2-pin socket is no longer made, and is un-available (apart from at electronics second hand dealers/junk shops). This is a problem for the enthusiasts requiring to retain the original appearance.

Valves / Tubes

The valves required in the TL/12 are: EF36 or EF37, ECC33 or 6SN7, KT66, and 5U4. The only valve that will be diffcult to obtain is the KT66. Original GEC ones have are no longer available, and fetch very high prices, as it is one of the best audio power valves ever made. However some suppliers do make equivalents.

For close to original appearance, the "Golden Dragon" KT66-R (retro-range) looks identical to the clear-glass GEC valve. It has ratings consistent with its use in the TL/12 (provided your TL/12 has the stated 445V HT on the output transformer. The KT66-R is rated for 425V screen voltage, and the KT66's in the TL/12 have about 405V between screen and cathode. The KT66-R is available from P.M. Components in the UK.

Sovtek make a replacement for the KT66. The Sovtek valve is closely based on the 5881, and does not look at all like a KT66. However it is a cheaper alternative, and could be useful if you wish to conserve your supply of KT66's until later! See Antique Electronic Components for this Sovtek valve.

Other valve types should be available from P.M. Components in the UK, or Antique Electronic Supply in the USA (see Suppliers Page).

Note: The 5U4 will give the correct 445V output from the power supply. It may vary with use of other rectifiers. (eg: 5AR4 has a lower loss, so the output will be higher, which is undesirable and it just stresses the valves more, for little change in performance.) Do not use solid stae diodes in place of the 5U4.


H. J. LEAK & CO. LTD.

Designers and Manufacturers of Specialised Electronic Instruments for the Communications Industries

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