Leak TL/25A (c. 1953)

Twenty-five watts of theatre-grade British valve power, built to order and still running decades after installation.

Overview

The Leak TL/25A Power Amplifier spent its working life in a commercial theatre, not a living room. Period documentation formally designates it the "POINT ONE" T.L./25A 25W. Amplifier. Photographed examples date from about 1953, and they were still in service in an Australian theatre some 40 years after installation. This wasn't domestic hi-fi that got babied on a shelf. It was a workhorse, and it looks the part.

The TL/25A arrived as a revision of the original TL/25 from the late 1940s, and Japanese sources note it was announced after the TL/12. The relationship to the TL/12 goes beyond chronology. The TL/25A shares the same chassis, which is even punched for TL/12 components. But Leak didn't just rebadge anything. The output transformer is vastly larger, looking more like what you'd expect on a 50-watt amplifier, and a sixth valve sits on the chassis between the output valves and the mains transformer. That's a big step up in iron and complexity.

The TL/25A is scarce. Information about it is very limited. It does not appear to have been advertised to the extent the TL/12 was, and it is likely these amplifiers were built to order rather than being available ex-stock. Commercial-grade build and low production numbers make it one of the rarer birds in the Leak lineup, and collectors have only recently begun to properly appreciate it.

Key Features

The Partridge Control Valve Screen Supply

The most distinctive piece of engineering in the TL/25A is the screen supply for the KT66 output valves. The original TL/25 derived screen voltage through a simple resistor or a cold-cathode tube. The TL/25A instead uses a KT61 operating as a cathode follower, with a voltage fed to its grid from a divider off the main 440V HT. This circuit was known as the "Partridge Control Valve," named for N. Partridge of Partridge Transformers. It regulates the screen supply dynamically, and it's one of the reasons this amplifier holds up sonically where simpler designs drift.

KT66 Output Valves in Tetrode Operation

The KT66 output valves are wired for tetrode operation. The power transformer carries an additional 6.3V winding to supply the filament of the KT61, which sits at about 300V above ground since it's tied to the cathode. Tested with 2 x new KT66 valves and a 5AR4 rectifier, the amplifier delivered its rated 25 watts with measured anode volts of 480V, running on a 233V AC mains supply with the voltage selector set to 230V, into an 8-ohm load.

Separate Feedback Winding on the Output Transformer

The output transformer includes a separate winding that derives the feedback signal for the amplifier, with the loudspeaker fed from another winding. This was common practice for amplifiers operating under wide or unknown load conditions, as it improves stability. The feedback loop measures 26 dB at 1 kHz, decreasing at approximately 6 dB per octave above 4 kHz and below 30 Hz. That makes the TL/25A forgiving of the load presented to it, which helps in a theatre where speaker impedances weren't always predictable.

Component Quality and Construction

Like the TL/12, the TL/25A uses first-rate components throughout. The transformers are large and oversized, the power supply uses oil-filled paper capacitors, and there are no electrolytic capacitors in the signal path. The measured frequency response at 1 watt relative to 1 kHz is flat from 10 Hz to 10 kHz, down just 1.3 dB at 20 kHz, and reaches -3 dB at 34 kHz and -6 dB at 62 kHz. That's wide bandwidth for an amplifier from the early 1950s. Distortion at rated output measured 0.1% at 1 kHz, rising to 1.8% at 30 Hz and 0.5% at 10 kHz. Pushed to maximum power, the amplifier delivered 34 watts at 1 kHz with 0.22% distortion, and 32 watts at 100 Hz and 10 kHz.

Historical Context

The original TL/25 from the late 1940s used cold-cathode tubes to derive its screen supply instead of the KT61 used here. The photographed units date from about 1953, and the documented installation that survives is a theatre sound system installed for the stage show "Kismet" in 1953 - six Leak TL/25A amplifiers, one Leak TL/12, and seven Leak Varislope III pre-amplifiers.

These amplifiers weren't competing for shelf space in a hi-fi shop. They were specified for professional sound reinforcement, a role they filled for over three decades. Leak didn't advertise the TL/25A to the extent it did the TL/12, which suggests a narrower, professional market, and the build-to-order production fits that.

Collectibility & Value

The TL/25A is rare - built to order, scarcely advertised, and never produced in the numbers the TL/12 achieved. But that rarity hasn't translated into the same level of collector awareness or documentation. There's no established price history, no well-trodden restoration community specific to this model, and no consensus on what a working pair should fetch.

Current listings show a pair of Leak TL25A 25W Point One tube amplifiers offered on eBay, with a Russian resale listing quoting approximately 2,814,790₽ for the same pair. The units that survive are often the ones that spent decades in commercial theatre service. The 44bx.com pair were in service for over 30 years and were in need of restoration when photographed. Amplifiers that come to market may need significant work.

The TL/25A offers something the TL/12 can't: genuine scarcity and a direct link to professional theatre use. The circuit is similar to the TL/12, so much of the knowledge base transfers, but the larger output transformer and the Partridge Control Valve arrangement are unique to this model. A matched pair is worth serious consideration, but budget for restoration.

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