Altec Lansing A340 Monoblock Amplifier Repair

This Altec 340A tube monoblock amplifier came through the shop for an overhaul. The owner acquired a stack of Altec gear in original condition and like any old equipment it was due for an overhaul before being put into service.

Altec is of course an important part of audio history, and these are nice little amplifiers. Each offers 35W of output, featuring 6550 output tubes fed by a power supply with active voltage regulation through a VR75 gas-discharge tube. There’s also a damping factor control on the rear, which adjusts some feedback in the amplifier to compensate for the behavior of attached speaker systems. It’s best left set to the normal position pointing straight up for almost all consumer applications, but some exotic systems like big woofers in ported or open-baffle cabinets might benefit from increasing the damping factor to offer better cone control, or smaller speakers in tightly sealed enclosures might benefit from having it reduced a little to ensure the speaker drivers will get their full range of extension. Once again, though, most speaker systems – even high quality ones – will work well with damping set to normal.

The bottom cover has a full schematic and parts list attached. There was a small drip of potting tar out of the transformer, likely from before the unit was being pulled out of service with some failing parts originally. The transformer itself tested fine, though, so nothing to worry about.

 

The wiring layout is open and accessible for service.

All of the capacitors are due to be replaced, as are most of the resistors, but Altec used some precision resistors of different construction which might not be bad. Every resistor was tested, and about half replaced as they were past their tolerance.

All new capacitors and many resistors. The 40 uF dual can capacitors got replaced with 47 uF 500V Nichicon capacitors with a long lifetime and high ripple current and temperature ratings.

A new power cord. This amp also needed a new 12AY7 and a new pair of 6550s. Then it was time for a series of slow power-ups with the variac. This model uses self-bias for the output tubes, so there is no bias adjustment required. Testing showed this amp is an exceptional performer.

Frequency response is very flat both through the normal range, and extended 10 Hz – 100 kHz.

The amplifier has adjustable input gain; with the level control set at maximum, the amplifier produces its rated 35W output with 1.2V of input signal.

Distortion is exceptionally low, too.

All set! After performance analysis and a 4-hour burn-in period on the bench, this amp is ready to go home. It sounds excellent when playing, and is dead silent with no input signal. It’s going to be a great performer for years to come!