I found this Altec A-324A audio amplifier at a hamfest (amateur radio swapmeet). The amp was in excellent cosmetic condition, although with incorrect knobs and no output tubes.
The A-324A was introduced in 1949. Click on this thumbnail image for two full-page cut sheet ads for the A-324A on the Altec pages of the excellent hifilit.com site.
The cut sheets lists frequency response from 20 to 20,000 cycles plus or minus 1 db. Altec Lansing is well known for quality public address speaker systems and audio hi-fi equipment. According to the Altec cut sheets;
The circuit is a fairly straight-forward class AB1 with cathode bias, inverse feedback and with a bass control in the feedback line. Except for a more complex front end, the circuit is nearly identical to Altec's 15 watt "General Purpose High-Fidelity Amplifier" model A-323B introduced in 1947. The A-323B is described in detail in Chapter 12 of the ninth edition (1949) of The Radio Amateur's Handbook by A. Frederick Collins. The A-323B was apparently available in kit form according the the Collins book. The A-323B schematic in Collins is useful since it includes the expected voltages at various points in the amp. Those voltages are not listed on the A-324A schematic but are roughly equal to what would be expected in the A-324A.
The A-324A has two transformer-coupled low-impedance microphone inputs and two line level (phono) inputs that can be faded from one to the other as if designed for use by a DeeJay. Most of the capacitors and resistors are mounted on a large modular terminal strip making servicing relatively easy. Resistors are mostly oversize 1 watt units that have stood the test of time well. Most of the capacitors were obviously wax impregnated and needed replacement.
The schematic is glued inside the amp's bottom cover. The schematic is also available as a pdf at this link.
The original output tubes were 6L6G with a 5U4G rectifier. I located original knobs and inserted a 6L6GB pair and matching 5U4GB. The chassis-mounted electrolytics reformed easily using a Heath PS-4 variable high voltage supply and series resistor to limit current. I immediately replaced all four of the plate-to-grid coupling caps. Those are critical in any audio amp as leakage can upset tube bias and damage tubes and transformers. All other wax caps were also replaced.
I then connected a speaker to the 8 ohm tap, provided an audio source, inserted the rectifier, and powered it up slowly while monitoring B+ voltages. The amp was working but with noticeable distortion at higher levels of volume. I followed up with an audio generator and scope. I could see flat topping on both peaks of the sine wave at relatively low power levels. I reviewed the schematic closely to determine the likely suspects and then noticed the small electrolytic in the screen grid circuit of the main 6J7 preamp. That 6J7 is directly coupled to the 6C5 phase inverter. The screen is fed by a relatively high value 1 megohm resistor. Any leakage of that small electrolytic would be in series with that 1 meg resistor and ground and pull down the screen voltage. That small electrolytic is 4 MFD and is out of sight under the amp's main terminal board. I thoroughly tested a new replacement for lowest possible leakage and ESR. Replacing the cap solved the distortion problem. Dummy load and scope testing now showed no noticeable distortion until the amp was producing at least 17.5 watts RMS, well within specs.
The proof is in the listening. The amp's somewhat unusual tone controls take some getting used to. The treble control is actually a treble cut control rather than boost. It is best left in the flat position. If a treble tone boost is desired, the amp should be used with an external preamp. I enjoyed listening to my MP3 player fed to the amp directly. No tone boost neeed. Not having Altec speakers, I used my smaller AR-8b speakers instead. It was obvious that the Altec is a quality amp that was now performing as it should.
The Altec with 6L6G tubes
It was suggested that I take a picture of the A-324A with 6L6G and 5U4G tubes to show how the amp would have appeared originally. Here are two pictures of the amp with a matched set of Sylvania "coke bottle" "G" tubes that I treat with great care.
The Heathkit GC-1A Mohican Receiver was the previous item on the bench.