Custom Altec 323C tube amplifier - stereo build

[Karel Mars]

I have been toying around with different amplifier circuits built around octal preamp tubes. I have a box full of EF37/6J7 tubes, and I needed something to make music.
The circuit that caught my eye, one I haven't built yet, was the Altec Lansing 323C using 6L6 tubes in push pull tetrode mode.

"The Altec-Lansing A-323B was inroduced in 1947. The amp is conservatively rated at 15 watts RMS from a pair of 6L6G output tubes in push-pull. Its frequency response is specified at 20-20,000 Hz plus or minus 1 dB. The amp is written up in Chapter 12 of the Radio Amateur's Handbook (ninth edition) by A. Frederick Collins and published by Thomas Y. Crowell, 1949. The book chapter is titled "A 15-Watt General Purpose High-Fidelity Amplifier" and includes the schematic and a couple of pictorials."

I have an amplifier chassis with transformers mounted that seems to be just right. The B+ is a tad high, coming in at 370V DC. This pushes the 6J5 slightly over it's cathode to heater rating of 90V. I'll tend to that later.

Here is the schematic:



I opted for separate cathode resistors for the 6L6's so I can use unbalanced 6L6's. I also went for SS rectifier. The transformers are 50watt rating 6K6ohm:8ohm with 12% cathode feedback.

Here is another version of the circuit with a few ideas that I thought were good:

1. I prefer the PSU decoupling for the input pentode and 6J5 (C14/C16)
2. I also prefer the shared 1K resistor on the 6L6 screen grids. I tried with, and without, and I prefer the slightly more controlled bass with the 1K screen grid resistor.
3. I left out all the sloping mods (R17/C13, R34/C19, R35/C20). The cathode feedback is doing it's magic and the sound is very pleasing.
4. Temporarily zero feedback - the sound is fantastic as it is - with local feedback on the 6L6's

 

[Karel Mars]

 

[Family_Dog]

Good stuff Karel, if it sounds as good as it looks - and I am sure it does - you have a winner!


-F_D

 

[MusicMan]

Looks good!
Interested in your thoughts on the options to drop the B+ down from 370VDC.

 

[johanhp]

Great work. Much better for those tubes to make music than lying in a box

 

[Karel Mars]

Looks good!
Interested in your thoughts on the options to drop the B+ down from 370VDC.

If I check the specifications of a 6L6, the max screen grid voltage is 270V, but in a lot of designs 360V and higher are common.



The front end (6J7 & 6J5) only draws about 3mA per channel. If I drop the screen grid to 270V, I'll need to create another node on the PSU to feed the front end. Currently the front end is fed from the screen grid node on the PSU.

 

[Agaton Sax]

It was winter. It was the very early 1990s. The grass had turned yellow-white and a bitterly cold wind from the far-away Lesotho mountains raked the leafless trees, making the scene more sombre and sad. We were standing on the steps of the National Women's Monument in Bloemfontein. The we being myself with the president , CEO or whatever they called the main peanut of Altec Lansing. Like all men who are facing memory of an atrocity we had a peculiar feeling and like all men we hid it by talking kak. In this case audio: "You know"he said " Every month, twelve months a year, our distributor in Japan charters a cargo 747 that gets flown from Tokyo to Anaheim. We load it full of A7 and A5 speakers and it flies back." He grimaced' " They are not gong to cinemas or auditoria, no they are going to Audiophiles. I mean, have you seen the average Japanese living room and you know the size of a Voice of the Theater!" He continued. " This distributor has a lot of clout and he is leaning on us to re-introduce a low powered Tube amp, designed in the 40s! He said it is the best amp he ever heard." It is difficult to look intelligent with your mouth open, so I said " A SingleEnded Triode?" SETs had been the rage in Japan in the 70s and 80s and was slowly percolating to the West. Being an Engineer above all else , he looked horrified: Tubes were bad enough but a SET! "NO" he said "A push pull job ,but low powerd. We are getting parts suppliers together and are going to build it at our plant in Mexico." Trying to look less stupid, as I thought I already knew the answer , I asked: "Why Mexico?' His answer dropped my jaw further. " The Mexicans are incredibly good with fine handwork. They still have skills that are disappearing from the US."

The amp never happened. In 1992 the slimiest of corporations bought the once mighty ALTEC. Sparkomatic, erstwhile manufacturer of low quality spark plugs and now lower quality car audio entertainment and rubbish paraphernalia , turned corporate raider, stripped Altec of all assets and heaped further torture on the dying beast by unloading all their debt on ALTEC. ALTEC died. As a final gesture , Sparkomatic ripped the once proud name from the carcass and put it on the crappiest plastic computer speakers and other junk.

I do not know if that amp was the 323. I do not even know why I am writing this. All I know is that the proudest name in all of audio, risen from the government destruction of Western Electric and unwilling parent to JBL is dead and its name destroyed.

 

[johanhp]

Capatilism does not care for heritage or history. Nor does it care for quality or excellence. Only people do. Corporations care about the bottom line and will stop at nothing to create shareholder "value".

Thanks for sharing that story Agaton. Who knows where it would have gone if things turned out differently for them.

 

[Karel Mars]

Great story Agaton. Tubes and tube equipment does make one think back to times when humanity was driven by different needs and perceptions. But I think the time is right to take a moment to put down the cell phone and listen to music.

 

[Family_Dog]

I was just thinking yesterday about how much I missed @Agaton Sax's stories that are always so excellently told so it was indeed a very heartening experience this morning to view the latest offering here. Thank you Mr. Sax, please spoil us some more, I'm sure we all love reading about your past real-life experiences.


-F_D

 

[Karel Mars]

Back to the amplifier under discussion. This weekend I had a chance to push the volume a bit. As can be expected from the split load phase inverter, the amp does not go loud, and sounds quite harsh when pushed. This circuit sounds fantastic at low levels. I think it can be optimised with a different way to bias the 6J5.

 

[Karel Mars]

I found a circuit of the ALTEC A324-A. It seems identical, except for the DC coupling between input pentode and phase inverter. I'll do some measurements on both circuits and report if there are any significant sonic differences between the two.