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Modernized Altec Lansing A-324A Clone

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B

baloneygeek

Member
Joined 2014
#1
Hi all,

I've been a member of this forum for a long time but just lurked, now it's time for me to muster up the courage to actually post something here.

I've always had a thing for Altec Lansing (their brand was quite popular in India in the late 2000s and early 2010s) and the first audio device I bought with my own money was an Altec Lansing 2.1 active speaker system when I was in middle school. Fast forward, I got into amplifier design (mostly chipamps and then discrete BJTs, all Class AB) and very recently built my first tube amp (the AE-1 from following the tutorials on the Analog Ethos website) and now I'm hungry for something a bit more advanced.

I found the schematic for the Altec Lansing A-324A and I'm given to understanding that the amplifier stage was common to quite a few Altec products of that era and was actually a decent-enough HiFi performer. So I thought, why not just build that circuit and I'll have an Altec Lansing tube amp, which would be quite a big deal!

Of course, things aren't that easy, and I have questions.
  1. The Altec TL-217-B output transformer is a 6600-ohm primary transformer. A modern equivalent I'd like to use is the Edcor CXPP60-MS-6.6K. However that OPT only has a single 8-ohm primary, and the A-324A seems to run its feedback circuit off of a 16-ohm output tap. I suspect that this will need a change to the feedback resistor R23. Could someone suggest what I could replace it with, or better yet explain the math so I can calculate it myself?
  2. The input pentode (6J7) is really hard to source in quantity - all I can do is buy NOS and hope they don't break - but I suspect a roughly drop-in replacement would be the 6SJ7 or the Chinese 6J8P. Do you have any other ideas of what else I could use? If it's something current production, all the better.
  3. I'd actually like to analyse this amp and see what the operating point of the input pentode is, what kind of gain it's configured for and if I can replace it with some other kind of current production triode (of course, then it wouldn't exactly be an Altec amp and will also sound different, but I'm also happy with an "inspired by Altec, designed by me" amp). Maybe use a 6SN7 or 6SL7 and use one half as the input gain stage, and the other half as the phase-splitter. But this math is way over my head (I'm a comp-sci major doing database storage engineering, I took an undergrad-level elementary analog electronics class and fell in love with electronics that I could hear, but this means some stuff that's obvious to the experts here is fairly alien to me). If someone has the patience to explain this math to me and walk me through figuring out the operating point of that 6J7, I'd be very grateful.
The objective here is to build the design (and maybe design a pretty case for it), and I don't care too much about using "audiophile grade" stuff. I don't have much of a budget, so I'll just order parts off Digi-Key that seem within a reasonable level of tolerance, and use Electro-Harmonix or JJ or Chinese current production tubes. If I really love how it ends up sounding, I can always rebuild the circuit with better components and/or tubes. The A-324A seems to also use a 400V B+ supply, so I'll also just re-use the power supply from the AE-1.

Thanks,
Boud

[1] Circuit Schematic: https://web.archive.org/web/20110924120419/http://www.theused.com/manuals/altec/altec_a324a.pdf
[2] Altec Transformers reference: http://altec.bayoucables.com/AltecTransformers.pdf
 
#2
baloneygeek said:
explain the math so I can calculate it myself?
Click to expand...
Convert to voltage. 8V in 8R would be 8 Watts. What is 8 Watts in 16R? 11.3V. So you need a 1.4:1 change of resistor.

6SJ7 is 6J7 with grid on bottom pin. Both are readily available in the US.
https://tubedepot.com/products/6j7-ef36
BTW, 6J5 is often a 6SJ7 with internal triode connection. In broadcast studio gear 6J7 was used everywhere, triode-strapped on the socket if that's what was needed. Good to have just one box of spares for 80% of your tubes.
Here, if I were in a big hurry and could not wait for a 6J7, I'd go mini and 6AU6, fiddle the 1000R bias for reasonable plate voltage.

A lot of "the Altec sound" in these amps is the output transformer. It was not magic, just Good Commercial Quality, but nobody today has a room full of calculators and women winding thousands of cores.
 

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#3
A bit of a brainfart on my end - when I said 6J7's were hard to source, that's because I really want glass tubes and both 6J7GTs and 6SJ7GTs seem to be either hard to come by or expensive. I'm located in Germany and the most affordable 6SJ7GTs I can get are here: https://www.tubeampdoctor.com/6sj7gt-vt-116a-nos-western-electric-usa. I don't know if I want to spend 65 EUR on just the input pentodes on a design that I'm not entirely sure will work immediately. Also I'd feel really bad if I destroyed these old tubes.

The 6AU6 is a great idea, I'm not entirely sure though how I'd tweak the resistors. I'm not fully sure how to calculate the operating point and gain of the input stage. Once I have that I could well build an equivalent circuit with the 6AU6 or (preferably) an EF86 (6267/6J32P).
 
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