Comments on: A Few Interesting DIY Audio Projects c. 1955

By: Merlin

Merlin — Thu, 30 Jan 2025 11:28:40 +0000

I located the eitions of some of these circuits, maybe this helps some readers.
Bass/Treble parametric equalizer: November 1954
Two Channel Mixer: February 1955
Cascode preamplifier: October 1955

By: Merlin

Merlin — Wed, 29 Jan 2025 21:00:20 +0000

Hey great page, I just discovered it! Any chance you could add which editions of the magazine these figure are from? I’d love to read the original articles.

By: juliand

juliand — Sun, 10 Apr 2016 15:29:01 +0000

gud day sir,… may i know what is the issue of the equalizer for bass and treble with two variable resonances diagram with no inductors, thanks

By: chris

chris — Sat, 12 Dec 2015 16:25:08 +0000

In reply to jazbo8. no clue, srry

By: jazbo8

jazbo8 — Sat, 12 Dec 2015 16:23:58 +0000

Do you recall which issue of the Audio magazine that the 6550 ultralinear amplifier shown above came from? I could not seem to find the article where the scheamtic was taken from…

Thanks,

https://www.preservationsound.com/wp-content/uploads/2011/05/6550_hifi_amp-1024×860.jpg

By: JWright

JWright — Thu, 01 Jan 2015 17:32:48 +0000

Chris, I looked for the original article detailing the Fig. 2 Equalizer, but could not locate it. I am just finishing a 6SL7 project, Ampeg B-15 Preamp and am looking at this as a possible bass guitar compressor on the front end. Can you tell me exactly where you found it? Thanks, J

By: J Laber

J Laber — Tue, 18 Mar 2014 21:07:12 +0000

The EQ looks like the boost and Q are affected by the same control. It also appears that there is a resistor missing on the treble Q wiper going to the grid.

The 50 watt ultralinear looks pretty sick with the regulated B+. SR1 is shown backwards though. I may build a pair of these when I get around to winding the transformers.

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By: Steve

Steve — Mon, 19 Aug 2013 18:01:47 +0000

In reply to chris. Silly me the answer is in the circuit diagram check out R61 which is 250R and R60 equals 120K. Which is about the ball park figure I thought it would be, I could always do the maths and work it out.

By: chris

chris — Mon, 19 Aug 2013 11:13:46 +0000

In reply to Steve.

hi steve. re: B+ : yes 250V or therabouts is good for this. 20ma should be more than enough, considering that the designer spec’d chokes rated at 6ma.

re: plate resistors for the input stage: that is a very good question! I have never seen a 12AT7 section with a 500 ohm bias resistor (ie two stages sharing a common 250 ohm bias resistor) so I have no idea. check the curves in a tube manual and see what the plate resistor should be when using a 500 ohm cathode bias.

good luck, c.