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Nodric 1st October 2012 12:37 AM

Another 6922/6AS7 OTL Project
So after much pondering I decided to give this design a go. Here are my initial observations and I welcome any feedback (no pun intended) :D

I used a Sylvania JAN6922 and an RCA JAN6AS7 as these are what I have in my stocks.

For all those folks out there thinking should I try this, it only took me a couple of hours to have it up and running on the bench. Building the case, and making a nice layout will take longer.

I used this circuit (not my own).

I wired all this up as you'll see below, lovely mess, just to test the design and to see how it worked. Even on on my prototype board with all its leads running here and there, it sounds awesome, so I can only imagine what it will sound like when it's finished. With no signal there is no hum/hiss/noise detectable by my ears :)

I did some measurements.

B+ was 156vdc
Plate dissipation on the 6AS7 was 24ma/1.85w

Temperatures measured using my Fluke Laser Thermometer
6AS7 151f 66c
6922 78f 25c
4k7 5w resistor 75f 24c
3k3 10w resistor 100f 38c (resistor not yet mounted)

Overall gain was a factor of 5. i.e. 1v input yields 5v output.

I used the suggested values, but wonder if they are over spec'd? e.g. the 120uf and 220uf caps are spec'd at 400v, but the max voltage across them is only 147/156vdc. Similarly for the 680uf output cap I only measure 80vdc across it. Could they be reduced in rating?

Caps are Nichicon and Panasonic, and all resistors are 1% Metal Film 1w

Any thoughts on the design and components welcome.

Zero Cool 1st October 2012 01:37 AM

I like your breadboard setup there! that looks handy! I think I need to make me something like that!


Nodric 1st October 2012 02:27 AM


Originally Posted by Zero Cool (
I like your breadboard setup there! that looks handy! I think I need to make me something like that!

I got inspiration from someone else who had made something similar. It make life easy, although I went overboard on tube sockets :eek:

I used a small nail on each set of connections to remind me which is pin 1 :)

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