Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

Re: Watching 2A3 burn , Help!

Posted by Perseo (A) on March 3, 2007 at 14:20:25

In Reply to: Re: Watching 2A3 burn , Help! posted by Les Hudson on March 3, 2007 at 10:04:37:

Matt has it right. Taken together, the power supply and amp schematic reveal a 2A3 amp with rhe grid referenced to ground and withe the cathode containing only the two parallel 100 ohm resistors to ground to create a bias.

Unlike Matt, I am not surprised it made music -- I once thought I had solved all hum and noise problems with a 26 tube by having the cathode and grid both pretty much on ground, until another inmate asked me where the bias went. It did make music.

I am glad you used half watt resistos - they acted like a little very slow blo fuse.

As I said in my post and Matt says, the two hundred ohm resistors need to connect (at their junction) to one end of a bigger resistor which connects to ground. That resistor would be say 750 ohms if each 2A3 has a separate filament supply. That would give you an operating point that is close to the text book 60mA, 250v plate to cathode (you have 300v of B+, minus something in the transformer, with the cathode biased up at 45v (48v if you get the full 60mA) -- that uses up all 300v). The 750 ohm resistor needs to be 5 watts or more.

Those schematics are bad if they left out the cathode resistor and referenced grid to ground rather than a negative supply.

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---