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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.
Follow Ups:
Here is how I now have it wired and it sounds great!
Here is the power supply now.
If you look at the schematic carefully, at the base of the 2A3 you can read 470(ohms) 2 Watt, those are supposed to be the cathode resistors to ground.
-- josé k.
Your post convinces me that I need to look more carefully -- the schematic was not in error. However, the 470 ohms resistors are in parallel with each filament (across the filament) and do not connect to ground. Rather, there is a 330 ohm resistor way on the right hand side of the schematic to ground, bypassed by a 470uF cap. That provides the cathode bias for 2 2A3s, and should be fine.I am not sure where Les's pair of 100 ohm resistors from filaments to ground went (in relation to the schematic), but think my analysis was correct. They burnt up because that were biasing the 2A3s with 50 ohms and therefore carrying a lot of DC bias current (plus a little filament current) and it was too much.
Like Homer says " Doooop"
Thanks again for your help. I actually rectified the 5V tap and ran the 2A3 filiments in series. I'll share the schematic when
I fin.
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