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In Reply to: Re: Watching 2A3 burn , Help! posted by Perseo on March 3, 2007 at 04:08:00:
Sorry here is the power supply
Follow Ups:
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.
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.
Les,
If this is exactly how you set up your 2A3 filaments I expect burning 100ohm resistors are the least of your problems. I don’t think that amp will work as drawn. I would not turn the amp on again until a few things are changed and you use a variac. You risk killing your power supply transformer as well as your output transformers and your 2A3s.Since a 2A3 has a filamentary cathode your high-tension supply (B+) can goes to ground through the 2A3 heater if you let it. In your case it’s about 300V through the 2A3 plate to cathode to 100ohm resistor to ground, which is enough to light those 100 ohm resistors up.
In addition I don’t see how you are biasing the grid on the 2A3? If you are attempting to bias the 2A3 with the pair resistors coming off your filament you will need to use much higher values. Or just hook up an 800 ohm resistor to ground from their junction.
Try looking at five or so other 2A3 schematics and see if you can figure out how they are dealing with DHT issues, mostly how they are setting Vbias and completing high-tension DC and AC circuits. Good luck, variacs really help.
Matt
Thanks so much for your help. I actually rectified the 5V tap and ran the 2A3 filiments in series. I'll post the schematic.
Your 2A3 cathodes/heaters carry music information so depending on how you have done things, you may have a pseudo-mono amp now with channel information mixed. Depending on how anal you are the grid bias for each tube is slightly different, the one on the positive end of the heater string has a higher positive potential over ground then the heater on the negative end of the string. This is manageable but something to consider if you are worried about such things.If you went to DC because of hum issues going back to AC with hum-pots may be an option, much simpler and better sounding to my ear. Again check other schematics to see how they did things. Series heater connected DHTs are not simple beasts.
Matt
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