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Old 27th March 2014, 03:50 AM   #1
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Default PP 2A3 Fixed Bias

Hey Anyone,

I'm currently building a push pull 2A3 amp for a friend. Its a mono amp that he wants to use to listen to 78's.

I've never worked with DHT tubes before and have a question. (The front end will be Pete Millet's Uniamp.) I want to use fixed bias because it results in a little more power and half the distortion. I know how to apply the bias voltage between the coupling cap the grid stopper. But how do you measure the current?

I mean the DC supply must ground through the 2.5 volt heater circuit right? And if I connect a resistor to ground between the two tube's heater supply any measurement across that resistor will be for both tube's DC component as well as some unknown quanity of AC cuurrent as well. Can anyone tell me how to do this? Or lacking that, point me to a PP 2A3 schematic?

Kevin
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Old 27th March 2014, 04:01 AM   #2
multi is offline multi  Australia
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You can use two DC supplies and 10 ohms to earth. I have 105Db speakers and no problem using AC with 2A3's.
You can always measure the Dc current after measuring the ohms from CT to plates. Or you can put 1 or ten ohms resistors to the plates.
Phil
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Old 27th March 2014, 04:10 AM   #3
AJT is offline AJT  Philippines
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2A3's are not prone to humming having built two SETS before...
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Old 27th March 2014, 04:10 AM   #4
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Thanks Phil,

Using the known resistance of the OPT primary half windings never occurred to me. (An elegant solution.) But what about a cathode resistor? Can I just let the DC ground through the power supply? It seems even If I put in a cathode resistor some of the DC will still go through the filament supply?

Kevin
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Old 27th March 2014, 07:51 AM   #5
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Hi,
If using fixed bias on the 2A3's, just tie the 2.5v transformer's Center Tap to ground. If your 2.5v transformer doesn't have a CT, then get two 10 ohm resistors and attach one to each of the 2.5v leads. The loose ends of these 2 resistors get tied together and this connection then goes to ground. You will need two filament supplies per channel if you want to measure current with a "cathode" resistor. The OPT winding resistance method is the easiest way to go to measure each output tubes current independently.
The DC will only go through to where your filament supply is attached to ground. The filament heater supply and 2A3's ground return path are 2 different circuit paths.
2A3's in PP with the driver RC coupled to the 2A3's will realistically give you about 8 watts out before it poops out. You can raise this to 14-15 watts out, if you use an interstage transformer or cathode follower to drive the 2A3's. All this is assuming class "A" operation at 250 volts B+ with fixed bias for the 2A3's.

Last edited by danFrank; 27th March 2014 at 08:01 AM.
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Old 27th March 2014, 11:27 AM   #6
AJT is offline AJT  Philippines
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in the two SET amps that i built out of the 2A3, one unit has 2.5 volts ac with center tap, and the other used a humdinger pot. in both cases i found that just setting the pot to center in one and just using the center tap in the other did not produce hum issues...

btw, my operating point was 250 volts plate and biased to -45 volts grid resulted in a plate current of 60mA, spot on with published specs for the pairs of Shuguang tube i used, truly matched characteristics....
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Old 27th March 2014, 12:21 PM   #7
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Thanks guys for all the help!
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Old 27th March 2014, 12:45 PM   #8
MelB is offline MelB  Canada
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Here's a nice Hammond transformer which just happens to have two 2.5v @ 2.5 amp windings and a bias tap and a 5v rectifier winding!
http://www.hammondmfg.com/pdf/EDB302AX.pdf
Should be available from digikey.
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Old 27th March 2014, 01:03 PM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by mr2racer View Post
Hey Anyone,

I'm currently building a push pull 2A3 amp for a friend. Its a mono amp that he wants to use to listen to 78's.

I've never worked with DHT tubes before and have a question. (The front end will be Pete Millet's Uniamp.) I want to use fixed bias because it results in a little more power and half the distortion. I know how to apply the bias voltage between the coupling cap the grid stopper. But how do you measure the current?

I mean the DC supply must ground through the 2.5 volt heater circuit right? And if I connect a resistor to ground between the two tube's heater supply any measurement across that resistor will be for both tube's DC component as well as some unknown quanity of AC cuurrent as well. Can anyone tell me how to do this? Or lacking that, point me to a PP 2A3 schematic?

Kevin
If you have experience with valve amps (as it seems to me) and do things carefully the easiest way is: put a 1 ohm precision resistor between each plate and its primary OTP connection. Read the voltage drop with a DC meter and each millivolt you get will equal to 1 mA.

Last edited by 45; 27th March 2014 at 01:07 PM.
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Old 28th March 2014, 01:22 AM   #10
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Hey All,

Having put the cart before the horse I'd already installed the taps and pots to adjust the bias voltage. (See photo) I've used this system before with non-dht valves. Problem is if I attach it to the center tap and the end taps the jacks will expose the user to @300 volts DC. So I have to attach it to the cathode side. I've also attached a drawing that I think will work without the high voltage. As multi suggested above.
Attached Images
File Type: jpg DSCN0434.JPG (704.6 KB, 284 views)
File Type: jpg Bias.JPG (52.4 KB, 271 views)

Last edited by mr2racer; 28th March 2014 at 01:37 AM.
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