I have just made a 2a3 Sovtek single plate PP fixed bias amp using 425 volts; sounds Great.
Would any one know the maximum voltage you can use with this tube?
The c core transformer i have has a 360 volt 300ma winding under test {180ma} get 480 volts; I can adjust this using a choke input and small polyprop caps.
Have seen this tube used at 500 volts.
Phil
Would any one know the maximum voltage you can use with this tube?
The c core transformer i have has a 360 volt 300ma winding under test {180ma} get 480 volts; I can adjust this using a choke input and small polyprop caps.
Have seen this tube used at 500 volts.
Phil
The original 2A3 was rated for 300V maximum plate to cathode, 15 watts plate dissipation. Most folks say you can run the old ones at 325V as long as you stay within the 15W max dissipation. Is the Sovtek flat plate 2A3 really that different?
I know there are some new 2A3-like formulations, like the JJ 2A3-40 (40W plate dissipation), which is probably their 300B with a 2.5V filament (a nice idea!). Now that 2A3's have become really expensive, that looks like a deal at "only" $120 each.
There was a thread about 2A3 characteristics. In it, you'll find a datasheet that some unknown entity put together for Sovtek 2A3. I don't know how accurate it is.
It says max plate volts = 450V
But max plate dissipation = 15W
I have a collection of vintage original 2A3's, so have never experimented with these things. I did have a set of Shuguang 2A3's (not the fancy ones), and ran them at 265V on the plate and at 18W dissipation. They lasted for more than 10 years before sounding a bit tired. They still work, just began to sound weaker.
FWIW...
I know there are some new 2A3-like formulations, like the JJ 2A3-40 (40W plate dissipation), which is probably their 300B with a 2.5V filament (a nice idea!). Now that 2A3's have become really expensive, that looks like a deal at "only" $120 each.
There was a thread about 2A3 characteristics. In it, you'll find a datasheet that some unknown entity put together for Sovtek 2A3. I don't know how accurate it is.
It says max plate volts = 450V
But max plate dissipation = 15W
I have a collection of vintage original 2A3's, so have never experimented with these things. I did have a set of Shuguang 2A3's (not the fancy ones), and ran them at 265V on the plate and at 18W dissipation. They lasted for more than 10 years before sounding a bit tired. They still work, just began to sound weaker.
FWIW...
2a3
Thanks for your reply.
Electra print have a cathode bias Sovtek 2A3 at 500 volts so I think after taking off the 60 volts or so of cathode bias would be about 440 volts.
I too have lots of RCA Dual plate 2A3's, have ordered several c core out put transformers so will give those a try too.
Have one single plate 2A3 will sell that to help cover costs of the PP 2a3 amps.
Will run the Sovteks at 400 volts.
Phil
Thanks for your reply.
Electra print have a cathode bias Sovtek 2A3 at 500 volts so I think after taking off the 60 volts or so of cathode bias would be about 440 volts.
I too have lots of RCA Dual plate 2A3's, have ordered several c core out put transformers so will give those a try too.
Have one single plate 2A3 will sell that to help cover costs of the PP 2a3 amps.
Will run the Sovteks at 400 volts.
Phil
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That Electra-Print circuit is actually a variant of the Loftin-White circuit. The 6AN4 plate is DC-coupled to the 2A3 grid. The total supply voltage is +500V, with point B at +220V to ground, as described in note B. That +220V is also the plate supply for the 6AN4 (through the 60H plate choke). There is probably about 40V grid bias on the 2A3 (grid -40V from the filament/cathode). I figure that puts the plate-to-cathode voltage for the 2A3 at about +240V. (240V x .06A = 14.4W, so just under the max of 15W for a NOS 2A3.)
Let us know if your Sovtek 2A3's survive +400V on their plates. I've heard they can do it, but I don't know if that high a voltage reduces their life span. The spec sheet says they still shouldn't be used at more than 15W plate dissipation, so with 400V on the plate that means only 37.5mA through the tube. That's probably not in a terribly linear operating point, but I could be wrong about that. These tubes are obviously not exactly like NOS 2A3's.
Let us know if your Sovtek 2A3's survive +400V on their plates. I've heard they can do it, but I don't know if that high a voltage reduces their life span. The spec sheet says they still shouldn't be used at more than 15W plate dissipation, so with 400V on the plate that means only 37.5mA through the tube. That's probably not in a terribly linear operating point, but I could be wrong about that. These tubes are obviously not exactly like NOS 2A3's.
Multi,
Best case is a 4K resistor. At 60mA it will drop 240V leaving 260V to drop between the 2A3 and output transformer. A UBT-3 has a DCR of 286R and will drop a bit over 17V, leaving 243V across the tube.
Worst case is with a 5K resisstor. At 60mA it will drop 300V leaving only 200V to drop between the 2A3 and output transformer. This is only 183V Vak.
It looks to me like they are running the 2A3 fairly conservitavly.
Best case is a 4K resistor. At 60mA it will drop 240V leaving 260V to drop between the 2A3 and output transformer. A UBT-3 has a DCR of 286R and will drop a bit over 17V, leaving 243V across the tube.
Worst case is with a 5K resisstor. At 60mA it will drop 300V leaving only 200V to drop between the 2A3 and output transformer. This is only 183V Vak.
It looks to me like they are running the 2A3 fairly conservitavly.
This polish maker can make cathode feedback Transformer, they are very good.
Tube output transformers - Shop Toroidy.pl
I don't think you need cathode feedback for 2A3's, the 2A3 already has very low Plate impedance and good damping. 2A3's are difficult to drive to start with, cathode feedback is best used with Pentodes, and Tubes like KT88, 6550's
Tube output transformers - Shop Toroidy.pl
I don't think you need cathode feedback for 2A3's, the 2A3 already has very low Plate impedance and good damping. 2A3's are difficult to drive to start with, cathode feedback is best used with Pentodes, and Tubes like KT88, 6550's
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