I know I need 2.5 V to heat my 2A3 filament but what's an appropriate current for it, especially if it's DC? 2, 3, 4 or 5 amp? Or, will it make a big difference between 2 to 5 amp?
Evenharmonics said:
The nominal filament current is 2.5A. Of course, that's likely to be very nominal since the 2A3 wasn't designed for series heater string operation. If you're using constant current heater power, it's best to adjust it while monitoring the filament voltage and adjust for 2.5V.
Sheldon said:
Did you use a 10 turn pot for adjustment? I didn't try that. It just didn't realy null at any point that I could tell.
For me, a 20dB reduction in this hum would be great.
zarniwoop said:
I actually built the 801 amp Steve used to describe the effect. I used as close as possible to his component values. With another circuit, things would be different, and would require playing with component values. I would try, say a 25k pot in place of the 5k. I used 10 turn pots. You should at least be able to make hum worse at some point. That will confirm that you are injecting a signal and have enough to work with. Can you post a circuit?
Sheldon
Thanks for the info.
I think I'll give this another try. Clearly, my setup was not optimal. I only got a slight increase in hum messing with the pot, which didn't assure me that things were working right. I'll post up a circuit next time I try this. It is a cathode biased 300B w/ 50R hum pot. I wasn't sure exactly the best way to adapt the grid components. Without this mod it was 270K grid resistor to gnd.
I think I'll give this another try. Clearly, my setup was not optimal. I only got a slight increase in hum messing with the pot, which didn't assure me that things were working right. I'll post up a circuit next time I try this. It is a cathode biased 300B w/ 50R hum pot. I wasn't sure exactly the best way to adapt the grid components. Without this mod it was 270K grid resistor to gnd.
Tweeker said:
My driver tubes (12A*7) are 0.7V above ground, and my output tubes (triode EL84) are 7.2V above ground. Does that mean I should increase these voltages? How do I do that?
The heaters have separate connections from the cathodes. They need to be biased up to a higher voltage, they should still have the same volts across them. A diagram would help Im sure, Ive not one handy.
For DC heating, are tube filaments in 300B and 2A3 polarity specific? Please bear with me, I’m novice.
I'm trying this kit with 8.5 V CT, 4 amp transformer and something isn't working.
http://store.qkits.com/moreinfo.cfm/FK809
Without load (no tube), I can adjust it to produce 2.5 VDC but when I put my 2A3 in place, it drops down to 1.7 VDC and won't go any higher even if I turn it up all the way. Not only that, the filament doesn't even glow and no sound comes out from speakers. Any suggestions?
http://store.qkits.com/moreinfo.cfm/FK809
Without load (no tube), I can adjust it to produce 2.5 VDC but when I put my 2A3 in place, it drops down to 1.7 VDC and won't go any higher even if I turn it up all the way. Not only that, the filament doesn't even glow and no sound comes out from speakers. Any suggestions?
The schematic shows quad diode (proper term?). Does that answer your question?
If 2A3 filament can be heated sufficiently with 2.5 VDC at 2.5 amp per tube, then 3 amp supplying one tube would’ve been enough, wouldn’t it? I didn’t check AC voltage coming out of transformer but I was able to adjust DC output from 0.2 to 6 VDC straight from output leads.
If 2A3 filament can be heated sufficiently with 2.5 VDC at 2.5 amp per tube, then 3 amp supplying one tube would’ve been enough, wouldn’t it? I didn’t check AC voltage coming out of transformer but I was able to adjust DC output from 0.2 to 6 VDC straight from output leads.
With no load, you will have a lot higher voltage than with the
2.5 amp load of one 2A3. Does the 8.5 V transformer have a center tap (most of the time they do).
If by chance you using the center tap you would only have 4.25 vac.... that could be the reason you have 6 vdc with no load and only 1.7 V under the full 2.5 a load.
Are you sure thr transformer was rated at 4 amps?
2.5 amp load of one 2A3. Does the 8.5 V transformer have a center tap (most of the time they do).
If by chance you using the center tap you would only have 4.25 vac.... that could be the reason you have 6 vdc with no load and only 1.7 V under the full 2.5 a load.
Are you sure thr transformer was rated at 4 amps?
It has a center tab but I’m not using it. I tried it earlier and yes, the voltage was too low so I switched it. The transformer is Hammond 166N8 8.5ct FL volts, 9.04 NL volts, 4.00 amps and I got it from Angela.com. Is there a test I can do to figure out amperage in real time?
The way to measure the current is with your volt meter on the "current setting" in series with the load....
Only test it for a short time.
Have you tested the dc heater supply on the tube but with the tube not in the amp?
How is the 2a3 cathode bypass Resistor and cap connected?
With a hum Pot?
With a pair of 22R resistors?
I'm thinking something else may be draining all tour current???
Only test it for a short time.
Have you tested the dc heater supply on the tube but with the tube not in the amp?
How is the 2a3 cathode bypass Resistor and cap connected?
With a hum Pot?
With a pair of 22R resistors?
I'm thinking something else may be draining all tour current???
I’m switching from AC filament heating to DC of this.
I’ll try that stand alone tube measurement and report back later.
An externally hosted image should be here but it was not working when we last tested it.
I’ll try that stand alone tube measurement and report back later.
Evenharmonics said:
Ok, you have an 8.5V, 4A center tapped transformer. If you are not using the center tap and taking the output from the ends of the transformer then, in effect, you have an 19V, 2A transformer. It will be current limited. If you want to use this kit, you need to rewire the diodes so that it is a full wave rectifier. That way, you can get adequate current from your transformer.
Sheldopn
2A3 with AC filament supply
Hi,
My 2A3 with ac filament supply is very quiet. The noise output is approx 1-2mV. I won't border switching it to DC at all.
You can further improve the hum adjustment by putting two 25-ohm resistors in parallet with the hum pot. This will allow you have finer adjustment which could further reduce the hum to minimal.
Johnny
Hi,
My 2A3 with ac filament supply is very quiet. The noise output is approx 1-2mV. I won't border switching it to DC at all.
You can further improve the hum adjustment by putting two 25-ohm resistors in parallet with the hum pot. This will allow you have finer adjustment which could further reduce the hum to minimal.
Johnny
DC on a 2A3 🙄
My opinion is I believe it's not the 2A3 (or the 300B) to be in focus, but somewhere else.
1. My JE Labs 2A3, my 45 are very quite on an AvantGarde DUO. Both of which has AC filament supply.
2. My 300B use to have audible hum from the listening position, also with the AvantGarde DUO, but when I changed the driver to a CCS load (diyAudio CCS board), the noise dropped significantly and I need to put my ears to the horn to hear faint noise. This amps also uses AC filament supply.
Note: AvantGarde DUO NOT mine 😀
ps.
I'm also happy with my pre-amps (WE 407A and 12B4A) and we drove a Mark Levinson power amp with no audible noise at full volume. Well, even my Foreplay is dead quite.
However, I gave up on the JE Labs 76 (a modified Berman) as I can't get rid of a loud hum! Even after communicating with Joseph Esmilla and I think I posted that pre in this forum. Dumped it.
My opinion is I believe it's not the 2A3 (or the 300B) to be in focus, but somewhere else.
1. My JE Labs 2A3, my 45 are very quite on an AvantGarde DUO. Both of which has AC filament supply.
2. My 300B use to have audible hum from the listening position, also with the AvantGarde DUO, but when I changed the driver to a CCS load (diyAudio CCS board), the noise dropped significantly and I need to put my ears to the horn to hear faint noise. This amps also uses AC filament supply.
Note: AvantGarde DUO NOT mine 😀
ps.
I'm also happy with my pre-amps (WE 407A and 12B4A) and we drove a Mark Levinson power amp with no audible noise at full volume. Well, even my Foreplay is dead quite.
However, I gave up on the JE Labs 76 (a modified Berman) as I can't get rid of a loud hum! Even after communicating with Joseph Esmilla and I think I posted that pre in this forum. Dumped it.
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