Hot Rod
Well, I figured out why it sounds so good and why I can easily drive it into distortion. With the filament set up like it is I believe that the current and dissipitation values need to be split between the two tubes as this is a series circuit. The voltage will be the same. I looked at a couple other 2A3 circuits and the cathode resistor is around 800 to 880 ohms. Gabe's design uses 330 ohms which is split between two tubes, but a little more than half of the usual cathode resistor. In my haste to build I subed this value out, rather ignorantly, with 300 ohms.
If my calculations are correct: 45v cathode/300 ohms= 150 ma/2tubes = 75ma.
Now here is where it gets a little fuzzy for me. Should I use my actual measured value at the plate x this current for plate dissipitation or subtract the cathode voltage from the measured value at the plate x the current.
Either way I am still over the PD value off the data sheet, but I'd like to get it right. I think this would explain why it is so easy to drive into audible distortion and I mean really audible. It is getting as loud as my 15 watt 6BQ5 but really distorted. I am going to go with a cathode resistor in the neighborhood of 350 to 400 ohms and tone it down a bit. This might also explain why the current limiting resistor on the HT center tap to ground is getting really hot.
Like I said it sounds really good, even with the 125ESE's. I just don't know how long I can expect it to keep running under these conditions. Any thoughts?
Well, I figured out why it sounds so good and why I can easily drive it into distortion. With the filament set up like it is I believe that the current and dissipitation values need to be split between the two tubes as this is a series circuit. The voltage will be the same. I looked at a couple other 2A3 circuits and the cathode resistor is around 800 to 880 ohms. Gabe's design uses 330 ohms which is split between two tubes, but a little more than half of the usual cathode resistor. In my haste to build I subed this value out, rather ignorantly, with 300 ohms.
If my calculations are correct: 45v cathode/300 ohms= 150 ma/2tubes = 75ma. Now here is where it gets a little fuzzy for me. Should I use my actual measured value at the plate x this current for plate dissipitation or subtract the cathode voltage from the measured value at the plate x the current.
Either way I am still over the PD value off the data sheet, but I'd like to get it right. I think this would explain why it is so easy to drive into audible distortion and I mean really audible. It is getting as loud as my 15 watt 6BQ5 but really distorted. I am going to go with a cathode resistor in the neighborhood of 350 to 400 ohms and tone it down a bit. This might also explain why the current limiting resistor on the HT center tap to ground is getting really hot.
Like I said it sounds really good, even with the 125ESE's. I just don't know how long I can expect it to keep running under these conditions. Any thoughts?
re: Gabevee
Ok 470 ohm it is. Now do I use the measured 300volts at the plate or do I subtract the 45 off the cathode to determine dissipitation. Either way I'm dissipitating 19.8 watts or 23 watts. It has actually been playing around 4 hrs a day for over a week now and I haven't noticed the plates turning red yet. I can not find any info on these sovtek 2A3's I got from uncle Ned but I hear their supposed to be stronger then the regular 2A3. I am blessed Gabe.
One other thing, I used a 250 ohm center tap resistor as I didn't have 220 ohm.
I flipped a coin as to which direction to go, up or down, as I also had a 200 ohm resistor. It does sound really good though Gabe.
Ok 470 ohm it is. Now do I use the measured 300volts at the plate or do I subtract the 45 off the cathode to determine dissipitation. Either way I'm dissipitating 19.8 watts or 23 watts. It has actually been playing around 4 hrs a day for over a week now and I haven't noticed the plates turning red yet.
I can not find any info on these sovtek 2A3's I got from uncle Ned but I hear their supposed to be stronger then the regular 2A3. I am blessed Gabe.One other thing, I used a 250 ohm center tap resistor as I didn't have 220 ohm.
I flipped a coin as to which direction to go, up or down, as I also had a 200 ohm resistor. It does sound really good though Gabe.
Hey Passfan,
The Sovteks sound very good. Almost like a 300b. Can you tell I really like the 300b?
I also have a pair of Billington (?) dual plates. They sound much warmer. They aren't mine, tho.
As for plate dissipation... I did the calc when I read your post. Yikes! I never generally go higher than maximum diss. I try to do things at no more than 80% of max recommended.
I am glad to hear others' comments about their use. So.. the 2A3 is pretty durable. So are the 45's FWIW. I tried my precious NOS Silvertone 45s and they also dissipate higher power than normal.
But... as you noted... no red plates. Try that with an EL84 and I hope you like cherries. I had about 370 volts on my EL84 once just because I was trying a NOS power transformer I acquired, its actually about 380 volts, but I cut it down to 370, and both the sound was too... solid state, and the plates glowed slightly red with the lights out. The tubes have a max dissipation of 12 watts, but they were dissipating 13.87 watts.
So, I would imagine that the 2A3's would bathe the room in red.
I can only guess that some tubes are rated more conservatively than others. I mean, I let the Sun, running at 300 volts, and 67 milliamps, for a 20 watt dissipation, break in for five days 24 hours a day. No reds, no burns, no errors.
I wonder how the 2A3 would behave in my antique radios using 45s. My only concern is actually filament current draw. I could always use an external filament transformer. This way I can save my rare and precious 45s.
BTW, the 45s sound very nice for some kinds of music. They do have that certain sweet sound. I prefer the 2A3's though.
As for driving it into distortion... what do you expect from 3.5 watts? That is why those who get 2A3 or 45 amps use 96dB speakers or better. Both the Sun and Afterglows distort at a moderate level on my 86dB speakers.
Gabe
The Sovteks sound very good. Almost like a 300b. Can you tell I really like the 300b?
I also have a pair of Billington (?) dual plates. They sound much warmer. They aren't mine, tho.
As for plate dissipation... I did the calc when I read your post. Yikes! I never generally go higher than maximum diss. I try to do things at no more than 80% of max recommended.
I am glad to hear others' comments about their use. So.. the 2A3 is pretty durable. So are the 45's FWIW. I tried my precious NOS Silvertone 45s and they also dissipate higher power than normal.
But... as you noted... no red plates. Try that with an EL84 and I hope you like cherries. I had about 370 volts on my EL84 once just because I was trying a NOS power transformer I acquired, its actually about 380 volts, but I cut it down to 370, and both the sound was too... solid state, and the plates glowed slightly red with the lights out. The tubes have a max dissipation of 12 watts, but they were dissipating 13.87 watts.
So, I would imagine that the 2A3's would bathe the room in red.
I can only guess that some tubes are rated more conservatively than others. I mean, I let the Sun, running at 300 volts, and 67 milliamps, for a 20 watt dissipation, break in for five days 24 hours a day. No reds, no burns, no errors.
I wonder how the 2A3 would behave in my antique radios using 45s. My only concern is actually filament current draw. I could always use an external filament transformer.
This way I can save my rare and precious 45s.BTW, the 45s sound very nice for some kinds of music. They do have that certain sweet sound. I prefer the 2A3's though.
As for driving it into distortion... what do you expect from 3.5 watts? That is why those who get 2A3 or 45 amps use 96dB speakers or better. Both the Sun and Afterglows distort at a moderate level on my 86dB speakers.
Gabe
Getting A Handle On Things
I have done some research into my bias situation and here is the latest update. I am now running at 328 volts on the plate with 53 volts on the cathode with a 450 ohm resistance for a current setting of 58 ma per tube for a dissipitation of 16 watts. That is 328v - 53v = 275v x .058 = 15.95w . The sound seemed to smooth out considerably and I am not sure wether I like it or not. I can't quite tell as I've been switching back and forth trying to break in a new set of OPT's on an EL 84 amp of mine so my ears are getting confused a bit. I see from the JE Labs site that Joseph runs his regularly at 17 to 18 watts plate dissipitation and frank is running his pretty high as well. Neither has had any problem with meltdown as of yet. Originally, through my mistake, mine were dissipitating just under 20 watts with no redness in the plates (WOW). They ran like this for 4 to 6 hrs a day during burn in and what detail. I am going to go back up to a 360 ohm bias point like frank and see what that gives me. I would probably not hesitate to go back to the 330 ohms that Gabe calls for as well. I will see how this sounds and let everyone know.
Gabe, what effect will different value resistors have on the center tap of the HT winding?
I have done some research into my bias situation and here is the latest update. I am now running at 328 volts on the plate with 53 volts on the cathode with a 450 ohm resistance for a current setting of 58 ma per tube for a dissipitation of 16 watts. That is 328v - 53v = 275v x .058 = 15.95w . The sound seemed to smooth out considerably and I am not sure wether I like it or not. I can't quite tell as I've been switching back and forth trying to break in a new set of OPT's on an EL 84 amp of mine so my ears are getting confused a bit. I see from the JE Labs site that Joseph runs his regularly at 17 to 18 watts plate dissipitation and frank is running his pretty high as well. Neither has had any problem with meltdown as of yet. Originally, through my mistake, mine were dissipitating just under 20 watts with no redness in the plates (WOW). They ran like this for 4 to 6 hrs a day during burn in and what detail. I am going to go back up to a 360 ohm bias point like frank and see what that gives me. I would probably not hesitate to go back to the 330 ohms that Gabe calls for as well. I will see how this sounds and let everyone know.
Gabe, what effect will different value resistors have on the center tap of the HT winding?
So if I go with no resistor, straight to ground, my voltage will go up? What happens if my resistor opens so that the center tap is floated?
So the smaller the resistor the larger the voltage and the larger the resistor the smaller the voltage? Right?
Thanks Gabe, you have another great sounding design. And, I saved money on the filament circuit.
Thanks Ditek
So the smaller the resistor the larger the voltage and the larger the resistor the smaller the voltage? Right?
Thanks Gabe, you have another great sounding design. And, I saved money on the filament circuit.
Thanks Ditek
Passfan,
If the resistor opens then you get no B+. But don't short the center tap to ground, because then the transformer will buzz, get too hot and may burn out.
I am so glad you are enjoying the sound of it. Now I have to try it myself... some day. 'Cept I am going to use my 1627SE's. Want that great sound right off.
So now, does the stereo separation suffer much?
Gabe
If the resistor opens then you get no B+. But don't short the center tap to ground, because then the transformer will buzz, get too hot and may burn out.
I am so glad you are enjoying the sound of it. Now I have to try it myself... some day. 'Cept I am going to use my 1627SE's. Want that great sound right off.
So now, does the stereo separation suffer much?
Gabe
Pass DIY Addict
Joined 2000
Paid Member
Awesome job!
Great work, Skippy! Thanks for sharing your experiences with us! I've been looking for a relatively simple SET amp to get my feet wet with and this just might be the one.
I was wondering if perhaps you or Gabe might be able to comment on the potential of removing the feedback loop from the 2A3 amp. Would this not be a good idea? If it is possible, what other changes would be necessary?
Thanks for your documentation efforts!
Eric
Great work, Skippy! Thanks for sharing your experiences with us! I've been looking for a relatively simple SET amp to get my feet wet with and this just might be the one.
I was wondering if perhaps you or Gabe might be able to comment on the potential of removing the feedback loop from the 2A3 amp. Would this not be a good idea? If it is possible, what other changes would be necessary?
Thanks for your documentation efforts!
Eric
Pass DIY Addict
Joined 2000
Paid Member
Seems like I looked too quickly and confused B+ with a feedback loop 
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