Hi
Just Finished Morrison Micro 2a3 . Has anyone idea how should this be biased correctly ? Mine has B+ about 420volts . With schematic given resistor values 6sl7 paralled has 195v on plate and about -25 on 2a3 185 volts over tube ,about 220 volts on 2a3 cathodes .So quite heavy currents about 90ma ?? Is this Correct ? Thanks for Info
papparazzi
Just Finished Morrison Micro 2a3 . Has anyone idea how should this be biased correctly ? Mine has B+ about 420volts . With schematic given resistor values 6sl7 paralled has 195v on plate and about -25 on 2a3 185 volts over tube ,about 220 volts on 2a3 cathodes .So quite heavy currents about 90ma ?? Is this Correct ? Thanks for Info
papparazzi
You adjust the cathode resistor on the input stage and/or adjust the cathode resistor on the output stage.
Cathode resistor on input 220 ohm ( then I got -37 on 2a3 ) maybe it must nearly ground to achieve desired -45 ? . Or liftin up to 3300 ohm on 2a3 ´cathode I achieve same -37 but I got then about 210 volts over 2a3 ??
http://i687.photobucket.com/albums/vv233/70vaughn/MorrisonMicro2A3.jpg
My output tranny is Trancender 3k Psu is 380 volts secondary with 47uf ,5h coil and 220uf
Papparazzi
The "Web Nazis" here at work limit me. So, I can't look at the schematic.
If your B+ rail is too tall, which I suspect is the case, change from CLC filtration to LCLCL filtration. You'll need an additional 10 H. choke, at the filter's I/P, that can take a beating. Don't forget a 10 Kohm bleeder resistor of an appropriate wattage rating across the 47 muF. 1st filter capacitor.
If your B+ rail is too tall, which I suspect is the case, change from CLC filtration to LCLCL filtration. You'll need an additional 10 H. choke, at the filter's I/P, that can take a beating. Don't forget a 10 Kohm bleeder resistor of an appropriate wattage rating across the 47 muF. 1st filter capacitor.
Hi .I made PSU also nearly same as orginal .Only exception tranny secondary gives 0.3A 380v and Hammond choke is 5h 150ma . With same capacitance as in Schematic it gives hum Although I get required voltage (415 v B+) i can not get 2a3 for required -45 . Trying a 1k pot instead of 680ohm 6sl7 i have to use 270 ohm reach bias -45 in 2a3 . Thats make input in 6sl7 -0.3 Not so good 🙂
Papparazi
Papparazi
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PM me if you would like me to get JC Morrison on here... he has an account, but doesn't get on much these days... might take him a few days to teleport in.
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Hi, When I built my micro 2a3 I had a similar experience to yours in that it looked as if there was way too much current flowing in the output tube. It did sound really nice though - it played tunes and the bass was surprisingly good for a small amp. I tried adjusting resistor values to get a 'traditional' 2a3 operating point and although this worked, the magic was gone and it sounded only average. There is a discussion thread online somewhere (can't find it right now) that J C Morrison has posted in. There he says that the amp is not intended to run at the usual operation points and that the higher current is intentional. At the same time, he cautions against using rare single plate 2a3s as they may not last long with this arrangement. My amp has been in daily use for over a year with a pair of Psvane tubes. They are running at 85mA and are showing no signs of distress. It still sounds great and is probably the best amp I have built to date.
I would try to keep the power supply as close to the original schematic as you can. I too tried bigger caps but found that this increased noise - I think there is some sort of cancellation going on with the noise from the 2A3 heaters.. I did have to reduce the value of the first cap slightly to get down to 400v. Because the values are low, I used motor run caps for everything - no electrolytics needed.
Good luck with your build.
I would try to keep the power supply as close to the original schematic as you can. I too tried bigger caps but found that this increased noise - I think there is some sort of cancellation going on with the noise from the 2A3 heaters.. I did have to reduce the value of the first cap slightly to get down to 400v. Because the values are low, I used motor run caps for everything - no electrolytics needed.
Good luck with your build.
Nice to notice that someone else have had same problem . Which are your critical voltages on 6sj7 and 2a3 where you ended in amp? .Amp look very nice ,thanks for pictures . There is oil caps "Made in Romania" What are these and where did you get them ?
papparazzi
That why I started this thread ,I dont know, and schematic drawing did not solve it ??
Papparazzi
Going by Martin's post above, it suggests around 200V across the 2A3, with idle current of 85mA and -25V bias - very far from the "ideal" operating conditions, and above the recommended max Pda of the tube.
Hi
So my construction was nearly right at start i thought there was something wrong at beginning when it sounded littlebit dizzy . Very special biasing My main , second PSU cap in back to 220uf .In 6sj7 I use 220uf cap in cathode ,sound better to me and I can use it as integrated . Now sound thick fullbodied maybe littlebit lazy , very detailed overall and well controlled bass . DC coupled sounds quite different whitout cap in series . Like OB speaker compared to BR lack of coloration and . Thanks for your advice ,additional tricks wellcomed also .
Papparazzi
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paralleled hi mu triodes will have huge miller cap. 404pF
i dislike whole schematic, second reason: big distortion and gain will be small here.
Morrison sounds littlebit uninspiring and distorted driving Voxatives and Pi4 speakers . Orginal circuit was in chassis Jelabs 6sl7/2a3 ,but I liked so much Fi X 2a3 sound so i had to make also similar type ,only changed construction Maybe this shishido better with 6sl7 or maybe SRPP version ??
Taken from rom this thread
http://www.diyaudio.com/forums/tubes-valves/258790-3-direct-coupled-2a3-amps.html
papparazzi
Taken from rom this thread
http://www.diyaudio.com/forums/tubes-valves/258790-3-direct-coupled-2a3-amps.html
papparazzi
2A3 Miller is high enough, to interfere with low gm, hi mu driver (less treble gain&more thd)
Reason why i have FET mu follower instead..😉
Reason why i have FET mu follower instead..😉
> Where did the -45V figure come from?
It's on the datasheet.... for a significantly different B+ and load. Of course we should expect different bias for different conditions.
> -45 60ma 250v over tube should be ideal for 2a3
For 2.5K load! (Not 3k.) And for Selling The Tube, not for seducing Golden Ears. In fact the condition is fudged to give exactly 5% THD, which was "accepted" at the time. Look at page 3 of Tung-Sol 2A3. A wide range of loads gives a lesser range of Watts and THD. (Conversely, for given load, a wide range of current is possible with lesser range of Watts and THD.)
"-45V bias" is a Serving Suggestion, for when you don't know how to use it. Like the cake-mix box tells you to add 2 eggs. The cake will be OK. You may like it better with 3 eggs or no eggs.
> quite heavy currents about 90ma ??
88mA, yes.
> how should this be biased correctly ?
Why do you care?
1) On many amps, too cool does not make "all" output power expected; while too hot burns-up the tube. But in this odd design the 2A3 can NOT idle over 16 Watts! (Assuming 400V supply and 2.5K cathode R. At 420V 17.7W is possible.) As 16W ~~= 15 Watt rating, you can not seriously cook the tube. That worst-case happens when cathode is half of B+. Go off that point *either* way, Pdiss drops.
2) For best power, approximate Tube + Load as 800 + 3,000 or 3,800 Ohms. Now figure voltage to make ~~15W in 3,800r. 239 Volts. Take that away from 400V supply, 161V on cathode. Check: 161V across 2,500r Rk is 64.4mA. 64+ma at 239V is 15 Watts.
A plot on the 2A3 curves suggests that 239V 65mA gives much wider swing and a couple dB more power output, with some rise in THD.
Your 220V instead of 161V at cathode seems high?
But wait. Taking my 239V 65mA with 161V at cathode, the grid must be swing down 86V or to 75V. Will the 2x6SL7 driver with 47K to ~~300V pull-down that far? Squinting the 6SN7 curves, I do not think it will. So where the 200V 88mA bias clips at high currents, the 239V 65mA bias clips (in the driver) at low current.
Ah, what to do?
When in doubt, try splitting the difference. 220V across tube and 76mA through it. Cathode voltage about 190V. Get there by diddling the 6SL7's cathode resistor. I would not worry over-much about what G-K bias it ends up at.
But consider also. We have two triodes both working near the limits they can swing. Large current variation, thus significant 2nd Harmonic Distortion. The 2A3 alone will approach 5%. The 6SL7 will be similar, but *opposite* phase. The two 2nd harmonic curves will tend to cancel. The combined 2nd Harm may be far less than 5% near full output. However "minor" harmonics begin to stand out. The exact "best sound" biasing would need brain-busting computation or tedious testing. And may be subject to personal opinion.
_I_ would start going closer to 70mA-80mA just cuz 2A3 is a costly tube and I want much of the Power I paid for. If my ears were better, and my listening levels lower, some other bias might be sweeter.
It's on the datasheet.... for a significantly different B+ and load. Of course we should expect different bias for different conditions.
> -45 60ma 250v over tube should be ideal for 2a3
For 2.5K load! (Not 3k.) And for Selling The Tube, not for seducing Golden Ears. In fact the condition is fudged to give exactly 5% THD, which was "accepted" at the time. Look at page 3 of Tung-Sol 2A3. A wide range of loads gives a lesser range of Watts and THD. (Conversely, for given load, a wide range of current is possible with lesser range of Watts and THD.)
"-45V bias" is a Serving Suggestion, for when you don't know how to use it. Like the cake-mix box tells you to add 2 eggs. The cake will be OK. You may like it better with 3 eggs or no eggs.
> quite heavy currents about 90ma ??
88mA, yes.
> how should this be biased correctly ?
Why do you care?
1) On many amps, too cool does not make "all" output power expected; while too hot burns-up the tube. But in this odd design the 2A3 can NOT idle over 16 Watts! (Assuming 400V supply and 2.5K cathode R. At 420V 17.7W is possible.) As 16W ~~= 15 Watt rating, you can not seriously cook the tube. That worst-case happens when cathode is half of B+. Go off that point *either* way, Pdiss drops.
2) For best power, approximate Tube + Load as 800 + 3,000 or 3,800 Ohms. Now figure voltage to make ~~15W in 3,800r. 239 Volts. Take that away from 400V supply, 161V on cathode. Check: 161V across 2,500r Rk is 64.4mA. 64+ma at 239V is 15 Watts.
A plot on the 2A3 curves suggests that 239V 65mA gives much wider swing and a couple dB more power output, with some rise in THD.
Your 220V instead of 161V at cathode seems high?
But wait. Taking my 239V 65mA with 161V at cathode, the grid must be swing down 86V or to 75V. Will the 2x6SL7 driver with 47K to ~~300V pull-down that far? Squinting the 6SN7 curves, I do not think it will. So where the 200V 88mA bias clips at high currents, the 239V 65mA bias clips (in the driver) at low current.
Ah, what to do?
When in doubt, try splitting the difference. 220V across tube and 76mA through it. Cathode voltage about 190V. Get there by diddling the 6SL7's cathode resistor. I would not worry over-much about what G-K bias it ends up at.
But consider also. We have two triodes both working near the limits they can swing. Large current variation, thus significant 2nd Harmonic Distortion. The 2A3 alone will approach 5%. The 6SL7 will be similar, but *opposite* phase. The two 2nd harmonic curves will tend to cancel. The combined 2nd Harm may be far less than 5% near full output. However "minor" harmonics begin to stand out. The exact "best sound" biasing would need brain-busting computation or tedious testing. And may be subject to personal opinion.
_I_ would start going closer to 70mA-80mA just cuz 2A3 is a costly tube and I want much of the Power I paid for. If my ears were better, and my listening levels lower, some other bias might be sweeter.
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