FireFly comes from all the LEDs in it.
One of the wife's powered speakers on her computer is on the fritz, and after seeing some of the other small SE designs (Fleathingie) I thought I'd try something similar as replacements.
I've got some FE103E drivers I'm building accompanying speakers from.
So far the prototype is sounding pretty nice. Volume is adequate for sound reproduction off a computer if you are only listening to occasional CDs etc. In this case, low output power is not going to be a problem.
A couple of minor issues I've encountered so far is the number of LEDs needed to set the bias point near 30mA. I'm purposely limiting the idle current (yes, I know it will increase distortion) so I can use a transformer I already have on hand (Allied 6K49VG).
The transformer is only rated for 50mA, but that is with a capacitive input filter. If I go to a inductor input filter I can increase this 40% to 70mA which will do for this amp. I've already run it for six hours at 70mA load (Plus two op tubes and the driver tube for filament loads) without any significant temp rise (only 23degree F) of the transformer so I think this will be fine.
I'm wondering how well simple shunt screen regulation would work to set the bias point.
So far I've not connected the GNFB and it sounds reasonable.
One of the wife's powered speakers on her computer is on the fritz, and after seeing some of the other small SE designs (Fleathingie) I thought I'd try something similar as replacements.
I've got some FE103E drivers I'm building accompanying speakers from.
So far the prototype is sounding pretty nice. Volume is adequate for sound reproduction off a computer if you are only listening to occasional CDs etc. In this case, low output power is not going to be a problem.
A couple of minor issues I've encountered so far is the number of LEDs needed to set the bias point near 30mA. I'm purposely limiting the idle current (yes, I know it will increase distortion) so I can use a transformer I already have on hand (Allied 6K49VG).
The transformer is only rated for 50mA, but that is with a capacitive input filter. If I go to a inductor input filter I can increase this 40% to 70mA which will do for this amp. I've already run it for six hours at 70mA load (Plus two op tubes and the driver tube for filament loads) without any significant temp rise (only 23degree F) of the transformer so I think this will be fine.
I'm wondering how well simple shunt screen regulation would work to set the bias point.
An externally hosted image should be here but it was not working when we last tested it.
So far I've not connected the GNFB and it sounds reasonable.
Single ended Red Light District? 
I don't know what you are running it at, but the 6N23P seems to be a plate current hog. How about 6N2P or 6N3P at about 3mA? That might lift some of the burden off the PT. A little more heater current though. I've been pleased with the 6N3P-EV as a SE driver on similar B+ as yours, with an IR LED for cathode bias (not regulator fed though).
I don't know what you are running it at, but the 6N23P seems to be a plate current hog. How about 6N2P or 6N3P at about 3mA? That might lift some of the burden off the PT. A little more heater current though. I've been pleased with the 6N3P-EV as a SE driver on similar B+ as yours, with an IR LED for cathode bias (not regulator fed though).
I missed that. It is currently running a 6N1P as the input tube. I started the schematic from the previous PP amp and must have used the 6N23P phase splitter as the input tube rather than the 6N1P input tube. 6N23P does not sound good at 3ma.
I will probably try a an IXCP10M45 current source in place of the present plate current source to see if I can hear the difference.
I will probably try a an IXCP10M45 current source in place of the present plate current source to see if I can hear the difference.
adjustable Regulated Screen for plate current adjustment
I'm finding some of the same issues being discussed in the RLD thread about biasing, with the FireFly.
The output is difficult to get biased without individually tweaking the number of LEDs and series resistor values for each tube (unmatched tubes for the two channels).
Instead of adjusting the bias by adjusting the cathode voltage, why not run fixed cathode bias and adjust the screen voltage to set the bias current?
Use the same voltage on the cathode via LEDs, CVS, or whatever, then use a simple pass regulator to control the screen.
I don't see any degradation in distortion by using this, and the sound is as good if not better than it was with just LED cathode bias.
What is the disadvantage of adjusting the bias this way, other than the few additional parts?
I'm finding some of the same issues being discussed in the RLD thread about biasing, with the FireFly.
The output is difficult to get biased without individually tweaking the number of LEDs and series resistor values for each tube (unmatched tubes for the two channels).
Instead of adjusting the bias by adjusting the cathode voltage, why not run fixed cathode bias and adjust the screen voltage to set the bias current?
Use the same voltage on the cathode via LEDs, CVS, or whatever, then use a simple pass regulator to control the screen.
An externally hosted image should be here but it was not working when we last tested it.
I don't see any degradation in distortion by using this, and the sound is as good if not better than it was with just LED cathode bias.
What is the disadvantage of adjusting the bias this way, other than the few additional parts?
I disconnected the Allied 6K49VG transformer / power supply and connected the HP6448B to try it out as I hadn't used it yet.
Power out just kept increasing until I hit Vka = 354V at Ik=61mA and Vg2=275V.
I could see a very slight red fin on the 6P1P-EV with the lights out (14 Cathode LEDs were providing lots of light).
Output voltage into a 8 Ohm resistor was 8vrms with noticeable distortion but no clipping.
8W SE!
I tried upping the voltage further, but it looks like the little AES P-T31 is saturating.
I'll have to wait for my Edcor opts to arrive before I can try to push it further.
Mean time I guess I need to get learn how to measure distortion to get a better idea of how the screen regulator is effecting operation.
When I reduced plate current it looked like distortion was more related to plate current (compression of the curve at lower current) than actually the screen voltage decrease itself.
So my conclusion is that reducing screen voltage does not directly increase distortion, but rather that the associated decrease in bias point is responsible for increased distortion. Therefore as long as I keep the plate current up I can use screen regulation to set my bias point without increasing distortion.
Power out just kept increasing until I hit Vka = 354V at Ik=61mA and Vg2=275V.
I could see a very slight red fin on the 6P1P-EV with the lights out (14 Cathode LEDs were providing lots of light).
Output voltage into a 8 Ohm resistor was 8vrms with noticeable distortion but no clipping.
8W SE!
I tried upping the voltage further, but it looks like the little AES P-T31 is saturating.
I'll have to wait for my Edcor opts to arrive before I can try to push it further.
Mean time I guess I need to get learn how to measure distortion to get a better idea of how the screen regulator is effecting operation.
When I reduced plate current it looked like distortion was more related to plate current (compression of the curve at lower current) than actually the screen voltage decrease itself.
So my conclusion is that reducing screen voltage does not directly increase distortion, but rather that the associated decrease in bias point is responsible for increased distortion. Therefore as long as I keep the plate current up I can use screen regulation to set my bias point without increasing distortion.
fleathingie ?????
Gimp -- interesting thread per usual -- but where is this fleathingie URL or did I get caught being dumb again. thx either way.
Gimp -- interesting thread per usual -- but where is this fleathingie URL or did I get caught being dumb again
. thx either way.
Fleathingie was a 50C5 SE amp by Mosquito. Type-oed it, Flea-Thingy
http://www.diyaudio.com/forums/tubes-valves/161193-flea-thingy-sounds-good.html
I actually have some 35C5 and 50C5s that I was thinking of building an amp with, but opted for the 6P1P instead as I didn't want to mess with odd filament voltages.
And I'm trying to stick with the 6P1P just so I'm learning on the same tube all the time. At least for now.
I did one other test with the screen regulator. I set the bias at 5 LEDs plus a 33 ohm resistor for about 9.5V, then set the B+ for a plate to cathode voltage of 250V. This resulted in a screen to cathode voltage of 253 (max adjustment). I got 55mA of plate current.
I then cranked the screen down till the plate current dropped to 25mA (too low for good audio production). It was down to 200V, so a 20% change in screen voltage resulted in nearly a 55% decrease in plate current. I need to go back and look at the screen drive plot of the 6P1P to see if I can better understand this relationship.
It looks like it won't take much shift in screen voltage to compensate for non-matched tubes in PP amps if this is used rather than using an adjustable cathode resistor. The MPSA42 is cheap, and the circuit simple so it looks to me like a good option.
I think the 250V limit of the 6P1P is advantageous in that it allows for the use of lower voltage parts (Cheaper) compared to say 6L6, EL34, KT88, etc. We don't get near as much power, but I'm still not convinced I need a lot more.
I think the determining factor will end up being distortion vs normal operating levels. The bigger amps will produce less distortion at the same output level as the smaller amps. The small amps are just working harder.
http://www.diyaudio.com/forums/tubes-valves/161193-flea-thingy-sounds-good.html
I actually have some 35C5 and 50C5s that I was thinking of building an amp with, but opted for the 6P1P instead as I didn't want to mess with odd filament voltages.
And I'm trying to stick with the 6P1P just so I'm learning on the same tube all the time. At least for now.
I did one other test with the screen regulator. I set the bias at 5 LEDs plus a 33 ohm resistor for about 9.5V, then set the B+ for a plate to cathode voltage of 250V. This resulted in a screen to cathode voltage of 253 (max adjustment). I got 55mA of plate current.
I then cranked the screen down till the plate current dropped to 25mA (too low for good audio production). It was down to 200V, so a 20% change in screen voltage resulted in nearly a 55% decrease in plate current. I need to go back and look at the screen drive plot of the 6P1P to see if I can better understand this relationship.
It looks like it won't take much shift in screen voltage to compensate for non-matched tubes in PP amps if this is used rather than using an adjustable cathode resistor. The MPSA42 is cheap, and the circuit simple so it looks to me like a good option.
I think the 250V limit of the 6P1P is advantageous in that it allows for the use of lower voltage parts (Cheaper) compared to say 6L6, EL34, KT88, etc. We don't get near as much power, but I'm still not convinced I need a lot more.
I think the determining factor will end up being distortion vs normal operating levels. The bigger amps will produce less distortion at the same output level as the smaller amps. The small amps are just working harder.
Did you change your name to/from George
I knew the 6P1P-EVs were tough, but that's over 21W Pd by my calculation.
What brand are your tubes? The ones I have here are SED I think (have a C in a circle with wings).
Push a Chinese 6P1 past 12W and be prepared for trouble.
Parallel SE with MOSFET follower drive might provide interesting results. A solid 10W in PSE at low distortion would be a great result for such inexpensive tubes.
Not really sure it matters, but there already is a DIY tube project called FireFly. It is a tiny guitar amp using 12AU7s in a self-split p-p topology, designed by a guy named Doug Hammond. firefly amp - Google Search
Yours is an interesting little amp too
Yours is an interesting little amp too
I'm using wing 'C' (Svetlana) 6P1P-EV tubes. I think this one was marked as 1974 production date, but I'm not sure. I can check to see. I've got a smatttering fo 1973 thorugh 1984 or so.
Thanks Metallo, hopefully Doug does not mind if I use the name as well. The AX84 site shows a copyright on the amp design, but not the name so hopefully he won't. Besides, this is not a commercial product, just another amp for fun.
I could change the name to "Fire-Fly" or maby "LightningBug", but the name just doesn't seem to have the same ring.
rotaspec, I only ran it about 10 minutes at that power level and I doubt it would last long with that level of abuse. I'll get back to abusing it again in a couple of months when the OPTs come in.
Mean time I'm going to try to figure out if I can use a computer sound card to drive the input and measure the THD. I'm hoping the THD of hte soundcard is << than the amp, so I should be able to get reliable measurements.
I'll be using a Soundblaster Audigy since my M-Audiophile 192 is dead and I still haven't received a response from the company about a RMA number to send it back for repare.
Thanks Metallo, hopefully Doug does not mind if I use the name as well. The AX84 site shows a copyright on the amp design, but not the name so hopefully he won't. Besides, this is not a commercial product, just another amp for fun.
I could change the name to "Fire-Fly" or maby "LightningBug", but the name just doesn't seem to have the same ring.
rotaspec, I only ran it about 10 minutes at that power level and I doubt it would last long with that level of abuse. I'll get back to abusing it again in a couple of months when the OPTs come in.
Mean time I'm going to try to figure out if I can use a computer sound card to drive the input and measure the THD. I'm hoping the THD of hte soundcard is << than the amp, so I should be able to get reliable measurements.
I'll be using a Soundblaster Audigy since my M-Audiophile 192 is dead and I still haven't received a response from the company about a RMA number to send it back for repare.
I got around to starting to take distortion measurements this weekend. I say "starting to" as our son got home from Germany Friday night, after being away for nearly 5 years in the Air Force.
So, (1) I didn't have much time to play with the amp, and (2) I'm still reading the manual on the HP333A distortion analyzer so readings are going slow and the first three attempts were way off.
I did run a couple of plots on AudioTester V2.2, and compared the calculated distortion with the HP333A.
I’m running with -6dB GFB, and measured the distortion with respect to plate current using the screen regulator to control plate current.
Input drive was adjusted for 800mVrms output into 8 ohms (80mW) in part because I didn’t’ want to overdrive the sound card I’m running with AudioTester V2.2.
Distortion goes up rapidly above 0.125W.
All dB measurements are minus, but the –sign is left off because it is used as a space setter for the table.
…………………….......25mA----35mA-----45mA---50mA
Screen Voltage….181-------205--------226------236
Vgk………………......10.42-----10.42-----10.56----10.59
THD-HP333A……..1.14%----0.92%---0.42%----0.32%
THD-Calculated….1.126%--0.71%----0.36%----0.287%
Fund……………….....3----------3----------3----------3
2nd Harmonic……..42--------46--------52---------54
3rd Harmonic……..63--------65--------67---------68
4th Harmonic……..95--------90--------92---------90
5th Harmonic………82--------100------92---------95
6th Harmonic………91--------95--------105-------98
7th Harmonic………85--------82--------84--------86
8th Harmonic………104------86---------92--------94
9th Harmonic………96--------91--------86--------84
10th Harmonic…….95-------93---------93-------95
Not really surprising results. Distortion decreases with cathode current. The more current the higher we are on the bias curve and the farther away from the compressed lower traces for higher negative bias conditions. However it looks like most odd harmonics are suppressed more with higher bias levels as well. This is more noticeable when watching the FFT while adjusting the screen voltage (and hence the plate current).
More current is better, but in particular below 35mA distortion really starts to go up.
So, (1) I didn't have much time to play with the amp, and (2) I'm still reading the manual on the HP333A distortion analyzer so readings are going slow and the first three attempts were way off.
I did run a couple of plots on AudioTester V2.2, and compared the calculated distortion with the HP333A.
I’m running with -6dB GFB, and measured the distortion with respect to plate current using the screen regulator to control plate current.
Input drive was adjusted for 800mVrms output into 8 ohms (80mW) in part because I didn’t’ want to overdrive the sound card I’m running with AudioTester V2.2.
Distortion goes up rapidly above 0.125W.
All dB measurements are minus, but the –sign is left off because it is used as a space setter for the table.
…………………….......25mA----35mA-----45mA---50mA
Screen Voltage….181-------205--------226------236
Vgk………………......10.42-----10.42-----10.56----10.59
THD-HP333A……..1.14%----0.92%---0.42%----0.32%
THD-Calculated….1.126%--0.71%----0.36%----0.287%
Fund……………….....3----------3----------3----------3
2nd Harmonic……..42--------46--------52---------54
3rd Harmonic……..63--------65--------67---------68
4th Harmonic……..95--------90--------92---------90
5th Harmonic………82--------100------92---------95
6th Harmonic………91--------95--------105-------98
7th Harmonic………85--------82--------84--------86
8th Harmonic………104------86---------92--------94
9th Harmonic………96--------91--------86--------84
10th Harmonic…….95-------93---------93-------95
Not really surprising results. Distortion decreases with cathode current. The more current the higher we are on the bias curve and the farther away from the compressed lower traces for higher negative bias conditions. However it looks like most odd harmonics are suppressed more with higher bias levels as well. This is more noticeable when watching the FFT while adjusting the screen voltage (and hence the plate current).
More current is better, but in particular below 35mA distortion really starts to go up.
I took a quick look at distortion on the output of the triode driver this morning before comming to work. With the cathode set for 35mA as previous, the screen drive shows 0.5% thd on the HP333A. I was expecting most of the distortion to have come from the output, but it looks like I need to work on the driver as well.
One thing I'm going to try on the output tube is to replace the parallel strings of LEDs with a PNP and a single string of LEDs from base to collector, then a 470 Ohm resistor from base to emitter. This configures the transistor as an emitter follower with the LEDs as the voltage reference. It should yield a lower impedance than just the LEDs with the balancing resistors in each string. I may try a bypass cap on the LEDs as well.
One thing I'm going to try on the output tube is to replace the parallel strings of LEDs with a PNP and a single string of LEDs from base to collector, then a 470 Ohm resistor from base to emitter. This configures the transistor as an emitter follower with the LEDs as the voltage reference. It should yield a lower impedance than just the LEDs with the balancing resistors in each string. I may try a bypass cap on the LEDs as well.
I tried changing the bias on the input 6N1P and it made little difference in the distortion. I suspect I'm at the limits of my power supply to swing through the plate curves. Lower distortion will require a higher B+ which will allow operation above the compressed curves.
Tonight I took several distortion measurements of the amp with different levels of NFB at full power (2.88W, 4.8VRMS 8 ohms just before clipping).
FB_______None__-6.75___-7.85___-8.9____–26.8
THD_____12%____6%____5.9%___5.4%___4.6%
And comparing bias schemes:
LED/EF___Resistor___Resistor/Bypass___LEDs
5.8%_____30%______5.9%____________5.8%
LED/EF - 5 LG M67K with 2N3638A Emitter Follower and 390R bias to LEDs.
Resistor - 266 Ohms
Resistor/Bypass - 266Ohms with 470uF bypass.
LEDs - three banks of cheap red leds, 5 per string, with 2R7 per string.
Feedback up to 6dB really makes a big difference in distortion. Beyond 6% it has a lot less measurable effect.
Just an unbypassed cathode resistor really produces distortion for some reason. I even dropped the input drive slightly to see if It was from clipping and it made little difference.
The remaining three bias mechanisms all had about the same level of distortion.
Next I think I'll set the feedback to -6dB and try listening tests with all four cathode bias schemes.
Tonight I took several distortion measurements of the amp with different levels of NFB at full power (2.88W, 4.8VRMS 8 ohms just before clipping).
FB_______None__-6.75___-7.85___-8.9____–26.8
THD_____12%____6%____5.9%___5.4%___4.6%
And comparing bias schemes:
LED/EF___Resistor___Resistor/Bypass___LEDs
5.8%_____30%______5.9%____________5.8%
LED/EF - 5 LG M67K with 2N3638A Emitter Follower and 390R bias to LEDs.
Resistor - 266 Ohms
Resistor/Bypass - 266Ohms with 470uF bypass.
LEDs - three banks of cheap red leds, 5 per string, with 2R7 per string.
Feedback up to 6dB really makes a big difference in distortion. Beyond 6% it has a lot less measurable effect.
Just an unbypassed cathode resistor really produces distortion for some reason. I even dropped the input drive slightly to see if It was from clipping and it made little difference.
The remaining three bias mechanisms all had about the same level of distortion.
Next I think I'll set the feedback to -6dB and try listening tests with all four cathode bias schemes.
Current Schematic:
Screen adjusted to 220V with:
Vak = 256V
Vgk=-10.18V
to achieve Ik=35mA
V3 was placed in the grid circuit of the output tube to make the screen and grid voltages come out right for the plate current at 35mA. Something wrong with the 6P1P model no doubt.
An externally hosted image should be here but it was not working when we last tested it.
Screen adjusted to 220V with:
Vak = 256V
Vgk=-10.18V
to achieve Ik=35mA
V3 was placed in the grid circuit of the output tube to make the screen and grid voltages come out right for the plate current at 35mA. Something wrong with the 6P1P model no doubt.
I read somewhere that an unbypassed cathode resistor means a dramatic change in the internal resistance of the tube compared to bypassed (but I can't remember which way it goes).
I have played with the bias on 6N1P until I am blue in the face, and I haven't found a plate current or load that I am completely happy with. Even without any real test equipment, I can tell by listening that there is some distortion there. A 6N3P seems to sound better in the same circuit (but it is a complete socket rewire job, so comparisons take some time). At least the mu is similar (36 vs 33), so NFB changes are probably not required. I would love to see spectra of the two of them under similar operating conditions.
How do I decipher the date code on these?
Mine have 8908 just below the 6N1N-EB (don't have cyrillic letters on my keyboard so that's as close as I can get).
They also have a rectangle with some cyrillic letters and 0490 .r, and also a diamond with 25 in it.
Is that 8th week 1989 production, and inspection of 4th week 1990?
They were not boxed specimens so I have no other data.
Gary
"Effect of Cathode Capacitor on P-P Output Stage" by Robert M. Mitchell shows it worse with the bypass cap than without if no GNFB is applied. With GNFB there is a cross over point where clipping is approached, beyond this distortion is worse for a bypassed cathode resistor.
I thought I had another article with similar testing but I can't find it.
Unfortuantly I don'at have any 6N3Ps or I'd do a comparison with the spectrem analizer and distortion analizer. I've got 6N2P and 6N6Ps if you would like any data I can get from them.
As far as date codes go, the tubes I have only have one 4 digit number so it is pretty clear which two are year and which two are week/month/lot or whatever. That is consistent with what I've seen on ebay at least.
I would think the numbers below the tube designation (8908) would be the date code and would represent 8th week of 1989.
I've been thinking about the distortion from the 6N1P, and wonder if it might be possible to reduce distortion by shifting the bias point but keeping the current source set point where it is. I tried changing the current from the current source but did not change the cathode bias.
I thought I had another article with similar testing but I can't find it.
Unfortuantly I don'at have any 6N3Ps or I'd do a comparison with the spectrem analizer and distortion analizer. I've got 6N2P and 6N6Ps if you would like any data I can get from them.
As far as date codes go, the tubes I have only have one 4 digit number so it is pretty clear which two are year and which two are week/month/lot or whatever. That is consistent with what I've seen on ebay at least.
I would think the numbers below the tube designation (8908) would be the date code and would represent 8th week of 1989.
I've been thinking about the distortion from the 6N1P, and wonder if it might be possible to reduce distortion by shifting the bias point but keeping the current source set point where it is. I tried changing the current from the current source but did not change the cathode bias.
I checked some date codes tonight. They are all below the tube designation which is below the wing C symbol. Typical date codes were 0286, 0985, XII 80, 0985.
I optimized the distortion of the input stage and got it down to half of what it was originally(near 1%). I'm loading the anode with a current source and the Cathode has an LED and 100R for the GNFB return point. The final numbers for the conditions for the 6N1P-EV were:
19.4Vp-p at the plate (to drive the output to 4.8Vrms into an 8 ohm load)
Va-k = 124V
Ik= 5.35mA
0.42%thd
Too bad it made very little change in the output distortion which is dominated by bias current effects at 35mA. I expect that in a PP amp or an amp with less distortion in the output stage, it would make an appreciable difference.
Distortion for the amp with about 7dB of GNFB stands at 5.2% at onset of clipping (4.8Vrms into 8 ohms).
At 2.8Vrms out (1W) it drops to 3%
At 1Vrms (0.125W) it drops to 0.95%
Soundwise, the wife is quite pleased with it, and wanted to know if I could build one and a set of speakers for her car!!!
Uh, sorry but no.
I optimized the distortion of the input stage and got it down to half of what it was originally(near 1%). I'm loading the anode with a current source and the Cathode has an LED and 100R for the GNFB return point. The final numbers for the conditions for the 6N1P-EV were:
19.4Vp-p at the plate (to drive the output to 4.8Vrms into an 8 ohm load)
Va-k = 124V
Ik= 5.35mA
0.42%thd
Too bad it made very little change in the output distortion which is dominated by bias current effects at 35mA. I expect that in a PP amp or an amp with less distortion in the output stage, it would make an appreciable difference.
Distortion for the amp with about 7dB of GNFB stands at 5.2% at onset of clipping (4.8Vrms into 8 ohms).
At 2.8Vrms out (1W) it drops to 3%
At 1Vrms (0.125W) it drops to 0.95%
Soundwise, the wife is quite pleased with it, and wanted to know if I could build one and a set of speakers for her car!!!
Uh, sorry but no.
Production dates on Soviet tubes are always month & year, sometimes reversed and sometimes with Roman number for month so 0985, 8509 and IX 85 would all be September 1985. Any stamps (OTK 'diamond', rectangle with numbers, etc) are QA (and/or military) inspection marks and can also contain dates but usually don't.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.