Hi,
I finished putting together my second Tubelab build (SE) and I was just going through the checkout procedure, step 1, no tubes, measuring B+, B- and 300B filament voltage.
No B+: Expected without a rectifier
Filament: 4.98V, good
Negative Bias voltage: -311VDC - this seems awfully high.
Hammond 276X power-tranny 640VCT
I know it's late, but I think I did everything according to the instructions.
What am I missing? According to the schematic, I should have around -150V, no?
Help, please!
I finished putting together my second Tubelab build (SE) and I was just going through the checkout procedure, step 1, no tubes, measuring B+, B- and 300B filament voltage.
No B+: Expected without a rectifier
Filament: 4.98V, good
Negative Bias voltage: -311VDC - this seems awfully high.
Hammond 276X power-tranny 640VCT
I know it's late, but I think I did everything according to the instructions.
What am I missing? According to the schematic, I should have around -150V, no?
Help, please!
Hmmm, the schematic lists -150V. I'd expect a little higher voltage without tubes in, but didn't quite expect it to be double.
I have to adjust my PS caps first, because B+ climbed up to 455V, which is probably on the high side for my 300B tubes.
Thanks for your input!
OK, I think it's time for the tube-noob to ask for help.
I have reduced the value of C4 to 4.7uF and still see B+ rise past 455V with just the rectifier tube in. I don't remember seeing the same effect when checking out my Simple SE.
The board is populated with the recommended values for 300B tubes. The only thing different I see is my use of the Hammond 193J choke. It only has about 75Ohms DC resistance; could that be the issue?
I had removed the choke earier and replaced it with the 150 Ohm resistor, but that didn't make a material change.
Can I expect B+ to drop by 60+ Volts once the tubes are in? I want to get at about 390V during normal operation.
Who is using 300Bs with his SE and would share their choices for PT, C4 and choke? What is the B+ voltage you are seeing?
I am worried for the life of my electrolytics (all 450V) and power tubes...
Penny for your thoughts...
The voltages on the schematic are for 45 tubes. Remember, that is what the amp was originally designed for.
Caps can handle some voltage overload for a short period of time. They are rated for this.
Since you have almost no load on the power supply, the B+ voltage will approach the theoretical maximum of the AC secondary voltage * 1.414. For you that is 320 * 1.4 = 448V. There is some load there from the FETs and input tubes, but Hammond iron tends to run hot anyway. Check your secondary voltage, but it sounds like it's working as it should.
Caps can handle some voltage overload for a short period of time. They are rated for this.
Since you have almost no load on the power supply, the B+ voltage will approach the theoretical maximum of the AC secondary voltage * 1.414. For you that is 320 * 1.4 = 448V. There is some load there from the FETs and input tubes, but Hammond iron tends to run hot anyway. Check your secondary voltage, but it sounds like it's working as it should.
The voltage should come down a little when the 5842's are installed. If you can set the plate voltage on them and you can adjust the grid voltage on the 300B's (without the 300B's installed) from -20 or so to at least -75 volts you are probably OK. Set the pots to the setting that gives tha most negative voltage on the 300B grids. Turn off the amp and install the 300B's. Power up the amp with a voltmeter connected to each 10 ohm plate resistor and after a minute or so warm up turn one pot slowly and watch the meters. One meter will begin to show voltage and the B+ will come down. Set the meter for about 0.5 volts (50 ma). Then turn the pot for the other tube until it also reads about 0.5 volts. The B+ will come down some more which will affect the reading on the first tube. Continue itteration until both tubes are the same (or within 2 or 3 mA) and the current readings are near 50 mA.
Leave the meters attached and play the amp for a few hours checking occasionally for signs that the current is changing or creeping upward (turn the volume down for an accurate reading). Some tubes may wander for a while but eventually settle down (especially old stock). Some tubes will tend to runaway and these should be avoided.
If everything is stable you can turn the current up to the desired value. The plate voltage will drop slightly as the current is increased and one tube will affect the other. If you have 400 volts of B+ I would stay under 90 mA (0.9V). I run my Chinese tubes at 80 mA with 400 volts and 90 to 95 with 360 volts. The optimum current depends on the tubes, the OPT, your speakers actual impedance and the type of music you play.
Leave the meters attached and play the amp for a few hours checking occasionally for signs that the current is changing or creeping upward (turn the volume down for an accurate reading). Some tubes may wander for a while but eventually settle down (especially old stock). Some tubes will tend to runaway and these should be avoided.
If everything is stable you can turn the current up to the desired value. The plate voltage will drop slightly as the current is increased and one tube will affect the other. If you have 400 volts of B+ I would stay under 90 mA (0.9V). I run my Chinese tubes at 80 mA with 400 volts and 90 to 95 with 360 volts. The optimum current depends on the tubes, the OPT, your speakers actual impedance and the type of music you play.
Thanks much, Russ & George. I did install the driver tubes yesterday and was able to adjust the 300B grid voltage to ~-115V and also set the 5842 plates to 175V. Both had little effect on B+; I am still reading around 445V. Changing C4 to 4.7uF also didn't affect B+ much.
I ran out of tinker time, because my son needed me to adjust the capacitance of his diaper
but I will continue the checkout with the 300Bs hopefully some time this week. If I still don't see B+ drop to about 400V, I may see if my other PT runs a little 'cooler'.
Maybe I'll just convert to 45s, since I have the stuff needed lying about and also picked up some tubes recently.
Anyway, I'll report back with results.
Cheers and thanks again.
Stefan
PS. George, I sure hope Irene decides to stay offshore and leave you alone. Good luck to you!
I ran out of tinker time, because my son needed me to adjust the capacitance of his diaper
but I will continue the checkout with the 300Bs hopefully some time this week. If I still don't see B+ drop to about 400V, I may see if my other PT runs a little 'cooler'. Maybe I'll just convert to 45s, since I have the stuff needed lying about and also picked up some tubes recently.
Anyway, I'll report back with results.
Cheers and thanks again.
Stefan
PS. George, I sure hope Irene decides to stay offshore and leave you alone. Good luck to you!
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I got a little more time on the amp today and continued the adjustments. Everything worked out fine, rechecked 5842 plate voltage and set 300B bias to 50ma. I still have about 430V B+, but I connected my signal source via a 100K pot anyway.
There should have been some music, but there was.....silence. No hum, though...
I can hear the music very faintly when I put my ear to the speaker.
Swapped out input tubes for another pair; same thing.
All heater voltages are where they should be, too; and all tube filaments glow.
I am going to give the board a complete review to make sure I didn't screw anything up when bottom-mounting CCS ICs and MOSFETs, although I checked that at least a gazillion times before soldering them in. Nothing is impossible.
Anyway, just wanted to give status.
Time for some shut-eye.
Stefan
There should have been some music, but there was.....silence. No hum, though...
I can hear the music very faintly when I put my ear to the speaker.
Swapped out input tubes for another pair; same thing.
All heater voltages are where they should be, too; and all tube filaments glow.
I am going to give the board a complete review to make sure I didn't screw anything up when bottom-mounting CCS ICs and MOSFETs, although I checked that at least a gazillion times before soldering them in. Nothing is impossible.
Anyway, just wanted to give status.
Time for some shut-eye.
Stefan
Me again...
Amazing what a good night's sleep can do...
Woke up this morning and just looked at my breadboard setup while sipping coffee when I saw that I didn't have my speakers grounded...
Had to run to work, but just addressed this little oversight and, voila, there is music! But...
My B+ is still at around 440V. The 300B look healthy, no glowing other than that of the filament.
Now I have to figure out whether to
1.) find another PT with a more appropriate voltage, or
2.) try a different rectifier tube (any drop-in replacement suggestions?), or
3.) scratch the 300B for now and convert board and iron for 45s
Besides that, it doesn't sound very 'clean'. I don't really know how to describe it other than a mix between foggy and almost scratchy; kind of like a speaker that has a broken cone foam (mine don't...).
As of now, NO comparison to my Simple SE build sound-wise.
Amazing what a good night's sleep can do...
Woke up this morning and just looked at my breadboard setup while sipping coffee when I saw that I didn't have my speakers grounded...
Had to run to work, but just addressed this little oversight and, voila, there is music! But...
My B+ is still at around 440V. The 300B look healthy, no glowing other than that of the filament.
Now I have to figure out whether to
1.) find another PT with a more appropriate voltage, or
2.) try a different rectifier tube (any drop-in replacement suggestions?), or
3.) scratch the 300B for now and convert board and iron for 45s
Besides that, it doesn't sound very 'clean'. I don't really know how to describe it other than a mix between foggy and almost scratchy; kind of like a speaker that has a broken cone foam (mine don't...).
As of now, NO comparison to my Simple SE build sound-wise.
2) Of the drop ins, a 5W4 has the most voltage drop - probably around 60-65 volts, but is only good for 100 ma or so; NOS 5Y3 will drop 55-60 volts and is good for 125 ma; 5R4 will drop a bit less than 5Y3 and you probably don't need to worry about its current limitations. Be advised that the Sovtek 5Y3 is not a 5Y3 - it's more like a reduced current 5V4.
A better way might be to remove the first capacitor in the power supply and use choke input - an LC power supply instead of CLC. That's how I ran my TSE.
Win W5JAG
A better way might be to remove the first capacitor in the power supply and use choke input - an LC power supply instead of CLC. That's how I ran my TSE.
Win W5JAG
I already went down to 4.7uF for C4 without seeing much effect.
Yes, I use a Hammond 193J choke. I haven't yet measured PT output AC voltage and haven't tried the 125V primary yet. I'll do both as soon as I get the time, which probably won't be before Friday.
As always, thanks for your helpful suggestions. I still have a lot to learn...
Russ, four out of three Germans are bad at math and theory... I am two of them.
No, I haven't used any simulation software recently. A little bit of Spice in school a long time ago. That may be a good way to learn, too, if that stuff runs on Mac or in a VM. I'll definitely try it.
Having said that, I couldn't help but try the 125V primary tap after my post last night and I am down to 395V. The distortion seems to be gone, too.
More testing and simulations to come.
I just downloaded PSUD2 and played around with it for a bit, plugging in my transformer and CLC components. The simulation seems to match reality pretty closely. This is going to be a really helpful [learning] tool for me, thanks very much for pointing it out, Russ!
And thanks to the generous soul that spent time designing & implementing this tool and making it available, wherever you are!
Can't wait to get home today to spend some time listening to the amp and play with the bias a little bit more.
THANK YOU for all your help and guidance, the newbie appreciates it very much!
Stefan
And thanks to the generous soul that spent time designing & implementing this tool and making it available, wherever you are!
Can't wait to get home today to spend some time listening to the amp and play with the bias a little bit more.
THANK YOU for all your help and guidance, the newbie appreciates it very much!
Stefan
All checks out, but distortion at higher volume
So, after a couple of weird events (R3 silently died, one of the two 5824s with it), everything checks out. B+ is around 390V, bias set to ~70ma for now and pretty stable.
Sound is good, but not as good as the Simple SE built earlier this year. Maybe I a just got used to tube sound...
But, when turning up the volume while feeding a sine signal, at about 3/4 of total volume, the output shows a distortion. It doesn't look like clipping, but when playing music, it doesn't sound good at all.
Also, I noticed that the current across one of the 300B grid resistors seems to go up quite a bit at higher volumes (>100ma), which seems strange and can't be good for the tube.
What does an output signal form like this indicate, is this to be expected?
So, after a couple of weird events (R3 silently died, one of the two 5824s with it), everything checks out. B+ is around 390V, bias set to ~70ma for now and pretty stable.
Sound is good, but not as good as the Simple SE built earlier this year. Maybe I a just got used to tube sound...
But, when turning up the volume while feeding a sine signal, at about 3/4 of total volume, the output shows a distortion. It doesn't look like clipping, but when playing music, it doesn't sound good at all.
Also, I noticed that the current across one of the 300B grid resistors seems to go up quite a bit at higher volumes (>100ma), which seems strange and can't be good for the tube.
What does an output signal form like this indicate, is this to be expected?
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