Tubelab SE checkout shows -300V Bias voltage? - diyAudio
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Old 22nd August 2011, 06:38 AM   #1
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Default Tubelab SE checkout shows -300V Bias voltage?

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!
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Old 22nd August 2011, 11:36 AM   #2
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Well,

I have the same, and it's not surprising, as the step down of the voltage is done with a 10K resistor. The minimum current that will flow will bring it down.

But my built is not very standard.

D.
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Old 22nd August 2011, 04:11 PM   #3
Green77 is offline Green77  Sweden
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I have -286 v out at C7 + and it's working as expected.. around -80V at the tubes if i remember right.. (300B)
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Old 22nd August 2011, 05:10 PM   #4
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Quote:
Originally Posted by Green77 View Post
I have -286 v out at C7 + and it's working as expected.. around -80V at the tubes if i remember right.. (300B)
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!
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Old 23rd August 2011, 12:25 AM   #5
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Quote:
Originally Posted by dagerman View Post
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...
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Old 23rd August 2011, 01:30 AM   #6
rknize is offline rknize  United States
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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.
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Old 23rd August 2011, 02:17 AM   #7
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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.
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Old 23rd August 2011, 02:54 PM   #8
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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!
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Last edited by dagerman; 23rd August 2011 at 03:00 PM.
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Old 23rd August 2011, 04:02 PM   #9
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Changing the cap won't have much effect because there is very little load on the power supply right now.
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Old 24th August 2011, 06:22 AM   #10
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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
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