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Old 21st April 2012, 10:20 PM   #1
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Default Am I loosing power somewhere?

Hello,

So, I've been trying to bias the left and right sides of my 805 SET tube amp. I can't quite get it right. One side is showing about a 10% difference which I can trace back to the power supply and/or/hoperfully the filtering.

The 390V tap is putting out aprox 360-370V and that makes aprox 860-870V on the B+ which ought to be 950V It's about 10% shy.

What is the likely cause?

Judging by the schematic Is there anything I could do?

Am I being too fussy???

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Old 22nd April 2012, 10:49 AM   #2
AndrewT is offline AndrewT  Scotland
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What is the voltage drop across 200r?

That will tell you the current that is flowing and will also tell you the voltage at the rectifier output.
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Old 22nd April 2012, 11:42 AM   #3
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Is B+ 2 OK ?
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Old 22nd April 2012, 08:57 PM   #4
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42 volt drop across the resistor, on both sides! However the one on the good side is on 998V and the low side is 937V

B+ 2 is also running a little low. When I set the grid bias even on both sides the voltage on the plates is lower on the low side by 25-35V or so.
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Old 23rd April 2012, 08:58 AM   #5
AndrewT is offline AndrewT  Scotland
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42Vdrop !

That's almost 9W and 210mA
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Old 23rd April 2012, 09:02 AM   #6
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If safe to do so, can you run PSU on its own with nothing attached to see if the doubler has a problem.

CHECK that diodes and caps are rated sufficiently high enough for NO LOAD operation.
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Old 23rd April 2012, 09:09 AM   #7
AJT is offline AJT  Philippines
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Quote:
Originally Posted by scutterflux View Post
Hello,

So, I've been trying to bias the left and right sides of my 805 SET tube amp. I can't quite get it right. One side is showing about a 10% difference which I can trace back to the power supply and/or/hoperfully the filtering.

The 390V tap is putting out aprox 360-370V and that makes aprox 860-870V on the B+ which ought to be 950V It's about 10% shy.

What is the likely cause?

Judging by the schematic Is there anything I could do?

Am I being too fussy???

Click the image to open in full size.
yes, i believe you are being fussy....

with a 390Vac transformer secondary, your unloaded voltage should be right around 1092 volts....now as you load your power supply, this voltage drops owing to the secondary dc resistance of your 390volt winding and the 200 ohm resistor in your psu.....

the thing you need to do is set the grid bias voltage to maximum negative...then when filaments are fully aglow, you can then set you bias voltage to get your plate currents to desired value.... you will notice that B+ starts going down with plate current....

if you can post the schematics of your 805, we can look further....

your B+ going lower than desired means that the traffo that you got is a tad lower than what you need, your primary voltage can also be lower so that secondary voltages are likewise lower.........with voltage doublers, what i do is to put several taps on the traffo secondary so i can adjust voltages.....

with tubes, voltage drops on the psu to 100 volts is nothing unusual.....
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Last edited by AJT; 23rd April 2012 at 09:30 AM.
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Old 23rd April 2012, 09:19 AM   #8
AJT is offline AJT  Philippines
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i notice there is something wrong with your grid bias circuit....as drawn....
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Old 23rd April 2012, 12:18 PM   #9
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Have you checked correct function of your diodes? Had the same problem once, turned out to diodes were out of spec....
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Old 23rd April 2012, 01:05 PM   #10
AJT is offline AJT  Philippines
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Quote:
Originally Posted by scutterflux View Post
42 volt drop across the resistor, on both sides! However the one on the good side is on 998V and the low side is 937V

B+ 2 is also running a little low. When I set the grid bias even on both sides the voltage on the plates is lower on the low side by 25-35V or so.
imho....nothing to lose sleep about....

are those plate voltages? the important thing is to get your plate current right....

they must be plate voltages....one having 937volts is drawing more current than the one with 998....
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