B+ Voltage sag for AB1 PP - diyAudio
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Old 23rd January 2010, 11:14 PM   #1
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Default B+ Voltage sag for AB1 PP

What's a reasonable amount of B+ voltage sag for PP AB1?

With a starting (no signal) B+ of 380V, the B+ voltage is sagging to 360V at the onset of clipping using a 1 Khz sine wave. This is a EL34 triode amp with a FWCT SS CLC supply, and a 360V 125ma rated transformer. Bias is 50ma/tube. So, about 20V of B+ sag.

I just tested it with a 350V/175ma rated transformer, and got about 15V of sag between no signal and the onset of clipping.
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Old 23rd January 2010, 11:33 PM   #2
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1.at how many watt's OP is the onset of sag (clipping).
2."have you a schematic".
3.may you could use a V big reservoir cap.
4.have you watered your earthing rod.
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Old 24th January 2010, 12:38 AM   #3
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1.at how many watt's OP is the onset of sag (clipping)?

It's an iteration Poinz' 6GK5/EL34 music machine. Appears to be about 12 watts output based on my beginner calc's. 10 Wrms into 8 ohms=12.5W

2."have you a schematic?".

See posts #6 & #7 here:

PP EL34 or 6L6 circuits?

My PS (for the EL34's) differs from the orig; it is a CLCRC with 2.2 uf, 8H, 220 uf

3.may you could use a V big reservoir cap.

bigger than 220 uf? I'm assuming that my transformer current rating is a little undersized........Is a 1Khz sine wave at the edge of slipping more severe than playing typical music at reasonable levels?

4.have you watered your earthing rod?

That's rather personal, isn't it?

with the amount of rain we've just got in So Cal, I'm sure my earthing rod is plenty watered........

Last edited by boywonder; 24th January 2010 at 12:40 AM.
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Old 24th January 2010, 01:04 AM   #4
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I think the power trafo lacks the "stones" needed. Your experiment with bigger "iron" illustrates the situation well.

FWIW, take a look at "El Cheapo". Notice the 100 VA trafo used to provide B+ to 4X "12" W. tubes. That PSU is no small part of the design's success.
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Old 24th January 2010, 01:36 AM   #5
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Quote:
Originally Posted by boywonder View Post
Is a 1Khz sine wave at the edge of slipping more severe than playing typical music at reasonable levels?
Yes, but a low-sag supply is worth it. I have switched over to Maida regulators for p-p outputs. My chokes are gone and my capacitors are much smaller. It's more complicated to build but cheaper and better.
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Old 24th January 2010, 03:01 AM   #6
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Forgot to mention that the 360V/125 ma transformer (and the 350/175 ma transformer) above are powering monoblocks.
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Old 24th January 2010, 03:35 AM   #7
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Quote:
Originally Posted by boywonder View Post
Forgot to mention that the 360V/125 ma transformer (and the 350/175 ma transformer) above are powering monoblocks.

The EC supply easily delivers 200 mA. to 4X "12" W. tubes. You have 125 mA. going into 2X "25" W. tubes. Guess where the advantage lies? Also, EC's supply is quite "stiff", thanks to (sic) 820 μF. caps. in the doubler stack.
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Old 24th January 2010, 04:37 AM   #8
pointy is offline pointy  United Kingdom
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if i've got this right......................your load lines are in an area of the graph were the lines merge in to crayon.
so....... have you tried one psu to one mono block and listen to it

{also put a thermometer on the power transfomer}
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Old 24th January 2010, 04:46 AM   #9
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380-360=20
(20/380)*100= about 5%

Why do you think it is a big sag?
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Old 24th January 2010, 08:43 AM   #10
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Did you say that you added an extra RC stage for the pentodes? This isn't going to help the output impedance of your power supply, especially if the R is comparable or larger than the DCR of your choke.

That said, a pentode is much more sensitive to changes in the screen voltage than the plate voltage, so if you regulate your screens, you can get away with quite a bit of sag on the plate.
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