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Old 24th September 2003, 08:39 PM   #1
tom1356 is offline tom1356  United States
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Default 300b at 3.35V after bridge replacement

I replaced the bridge rectifier in one of my 300b amps with some fast soft recovery controlled avalanche rectifier diodes I had on hand. The voltage has dropped to 3.35V from the normal 5.0V. It seems to work very nicely but with reduced gain. Any ideas as to fixing this? Does it hurt to run the 300b's at 3.5V or less?
During warm up, after a few mins. it did get a little louder, but not the way it used to be.
The tubes are TJ mesh plate golds.
Thanks in advance
Tom
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Old 24th September 2003, 09:13 PM   #2
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Hi,

That's really odd.

You could try to use a bigger filter cap but this voltage drop shouldn't have happened.

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Old 24th September 2003, 09:18 PM   #3
tom1356 is offline tom1356  United States
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Frank
Yes it's strange. It's just over 3.5V now and rising very slowly. Sounding very sweet though.
I'm tempted to do the other just to see what happens but if I get the same drop then what?
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Old 24th September 2003, 09:20 PM   #4
tom1356 is offline tom1356  United States
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The cap across the 5.0 volt (suposedly) wires is 10,000uf.
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Old 24th September 2003, 09:24 PM   #5
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Hi,

Quote:
Sounding very sweet though
I'm not surprised, you're starving the 300B which actually linearises the bottom end of the curves.

Quote:
I'm tempted to do the other just to see what happens but if I get the same drop then what?
If you have a DVM, set it to lowest resistance and measure the series resistance of the diodes before you do the other channel.

Maybe you received the wrong kind of diodes?

If anything, I'd expected a slight rise in voltage, not a drop.

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Old 24th September 2003, 09:30 PM   #6
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Hi,

Quote:
The cap across the 5.0 volt (suposedly) wires is 10,000uf.
Should be more than ample....strange, really strange.

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Old 24th September 2003, 09:31 PM   #7
dhaen is offline dhaen  Europe
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Hi,

The Fast-Soft Recovery Diodes that I have come across have high forward voltage drop. Schottky diodes are more suitable for low voltage applications.

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Old 24th September 2003, 09:33 PM   #8
tom1356 is offline tom1356  United States
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"If you have a DVM, set it to lowest resistance and measure the series resistance of the diodes before you do the other channel."

Could you clarify at which two points do I take the measurments?

Maybe as you say these diodes are not suitable for this application.
Is there any long term problem with using the 300b at 3.5V?
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Old 24th September 2003, 09:41 PM   #9
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Hi,

Quote:
The Fast-Soft Recovery Diodes that I have come across have high forward voltage drop.
But of course, I forgot about that entirely...in HV sullies this doesn't really matter but at 5V it sure would.

Good of you to remind us of that.

Quote:
Maybe as you say these diodes are not suitable for this application.
See D'Haen's reply...you're losing too much voltage.

Quote:
Is there any long term problem with using the 300b at 3.5V?
Not a problem as such but it won't exactly be a 300B you're listening to...could be better, could be worse.

I'm not going to take side on this one....

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Old 24th September 2003, 10:04 PM   #10
tom1356 is offline tom1356  United States
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Well that explains it. Thank you both for the help.
I think I'll listen to it this way till I can get the shottky's.
It's warm and thick like a quilt on a winter morning.
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