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Old 22nd December 2005, 08:45 PM   #1
JZatopa is offline JZatopa  United States
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Default How can I lower B+ of power supply?

I have built a simple EL84 amp using parts from an old tube amp and the schematics from diy paradise. Now that it's built I am getting very little output. I think one of the problems is that my b+ after rectification is about 400V and it needs to be 300V. Can I fix this and if so how? I am not willing to unwind the PT.

If this is impossible could somone tell me what I could do to salvage what I have built into something useful? I have a total of 6 9 pin sockets.
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Old 22nd December 2005, 08:47 PM   #2
SY is offline SY  United States
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Use a choke input supply filter.

It's doubtful that this is the cause of low output. But you need to fix it anyway...
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Old 22nd December 2005, 08:47 PM   #3
cerrem is offline cerrem  United States
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You can make a simple voltage regulator....

Chris
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Old 22nd December 2005, 08:58 PM   #4
JZatopa is offline JZatopa  United States
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I need this drawn out for me as I am a compleate noob. I don't have the $$$ to use chokes. I'm looking for something cheap and or simple if possible.
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Old 22nd December 2005, 09:12 PM   #5
dhaen is offline dhaen  Europe
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Post or link to your schematic please.
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Old 22nd December 2005, 09:34 PM   #6
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There is a pretty in depth thread here somewhere from the last month or so with some great ideas. I had similar problems with a transformer that had it's voltage specs mislabeled.
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Old 22nd December 2005, 09:52 PM   #7
JZatopa is offline JZatopa  United States
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http://diyparadise.com/simpleel84.html

I am using an el84 for input tubes.
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Old 22nd December 2005, 10:19 PM   #8
jazz is offline jazz  Netherlands
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Hi djnubz,

You say you used old parts, is the power trannie rated according to the schematic?

If so i would suspect you are not drawing current somewhere. Hence i would start by checking you're wiring. please check the pinouts of all tubes and then double check them.

Do some measuring to, check the grid and cathode voltages on the various tubes. Given the high supply voltage values can be a bit off but shouldn't be off to much. please report back.

If it really would be due to the psu voltage, try this for starters. replace the first cap in the psu with a 0.1 uf cap. Next add a 100R resistor in series with the second cap and place the 100uf cap you took out in the first stage of the psuafter the resistor. This should at least lower you're psu voltage.

Oh and just try connecting a 500k to 1m resistor from the input tubes grid to ground.

hope this helps
Joris
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Old 22nd December 2005, 10:20 PM   #9
dhaen is offline dhaen  Europe
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Are you saying you haven't used a choke?

NB The smoothing circuit is a bit unusual, being a resonant type, and it is designed for 50Hz mains.
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Old 22nd December 2005, 10:50 PM   #10
anatech is offline anatech  Canada
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Hi DJNUBZ,
You need what the original power supply calls for. That means a choke. You can not use dropping resistors, the current variation is too high. If you don't want to use a choke, you need another power transformer with lower output. Your hum level may be too high if you go that route.

An electronic regulator for your B+ may work. It may possibly cost more than the choke.

You are also using the wrong tube type for the voltage amplifier. Try to stick as close as you can to the original diagram.

-Chris
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