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Old 26th May 2010, 02:32 AM   #1
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Default Going through check out Tubelab SE

Finally got to put the multi-meters on the board and power the amp up. This is my first build so was a little nervous powering it up. Thankfully, nothing happened except a slight hum from the transformer. My initial readings were -14 vdc at B+, -220 vdc at B-, and 2.48 volts at the filament. I am using this amp with some 2A3C output tubes so the filament voltage was right on. After putting in the rectifier tube B+ rose to 407 vdc. Being the voltage that the 2A3C want is in the 250 to 300 volt range, should I change C4 to a different value? If so, by how much? I am using a Triad C-14X choke in place of R-4 and using the 47uF 450V cap. in C4. Thanks,

Matt
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Old 26th May 2010, 05:14 AM   #2
rknize is offline rknize  United States
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What power transformer are you using? B+ will sag under load, but not 100V worth.
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Old 26th May 2010, 05:53 AM   #3
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I'm using Hammond 270HX. I've run PSUD and there doesn't seem to a rating for c4 that would get me below 300. How about a resistor before the rectifier?
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Old 26th May 2010, 01:34 PM   #4
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From George's page on the TubelabSE:
Tubes and Transformers

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The 2A3 is essentially two 45's in parallel in the same envelope. The filament current is 2.5 amps per tube. The 2A3 also has a 300 volt maximum plate voltage rating, although several new production "2A3 types" have higher ratings. The higher plate current drawn by the 2A3 will bring the plate voltage to about 300 volts when used with a 550 volt transformer and a choke in place of R4. The choke should be rated for 150 to 200 mA. I use the Triad C-14X or the Hammond T193J. The B+ voltage is then fine tuned to the desired value by changing C4. Start with a very low value for C4 (4.7 uF at 450 volts) and increase if needed.
I'd start with the recommended 4.7uF cap for C4. I think the 47uF part you have in there is too large, and my experiments with PSUDII suggest you'll hit a voltage over 350 the way you have it set up now. Something else to keep in mind - Hammonds have a reputation for running at higher than specified voltages. Perhaps this is because they are designed for 115VAC on the primary, and nobody has a line voltage that low. You might even want to start with a 4.0uF or 3.3uF cap at C4.
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Old 26th May 2010, 04:40 PM   #5
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Originally Posted by matt82609 View Post
Thankfully, nothing happened except a slight hum from the transformer.
Which transformer, the output or power? If it's power, that's typical of Hammond. If it bothers you, you can try Edcor or toroidal transformer from Antek like I did.
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Old 26th May 2010, 05:35 PM   #6
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Like Ty said, Hammond's 200 series are notorious for running hot. You'll get 50-100V more than you are expecting. Their 300 series are better. You can add series resistance to the choke, but then you lose regulation. Your best bet is to lower C4.
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Old 26th May 2010, 05:38 PM   #7
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Also, Lowering C4 taxes the rectifier tube less (which is good) but it taxes the choke more. More henries will do a better job of filtering so I would use the 193J over the Triad.
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Old 26th May 2010, 05:50 PM   #8
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Thanks for all the help. Yeah, the Hammonds are putting out 304 volts, when their rated for 250 volt. I've found a Jantzen 3.3 uF capacitor that I'll hope will get me to where I need to be. Thanks again for all the help!
Matt
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Old 26th May 2010, 05:51 PM   #9
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Could you just add another external RC network for improved filtering and voltage reduction?
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Old 26th May 2010, 06:15 PM   #10
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Not sure where or how I would do that. But I'm open to suggestions. My first thought (no electrical background here) is to put a non-inductive resistor between the board and the power transformer to reduce voltage, and adding a capacitor I suppose would help filtering too. Any idea how to calculate the voltage drop with a 4.7uF instead of a 47uF capacitor (Hard to find the esr value for capacitors by manufacturers)? PSUDII needs the resistance value of the capacitor to do its math.
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