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In Reply to: RE: Resistor dissipation Question posted by timp on September 15, 2012 at 20:26:39
Timp, can we have a look at a schematic of your power supply? Might there be another way to shed that 45 v without creating a burrito warmer?
Follow Ups:
Thanks everyone for helping me. I was up till 1 in the morning to lower the heat.
Here is the PSU design using Duncan's program. The R1 is the burrito heater that's cooking my amp right now.
FIY, this is for my 6bg6g Push Pull power amp using 8k OPT.
Edits: 09/16/12
Did you mean to draw this with a half wave rectifier? If you use a full wave bridge (solid state) and terminate C2 with a 160mA load, the supply will output about 385 volts of nicely filtered B+ for the power tubes. It will unfortunately also exhibit an impedance "bump" at about 15 Hz, but this may or may not be problematic. The bump can be smoothed if you interchange the first two filter caps so that C1 = 23u and C2 = 47u.
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Buy Chinese. Bury freedom.
I bet he was "meaning" a full wave center tapped rectifier circuit.
No matter what, I would swap C1 and C2, I agree with you.
I don't think there is any getting around a new power trannie to do the amp properly.
Jeff Medwin
You guys are amazing! Ok. I come clean! Jeff you are absolutely right. I have a full rect of b+ 386 per side with a center tap. I was lazy that I did not measure the tranny value. From one b+ to the other b+ measures
83 ohm. Did I measure it right?
TRY to "input" into PSUD2 what is really going on, to the best of your knowledge.
Insert a current tap before R2 and assign 150 mA. as its value.
Change the current tap at the end of the filter to 50 mA. OR better yet, add current taps after R2 and R3 to accurately reflect actual current draws, at different locations along the filter.
If 83 Ohms is the DCR across the entire winding, use half of that ( 42 Ohms) as a ROUGH input figure in PSUD, which will be ( not exact but ) an improvement over the 20 Ohms and 83 Ohms you have inputed.
Hi Jeff
Thanks for helping me. I am extremely thankful. And I learned a lot in the process. I am still confused... I just read voltage on the plate and it says 377.
Somehow, Duncan's PSU says that the reading on C2 is 350ish. So, I may be doing something wrong?
To add more info,
I am using 2 12sn7gt as phase inverter
and 1 12sl7gt as a premp section.
Then 4 6bg6gs as power tubes.
In short, looking at my PSU, do you see any problem using it for a pp amp?
Thanks.
Tim,
That PSUD2 simulation above looks mighty fine to me, and WOW, you have come a long way on this in a very short time.
I would NOT worry about the difference in VDCs of 27 VDC on a 350 VDC supply, that is less than 10%. Is 377 VDC what you wanted ?
What you did is totally opposite of how I would approach the filter to the finals. See my NEW post above, " Pinging ...timp."
Cheers,
Jeff
Question,
Are your Finals (output tubes) fed from C2, and your front end tubes fed from C3 and C4 ?
If so, you need to PSUD input the Finals current draw if ( 180 mA??) right after C2, and PSUD input your Front end draws after C3 ( 15 mA??) and C4 ( 5 mA.??).
Jeff. You are right! C2 is the for the power tubes.
You might consider using a 5R4 to drop more voltage in the rectifier. Also lowering the input capacitor value bfore the 6H choke will make a difference. Try 4.7 to 10 uF with a motor run cap. One other idea is to use a CL-150 inrush limiter on each leg of the transformer, as this will also drop a few volts.
Jeff,
Nice post. I agree with much of what you said. However, the large voltage drop in a lovely sounding ( directly heated !!) 5R4 comes with a price to pay, higher Z.
A single 5R4 is best used in a preamp. Too whimpy for a stereo P-P amp seeking to play back dynamic expressions.
Jeff
The addition of 250 Ohms resistance after the rectifier tube, and the use of a 122 Ohm DCR choke, prior to the Finals, are causing a loss in dynamic capability of your audio amplifier.
There is NO combination of chokes ( that I can see ) which will get you close to the 377 VDC you seem to want to use, when you start with a 386 VAC 20 Ohm power transformer, IF you seek to retain a tube rectifier.
Off the top of my head, to optimize the amp would take a lower VAC secondary on the Power Transformer. Ugh, got milk ??
Jeff Medwin
Right now the 4 6bg6g are drawing 150ma.
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