A few other thoughts:
If you stack 300V (or greater) rated caps in series to get more voltage rating you also need to put 220k/2W (or thereabouts) resistors across each cap to assure that they share the voltage load evenly.
Many low ESR 500V rated electrolytics have a surge rating of 550V, so you can also go that way.....Panasonic TS-UP series will work, but unfortunately, Digi-Key no longer stocks many values. You can check out Allied & Mouser for 500V caps as another alternative. IIRC Allied stocks Cornell Dublier.
You can also add additional RC or LC stages in your PS circuit; they will drop the B+ and ripple voltage further, but will increase the output impedance of the supply as Sgregory already mentioned.
Also, as already mentioned, a 5U4 rectifier will drop additional volts.
If you stack 300V (or greater) rated caps in series to get more voltage rating you also need to put 220k/2W (or thereabouts) resistors across each cap to assure that they share the voltage load evenly.
Many low ESR 500V rated electrolytics have a surge rating of 550V, so you can also go that way.....Panasonic TS-UP series will work, but unfortunately, Digi-Key no longer stocks many values. You can check out Allied & Mouser for 500V caps as another alternative. IIRC Allied stocks Cornell Dublier.
You can also add additional RC or LC stages in your PS circuit; they will drop the B+ and ripple voltage further, but will increase the output impedance of the supply as Sgregory already mentioned.
Also, as already mentioned, a 5U4 rectifier will drop additional volts.
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Is there anything bad about increasing my impedance, because I like the idea of adding more filter stages.
You want the power supply to have the lowest impedance possible. That way it can handle transients better. I wouldn't get too extreme with ripple, somebody check me here, but the baby huey has a pretty decent PSRR. If you want a second filter stage, keep C1 low to keep the voltage down and drop R1 by 75R and the install a RC section after the LC with the R being 75R. You can reduce your impedance further by increasing the capacitance of C2 and C3.
Low output impedance means that the power supply can provide the current when called for, ie drum whacks, transients, etc. That's one of the downsides of using a transformer that has a higher secondary voltage than needed. You can certainly reduce the B+ and ripple voltage, but it comes at the expense of output impedance.
If the PS has high impedance, the B+ voltage will sag when you drive the amp hard also. Many guitar amps have marginally spec'd transformers & PS components (originally to save mfr costs), and when driven hard, the B+ sag is part of what gives the amp it's distortion & "tone". When people talk about a stiff power supply, that means low impedance; that's what you want for Hi-Fi, as the amp will sound "punchier".
Using a large value final cap with low ESR also helps with the ability of the PS to supply the current when needed, since the amp circuit will mostly be drawing power from that last cap, especially for transients. It's also nice to have the stuff before the last cap able to provide the juice, so that when really cranking the amp the last cap can stay charged up, ie minimal B+ sag.
If your transformer was rated at 275-300V or so, you could use SS rectifiers (they drop way less volts than tube rects), and less series resistance; these are the main things to reduce output impedance. Larger transformers with high current ratings also have less coil resistance for the same reason; they are designed to source more current. If you modify your PSUD model an make the transformer secondary resistance very high you will see your B+ start to wilt......
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An externally hosted image should be here but it was not working when we last tested it.
Ok, so I've tabled the amp project for the past couple weeks while I finished up my last semester at school. I'm glad to say that I graduated, and I am moving to florida to start a new job in about two weeks. Anyhow, I just got back to the amp today, and I have another question. I hooked up the tube heater circuit that runs through all the tubes. The rectifier and the two el84 tubes light up and get hot, but the 12ax7 tube seems to stay cold. It has 6.3 volts connected to pins four and five, and pin nine connected to the center tap. Have I done something wrong, or do 12ax7 tubes just run cold?
As always, thanks for all the help guys.
-Matt
I think you need to hook pins 4 and 5 together and the other leg of 6.3v to pin 9, you are putting 6.3v over a 12.6v heater, use the center tap
I have sound! But I also have a few problems. I finished the wiring and hooked up a speaker. To my surprise, it produced music, and it sounded really good. I have a few issues though. For one, one of the el84 sockets causes whatever tube is in it to glow bright orange. Not just the filament, but the whole structure inside the tube gets bright orange. Also, the amp blows a fuse after about five to ten minutes of operation. Do any of you guys have any pointers as far as what the problem could be?
Thanks guys
-Matt
An externally hosted image should be here but it was not working when we last tested it.
Thanks guys
-Matt
Well, you are red-plating that tube, so leave it out until you figure out whats going on. Otherwise, you risk toasting the tube, and possibly a few other things. That tube is pulling lots of current, possibly because of a problem with the bias block, or something else....
Do you access to a variac or other means to bring up the AC mains voltage slowly (or reduce the AC voltage)?
Can you measure the voltage from pin 3 to ground (at idle) for each EL84? You'll have to do this with the tubes in place so use clip leads for your meter if you have them and set everything up then turn it on and turn it off as soon as you get good readings.
Also measure the voltage from pin 2 to ground for each EL84.
You could also swap the bias blocks between channels and see if the problem follows the bias blocks. Retesting each bias block on the bench may also be a good place to start.
Do you access to a variac or other means to bring up the AC mains voltage slowly (or reduce the AC voltage)?
Can you measure the voltage from pin 3 to ground (at idle) for each EL84? You'll have to do this with the tubes in place so use clip leads for your meter if you have them and set everything up then turn it on and turn it off as soon as you get good readings.
Also measure the voltage from pin 2 to ground for each EL84.
You could also swap the bias blocks between channels and see if the problem follows the bias blocks. Retesting each bias block on the bench may also be a good place to start.
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Replace the bias block with a resistor sized to give you about 35ma of idle current.
Use the voltage measured at pin 3 on the good tube to calculate the value of the resistor.
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I replaced bad bias block with a 285 ohm resistor group. Now it runs fine and the tube is no longer red. I think I may just replace the other bias block with a resistor as well. I want to get this project wrapped up before I move down to Florida this coming tuesday. I am getting a fair amount of hum from the amp, but I think that neatening up some of the wires may help.
You will want to replace the bias block. That is an important part of the amps character. It also is what keeps the output tube at is best operating point. A resistor will not. you could use a trim pot as a temporary measure, but you will need to accurately measure the bias current and manually set it.
Without proper and equal bias the amp won't perform close to it's potential
Without proper and equal bias the amp won't perform close to it's potential
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