Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 3choplex on February 22, 2011, 02:17:46 pm
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I'm using what amounts to a 5e3 power amp with the magnetic components vintage spec iron. I'm using a SS rectifier, and want to use a sag resistor on the CT. The value I hit on was 390R, so I could also get the voltage closer to what I would get with the tube rectifier.
Is there any disadvantage to using a sag resistor that big? I've seen a couple of places where people mention that they wouldn't want to use over about 150R, but there's no explanation of why.
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Heat. How many volts are you dropping across that resistor? From there you can calculate current draw and then wattage. Also hum can come into play. Did you put a cap in parallel with the dropping resistor? If not, remember that the center tap is now more negative than ground, (+ to ground, - to CT). Voltage of C> than the voltage across the dropping resistor. Calculate the value of C by deriving from the equation F=1/2piRC where F is in the neighborhood of 1Hz.
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I was in a similar situation with an amp I recently built.
I used a Magnetic Components 40-18016: 330-0-330/120mA
SS rectifier.
I put a sag resistor between the rectifier and the reservoir cap, I didn't have a suitable 10W in the drawer, so I used two 5W in parallel to get to the 350R neighborhood.
With two fixed bias 6V6s, and 3 12AX7's, I ended up with 405V on the plates of the 6V6's and it sounds great to me.
I did clip a voltmeter across those resistors, cranked up the amp and hit some open-E chords. I calculated the dissipation. I don't remember the result, other than that I was safe with a 10W resistor.
There's a gut shot of it in the gallery where you can see the blue 5-watters on the left-hand side by the three radial filter caps. Look for "Blackjack" in the gut shot gallery.
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Out of curiousity, why put the sag resistor on the power transformer's center tap instead of between the rectifier(s) and the reservoir cap?
Having used a BIG zener diode on the center tap to drop almost 50 volts one time, I learned the hard way that you have to be really careful about grounding. Ground reservoir cap on "top" of your sag resistor (same point as PT CT). Ground the rest of the power amp at other end of your sag resistor. At least that's what worked for me.
Chip
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Out of curiosity, why put the sag resistor on the power transformer's center tap instead of between the rectifier(s) and the reservoir cap?
I figured bolting it directly to the chassis would eliminate some heat. The way I'm reading your post, it sounds like I can avoid some potential noise issues by putting it in the B+ line between the rectifiers and the reservoir cap--am I reading that correctly?
I'm using a 25 W resistor, so I'm comfortable with the rating.
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That big zener diode uses the chassis as a heat sink too - but it's dropping almost 50 volts.
You can put the sag resistor on the CT. I just wanted you to be aware that there are some grounding issues which may cause problems. One way to reduce those potential problems would be to add a C-L-C C-R-C or "pi" filter before the power rail node supplying the power tube plates. IOW add another filter cap plus a small isolation resistor between the rectifier(s) and what is now your reservoir cap. That way, the new reservoir cap is the only one you need to ground on top of your sag resistor with the CT.
OTOH if you do that, the "small isolation resistor" might as well be your sag resistor...
Chip
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Fresh start could you like draw that on paper so stupid people like me can understand,(LOL) No really a visual aid would be helpfull. Thanks Bill
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I wanted to revisit my post. I wasnt trying to be sarcastic I was really interested in learning about sag resisters and I thought all of your ideas were really good. I have sleep problems so when I get tired i have a habit of coming off the wrong way or saying something thats easily missinterpreted.
When I really enjoy everyone here on the forum and learn so much from you guys.
Thanks Bill
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Bill - no problem with your first post. I understood... I think. When I was trying to describe the "extra" C-R-C filter, it would look like the start of the power rail in the attached schematic except you would replace the choke with your sag resistor.
Sorry I typed "C-L-C" when I was thinking "C-R-C" before.
Cap(to ground)=>Resistor=>Cap(to ground)
Does this help?
BTW, you can also use two sag resistors between the power transformer's AC secondaries and your rectifier(s). That lowers the wattage rating you need because the current is split between the two resistors.
Chip
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Thanks