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Old 9th January 2013, 06:07 PM   #21
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Quote:
Originally Posted by cyclecamper View Post
Rod, the Studio Bass, Super Twin, and Super Twin with Reverb are all very similar except the obvious (like reverb, or the super twin not-so-super distortion knob). Except the Studio Bass uses a lower B++ (they all use the same 500V B+ for 6 6L6GC) and different caps in the rotary graphic EQ.

What's PSUD2, simulation software? Wouldn't those have to be some pretty leaky caps to draw more current (except momentarily at start-up of course). If they made that much difference that would kind of prove how badly they were needed.

So some caps right at the rectifier; more right at the load.
Hi,

Here's PSUD2:

PSUD2

Power supply Designer modelling.

Runs native on Windows, Works perfectly with Wine on GNU/Linux platforms.
It's highly recommended - both to check the effect of Choke and caps on Ripple voltage, plus you can also see the current draw in the transformer.

Transformer current should be carefully inspected, because changing the value of the cap for the rectifier will increase the peak pulse current in the rectifier/trafo, and higher peaks mean that the rms current is higher (crest-factor effect). Higher rms current means more stress, which the old transformer may not withstand. Also, the higher effective load means there's less power for the speaker.

With PSUD2, you can simply model all this in a few seconds. Next, try all the values of C you have in mind, add chokes, etc.

Maybe a choke of something like 1 to 2 Henry, 600mA rated would be a starting point for a choke (will be quite large, but they are no use in the main circuit unless they can take the current).
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Old 9th January 2013, 06:14 PM   #22
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For a rough estimate of the dc supply current check out the Tung-Sol 6L6GC data sheet.

for 450V B+, and 55W output power (single pair), the dc current is 210mA in the anodes. so, allowing for the screens and the preamp, three pairs of 6L6s may take as much as 750mA for 180W out, using higher B+.

It's an unusually high current - may make the choice of chokes smaller than other power levels.
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Old 9th January 2013, 07:01 PM   #23
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I'm starting to wish I'd removed the entire power supply, and put it in a second chassis mounted on the bottom of the cabinet. That would allow very sensible layout, move the power transformer field and power wiring etc. farther away from any sensitive circuits, allow me to mount decent screw-post electrolytic filter caps via clamp mounts, mount multiple chokes, have short wires all around the bridge rectifier, allow me to experiment with power supply seperately from the real amp circuit, and leave lots of room in the main chassis for new features (tetrode/ultralinear/triode mode switch, 1/3 power switch, voltage drop switch, seperate bias pots), put a heatsink on the bridge rectifier, and keep both chassis cooler. A power relay could even keep the 120 V out of the amp chassis if I wanted. I could add a 110/220 switch and universal cord socket, some line MOVs. And it wouldn't be quite so top-heavy anymore. It would certainly keep the power tubes cooler with the transformer out of the way of airflow to those two last tubes, and keep the power transformer cooler without the power tubes radiating heat directly onto it. I just wish I'd made that decision earlier and made an even taller cabinet shaped more like the tall "Showman With Reverb" cabinet to maximize the benefits.
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Old 9th January 2013, 08:11 PM   #24
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Or it might be possible to remove the power transformer and filter capacitor pan, and mount a daughter chassis directly onto the stock chassis. More work, less flexible, harder to work on, less benefit.
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Old 9th January 2013, 08:19 PM   #25
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Quote:
Originally Posted by Rod Coleman View Post
three pairs of 6L6s may take as much as 750mA for 180W out, using higher B+.
Plus there's the preams/splitter and not 'choking' off the necessary supply of current to replenish and maintain good regulation. I was thinking 700 ma to 1A conservatively derated to do no harm, if possible. For 1H that's do-able, but if I want 10H and low Dc resistance, it's starting to approach the size of a toaster. Which is why the mfgrs don't throw in a 20-pound $100 choke when there are cheaper lighter options.
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Old 9th January 2013, 09:56 PM   #26
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Originally Posted by Rod Coleman View Post
It's an unusually high current - may make the choice of chokes smaller than other power levels.
No lie. I found only one. It's only 1.35H 1.1amp and 20 pounds. But there are a LOT of 500ma chokes with much more inductance. Maybe I should consider parallel inductors, but I would think there would be ringing issues which might require damping resistors. I'd make seperate parallel supplies for pairs of output tubes, but it gets complicated and I want the B++ down-ladder from the B+ so if B+ does sag B++ will sag too, and never cause a grid to go more + than plate, so B++ would hang off only one B+ if I had multiple B+. So I'd rather use parallel inductors. There's no easy way around the weight, as a certain amount current requires a certain amount of iron.

Last edited by cyclecamper; 9th January 2013 at 10:00 PM.
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Old 10th January 2013, 02:46 AM   #27
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OK I bought the huge 20-pound choke. Now both my bigger guitar amps will be the only ones on the block with filter chokes for the full B+ rail. Yikes what have I done. I'm waiting to inspect & measure it. I don't know its DC resistance, but it's got to be pretty low or it would have more inductance; its got to be wound with pretty thick-guage wire. It's sure got a lot of iron, so the 1.1 amp rating is no lie! That will handle 550 watts. It's certainly got way morethan enough surface area to dissipate any heat from resistance or doing its job.
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Old 10th January 2013, 05:11 AM   #28
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Oh now I know I'm a nerd. I've been playing with that PSU designer 2 four hours and having a real blast. Often I get unexpected results and have to investigate and learn something.

But I have no clue what the real resistance of mhy capacitors is...I assume that the ESR but I have no idea what a typical real value is. I guess I should dig in a catalog.

And my transformer source resistance...I'm working that backwards from the behavior I want, so this is not much of a real simulation.

As suspected, nothing beats adding another filter stage for getting rid of ripple.
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Old 10th January 2013, 06:33 AM   #29
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Hmm, it looks like using a really high-quality choke and really high-quality capacitors in a circuit meant to not drop much voltage in the filters means that I have to upgrade the rectifier diodes or devise a soft-start circuit. I guess that's not much of a surprise.
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Old 10th January 2013, 06:44 AM   #30
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Quote:
Originally Posted by razorrick1293 View Post
Also, try adding a 0.22uF capacitor accross the primary of the power transformer.
As a noise filter on the wall-power? How did you choose that value? Is it related to the primary inductance at all?
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