Bottlehead Forum
Bottlehead Kits => Kaiju Stereo 300B amp => Topic started by: Deke609 on August 23, 2019, 11:32:59 AM
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Mouser sells some Kemet AC filter metalized polypro caps rated 700 VDC and either 330 or 440 VAC rms that i think I might be able to make fit. Do I need 700V AC/DC or just DC? Once rectified, I thought the current would be considered DC for cap purposes, since it never goes negative. But I'm not certain.
cheers and thanks, Derek
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700DC is fine. This is just to replace the ones that come after the doubler?
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Many thanks PB. Yes, I'm not touching the doubler.
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Even a 600V rated film cap is probably fine in those positions. They don't really take much of a beating in terms of AC current, though the doubler caps sure do!
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Awesome! Thanks PB. That may make some smaller diameter caps an option.
cheers, Derek
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And here they are. 100uF 700VDC each. So a little shy of the stock capacitance. Mounted but not yet connected. I need to cut and router some simple risers to raise the (still unfinished!) base by about 2 inches.
One of the new caps is likely only visiting. I am going to experiment with replacing each of the 120Rs with chokes. A 4H 65 DCR 225mA rated in place of the first R, and a 6H 150R 200mA rated in place of the second R. But PSUD shows that the combination of the chokes and the 2 new caps (or even the stock 110uF effective value caps) results in initial B+ overshoot and then dip -- and from what I've read that looks like a recipe for ringing. So I played around with different configs and found that a 200uF cap in last position completely eliminates it. In PSUD ripple is reduced to about 0.02mA pk-pk! For reference, PSUD shows about 20mV pk-pk ripple with the stock filter config, and approx. 0.8mV pk-pk with the last RC replaced by the 6H choke and a 100uF cap. We'll see if any of this actually works out in reality. I'll scope the amp this weekend to get some baseline measurements before making any changes.
[edited to add PSUD results for the stock filter config and for when the last RC stage is replaced by an LC]
cheers, Derek
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I played around with PSUD some more this morning and found what looks to be an even better looking LCLC config: 4H-10uf-6H-200uF.
I attach screenshots of the various configs: my rough estimate of stock (2X 120R 110uF); the overshoot/dip with two chokes (4H-100uF-6H-100uF); my first attempt at correction (4H-100uF-6H-200uF); and finally what looks to be a pretty good solution (4H-10uF-6H-200uF). That saves me from having to buy a 200uF cap as i can just connect the two existing 100uF caps in parallel.
Does anyone see/foresee any issues with the 4H-10uF-6H-200uF config? Anything in particular that I should be watching out for?Any big problemswith my PSUD modelling?
cheers and thanks, Derek
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FYI in the model, I would use a 160mA constant current load rather than the 10K resistive load you have, though this shouldn't really change your results.
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Thanks PB. I ran the models with 160mA constant current as you suggested and the only change I noticed was about a 30V drop in B+. But since the drop is the same for both stock and modified, I figure that's ok.
cheers, Derek
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These are such interesting results - thanks for sharing the sims. In the Class-A solid state world, the smallest ripple tends to result from equal size caps in the pi filter. I would not have thought about such a "lopsided" approach to cap size for a tube build. I have not yet measured PSU ripple in my 300B prototype. That will happen once I am satisfied that it makes music.
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In the Class-A solid state world, the smallest ripple tends to result from equal size caps in the pi filter. I would not have thought about such a "lopsided" approach to cap size for a tube build.
Eric - I suspect there's no difference between SS and Tube worlds for ripple. The lowest ripple I got from my models was 0.02mV with the combo of 100uF and 200uF. But the shape of the curve was still a bit wonky and I fear oscillations/ringing, so I was looking for a smooth voltage rise. My latest config with 10uF and 200uF results in about 0.5mV ripple - so 25 times more. Based on my playing around with PSUD it looks like ripple is largely function of total capacitance. For S&G's, I modeled the chokes with 2 X 500uF and got negligible ripple well under 0.01mV. But the voltage rise time was not so good - taking a lot longer to get to B+. I figure (but do not in fact know) that rise time may be relevant to transient response - so I figure the faster the better (provided it is smooth).
The 10/200 uF combo appears to give me faster rise time than stock with about 40 times less ripple - and this to me looks like a good compromise between fast response and low ripple.
But I'm just guessing here. I'm still new to all of this :)
[edited to correct my lousy arithmetic: 20mV ripple (stock) / 0.5mV ripple (modified) = 40 times, not the 80 I initially stated - sheesh!]
cheers, Derek
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Looks like one of your goals is to prevent ringing on B+ when power is applied? A resistor or current source load to model the rest of the amp probably (obviously?) isn't going to show what's happening on B+ since the rest of the amp has a "wake up" phase (tubes warming up, etc.) Is it possible to load your PSU model with a piecewise current source that has a ramp up period? I guess the difficulty with that is modeling the ramp up behavior. I suppose you could scope your amp's B+ to get some approximation.
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Thanks Raymond. Yes, I am trying to avoid ringing. I have two mostly uneducated thoughts about this: (1) B+ ramp up on turn on is representative of what may happen when there is sudden spike in demand from a loud transient; and (2) ringing in one part of the circuit could cause unwanted effects in other parts of the circuit. One or both of these thoughts may be wrong.
PSUD does allow for a two-step current load. One can set the initial current load and specify a second current load and when it is to take effect. I haven't tried it with the recent models. Do you have any suggested values I should try? Edit: Oh, I see that you;ve already suggested that I scope the amp to estimate this. Thanks - I will do this tomorrow when i am playing around with the scope.
many thanks, Derek
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And here's my four-footed Kaiju. The feet were intended only to be provisional and I had low expectations for how they'd look. But I actually like the look of the thing, even in its unfinished state - which is too bad b/c it is extremely hazardous: the large gap makes for a easy path for finger and power supply to meet. The new caps are still not hooked up , so no danger at present, but I will have to figure out how to make it safe before making my intended changes. Happily, the feet are just press-fit with one retaining screw each from the backside - so they can be removed and reused in a different configuration.
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That's clever and very nice looking! I've been thinking about feet under my chassis and found some on ebay that look decent, but I was wondering if they would hold up under the weight.
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The feet (or legs, to be more precise) are cut from a piece of true 2" x 2" maple. All I did was router a channel in each the width of the base panel thickness. They should be good for at least 2000 lbs or more!
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I like the look of the wooden legs! A short while ago, I was looking at something like these on ebay, but not entirely sure if I want to use them or not. Not sure how much weight they'll support since it seems they are intended for jewelry boxes... I'm figuring a completed chassis will weigh in somewhere near 40-50lbs.
https://www.ebay.com/itm/4x-Antique-Brass-Jewelry-Gift-Box-Wood-Decorative-Feet-Leg-Corner-Protector-LY/163425802030 (https://www.ebay.com/itm/4x-Antique-Brass-Jewelry-Gift-Box-Wood-Decorative-Feet-Leg-Corner-Protector-LY/163425802030)
https://www.ebay.com/itm/8X-Antique-Brass-Jewelry-Wood-Box-Decorative-Feet-Leg-Corner-Brackets-Protector/333228809550 (https://www.ebay.com/itm/8X-Antique-Brass-Jewelry-Wood-Box-Decorative-Feet-Leg-Corner-Brackets-Protector/333228809550)
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Eric - Have you checked out Lee Valley? http://www.leevalley.com/us/Hardware/page.aspx?cat=3,40993&p=41282 (http://www.leevalley.com/us/Hardware/page.aspx?cat=3,40993&p=41282)
They're local to me (Toronto), but have a U.S. online sales site (linked to above). I've found their hardware to be pretty good.
cheers, Derek
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Those are nice looking feet! They certainly look more sturdy than the ones I found on Ebay. Thanks for the link, Derek. Too bad we didn't "meet" sooner, I was just in Toronto about 5 weeks ago for a conference...
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Looks like one of your goals is to prevent ringing on B+ when power is applied? A resistor or current source load to model the rest of the amp probably (obviously?) isn't going to show what's happening on B+ since the rest of the amp has a "wake up" phase (tubes warming up, etc.) Is it possible to load your PSU model with a piecewise current source that has a ramp up period? I guess the difficulty with that is modeling the ramp up behavior. I suppose you could scope your amp's B+ to get some approximation.
This can be pretty hard to model accurately. Some time ago I built a pair of all directly heated 300B amps, and they passed signal within a second or two of being on, but this amount of time depended significantly on the specific 300Bs I used.
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I'm going to try to address to the possibility of transformer ringing with snubbing. A clever guy over on DiyAudio put together a little pcb circuit for "no math" snubber calculations called "Quasimodo the bell ringer": https://www.diyaudio.com/forums/power-supplies/243100-simple-math-transformer-snubber-using-quasimodo-test-jig.html (https://www.diyaudio.com/forums/power-supplies/243100-simple-math-transformer-snubber-using-quasimodo-test-jig.html) Essentially, it puts an adjustable snubber across the secondaries, rings the transformer, and you adjust a pot to determine the most effective damping while scoping the transformer. And then you measure the pot resistance to find your snubber value. I picked up the board and parts, but haven;t put it together yet, so I can't vouch for it. But it seems to be pretty well received.
Just something fun to learn with.
cheers, Derek
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This can be pretty hard to model accurately. Some time ago I built a pair of all directly heated 300B amps, and they passed signal within a second or two of being on, but this amount of time depended significantly on the specific 300Bs I used.
Yeah, I think PSUD is a good tool for analyzing quiescent states and not good for anything else, such as power up ringing in LCR circuits or transient response. For all that other stuff, you really need full blown circuit simulator (e.g. HSPICE), but even that won't simulate variations in tube "wake-up" times.
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Derek - break out the soldering iron and build Mark's Quasimodo board! It works great and helped me quiet down a 750VA transformer in my modular PSU build for various First Watt amps. Results on the scope are quite striking as you adjust the pot.
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Cool! Thanks Eric. This should be fun.
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I can't believe it worked! The install of the new LCLC power supply filter config went without a hitch. I thought for sure that this was going to be the time that something went pop and boom. But it worked! I went with the 4H (65DCR) - 10uF - 6H (150DCR) - 200uF config I previuosly posted about. Voltages checked out and it sounds great. At the very least, it definitely doesn't sound worse. And I'm pretty sure it sounds a bit better. :)
cheers, Derek
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Congrats on the successful mod, Derek! Looks like tonight is a good night for projects :)
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Thanks Eric. I saw that you've made progress with your project as well. Good stuff! Looking forward to reading about the final implementation.
cheers, Derek