Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: Deke609 on July 23, 2019, 08:32:44 AM
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@PB - I've got the Beepre up and running again. With today's 119.5 line voltage, PosIn on both boards are measuring at approx. 13.45V (previously 13.05V and 13.15V @ 118V AC in). [Edit: and PosOut values remain unchanged: 9.82V on one side, 9.85V on the other]
You previously suggested that I could try bumping up the 931R to 1K. Is that still your suggestion in light of the new PosIn voltages?
Many thanks,
Derek
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Follow-up on my previous post. My Mouser package arrived this aft, so I had to try it. I added 68R 1/2W in series with the 931R Rset2 to get approx. 1K. Result: 10.48V out of the filament regulator and 5.05V dropped across the filament. Great success! But the schematic shows +10V bias applied to the cathode / 10V dropped across filament and heater resistors - with no approx. symbol. Is the 10V a hard/optimal target? If so, I will try a 30R in series with the 931R.
many thanks, Derek
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Further follow-up. I added 28.7R 1/2W in series with the 931R Rset2 and now get approx 10.1V out of the filament reg board (maybe closer to 10.05V with the EML
45Bs 300Bs - I got 10.1V with the EH 300Bs). Tweaking the cathode resistor values a bit (adding a 68R to one side and a 75R to the other), I now get approx. 4.95V dropped across the EML 45B 300B filaments.
Unless someone tells me this is overtaxing the SR circuit or doing something else bad, I am calling it good -- in fact great!
[Edit - mixed up my amps in my head and mistakenly referred to EML 45Bs when I meant the EML 300Bs in my Beepre]
cheers, Derek
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Provided the regulator isn't dropping out (in which case you'll hear hum), there isn't much harm in running a BeePre like this.
-PB
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Many thanks PB. No dropouts yet.
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@PB or @PJ: what is the current rating for the 6.3V secondaries of the PT-7?
many thanks, Derek
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3.5A AC.
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Many thanks PB.
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@PB: when a filament voltage regulator drops out and starts humming - does the output tube continue to amplify the signal (music) or does the music cut out and get replaced with hum?
I have an intermittent hum issue that I'm trying to troubleshoot that I thought for sure was the reg dropping out, but now I'm not so sure - I'll post more about this if I fail to figure it out myself. For now, it would be really helpful to know the precise sonic symptoms of the fil reg dropping out so I can compare them with what I'm hearing.
many thanks,
Derek
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It's hum on top of the signal. Intermittent hum may mean that you're too close to the dropout region and as your line voltage wanders a bit, you're passing in and out of dropout.
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Shoot. Yeah, that makes sense. It also may explain why I can't get it to happen now. Gonna keep working on it - I did a fair bit of work taking out that PT last wknd - it's possible that something wasn't resoldered properly -- or maybe I toasted one of the B+ UF4007 rectifier diodes by desoldering and then resoldering them - I have some new ones that I can put in.
It would be a real bummer if I couldn't use the EML 300Bs - they sound great. I've reverted to stock values for Rset2 on the reg board and for the cathode resistors - which puts me shy of the 4% filament voltage spec - but even with that I was still getting hum early today - but for the past 3 hrs, nothing.
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You could also split the difference between the original resistor value and what you have now. The UF4007 diodes are in the HV circuit, and if you have a damaged one the raw high voltage rail voltage will drop way down.
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Thanks PB.
You could also split the difference between the original resistor value and what you have now.
I tried that earlier today - I brought fil voltage to 4.8V -- exactly 4% low -- and still had intermittent hum.
The UF4007 diodes are in the HV circuit, and if you have a damaged one the raw high voltage rail voltage will drop way down.
Yeah, I get that. I was just speculating that a slightly toasted UF4007 might still function, but become noisy - pure uniformed speculation on my part.
cheers and thanks, Derek
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Just trying to think things through here.
If the problem is the 1.3A draw of the EML 300B filament @5V, then aren't I a bit stuck unless the Shunt Reg can be made to handle more current? If the filament resistance is fixed, and I'm not supposed to go lower than 4.8V dropped across the filament, then I am stuck with a minimum current draw.
If the current requirement of the EML 300B presently exceeds the current limitation of the shunt reg, is there a way around this ? E.g., adjust the C4S to allow more current - e.g., 1.4A? and then tweak the Shunt reg board as needed?
cheers, Derek
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You're mixing up the high voltage part of the circuit and the low voltage part of the circuit. The high voltage shunt regulator has no idea what's going on at the filament.
If the filament is drawing 1.3A, then yes, you could trim down the paralleled 10W resistors to look more like a 3.8 ohm resistor instead of a 4 ohm resistor, but the additional current draw will also lower the output of the filament regulator, hence the desire to bump it up just a little bit.
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Many thanks PB. I think I understand what you're saying. I had wrongly assumed (without carefully looking at the schematic) that the fil reg was fed by a C4S, but I see that the C4S is used for the HV anode load. Apologies for that, I should have checked the schematic first.
So I think it follows from what you've written that the fil reg board is, subject to heat dissipation limits, not current limited, other than by the 3.5A ratiing of the 6.3V secondaries - which is plenty more than I need.
I will retry adjusting the Rset2 resistor. And it just occured to me that something else may be contributing to the hum issue. On one channel I installed a pcb solder cup pin on one leg of the Rset2 (because I was testing only one side at first, and expected to make many resistor changes, and so feared ripping out the trace). The pin diameter is a bit small so it's possible that during my previous Rset2 trial I unseated the pin, leading to a noisy contact. I will check and fix today.
Is there a max PosOut voltage that I should stay under? I ask b/c my first attempt at bumping up Rset2 resulted in about 10.45V out (IIRC) and I don't think I heard any hum at that setting - but did hear hum when I dropped PosOut to 10.1V. [Edit: I realize that in changing PosOut I am changing the bias, and am not sure how tight the acceptable bias range is]
Many thanks again for all your help, Derek
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Regulator drop out should occur as PosOut voltage is increased, as the dropout occurs when the difference in voltage between PosIn and PosOut is not large enough. As your bring the PosOut voltage up, the heat dissipated in the regulator will decrease.
I wouldn't worry too much about the bias voltage being off by a few tenths of a volt. That will move the plate voltage around a little bit, but it shouldn't be very significant.
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Ah, many thanks. So the reg board "reads" and regulates the difference between out and in?
I must have been wrong about the 10.45V PosOut not humming, b/c I just tried 10.3V out and got instant hum on both channels - and both were dead silent before.
I am going to try to hit exactly 10V out and then play with the cathode resistor value.
cheers and thanks, Derek
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Ah, many thanks. So the reg board "reads" and regulates the difference between out and in?
The 1085 just regulates its output, provided the voltage at its input is high enough to do so.
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Alright - so I'm guessing that the 1085 drops a minimum voltage doing so, and hence the need for a min difference between V-in and V-out. Increase V-out too high, and the regulator starves for voltage.
No need to correct me if I'm wrong - fixing all my misunderstandings via forum posts could take a lifetime ;D
cheers and thanks again, Derek
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Alright - so I'm guessing that the 1085 drops a minimum voltage doing so, and hence the need for a min difference between V-in and V-out. Increase V-out too high, and the regulator starves for voltage.
Sure, though when the regulator drops out, you'll still get decent voltage out of the regulator, but the DC regulation will become poor and the noise rejection abilities of the regulator will follow.
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Adding 18.2R in series with the 931R gets me 9.97V-out on one side and 10.02V-out on the other. PosIn is about 13.2V on both sides. Approx. 4.7 V dropped across the fil. No hum at start-up. Going to bring the fil voltage up to approx. 4.85V or so -- just within 3% of target. If that works and no hum after 2 hrs, I will call it good.
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Hmm. The right channel is humming - not much, but it's there intermittently. I am going to measure the difference between Pos In/Out on both boards to see if the right channel has the smallest difference - as I expect. And then tweak Rset2 to get the same difference as the other channel.
Failing that, I will simply reduce Rset2 on the right channel to where there's no hum.
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Where you get no regulator hum today may be different than where you get no regulator hum tomorrow unless you're on a regenerator. The big challenge is that if you lower the 8 ohm filament bias resistors, then you will both increase the voltage across the 300B filament (a good thing) and increase the filament current (not a great thing for proper circuit operation). The extra current will drop more voltage across the active noise filter at the output of the regulator, can increase the thermal stress on the 1085 regulator, and can draw down the raw DC voltage available to the regulator in the first place. It's a pretty delicate balance!
The BeePre is just not a particularly flexible circuit when it comes to altering it to work with a tube that isn't a 300B, and unfortunately the EML 300B doesn't meet the factor specs.
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It's a pretty delicate balance!
Yeah, you're not kidding there!
I've swapped the 18.2R for a 10R in series with the 931R on the offending right channel, bringing Pos-Out down to about 9.93V from 9.97V which gives me almost exactly 4.8V across the filament (so just inside the 4% target, exactly where you recommended that I start over a weeks ago!) with a 120R added in parallel with the stock 4R. A tiny bit of hum of intermittent hum in the first 10 seconds after power-on, but no hum since then. I'm going to let it play for about 4 hrs and then check again.
If need be, I'm even willing to run the tubes with a little less than 4.8V across the filaments. This will undoubtedly hurt tube life, but for me it's worth it. To my ears the EML 300B has more energy and detail than the JJ 300Bs, and havign heard them in the Beepre, it would be hard to go back.
Well ... regardless of what ends up working, I can rest assured that I made some serious efforts to get the amp dialed in and that whatever "solution" I end up with is pretty much the best I can do. Plus, I learned a bit about Schottky diodes and the Beepre circuit - which is great.
The Kaiju fil reg circuit will be a little more forgiving, right? Please say "yes".
cheers and thanks again, Derek
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Just to cap this off: it's day two and no hum. I ended up with only 10R in series with the 931R Rset2 on both boards. That brought PosOut down to 9.88V on one side and 9.93V on the other, @117V AC mains. That's still about 60 mV higher than where I started before installing the IXYS diodes and tweaking the Rset2 value. With a 100R in parallel with the 4R cathode resistance on one side, and 120R in parallel on the other, I get very slightly over 4.8V dropped across each filament.
My present mains voltage of 117V AC is the lowest I've ever seen it. We are having a hot and humid summer in Toronto this year and I suspect that everyone is running their air conditioning pretty hard and causing some voltage sag. I expect that by the end of September my mains voltage will be back up to 120V AC. I'm pretty sure I've seen it go as high as 124V AC in non-summer months. So I expect the risk of shunt reg dropout to be limited to summer ... but so far it looks like I have a winning combination even for the low voltage months. Very pleased. These tubes sound great to me, with more goodness to come as they are still breaking in.
cheers, Derek
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Well, I've only heard the regulator drop out once since finalizing the tweaks for the EML 300B. And I expect that with cooler fall temperatures, my mains AC V will bounce back up and it won't be a problem.
But ... I like playing with stuff, so I've been thinking. What about heating each filament with 2 x adjustable LM1085 reg boards, paralleled, each set for 5V out? I.e., split the work between them and stay well under any current and heat dissipation limits? Further, if the current used by two boards after voltage doubling and rectification, when added to the 1.3A filament draw, would exceed the current rating of the existing heater winding, the boards could be powered by a new separate heater PT.
AND ... if that's workable in principle, then would it not be possible, with the right heater PT, to use EML 300B mesh tubes with 1.5A 1.4A filament draw?
I have in mind a separate enclosure to house one large-ish heater PT with dual heater secondaries, rectification and regulation (four LM1085 boards). I figure I could put together the LM1085 boards on protoboard.
Would this be workable in principle?
[edited to correct misstated current draw of the EML 300B Mesh filament]
cheers and thanks, Derek
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Paralleling regulators doesn't do anything helpful in this scenario. Regulators don't split work well.
Buying an AC regenerator would definitely be the way to go.
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Thanks PB.
Paralleling regulators doesn't do anything helpful in this scenario. Regulators don't split work well.
I was thinking this might help if the reg was hitting its thermal or current limits - not as a solution for my low summertime mains voltage (which just got worse! The city repplaced the line on my street today and now I only get approx 115V! Hoping it's just everyone's fridge and ac working overtime after being off for 8 hrs and that mains V will go back up to at least 117V tonight).
But I did some reading and see your point about regs not splitting current well. Plus, it looks like the current limit of the LM1085 isn't an issue anyway - since it's rated for min. 3A output.
Question: If the current limit of the LM1085 doesn't limit the output current available, what does (assuming, hypothetically, that one has more than enough VA from the
heater winding)?
Buying an AC regenerator would definitely be the way to go.
Yeah, it's looking that way. But they are crazy expensive (PS Audio, Monster Power, etc.). Tripplites with mains regulation are a lot less expensive, but they only kick in at 111V, and just pass 112V - 127V straight through -- so they're no help. You'd think that a mains voltage regulator would be an ideal diy audio project, but so far my search for a schematic or build guide has turned up nothing. But I will keep looking. Maybe the fact that it involves mains power keeps people from posting about it? For liability reasons? But i find that hard to believe since there are lots of build threads/schematics for 211 and 845 amps that operate at really high B+. Dunno.
In the interim, I'm using my variac to bump up the mains voltage - but it's only good for 6 hrs max continuous use and needs frequent monitoring. So I will need to find a better, permanent fix.
cheers, Derek
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You could also get a 115:120 autoformer or power transformer to sit ahead of your BeePre.
Dropout occurs when input voltage and output voltage get too close together. That's why low line voltage is an issue. The regulator will not overheat with some other major problems going on.
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You could also get a 115:120 autoformer or power transformer to sit ahead of your BeePre.
Excellent idea! Thanks PB. And I can install a cheap analog panel mount voltmeter to monitor voltage.
Dropout occurs when input voltage and output voltage get too close together. That's why low line voltage is an issue. The regulator will not overheat with some other major problems going on.
OK, so I think I understand this part, b/c you've explained it to me before and I've read up a bit about regulators. But then why/how does a draw of 1.3A cause dropout? And why couldn't the heater circuit handle the 1.4A draw of an EML 300B Mesh? The LM1085 is good for at least 3A output - so I don't think the 1085 is the limiting factor. Just throwing out guesses here: Are the transistors current limited? Or does voltage output of the secondaries sag as the Volt-Amps demand approaches the VA rating of the secondaries? Is the power demand of 13.5V (after rectification) @1.3A just too much for the PT to handle? Or does something else explain this?
If the issue is limited VA output of the secondaries, then a relatively simple solution would be to get a separate heater transformer with higher VA rating. Does that make sense?
cheers and many thanks, Derek
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But then why/how does a draw of 1.3A cause dropout?
The voltage available from the raw DC supply drops, not necessarily only because of the 1.3A, but also because of low line voltage.
And why couldn't the heater circuit handle the 1.4A draw of an EML 300B Mesh?
The noise elimination circuit also has some series resistance, and drawing extra current through this will reduce regulated voltage.
If the issue is limited VA output of the secondaries, then a relatively simple solution would be to get a separate heater transformer with higher VA rating. Does that make sense?
The 6.3V secondaries have some extra current to give. If you bought a power transformer that had two 7V/4A windings, that would bring up the raw voltage a bit as well.
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Awesome. Thanks PB. 7V/4A windings maybe hard to find. But I figure I can go with 10V/4A and use dropping resistors to dial in the voltage.
cheers and thanks, Derek
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10V/4A with a bridge instead of a doubler might also work.
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Cool. Thanks PB.
Along these lines, I might also be able to go with 12.6V/3A with full wave and skip the doubler. I can play around with PSUD to see what works.
Is there a rectification advantage to using a 4-diode bridge instead of two-diode full wave other than not needing a center tapped secondary? I.e., does a bridge produce less ripple?
For modelling the heater circuit in PSUD, do you have a recommended load resistor value? Or can I just use PSUD's default value (I forget what it is)? Guessing that the answer is: add up all the resistance in the heater circuit and plug that in - but just want to be sure.
Edit: one more question - I plan to install switches on the transformer to allow for nominal +5V, 0V, -5V changes to the mains voltage. Shorting or non-shorting? I have no idea which is best for changing between windings.
Apologies for all the questions - I'll try to stay quiet for a while after this ;D
cheers and thanks, Derek
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In PSUD, you can change the resistive load to constant current. Most of the questions you're asking can actually be pretty well answered by doing some modeling in PSUD and looking at the power transformer current, ripple, output voltage, etc.
If you're asking about +5, 0, and -5V switches for a step-up before the BeePre, I would just leave it at +5, there isn't much penalty for having a little extra AC voltage. Otherwise I would recommend non-shorting switches.
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Cool. Many thanks PB. that constant current feature will sure make things easier.
cheers, Derek
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I'm going to try using an autotransformer to adjust mains volatge going to my BH amps. Since it's an autotransformer, it offers no isioation from mains power - but I figure this is ok since all BH amps have their own isolating transformers.
I'm going with the Hammond 168J - rated 2000VA and weighing 23lbs! I plan to use a 10A fuse on the incoming mains power - so run it at a max of 1200-1250VA (depending on mains voltage variations).
And so here's my question: Am I correct that running the 168J well below its max VA rating gives me some latitude on how to wire it? Specifically, the Hammond schematic shows incoming hot wired to 115V with options to take 85, 95, 105, 110, 115, and 125V out. I want to take power from the 115 tap and use switches to connect mains incoming power to either 105, 110 or 115 for nominal +0V, +5V and +10V output options. I figure I can use 2 X SPDT switches (high voltage/current rated) - please see attached wiring scheme.
Does anyone see a problem with this?
many thanks, Derek
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I'm going with the Hammond 168J - rated 2000VA and weighing 23lbs! I plan to use a 10A fuse on the incoming mains power - so run it at a max of 1200-1250VA (depending on mains voltage variations).
Is this just for the BeePre?
And so here's my question: Am I correct that running the 168J well below its max VA rating gives me some latitude on how to wire it?
You may find that the listed input/output voltages are not at all accurate with that transformer lightly loaded.
I figure I can use 2 X SPDT switches (high voltage/current rated) - please see attached wiring scheme.
What you're doing would work, but something like this will be a lot simpler:
https://www.mouser.com/ProductDetail/CK/R10307RN02Q?qs=sGAEpiMZZMvNbjZ2WlReYgfl1vEqXQaLTI2c32B8PEc%3D (https://www.mouser.com/ProductDetail/CK/R10307RN02Q?qs=sGAEpiMZZMvNbjZ2WlReYgfl1vEqXQaLTI2c32B8PEc%3D)
The Hammond 168B is a far better choice to use with the BeePre. This is an instance where spending 5x the money will net you far worse performance.
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It's for all BH amps - so, at minimum the Beepre and either Kaiju or an SII. And down the road, it might be running the Beepre, Kaiju and both SII's off of it at the same time.
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So 168C, or maybe D. The Kaiju and SII are quite a bit more tolerant to line voltage fluctuations and have the universal power transformers that allow you to wire for lower line voltage if that comes up, so you may want to consider making this a BeePre only effort, as you could really dial things in if you only used the BeePre on the autoformer.
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Many thanks PB. Good to know that it should at least work in principle. I will weigh my options. I do like the idea of having everything plug into one unit. Maybe even my dac as well. The bigger plan is to incorporate 3-4 combo common/differential mode filters (already built), each rated 3A, and have 3-4 pairs of outlets - with switches for bypassing filtering and possibly even the autotransformer. I'm even contemplating using some high current/voltage schottky's with 1.5V forward voltage drop on switches to better calibrate the voltage. Plus an analog voltmeter dial to monitor what's going on.
cheers and thanks, Derek
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I'll add that much of fun for me is being able to think something up myself, build it, and have it work reasonably close to as planned. I 'm ok with the prospect that years from now (or perhaps much sooner) I'll look at the thing and realize that I could have achieved the same or better results for 1/5 the price and 1/4 the effort. And if I later choose to dismantle it, I'm pretty confident that the parts will get used in other projects.
cheers, Derek
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I have a working breadboard rebuild of the BeePre and am working on dialing in the filament regulator (LM1085 board). I have a bunch of questions that I'm hoping @PB and @PJ can help me with. Any directions/pointers you can provide will be greatly appreciated. I am taking my time with the BeePre rebuild and really want to find an implementation that works well with my EML 300B tubes.
In stock configuration the reg board is set up so that Vin - Vout = approx. 3.5V. Based on my best attempt at making sense of the TI datasheet for the LM1085 (attached), it looks to me like the LM1085 only needs about 1.5V difference between Vin and Vout, and even @1.5V there's should be at least a 200mV cushion at typical operating parameters. But the stock config has an additional 2V of difference - which would otherwise suggest to me that there is huge cushion to prevent reg dropout, but my past experience suggests that this isn't necessarily the case and that the reg can drop out even where Vin - Vout > 1.5V.
So my first question is: is some other part of the fil reg circuit dropping the additional 2V? As best as I can tell, the first half of the fil reg circuit is identical to the TI "ripple rejection enhancement" model found at 8.2.9 of the datasheet, except for the use of 33uF caps where TI has 10uF caps. But the last half of the reg circuit looks like its based on what I've learned is called a "Sziklai pair" - close cousin to the "Darlington pair" but with reported thermal regulation, impedance and possible lower voltage drop benefits over the latter -- I dunno about any of this: trying to read about it made my head spin. Related second question: What does the second half of the fil reg circuit do? / why is it there?
My next set of questions is about noise and ripple rejection. When the regulator drops out, where does the resulting hum come from - is it power supply ripple being passed to the filament? or is it generated by the reg circuit itself? If the former (passed power supply ripple), then my separate filament trafos with CLC filters that reduce ripple to a modeled (PSUD2) 8 mV pk-pk should help to keep things quiet if the reg drops out (for comparison, my PSUD model of the BP stock voltage doubler/rectifier shows about 2.25V of ripple being fed to the inputs of the reg board - all of which is eliminated by the reg circuit). But if the reg circuit is the source of the dropout hum, then no amount of ripple rejection before the reg circuit will make a difference.
To minimize the incidence of dropout, I'm thinking of increasing Vin to 14V instead of 13.5, while keeping Vout at 10V. I can't tell from the datasheet whether this decreases ripple rejection / increases noise. Does it? And if it does, do you think some or all of the increased noise would be offset by the increased ripple rejection of my CLC filter? If my basic arithmetic and use of an online dB calculator is correct, then 2.25V pk-pk (stock) : 8 mV pk-pk (CLC) is 281:1, or about -24.5 dB.
I anticipate that increasing Vin by 0.5V will put more thermal stress on the regulator. I plan to deal with heat stress to the LM1085 via (a) replacing the stock mica spacers with "kerafol keratherm" insulators that are reported to be more heat conductive than mica and comparable to a combination of mica and thermal grease; (b) forced air cooling @ 400 cfm. But this only takes care of the LM1085 (hopefully). Might other parts of the board - e.g., the transistors, need additional cooling if I bump up Vin?
Apologies for the numerous questions, but I really want to get this right.
cheers and many thanks in advance, Derek
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Congratulations! You are discovering the large amount of work involved in developing a detailed design! I'l offer a few comments,, but you'll still have to do the work.
There is some headroom on the dropout to allow for variations in powerline voltage, plus more to allow for ripple on the input line.
The second part of the BeePre regulator is a broadband noise cancellation circuit. You should be able to estimate the voltage loss.
Power dissipation is easily calculated if you know the voltage and current in a component. Temperature is more challenging due to variation of the environment.
In my experience, you want to keep the junction temperature of silicon components below 100C for longevity, even though a specified maximum temperature is usually around 150C.
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Many thanks for the pointers PJ. I am happy to do the work.math myself - otherwise I won't have a clue about what's going on.
Congratulations! You are discovering the large amount of work involved in developing a detailed design!
Well, what I'm trying to do is a far cry from designing a filament regulator - but even trying to tweak the values of your regulator is proving to be a sizeable research and learning endeavor. :)
I'll look into the voaltge drop of the broadband filter and do some power.heat dissipation calculations as suggested.
Just one repeat question: when the reg drops out, is the resulting hum power supply ripple passed to the filaments or noise produced by the regulator itself? This is important b/c if it is power supply ripple, I will be sure to find room for my CLC filter; but if the hum is from the reg itself, then I'll focus even more on preventing reg dropout and consider leaving out the CLC filter if space is limited.
cheers and many thanks, Derek
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If you can't deduce that from the data sheet, then you'll have to do the experiment to find out.
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Well ... I couldn't figure it out from the datasheet, so I did a listening test on the breadboarded BP. The filament heater circuit of the right channel has a CLC filter that delivers 13.9V to the LM1085 board, and the equivalent of a 3.87R cathode resistor that results in 4.85V dropped across the EML 300B filament (I hope to bump that up to 4.9 by tweaking Rset2 to bring Vout to 10V from existing 9.91V). The left filament heater circuit is stock, and delivers approx 13.45V to the LM1085 board, and has a 4R cathode resistor, resulting in 4.75V dropped across the EML 300B filament.
I listened to one channel at a time - starting at 120VAC mains via a variac and then dialed down the mains voltage until I got hum (dropout).
Right channel: hum started at 107VAC - hum is fairly quiet and doesn't get much louder if I dial down the mains voltage even further (e.g., to 100VAC).
Left channel: hum started at about 113.5VAC - hum is loud and buzzy, and gets much louder as mains voltage is dialed down more (e..g, to 108VAC, at which point the hum was really loud) [This is a bit surprising b/c in the summer I was getting dropout at mains VAC < 117]
So my provisional conclusion is that at least a good part of the hum from reg dropout is either (a) power supply ripple itself, or (b) caused by power supply ripple. But I can't rule out the reg producing some of the hum, independent of power supply ripple.
And for building purposes I've learned two things: (1) a modest bump in voltage feeding the LM1085 board (in this case, just less than 0.5V) gives me a huge buffer for dealing with variances in mains voltage and preventing dropout ; and (2) in the event of dropout, the CLC filter tames the hum considerably. So I plan to incorporate both.
I haven't done any heat dissipation calc's yet, but I'm hopeful and somewhat confident that the combination of new heat transfer compound (in place of the mica) and forced air will do the trick. I seem to recall the Ohmite datasheet for the heatsink showing that forced air results in significantly cooler temps. I'll have another look.
cheers, Derek
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I would also mention that it took PJ (and me to a lesser degree) months to figure out how to make the BeePre sufficiently quiet. Don't be surprised if it's a long road.
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It's already been a long road! ;D I've been at this since July.
Based on today's listening test, I think the filament reg setup is looking good and all I need to worry about is thermal overload. I'm turning my attention to that now.
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Okay. I've done some very rough heat dissipation calculations.
I started from the following formula set out in the Ohmite heatsink datasheet:
TJ = TA + PD (θJC + θCH + θHA)
where,
TJ is junction temperature,
TA is ambient temperature,
PD is the power consumption,
θJC is thermal resistance of junction to case (stated on datasheet to be 0.7 C/W)
θCH is thermal resistance from case to heatsink - I'm assuming this is the thermal resistance of the insulator
θHA is the thermal resistance of heatsink to ambient air
I've also made three simplifying assumptions:
(1) that TA (ambient air temp) is the same in the stock BP as in my future rebuild. I think this is a conservative assumption since I will be adding a lot of vent holes on the top plate around the tubes and the pc boards, the new chassis is much larger than stock and will have more open space, there will be at least an inch air gap at the bottom of the chassis (min 1 in feet), and the two 140mm x 140 mm fans will draw cooler air from outside the chassis
(2) that the total difference between Vin and Vout is dropped by the LM1085
(3) that the reg board draws a current of 0.5A [correction: 0.05A] in addition to the current drawn by the filament (this seems high - but I figure it will do for rough calculations) and that all current passes through the LM1085
Assuming the above, this means I need only compare PD * θ-total for stock and rebuild.
Stock w/ Regular 300B w/1.2A fil current
θJC (junction to case) = 0.7 C/W (from datasheet)
θCH (case to heatsink) = 0.5 C/W (can’t find hard figures – 0.5 was the lowest stated I found for a mica insulator w/o thermal grease/paste)
θHA (heatsink to ambient) = 3.5 C/W @ 100 linear feet per minute (can’t find estimate for natural convection, so I am using 100 lfm from the heatsink datasheet, which is the lowest forced air rate indicated – this seems high to me)
So PD (θJC + θCH + θHA) = 3.5 * 1.25 * (0.7 + 0.5 + 3.5)
= 20.56
Rebuild w/ EML 300B w/ 1.3A fil current
θJC (junction to case) = 0.7 C/W
θCH (case to heatsink) = 0.09 C/W (thermal resistance of Keratherm Red. This seemed insanely low to me until I check the thermal resistance of Kerafol's other types of Keratherm insulator pads. Red is their best. Their worst thermal performer is "Keratherm Brown" @ 0.44 C/W, - so I am inclined to trust the specs)
θHA (heatsink to ambient) = 2.5 C/W @ 200 lfm (reduced from my calculated 291 linear feet per minute which gives 2 C/W - just to be conservative)
PD (θJC + θCH + θHA) = 4 * 1.35 * (0.7 + 0.1 + 2.5)
= 17.82
So, based on this admittedly rough estimate it looks to me like it might work.
If I've mucked this up, I'd appreciate someone telling me :)
cheers and thanks, Derek
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After doing the calculations, then you have to build it and measure the temperatures and compare the differences.
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Just a note - the National Semiconductor data sheet specifies thermal resistance as 0.7 C/W for the control section, and 3.0 C/W for the output section. TI seems to have changed how they do thermal calculations, and in a quick look I don't see them making the distinction. I've always used the 3.0 figure, which is compatible with most other TO-220 packages.
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Many thanks PB and PJ.
Using 3.0 C/W results in the following estimations: stock 30.65, rebuild 30.25. Since I think I was fairly conservative in estimating cooling for the rebuild and more liberal in estimating cooling of stock, this gives me enough confidence to proceed with the rebuild. I have the great advantage of having a proven comparator (stock) that I know was designed to perform within the proper thermal operating range of the LM1085. So all I need is a temp rise ≤ stock and I should be good. At the very least, my rough estimate suggests that actual thermal performance of the rebuild will not be significantly poorer than stock. Once it's built I can use my cheap thermocouple to measure case temperature and use the reading to estimate junction temp. If it's below 125C, all is good. If it's below 100C, all is great.
If more cooling is needed, I figure I can bump up airflow by adding a funnel to each fan to direct more/faster air at the LM1085 heatsinks.
And now back to trying to figure out layout - you'd think that having more than twice as much area to play with (320 in-sqr versus 144 in-sqr) would make layout a breeze, but it's not! The CLCLC HV and CLC LV filters take up a quite a bit of room. And I still need to figure out how I want to power the fans - I have a separate trafo that would do the trick, but that will take up even more space. So I'm considering taking power from the Pt-7 LV windings. And either way, I'll need a small CLC filter to get proper DC.
But I'm having fun. I got my second wind after hearing the hum-free breadboarded BP. Man ... that hum issue drove me insane - example: removing, filing, cleaning and reinstalling the house ground connection to the water supply pipe at 2AM on a Saturday night! Did nothing, of course, but it was badly corroded and loose, so in the end I'm glad I did it.
cheers and thanks, Derek
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... I did a listening test on the breadboarded BP. The filament heater circuit of the right channel has a CLC filter that delivers 13.9V to the LM1085 board, and the equivalent of a 3.87R cathode resistor that results in 4.85V dropped across the EML 300B filament (I hope to bump that up to 4.9 by tweaking Rset2 to bring Vout to 10V from existing 9.91V). The left filament heater circuit is stock, and delivers approx 13.45V to the LM1085 board, and has a 4R cathode resistor, resulting in 4.75V dropped across the EML 300B filament.
I listened to one channel at a time - starting at 120VAC mains via a variac and then dialed down the mains voltage until I got hum (dropout).
Right channel: hum started at 107VAC - hum is fairly quiet and doesn't get much louder if I dial down the mains voltage even further (e.g., to 100VAC).
Left channel: hum started at about 113.5VAC - hum is loud and buzzy, and gets much louder as mains voltage is dialed down more (e..g, to 108VAC, at which point the hum was really loud) [This is a bit surprising b/c in the summer I was getting dropout at mains VAC < 117]
So my provisional conclusion is that at least a good part of the hum from reg dropout is either (a) power supply ripple itself, or (b) caused by power supply ripple. But I can't rule out the reg producing some of the hum, independent of power supply ripple.
And for building purposes I've learned two things: (1) a modest bump in voltage feeding the LM1085 board (in this case, just less than 0.5V) gives me a huge buffer for dealing with variances in mains voltage and preventing dropout ; and (2) in the event of dropout, the CLC filter tames the hum considerably. So I plan to incorporate both.
@PJ or @PB - I may have found an error in my reasoning. I previously concluded (above) that the extra 0.5V DC input to the LM1085 reg board greatly lowered the mains VAC dropout point. But is it just the extra VDC? Or does a significant reduction in incoming ripple account for some or much of it? My previous conclusion was premised on the idea that the reg board acts as a filter that smooths the voltage supply - but it just occurred to me that maybe it "shunts" the ripple (still not sure how the reg circuit works), in which case incoming ripple voltage reduces max regulated DC output. If it "shunts" the ripple, then I I think it follows that I can reduce the input VDC to 13.5 (as provided on PJ's schematic), and still have a lot of dropout headroom, AND run the reg board cooler - b/c I'm using a CLC filter in front of the reg board that knocks down ripple considerably.
Am I right about this? I'd test it myself, but the wood chassis mock-up rebuild is almost done and I don't want to risk damaging parts with any more desoldering/resoldering than is necessary. But if the above makes sense, I will tweak the incoming VDC of the reg board once the amp is up and running again.
many thanks in advance, Derek
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When I calculate regulator performance, I look at the input (DC with ripple) and take the lowest voltage - i.e. average DC minus peak ripple - then allow further margins for dropout and line variations.
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... I look at the input (DC with ripple) and take the lowest voltage - i.e. average DC minus peak ripple ...
Many thanks PJ. I think that confirms my suspicion. Once the mock-up of the amp is functional, I'll do a bit of experimenting with dropout. If I'm lucky, the CLC filter in front of the reg board may even allow me to go lower than 13.5V input and still be dropout-proof down to 115VAC mains.
cheers and thanks, Derek