Bottlehead Forum
General Category => Technical topics => Topic started by: Paully on April 06, 2012, 10:12:33 AM
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Excuse the cross-post from Tube DIY, and hopefully Doc won't ban me for mentioning a tube rectified power supply, but here it goes. Grainger and I are going to build an 866jr full-wave (two tubes) rectified stereo amplifier. Why? Because they glow! 45 outputs with a 76 driver. For the power supply according to the tube manual (link below) if our max input voltage is 500V we should choose a choke input to follow the rectifiers with a minimum value of 2.1 Henries. That is followed by a capacitor with a maximum rating of 4.2uf. The manual says we can increase the inductance and then increase the capacitor in proportion. We are planning on using an LCLC setup of a 5H 400V choke / 10uf cap / 5H choke / 10uf cap being fed by a 375-0-375 power transformer. It is going to be a stereo amp, not monoblocks, with four of these rails, one for each driver and output. Sound good?
Last question, I obviously have the filament chokes in my Paramounts. They seem like a very good option. Any reason not to use them with 45's in the new amp?
http://www.tubebbs.com/tubedata/sheets/084/8/866Jr.pdf
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Are you really planning to use mercury vipor in your living room?
Wikipedia:
Mercury compounds are toxic, highly persistent in the environment, and present a danger to humans and the environment. The use of large quantities of mercury in fragile glass envelopes presents a hazard of potential release of mercury to the environment should the glass bulb be broken. Some HVDC static inverter stations have required expensive clean-up to eliminate traces of mercury emitted from the station over its service life. Steel tank rectifiers frequently required vacuum pumps which continually emitted small amounts of mercury vapor.
Also, just curiosity, what are the objectives of this project ? Are you after the best SET 45 sound or there is something else ?
Thank you,
Alex
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From what I have read, and its worth what its worth, there is less Mercury in one of these vacuum tubes than in a typical CFL light bulb. Maybe, maybe not but I have plenty of CFLs in the house with plenty of mercury and don't worry about accidentally breaking one when I have them throughout the house and I am more likely to break one of those than one of my precious vacuum tubes. Not worried about continual release during operation, its not like the vacuum tube is going to work if the vacuum seal is broken.
The point is to build a vacuum tube rectified SET amp and not much more, whether it will sound better than solid state is open to question. I wouldn't want to state that it would when I have no idea and the guys around here who design the amps think SS rectification works much better in the bass. I want to play around and have some fun with it. And the MV rectifiers look interesting.
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Personally, I'm using tube rectifier with my 300b set. It's Mullard cv378 (version or 5u4g). I tried number of SS rectifiers (soft, fast, whatever) and ended with tube rectifier with CLC (20mkf/30H/660mkf) filter.
I did not calculate any values for the design you've described, but looks like with the values you have, there will be 120hz buzz and LF roll off , due to very low value of capacitors, and these values are direct result of rectifier tube selection ( lets not to worry about mercury for a moment).
By looking tube data, seems like 5y3, 5u4 etc. are better candidates. Also you can easy compare tube with SS.
Also there are a lot of other things to consider and compare - driver tube/topology, autobios vs. fixed one, direct coupling vs. cap/transformer, output transformer/topology. All this will probably change sound more than type of the rectifier. Just built oversized power supply and worry about other things. That is my 2c ...
Google please, Flesh and Blood by Reichert - he has pretty good advice on PS.
Good luck with project - 45 is fun!
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Paul owns the 866JR tubes already. The last C in the LCLC filter should be increased to give a larger storage capacity as well as better filtering. The choke input for the filter is required for the 866JR.
After the tubes for rectifiers there will be separate LCLC filter circuits for each 76 and each 45.
Both the driver and output tubes will be cathode biased, much like my Paramours.
Paul has also run his Paramounts with 45 outputs and this is why he wants to scratch build this amp.
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That is why I asked what are the objectives. Ok, so the objectives are to utilize what is avalability and built paramount with the glow. Correct?
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The circuit will resemble the Paramour and Paramount. A driver capacitor coupled (possibly not) to the output. Parafeed output, the iron is already bought. It is the power supply that is still up in the air.
The post above is modified with a little more information.
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Again, it's just an idea, nothing more: why not to built on the sheet of plywood outboard ps and try different tubes, capacitors etc. with the paramount. That will give you very good reference point - I guess you know sound of you amp. well enough.
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The inductance from the data sheet only ensures that the peak rectifier current is not exceeded. To get good regulation, the first inductor should be at least equal to the kOhms of the load. For a 375v transformer you will see about 335v, and if the whole amp draws 80mA, the equivalent resistance is 4.2K, so you need at least 4.2 henries. If on the other hand there is a separate LCLC feeding one driver at 5mA the the equivalent resistance is 67K and the choke must be 67 henries. The voltage will still rise to over 500v at startup, until the tubes draw full current. Traditional practice was to use a bleeder resistance to set a minimum current (usually 10% to 20% of the total) and size the inductor for that current, so that the voltage won't rise that high even if a tube goes out.
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Sounds like a mess. So by having four filtering circuits, one for each major tube, I drop the current for that section from the total for the amplifier and therefore have to raise the inductance of the inductors in that section. I am just starting to look up equivalent restance, but say for a single 45 drawing around 30 ma, we are talking about a choke somewhere around 30H, give or take, if I am starting to get the gist of this plus it needs to be around 600V And that doesn't even begin to show what size caps we need which will be large. Ugh. Anyway, I will look up the equations you used to figure out the Henries and decide if this is still worth the playing around given the cost.
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Remember, for small-current stages (drivers) you can use small-current chokes (small and cheap).
Power supply filter theory is fairly simple and widely documented; google "choke input filter" for instance.
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I appreciate the input, thanks. Time to do some more research. Thanks again!
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With just 2, 45's and 2, 76's. your not drawing a lot of current. Maybe try 2 power supply's, one for each side, or one for drivers, one for outputs...John
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I read on a DIY site that 83 mercury vapor tubes which are similar to 866's glow in proportion to their current draw. Someone who had an amp that drew 30ma was complaining that the 83 rectifier didn't light up much. He was referred to a youtube video (Bach Toccta D-moll on a 1000w audio tube), which showes an impressive array of 83's with a bright blue modulating glow. More current more glow less current less glow.
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A dummy resistor could be used to increase the load current. Maybe after the initial LC filter?
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Gave up the mercury rectifiers, just too much trouble. Modelling on PSUD a Pi filter with the following parameters to feed a 45 amp with 76 drivers and a voltage regulator with a GZ33. Planning on a rail for each driver tube and output tube, which is hard to model on PSUD but essentially the first cap totals to 50u for all rails combined. Going to use a 475-0-475 transformer (need about 425V out on each rail for what we are planning. The CLC filter for each 45 is 15u/20H(181 ohms)/20u. For the 76 it is 10u/10H(270 ohms)/20u. PSUD doesn't seem to have any issues with what we have done for warnings. Anyone see a problem such as too taxing on the rectifier, etc...? Any thoughts are welcome.
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Anyone trying to check this with PSUD II will need to use a 50uF cap for the first cap in the string. Without that the instantaneous current will be too high. I figured that was only a little cheat.
To clarify there are 2 rails/filters for the output tubes and two rails/filters for the driver tubes
We are hoping that the 76 will have enough voltage swing to get the 45s to a comfortable volume (note not to clipping) with Paully's speakers, 100dB+ sensitivity. He doesn't listen loud at all. We do not ever want the 45s to produce maximum power.
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According to GZ33 datasheet C max = 60uF. Also with 475+475 and capacitor input it will be around 465v DC on the first cap. Looks like HT for 45 is OK. 10H choke probably is not enough for 76 to filter properly. You may need to enlarge capacitors and add load to get more current through the choke.
Here is the datasheet http://www.r-type.org/pdfs/gz33.pdf
BTW, why 4 HTs ?
HTH
Alex
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The plan is a choke for each rail so we have 4 chokes (2 for each 45 and 2 for each 76). Does that explain the 4 HT (I am drawing a blank on HT, sorry)? We will do a lead off 15u cap for each 45 and a 10u for each 76 (total of 50u leaving us well under the 60u total). Do you think bumping it up to 20H on each 76 would be enough? We can certainly bump the end capacitors to get more smoothing on each rail as well, I don't think that would hurt anything. I will go try that on PSUD. Thanks!
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Ok. Let's talk about one amplifier. I presume we are talking about 2x mono,right ?
So, you have one full wave rectifier (correct?),capacitor loaded ( that is the cap. With max value 60uF). What's next ? L-C-L-C where first half of the filter loaded by 45 and 2nd by 76 ? Or parallel lL-C for 45 and L-C for 76 ?
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Alex,
This is a stereo amplifier.
There will be one power transformer for the B+ (500V-0-500V) feeding a single GZ33 (dual rectifier). That will feed four filters, driver and output tubes, right and left channels dual mono after the rectifiers. This is an old Bottlehead trick of PDMPS (pseudo dual mono power supply).
So when we talk about the 50uF that the rectifier "sees" it is the total of the four input caps. Then each string will have a choke and a film cap to feed the voltage to the tubes.
Does this make sense???
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Sorry I missed the switch - you are now looking an a CLC filter instead of LC or LCLC? That will increase the power supply voltage 35-40% - like, 600v or more from a 500v winding. That's a LOT, and way too much for 76s or 45s.
Am I missing something?
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Grainger,
Now I'm lost .... How rectifier " sees" four capacitors ? If rectifier is loaded by capacitor, it's one capacitor, never mind if it is made out of four parallel connected capacitors. If rectifier is loaded with LC, there are no capacitors at all, rectifier "sees" inductors first. Also, with regards to dual mover supply (psudo or otherwise), I'm not clear why driver and output tube need to have separated filters.
Speaking of output voltage, "Typical operational conditions" for GZ33 shows with 500v AC and C load (from 8uF to 60uF) output voltage 479-493 v DC with 250 ma (!!!) current.
I not really comfortable making recommendations, i hope Paul will comment, but with 1000vCT transformer, the way to go is LCLC or LCRC for each side with additional R for bleeding.
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Paul,
We modeled this on PSUD II with a GZ33 rectifier and the 500V transformer. We didn't get that high a voltage out at all. We had started with a 375V transformer and had to keep increasing the voltage.
I think it would be best if I can somehow post the circuit from PSUD II.
Alex,
The four capacitors are the first caps of the CLC filter. They are all across the rectifier's output so they sum to 50uF. If we model with a 15uF cap the rectifier has too high an inrush current. So we were modeling with the 50uF as the input cap. I made no mention of an LC filter above.
Here is a link for the PDMPS on VoltSecond's site to catch you up:
http://www.siteswithstyle.com/VoltSecond/PDMPS/pseudo_dual_mono_foreplay.html
I don't understand your emphasis on 250mA current, is it necessary to draw full current to get the 479-493V output?
There will be a ~0.5M ohm bleeder on each B+ leg.
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Not sure what PSUD II shows. Did you design four separeted CLC filers with it ? Anyway rectifier GZ33 with 50uf input and current much less then 250 ma will give you from 1000vct transformer voutage higher then you need.
250 ma is important to get voltage drop on internal resistance of rectifier. Lower current, smaller drop.
Put GZ33, 1000vct, 50uf and current of 2x76 +2x45 into PSUD and check what voltage you will get.
Bleeder resistir: 500k - it's less then 1 ma , won't help to much with rectifier/ filter load.
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Since there are pairs of rails, one pair for drivers one pair for 45s, we only looked at two CLC filters, so we did cover all four.
So you are saying that we need more current drawn through the rectifier to get a lower voltage, right?
I'll try to post PSUD shots later today.
We are not looking for the bleeder to be part of the load, just a safety device. Most Bottlehead equipment, maybe all, has a bleeder of 400k+ ohms. It is simply for bleeding the PS dry when you turn it off.
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Grainger, just look at the first stage of the power supply. You have 2x500v AC, full wave rectifier loaded by 50uF capacitor. If you have no load and no losses in the rectifier, you will get ~700v DC on the capacitor. Now you're loading this PS with 4 tubes via LC filters. That's were current starts to play a role. All of the voltage drop will be on the rectifier (there are relatively small voltage drop on the DC impedance on a choke in CL filter, as opposite to LC).
I downloaded this PSUD software. With 1000VCT/GZ33/47uF/3H/47uF/120ma load PSUD shows 600v output.
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Alex,
Thanks, I have decided, for some reason, PSUD is not computing properly on my computer. When I "simulate" I can not read the scales, neither the left side or the bottom. I see what you are saying. The resulting voltage is too high. And I'm with you, there is only a drop in voltage across the inductor if there is appreciable current. Since there is not much current total here. I would guess less than 100mA. We wanted a Beefy power supply.
Since I can't run PSUD well on my computer, what transformer voltage would you suggest for a 425V output?
Your trace is on the first cap and not after the inductor so I won't worry about the ripple. How did it look on C2? Was it smooth?
BTW, how did you get a JPG of the screen??? I'm not much of a computer guy. I'm old!
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Ok. Last thing first. Whatever screen is active, you press alt+contr+shift+print_screen and after that go to whatever image apps you like (Microsoft Paint will do) and paste into into apps (could use contr+v)
Control+shift+print screen will cupture entire screen, with ALT only wiindow "in-focus"
For 425vDC after gz33 with C input, gz33 datasheet shows 2x375v AC (750vCT).
I installed PSUD in the office. Will let you know what's data I can get from it tomorrow.
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Thanks for it all!
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here we go ...2x475v AC
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Beautiful! It never reaches 500V, the rating for the inductor's insulation. Looking at the RMS for R1 it is up at 379V. Interesting, the graph shows different voltage than the RMS value in the chart next to it.
This is part of my confusion about PSUD II. The RMS voltage should be the steady state supply voltage. That is why I'm so befuddled with PSUD.
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Ok, I tried with all the same components you have and I got much different results.
WOW That is tiny. Click and it will open in a window. As you can see I have ripple across R1. I don't think any of the voltage and current values are the same as you have.
This is why I get frustrated with this program.
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Grainger, your first capacitor is 50 farads, not microfarads. That will make a difference!
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I'll look at it again, Thanks!
We are still wondering, looking at Alexz's example above, how do you start with a transformer that puts out 750V and R1, the load has a calculated RMS voltage of 379V? The graph of Vc1 shows DC of 480-490V.
What am I missing?
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Second try got almost the same numbers as Alex.
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Oh yeah - I forgot to post this one. It's bit me in the backside more than a few times!
The average, rms, max, min, etc. cover the entire reported interval. If you don't use a delay, then it includes the initial start transient. A ramp from 0 to 500 volts is an average of 250 volts. You have to use the "after an initial delay of..." if you want to know where the voltages etc. end up.
By the way, there's another hidden time bomb. If you use a constant current load instead of a resistor, it offers the option of changing the current after a delay time. Very good for approximating tube warmup - but in my experience this gets introduced without your asking for it, at seemingly random times. If it looks weird, re-check the current load specs!
Oh yeah, here's another one. The transformer is described by its UNLOADED voltage plus its DC resistance. Transformers are rated by their LOADED voltage, after the resistance has dropped the voltage due to the entire rated current. The difference is usually at least 5% - so if you use a generously over-rated transformer the voltage will be at least 5% higher than the rated voltage. Plus, most transformers today are still rated for 115 volts input, and most houses have 120 to 125 volts. There's another 5% to 10%. So a transformer rated 375v may have an unloaded voltage of as much as 430 volts. And by the way, the resistance is the secondary resistance PLUS the primary resistance multiplied by the turns ratio squared. There's a calculator hidden in PSUD, but a lot of people aren't aware they need to find it and use it. Bottom line, you need to buy the transformer, measure the heck out of it, and do some calculations before you can actually put it in PSUD and expect realistic results. You can't just use the specs in the catalog. Serious pain in the aforementioned backside.
It's a really cool simulator and very useful, but it does have a lot of traps for the unwary! You know how, in house painting, the surface preparation takes more time than the actual painting? Same thing with PSUD - getting all the numbers and getting them into the program without triggering any hidden "features" takes more time than using it to get useful results.
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Paul, Thanks! I am beginning to see the light at the end of the tunnel. I added the delay and the voltage plot is a sine wave. But the ripple goes from 476.58V to 476.56V, 20mV. Additional resistance should bring that down, right? Or is 20mV good?
PSUD is a free program and so is EAC. Neither seem well documented. But I got what I paid for.
If we need to shed voltage do you suggest a resistor before the tube Anodes, after rectified or in series with the inductor. I am thinking in the AC legs will soften the blow on everything.