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PostPosted: 09 Dec 2017, 12:04 
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Joined: 17 Mar 2016, 07:26
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Location: Prêles, Switzerland
Dear Bruce and builders,

I have mounted the PSU as in the scheme, and the tests look good. I started with a Variac, taking up the voltage up to 230-240V, all ok, no smoke, and good 6V and B+ around on both channels. I did a few things right...

... however, I have a bit too much B+ voltage I think. With a load of 6'600 ohms on one channel (3 power-R 2K2 in serie), I still get about 180V on B+. Input is 230V. What kind of load does the amp present to the PSU, so I can check if I'm correct before frying other stuff ?

Thx!
Charles


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PostPosted: 09 Dec 2017, 14:18 
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Hi, The CCS governs the current and as shown with a 50 ohm resistor sets it at 25ma. The thing to watch is the dissipation of the tubes. It is 1.8 watts per triode (3.6 total) this makes the max acceptable voltage at about 145 volts. More will probably be OK for a while, but will shorten tube life.

Good listening
Bruce

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PostPosted: 09 Dec 2017, 15:26 
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Location: Prêles, Switzerland
Bruce, thanks for your answer and sorry for being such a noob with tubes... ok, so I understand each channel of the PSU will see about 25mA flowing out, and because I don't want to dissipate more than 3.6W, it means I really want my B+ to be <145V, ok so far.

It's a bit too late in my timezone to power my circuit again to verify how much I get after the diode bridge, but your schema mention 270V so let's take it as a base for my calculations. Note I'm using the exact same power transformer (Edcor XPWR083A-120/240) wired for 240V.

If I apply ohm's law across the R-circuit from the 270V right after the diodes bridge down to the B+ outputs of the PSU, I get the following data :

- assuming a constant charge (at least during my testings) of 25 mA pulled by the CCS
- assuming each PSU channel outputs 25 mA, it means :
- each R2 sees 25V across it (1000 * 0.025)
- each R1 sees as well 25 V across it (500 * 0.05)
- so I get 3 times 25V drops across each R (R1, R1, twice R2)
- so the complete circuit "loose" about 75 V
- 270 - 75 is 195, about what I got in my measure of B+

So I'm not sure why your circuit is supposed to present a B+ of 135-150V with these load values... The 180V I got with my 6600 ohms load is more in line with my calculations. I'm lost... I suppose I will have to rise the R values here and there, but I would like to understand where I'm wrong.

Edit: if my calculation is correct and I replace both R1 with 1K, I get 125V drop, so 270-125 = 145V.

Kind regards,
Charles

P.S. electronic engineer with 28 years in IT doesn't help to understand tubes !


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PostPosted: 09 Dec 2017, 16:25 
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Hi, A case of simulation not being equal to the actual at least in my build. The actual B+ (in theory) would be exactly as you indicate. Filter efficiency is what probalby brings mine down. So...since you really need to get it down to the right range, just increase one or more of the resistors. That is part of what DIY is all about.....recognizing when something is not correct and knowing how to adjust it.

Good listening
Bruce

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PostPosted: 18 Dec 2017, 17:09 
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Hey Guys, Bruce, et al...

I'm almost finally there with this project and I have a few questions. Have been away from a DIY audio project for quite a while, so I'm a bit rusty, so bare with me...

In terms of initial testing, do I need to have the tube board (with tubes plugged in) hooked up to the power supply PCB when obtaining B+ or Heater voltages? Also, Muskyhuntr mentioned in a PM that with no load the power supply will retain voltage for quite a while after the supply is shut down. What is recommended to drain that extra voltage out for safety? I'm assuming I can short to ground to drain the power? Also, couldn't a resistor also act as a way of putting a load on B+? Would 10k ohm be the right value since that's what the B+ normally sees?

On another note, reading through the thread someone mentions that center tapped secondary for is 37.5Ω for the 10k/75 trafo, I'm assuming the same applies to the 10k/600 trafo... thus 150Ω for the CT secondary.

I'm using the boards that Muskyhuntr came up with that mimicked what Vixr had initially created. I have some more questions about hookups into and out of the OTs. But I'll save that for when I get closer to completing this project e.g. once I have my chassis in place and ready to go.

I intend to follow Bruce's guide on first start up... fingers are crossed!

Thanks,
Scott


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PostPosted: 18 Dec 2017, 18:01 
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Hi Welcome back, I usually don't put bleeders on the B+ as I fully enclose my projects. The only person to open them is me. I was made well aware of the voltages in things a long time ago and do not want to have the experiences refreshed. However you can install bleeders if you chose. I like in that case to see about 500 to 1000 ohms per volt of unloaded B+. 100 volts would need about 50-100K, 5 watters for up to about 100V and 10s to 200V. More wattage is OK. This will give a drain of about 1-2 ma and bring the voltage down fairly quickly. Still it is good practice to let things sit a while and when possible check with a meter that the circuit is actually at a low and safe level. With the tube in place (and having been fully warmed up first) this would happen in just a few seconds.

The 10K load is for AC signals not the DC current. The current is constant and controlled by the LM317s. So 10K is not an accurate value for this purpose.

In order to measure the voltage levels in the project you need the tubes in place. They significantly load down the power supply to operating levels.

Your thinking on the transformer is fine...the math seems off 1/2 of 600=300 not 75.

Good Listening
Bruce

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PostPosted: 21 Dec 2017, 17:03 
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Joined: 13 Jul 2017, 21:24
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Hi Bruce,

I have put together your headphone amp based upon the commercial boards noted here. I am posting so as to offer potential solace to other idiots who find themselves in similar predicaments. For those unfamiliar with the boards, there is a power supply board and a smaller board for the tubes. I was so impressed with Tyger23's build I have essentially copied his arrangement with a headphone stand. Only I haven't done it with much skill as mine has issues...

First of all it does amplify and the volume knob makes the sound level go up and down. However, there is considerable hum and hissing and cracking going on.
The voltage at the two B+ feeds to the tubes are sitting at 300V. The filament voltage is 6V.

Can you tell me how you would run the return wires
PSU to xfers have B+ and need a return
PSU to tube board have filament supply and need a return
tube board has signal in and feeds out to the taps on the xfers

With many thanks, yours ignorantly
Beardy


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PostPosted: 21 Dec 2017, 17:43 
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Hi, I really can't comment on PCBs from other sources or builds since I have none here to compare to. One thing for sure is that 300B+ on the tubes is way too much and will cook them rather quickly. The LM317 will keep the current in check, but the dissipation of the tubes caused by the over double voltage will seriously heat them up. At half that voltage they are close to max ratings. You need to post photos of what you did so perhaps we can sort it out for you.

Good listening
Bruce

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PostPosted: 22 Dec 2017, 18:34 
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Ok then I won't go with bleeders if you don't think it's really necessary. With everything in place how long does it take voltages to drain? Hours?

So I guess the idea is to assemble everything then measure voltages between B+ and gnd and heater and gnd. Are there any other tests that would be appropriate?

I was under the impression that the transformer secondary math was reworked based on this post - viewtopic.php?f=9&t=4161&start=260#p42351


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PostPosted: 22 Dec 2017, 21:24 
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Hi, with the tubes in place and warmed up...then turning off the power the capacitors discharge in seconds. If the tubes are not warmed up it can be as much as 30-45 minutes depending on the quality of your capacitors. If you need to discharge them without the tubes warmed up I would use a heavy guage (20 or thicker) wire with a ground clip on one end and an insulated probe on the other and "zap" the string at least twice.

With the tubes in place the voltage should stabilize in about 30-45 seconds.

I believe the formula to be correct...but the transformer is a center tapped 150 ohm one which I would expect each half to see 75 ohms like 75-0-75. If you could phase the the 75 taps together (they are 180 degrees off) then I would buy the 37.5 value for their junction to the ground. The math would be correct if the transformer was a 0-150 with a tap for 75. It seems illogical, but when you actully take into the account of how both types are wound you will see it works. Another example of this is in 4,8,16 ohm output transformers. Each winding builds on the earlier one, but the difference between the impedance between 0-4 and 4-8 is not what you would expect. A long time ago I built a project with the phase wrong on the transformer. I figured no problem, just use the 8 ohm tap for ground and the 4 ohm one for 4 ohms. When it didn't work like I expected I did a bit of research and discovered the difference between transformers with center taps and ones with series windings. Funny thing about inductors ...it would seem if 100 turns had an impedance of say 10 ohms it would need 200 to get to 20 ohms, Not the case..the added turns will be significantly less than guessed.

Good listening
Bruce

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