Would someone have a look at my new tube psu?

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rafafredd

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Jun 3, 2004
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OK, I´m making a new micpreamp based on the Pultec MB1 Schem and I want to use a unregulated LC B+ and a regulated floating (because of the cathode follower stage) heater. I´ve did a drawing but it doesn´t simulate as I expected... Here it is:

tube_PSU_floating_heater.GIF



In the simulation t sucks too much current in the B+ part. Do you think I should simplify it by getting rid of R2, C1? As I´m using a big choke, I don´t think I will need those in the end...

Also, does the floating heater supply looks ok?

Thanks!
 
Eliminate R2.

Change places:

L1>R1
R1>L1

Make R1 4.7K

Make R6 220K

Make R3 200K
Make R4 100K

Floating supply-hmmm.

Or should I say Hummmmm.

You could put a 1N4004 diode in the plus 12 line after the reg. to keep any leakage paths from nuking the reg with that 100 vdc.

I usually get humm with a floating heater supply, but give it a try.
 
Thanks CJ.

I forgot to say that it will feed 2 channels of MB1 and two channels of a modified version that´s not a MB1 anymore. May calculations says aprox. 80mA for the eight tubes, so I think 4k7 for R1 will be too high of a value, no? I agree that 100R is too little, so maybe I will change for something in between...

R6 is there just as a load.

I got an good idea about the heater supply. I´ll use AC heater for the four cathode output tubes (12AU7 cathode followers) and use a regulated supply for the 12AX7 input tubes. So, I think I can use regular 7812 for each pair of channels (I can´t get 78S12 easily here). I think 7812 will supply two 12ax7 easily, if properly heatsinked.

On the output tubes I´ll use the original AC heaters center tapped to 100v for floating.

The first pair of MB1 I´ve built had AC heater floating above 100v for all the tubes. No hum in there. It´s in fact really quiet for a 12ax7 mic preamp.
 
OK, I think it´s a matter of vocabulary (or my lack of vocabulary) here. It´s not really floating, it´s referenced to 100v in that voltage divider instead of referencing to 0v, to keep heater-cathode potential closer...
 
"Floating" is an unfortunate choice of words in this case. Floating heaters WILL cause hum, but this supply is not floating. It's biased above ground by the voltage divider R3 and R4. As a matter of fact, it's really biased too high above ground. Rafa used the same values for the voltage divider as found in the original MB-1; but note that in the MB-1, the pickoff for the biasing voltage appears after a tube rectifier and two 1.5K resistors. In Rafa's scheme, the solid-state rectifiers, the small resistor and the choke mean that the DC at this point will be higher than in the original design.

Maximum positive heater-cathode voltage for the 12AU7 is 100VDC or 200V total DC + peak signal. The MB-1 cathode follower's cathode sits at 163V. So we'd like to bias the heater to about +100V above ground. (Pultec chose 115V). So the resistor values may need to be altered from those in the original schematic, but I'm not going to do the math for you :wink: If you need to increase the division, better to increase the value of the upper resistor. The heaters are not drawing current from this voltage divider, so the impedances can be pretty high, to avoid loading down the plate supply unnecessarily.

You should avoid biasing the heater supply much more than +100V above ground because you don't want to exceed the negative heater-cathode rating of the first tube.

Eliminate R2. Its value is too low and it's not needed anyway because the heater biasing voltage divider will act as a bleeder. Also, to achieve full cathode temperature and emission from the tubes, insert a rectifier diode between the 12V regulator and the "low" side of the heater circuit (anode to "com", cathode to the heater low side) to bring the heater voltage up to 12.6V.
 
Rada, you will be on the hairy edge of filament regulator dropout with the value of peak voltage going in as shown (15 - 1.4), especially when you take Dave's good suggestion of adjusting to 12.6V. I would get a trafo with a bit higher voltage. Or are you possibly getting taken in by Circuitmaker's convention of specifying the generator amplitudes at peak voltages rather than rms? If the trafo is 15V rms then you'll be in good shape (in fact that would be overkill).
 
If you need to increase the division, better to increase the value of the upper resistor

I´ll use separate heater supply for each pair of tubes. I´m thinking about four 7812s... two biased above 100v (for the four 12aU7 tubes), and two referenced to ground (for the four 12AX7 tubes). Note also that in the original Pultec Mb1, the power transformer has a 600v secondary, so after the tube rectifier and the RC network, B+ sits at +345v... So, the refence point for the heaters sits at 115v. As I´m going to use the biased heater supply for the cathode follower tubes only, I think I´m safe at this point, no?

What gets my attention is that the voltage divider and the filter cap for the reference point forms a ramping circuit, and with that big cap (230uF), it will take some seconds for it to reach a safer potencial for the heater refence. If I go higher values for the resistors in the voltage divider for a lower impedance, it will take even more time to reach the desired bias point. Do you think it´s dangerous? I mean, Is it possible for the cathode and heater to short before the bias point is reached? Or am I just getting paranoid? I could also go lower with that cap value for a faster rising time.

to bring the heater voltage up to 12.6V

Looks better now?

tube_PSU_biased_heater.GIF


If the trafo is 15V rms then you'll be in good shape

OK. I´ll use a 15vRMS trafo. Thanks for noting this.
 
I just simulated a power supply using Duncan's Power Supply Designer 2 software. http://www.duncanamps.com/psud2/index.html.

mb1_clc_ps.jpg


The 270 ohm resistor before the first filter cap drops the voltage from the bridge about 55V, leaving 410VDC on the first filter cap.

My PSpice simulation of an MB-1 shows ~10 mA current draw per channel, which agreed with the value measured in my build almost exactly. So four channels are going to draw 40 mA, not 80 mA.

PSUD2 can't simulate the way you would do 4 parallel loads (one for each preamp) of R2, C3, and a 10 mA current sink after C2.

I'll see if I can PSpice this up

-Jon
 
10mA each?

mmm... I was thinking about isolating the channels with an extra 1k/230uF filter section for each channel. So, from that point where R6 is connected I will put four 1k resistors and four 230uF filter caps, each feeding it´s own channel.
 
Don't worry about the ramp up time. You can get away with no bias on the heaters at all if you want. Ask any G9 owner.

If your worried, take out that cap. It really dosen't do anyting.
 
So, are you saying that all that story about cathode and heater shorting is kind of bullshit?

I may just use a lower cap instead. 10uF or 22uF will probably be OK.

I forgot about the protection didode but I´ll add this when building...
 
Well, they talk about cathode stripping, but 12AU7's are a dime a dozen so no big deal.
Ask Jakob how many failures he has had.
Also, all the LA2's out there. Even the stock model had no bias up on the heaters. Not to mention all the DIY LA2's. I don't use a bias on mine.
 
What about a 12v one? 12v rectified would be 16.8. Wouldn´t this be enought for the regulator? A 12.6 volt one is hard to find for me.

Also, I was thinking about using a 110:48 wired in reverse as 110:252v. 252 rectified will lead me to 352. High enought. I don´t think it will be a problem, as it´s a 1A transformer, and the plate load will be something like 40mA. With the rectifier, the filter caps and the extra biasing load, it will probably recach 250mA at 252 volts. If the transformer gets warm I´ll have to buy a bigger one.

Thanks for all again guys!
 
12 would be fine. Usually 1.3 times RMS will get you in the ballpark.
12 times 1.3 = 15.6, right?
darlington output on 78xx series requires only 1.5 volts to get away with murder.
so figure 13.5 which allows for brazil nut falling from large non constricted tree which kills 3 toed sloth which falls on pwr line which sags pwr to rio 10% = 1.56 from 15.6 =14 volts, so you win by little more than 1/2 volt.
 
[quote author="rafafredd"]10mA each?

mmm... I was thinking about isolating the channels with an extra 1k/230uF filter section for each channel. So, from that point where R6 is connected I will put four 1k resistors and four 230uF filter caps, each feeding it´s own channel.[/quote]

I just PSpice'd the whole thing with BeigeBag Spice A/D v4.2 Lite...

Each channel is connected in parallel off the 20H choke/2nd filter cap,
with a 6.8K resistor and 47 uF cap. There's no need for any more capacitance. Each channel gets 342V, with less than 2 mV ripple. The 22K/22 uF filtering for the 12AX7 reduces that further.

-Jon
 

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