PSUD newbie: modeling Tubelab SE

[PaulyT]

Hi! I'm just starting to play with PSUD (II), and would like to model the Tubelab SE that I'm building. Has anyone done a model for this amp?

I'm not quite sure how to translate the TSE schematic into the components available in PSUD. I am using a choke in place of R4. Do I need to include R30 somehow? What is a reasonable value for a resistive or current load? I'm not sure how to estimate that.

Attached is what I've got so far. The sim stabilizes at about 410V, so I'm sure I'm not doing it right... any suggestions?

 

[boywonder]

Looks good to me. R30 is just a bleed resistor for the caps so when you power down the amp, you don't get zapped; you don't need to include it in the model, as it's drawing a mouse-fart's worth of current.

Your current load looks good, it should be the sum of the currents of all of the tubes. Most of the current is for the output tubes, the input tubes draw a little. Tubelab George has a page for Voltage and current recommendations for different tubes, so the total current load will depend on if you are using 45's, 2A3's, 300b's etc. the page is here:

http://www.tubelab.com/AssemblyManualTubelabSE/TubesAndTransformers_TSE.htm

Two things that you can tweak:

Adjust the value of C1 to change the B+ voltage. Lower C1 lower voltage, higher C1, higher voltage.

You can also use the "stepped load" function in the current load box. Set it for "stepped load" and change the current say 20 ma or so up or down after 4 seconds or so. You want the step to occur after the B+ voltage settles out.

Check for ringing (oscillations) of the voltage when you step the load. You should have no ringing, ie an overdamped system. You can also adjust the cap values to attenuate ringing.

Zoom in on the flat part or the B+ waveform to see how much AC ripple remains on the DC voltage. Adjust C's and R's to minimize this.

Adding R/C sections or R/L sections greatly reduces ripple but drops volts and increases the PS output impedance.

You can also try different L's: I use Hammond's data sheet to estimate the R for a given L.

 

[boywonder]

I just duplicated your model. It appears that you have a some overshoot/ringing on startup, and you B+ is a little high (assuming 300B's). If you are planning on running 45's, it's alot high.

The stepped response looks fine.

Reducing C1 to 10u or less reduces the startup overshoot and gets you away from the ragged edge of exceeding the max forward current of the 5AR4. If you see the "forward current has been exceeded" message, the current rating of the rectifier is being exceeded, and the rectifier tube can arc over under these conditions. The current is high at startup, since the caps are discharged and have low resistance until they get charged up.

Dropping the value of C1 below 10u or so also starts to reduce the B+ voltage, so that's how you adjust B+. Assuming 300B's, try a C1 of 10u and a stepped load of 120ma stepped to 140ma after 4 seconds.

For additional fine tuning, measure the resistance of the transformer secondary, and input that R value in the transformer box.

 

[Sheldon]

Your power transformer spec's look suspiciously like the stock sample for the software. Make sure you use values that reflect the actual part you will be using. Someone here will have measured the unit. You need the unloaded output voltage, and resistance of the primary and secondary. Edit the transformer values, by clicking on the Ohms button in the transformer edit box.

Sheldon

 

[luvdunhill]

For additional fine tuning, measure the resistance of the transformer secondary, and input that R value in the transformer box. [/B]

If you have a center tapped winding, do you measure this resistance from the center tap, or across the entire winding?

 

[Sheldon]

If you have a center tapped winding, do you measure this resistance from the center tap, or across the entire winding?

For a CT full wave, only one half of the secondary is conducting at any given time, so both voltage and resistance should be measured from the CT to either end.


Sheldon

 

[luvdunhill]

Ok, I measured things. Perhaps someone can check my numbers as well It's a RCLC supply

T1 is 365V, 241 ohms (unloaded, actual unloaded voltage). Center tapped

D1 is 1N4007x2 full-wave SS

R1 is 4.7 ohm (not sure if this is needed, but series resistor)

C1 is 100uF cap

L1 is 40H, 603.488 DCR (actually measured)

C2 is 100uF || 47uF caps

load is 40mA. I'd like to get 450V, but I'm not thinking that is possible Somewhere around here is fine.

 

[boywonder]

Ok, I measured things. Perhaps someone can check my numbers as well It's a RCLC supply

T1 is 365V, 241 ohms (unloaded, actual unloaded voltage). Center tapped

D1 is 1N4007x2 full-wave SS

R1 is 4.7 ohm (not sure if this is needed, but series resistor)

C1 is 100uF cap

L1 is 40H, 603.488 DCR (actually measured)

C2 is 100uF || 47uF caps

load is 40mA. I'd like to get 450V, but I'm not thinking that is possible Somewhere around here is fine.

I just modeled it (I'm at my desk on my lunch break ) and I got a B+ of 440V. I usually use the loaded/rated voltage of the transformer in PSUD, so using the unloaded transformer voltage is new to me, but I'm no expert.

Everything looked fine except that the forward voltage of the 1N4007's was exceeded (1002 volts, PIV for 1N4007 is 1000V)

What the rated voltage of the transformer? (Assuming the transformer is rated for around 40 ma)

 

[Sheldon]

Ok, I measured things. Perhaps someone can check my numbers as well It's a RCLC supply

T1 is 365V, 241 ohms (unloaded, actual unloaded voltage). Center tapped

D1 is 1N4007x2 full-wave SS

R1 is 4.7 ohm (not sure if this is needed, but series resistor)

C1 is 100uF cap

L1 is 40H, 603.488 DCR (actually measured)

C2 is 100uF || 47uF caps

load is 40mA. I'd like to get 450V, but I'm not thinking that is possible Somewhere around here is fine.

Is the transformer figure for the secondary, or the calculated value for the source resistance?

With those figures, I get about 360V. If it's the source resistance, that added to the inductor resistance is about 850R. At 40mA, you are dropping a good bit of voltage.

Sheldon

 

[luvdunhill]

I just modeled it (I'm at my desk on my lunch break ) and I got a B+ of 440V. I usually use the loaded/rated voltage of the transformer in PSUD, so using the unloaded transformer voltage is new to me, but I'm no expert.

Everything looked fine except that the forward voltage of the 1N4007's was exceeded (1002 volts, PIV for 1N4007 is 1000V)

What the rated voltage of the transformer? (Assuming the transformer is rated for around 40 ma)

I meant to say that I used the "1N4007x2" option or whatever it is, to help with the PIV

well, the help button says to use the unloaded voltage, so that's what I did The transformer is rated at 300-0-300 @ 95mA, so it will be unloaded somewhat.

 

[luvdunhill]

Is the transformer figure for the secondary, or the calculated value for the source resistance?

With those figures, I get about 360V. If it's the source resistance, that added to the inductor resistance is about 850R. At 40mA, you are dropping a good bit of voltage.

Sheldon

It's the source resistance after invoking the calculator. I can try and recalculate things tonite and double check the numbers.

 

[PaulyT]

Well, I don't have all the details because I don't have the transformer yet, so I can't measure the resistance on the windings, but the outer winding is 330V. I've also included in the sim that I'll have an external motor run cap in parallel with C5 on the TSE (C2 in PSUD). Now the simulated load voltage is a little over 380V, which is on target for the 300B.

Now the influence of the capacitance of TSE C4 (PSUD C1) is pretty small, which is really the question I was trying to answer, to decide what value cap to put there. So I'll stick with what's in the specs. (Maybe George knows what he's doing after all... )

 

[rknize]

Is this the "Universal" Edcor for the Tubelab SE? I can measure the DCR of mine for you, but you can get by with the default for now...

 

[PaulyT]

Yup, the Edcor XPWR131. Thanks! Yes, it'd be good to know this more precisely, as the T1 resistance in PSUD has a pretty substantial effect on both transient rectifier current and the final output (B+) voltage.

Obviously it'll be interesting to compare this model to the real thing, but it'll have to wait a month or so for the Edcor to ship... sigh.

 

[rknize]

The 660 winding has a DCR of 88 ohms. Across the 520 taps is 70 ohms.

 

[Sheldon]

Well, I don't have all the details because I don't have the transformer yet, so I can't measure the resistance on the windings, but the outer winding is 330V. I've also included in the sim that I'll have an external motor run cap in parallel with C5 on the TSE (C2 in PSUD). Now the simulated load voltage is a little over 380V, which is on target for the 300B.

Now the influence of the capacitance of TSE C4 (PSUD C1) is pretty small, which is really the question I was trying to answer, to decide what value cap to put there. So I'll stick with what's in the specs. (Maybe George knows what he's doing after all... )

If you don't have the actual measurements, you can usually get pretty close by clicking on the RMS V button in the transformer edit dialog box. Enter the rated voltage and current. Assume the default value for regulation of 5%. Assuming the manufacturer rates their transformers at about 5% regulation, this will be fairly close.

C1 will have a big influence on B+, but only once you get below around 10uF for this application. Try it at the limit (delete it) and see what you get. Nice thing about the sims is that you can play with a wide range of values to get a feel for the relative affect of various changes, including ripple and ringing, and diode current profiles.

Sheldon

 

[Sheldon]

The 660 winding has a DCR of 88 ohms. Across the 520 taps is 70 ohms.

I'm assuming that's end to end. For a center tap, you'd use 44 Ohms. You need primary resistance and voltage too.

Sheldon

 

[PaulyT]

Ok, if I use 44ohm at T1 in this model, the load voltage drops to ~375V (too low). But then if I switch to a resistive load of 5kOhm (since I'm using a 5k OPT), the load goes to 412V (too high). I can compensate by lowering PSUD C1 to 1uF, brings it back to about 400V.

My conclusion so far is that this is a great way to figure out the relative effect of changing component values. But the absolute B+ end value is highly dependent on these input parameters which may or may not really reflect the real circuit... Obviously actual empirical measurement will be the deciding factor, but at least now I have some better idea how to effect a change if the end result isn't what I want.

 

[DougL]

But then if I switch to a resistive load of 5kOhm (since I'm using a 5k OPT),

The 5K value is not something to use. Go back to the current source. If you are using 6l6GB biased at 40 mA, use 40 mA current source, and PSUD will be reasonably accurate, Switch to KT88 at 90 mA, that works too.

Yes, PSUD is a bit of a handful at first. But you do already get the major lessons.

Doug

 

[rknize]

This is a Tubelab SE. For a pair of 45s, you need to figure 25mA per 45, plus 12mA per 5842, plus about 10mA for each MOSFET...so say 100mA total. For 300Bs, you need to figure up to 75mA per tube, which brings you to about 200mA total.