Tube PSU Question

[philipbarrett]

Does anyone know of a data sheet comparing various tube rectifiers? So far, everyone I've found has an output voltage around 380 (assuming a high enough input), is this typical or are there other types with less or more voltage loss?

This is my first foray off a SS PSU design.

Thanks in advance for any help.

PB

[Eli Duttman]

TDSL is a GREAT resource for tube data.
TDSL

The forward voltage drop is low in the 5AR4/GZ34 and damper diodes, like the 6AU4. The forward voltage drop is large in the 5Y3 and 5R4.

A 650 V. B+ rail is within a 5AR4's capability. KV. rails can be done with damper diodes.

[Joel]

The PSUD2 program is the standard power supply sim for the DIY'er.

http://www.duncanamps.com/psud2/index.html

I routinely get within 0.5V of the predicted value with it. The key is measuring the DCR of your power transformer primary + secondary, and your line and off-load voltages. You then enter these into the program. It's a great tool.

Joel

[Miles Prower]

Here's something else that will come in handy for this: Rectifier Applications Handbook

[philipbarrett]

Thanks for the advice everyone, I downloaded the PSU designer and entered my prototype. I cannot figure out how to present the PSUD data here, however, I'm using a 250VCT transformer, 6CA4 rectifier, a C filter section at 100uF followed by an LC section at 20H/100uF supplying a load at 5K.

I meter the unloaded output of this at 385VDC which seems right inline with the 6CA4 datasheet.

However, when I run the simulator I get dire warnings about current limits and the final curve shows an output voltage of 600V !

Any ideas what I'm doing wrong?

Thanks in advance - PB

[aletheian]

Sometimes in that program, I get a nasty spike at startup, you can use a soft start or stepprd startup, or just use a report delay of a second... usually works for me.

[Joel]

Yes, you have a 100uF input capacitor. I don't think any standard rectifier can handle that large an input cap without a lot of added resistance before it.

Joel

[Eli Duttman]

The 6CA4 is a GOOD tube. What you've built is proof. 100 muF. in the 1st filter cap. position is TOO big and exceeds the limits given in the data sheet. That you don't get arcing over (VERY BAD) at start up is a matter of luck. When using vacuum rectifiers, the 1st cap. in a CLC filter needs to be kept relatively small. Some tubes can tolerate only 4 muF. max. The 6CA4 is quite tolerant of capacitance for a vacuum rectifier, but it should not be over stressed. 20 muF. in the 1st position is GOOD, as it will keep the rail voltage up, without exceeding published limits. The choke protects both the rectifier and the power trafo (I^2R heating) from the 2nd filter cap. Piling up the energy storage in the 2nd position is OK.

[Jay]

100uF is too big??? 4uF???

[Miles Prower]

"100uF is too big???"

Yes it is. Unlike solid state, you're dealing with high voltage, low current devices. A VT power diode doesn't have the ability to source the charging current a 100uF filter capacitor will demand. Back in the bad ol' days, some cheap equipment did have filter capacitors that big. This would overly stress the power diodes, however, it was assumed that you could just hop on down to the corner 7 - 11 and get a new one, and hope that the untech savvy customer wouldn't notice that his PS diodes were going bad too soon.

Since VTs aren't so easy to get, it pays to work them within the specs. Can't do that running them into such big filter capacitors.

Either go solid state, or use a ripple filter to clean up the AC mess.