Need Help with First Tube Build - WE91 300B Parafeed Derivative

[Deke609]

I'll get a color assortment from Steve at ApexJr.com. 

+1 for Apex Jr (no affiliation). I'd never heard of them before. But after reading Eric's post I checked them out and ordered 200 ft of 20 awg teflon silver plated copper wire for $56 USD - 100 ft red, 100 ft black.  It just arrived. I'm no metallurgist, but after cutting it, stripping it and examining the gauge, it looks pretty good. It's unbranded. Judging by the couple of cuts I made, the silver plating looks to be substantial.  A good deal.

cheers, Derek

[EricS]

Good to hear!  I've bought tons of stuff from Steve over the past decade or so - never been disappointed.  I haven't put in my order for wire yet, been busy with work over the past few weeks.  I also need to visit my local metal guy and order some aluminum plates so I can start drilling holes and working out the mechanics. 

[EricS]

I've been exploring various resistors for trimming rectifier filament voltage and I came across a notation that I've never seen before.  For these resistors, the "Maximum Working Voltage" is listed in the data sheet as the square root of (3xR) - circled in red in the attachement.  If the ohmic value of the resistor is 0R05, does the mean that the maximum voltage that can be applied across the resistor is the square root of 3 * 0.05, or 0.387v ??

This seems VERY low for a 3w rated part.  I'm a little reluctant to put them somewhere where they are likely to see 450+ volts across them while the PSU comes up at power on.  Am I interpreting this correctly?  Something seems wrong to me...

Thanks for helping me understand!

[Paul Joppa]

Ohm's Law: current = volts / resistance = 0.387/0.05 = 7.74 amperes

Power is volts times amps = 0.387 * 7.74 = 3.0 watts

It's just another way of saying it's a 3-watt resistor. Your rectifier is not going to draw 7 amps, so no sweat.

[Paul Birkeland]

I've been exploring various resistors for trimming rectifier filament voltage [...]  I'm a little reluctant to put them somewhere where they are likely to see 450+ volts across them while the PSU comes up at power on. 

Filament voltage is 5V.  You're using these to trim down a 5V supply.  They will not see 450V across them.  A 0.05 ohm resistor across a 450V power supply would dissipate 4 million watts

(In reality your fuse will just vaporize instantly if you tried this)

[EricS]

Got it, my thanks to both of you.  My primary concern was that the heater circuit also carries the B+ supply from the rectifier to the PSU caps, so it will have ~450vDC across it until the caps ramp up to steady state.  I didn't want to put a resistor there that was not rated for that voltage difference...

[Paul Birkeland]

My primary concern was that the heater circuit also carries the B+ supply from the rectifier to the PSU caps, so it will have ~450vDC across it until the caps ramp up to steady state. 

The heater circuit is floating and it has 5V AC across it.  It is completely unaware of the DC voltage in question.  It is true that the DC current needed to charge those caps will have to pass through these resistors, but that current is limited by the power transformer, the 5AR4, the size of the caps/chokes, etc.  When you turn the amp on, there will be 0V DC at the first filter cap and the 5AR4 will have zero emission (no current flowing) to charge up that cap.  The voltage seen by the filament dropping resistors at this time will just be 2A*0.05V=0.1V.  You may see a bit more than that since the 5AR4 filament is cold and will draw a bit more current until it's hot.  Without the filament being hot, the high voltage AC humming along on the rectifier plates is not in the equation.

A quick simulation in PSUD (which does not model the slow warmup of a tube diode) shows the current required to charge the power supply peaks briefly at about 2.25A, then settles down to 164mA almost immediately.  I worked on bigger amps with 1A fuses between the rectifier and the first filter cap as an extra safety measure, which is another indicator of how gnarly the charging current is when a tube rectifier is used. 

[EricS]

Ah - now all of the pieces are coming together for me!  There will be a current and voltage peak across the 0R05 resistor when the empty caps are first charged, but because of the low resistance of the resistor and the "slow" voltage ramp up of DC voltage out of the 5AR4, there will never be a 450v delta across the resistor...  This was my mistaken assumption.

I can't quite get the PSUD simulation to duplicate the exact voltages that I've measured in the amp (I'm unsure about resistance in the B+ winding in the transformer and I've estimated the reactance of the caps at 60R for 120Hz), but it seems close enough that I have a better picture of what is going on now.  The peak current on the 0R05 resistor is the same as the peak current going into the first cap of about 2.4A.  The voltage across this resistor closely follows the pattern of the current draw, which settles down to a very small amount once the PSU achieves steady state.  I'm seeing a peak voltage across this 0R05 filament resistor of less than 0.15vDC before it settles down to somewhere in the 0.02v range for steady state.  Even if I'm off by a few orders of magnitude (which I'm pretty sure I'm not), things still look good to me.

Many thanks, Paul and Paul!  I keep thinking I should go take an intro EE course from my colleagues across campus...  I've got many of the concepts in my head but sometimes I don't quite see how they all interact with one another.

Next up is re-reading Morgan Jone's chapters on transformer mounting to the chassis plate and looking over some of the great builds I've seen documented here so I can order all of the mounting hardware, grommets, shoulder washers, etc that I'll need next.

[Deke609]

That's pretty cool. A couple of milliseconds of 2+ amps @ 120 mV - so approx. 1/4W.  And the quoted specs for your resistors indicate that they can withstand up to 15W of dissipation for 5 seconds and a constant 3W dissipation.  You're golden!

[EricS]

Yeah, that was totally non-intuitive for me: any old 1/4w resistor will work for this application!  It's my strong desire for way over-engineering things that pushed me toward a 3w part for this purpose. 

The pair of 0R5 Mills MRA12 wirewound resistors that I put on the 300B filament to tame the voltage get toasty warm at 60c.  1.5A of current pass through those guys, so they dissipate about 1.125w of power each.  Derating for temp, they're still good for 75% of their rated power at 100c, so things should be good here.  The 300B filament PSU is a CLCR (1,000uF, Hammond 155B, 10,000uF, then 0R5) and I keep wondering whether the better solution is to torture the resistors at the end or torture a smaller first cap in order to hit a 5.0-5.1v target.  Using the CLCR approach, ripple on the heater is about 9mV - this wasn't really my goal, but is a nice side effect of clamping down on the voltage.
 

[Paul Birkeland]

You also have the DCR of the power transformer high voltage winding limiting current, as well as the impedance of the tube rectifier itself.

For the amp I last worked on with the rectifier to B+ fuse, it had blown because the owner had the amp plugged into a power strip and he turned the power strip off with the amp running, then back on a few seconds later.  This was enough time to discharge the caps but not so much time that the tubes actually cooled off much.  Still, this was a 1A fuse fed by 5AR4s (two per amp) into a 100uF filter cap, so I have reasonable confidence that in your particular amp, you're unlikely to even see 1A of peak draw.

[EricS]

That's interesting - there is a good deal of value knowing that the simplifications built into PSUD actually present more of a "worst case" scenario that we are likely to find in actual application.   Cool stuff!

[Paul Birkeland]

They can go the other way too.  There isn't really anything in PSUD to track how hot a solid state diode will get.  I've had some actually fall out of a PC board because they got so hot and melted the solder that was holding them in.  The same goes for ripple current rating in capacitors, PSUD will at least give you a number for that, but you have to pay attention to the parts that you're buying or you'll let the stinky goop out. 

[EricS]

Ha- shoulda figured the error would cut both ways 

[EricS]

I just received the aluminum sheets that I'm going to use as the top plates for the amp.  I ordered three that are 11" x 17" - one to prototype and make a mess of, and two for the finished amps.

Now that the amp is verified working properly (no longer using a bad driver tube), I made some additional measurements of bandwidth and gain.   The bandwidth measurements are particularly interesting:

With the 250k grid leak resistor: -3dB bandwidth points are 20Hz to ~17kHz - not really sure that I can hear above 17kHz ;-)
With the "Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 21Hz to 11kHz - wow, what a nosedive!
With the "High Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 23Hz to 11.5kHz - same dramatic dropoff
Not sure if the sound quality from the chokes is better, but based on bandwidth, the 250k resistor will stay for sure!

Clipping behavior is also pretty interesting with the various configurations for grid leak:
With 250k resistor, max input signal is 0.68v AC RMS before onset of clipping: max output is 7.43v AC RMS into an 8-ohm load
With both Nickel and High Nickel Grid Chokes, the max input signal before clipping is 0.47vAC RMS, but I got just a pinch more output at 7.50v AC.
Curious that the chokes deliver (marginally) higher input sensitivity and higher output than the Grid Leak Resistor did...

Overall output level is just shy of 7w into an 8ohm load (~20dB gain).

Overall noise/ripple measurements are very nice!  All of the following measurements were made with the RCA input shorted:
Speaker output: 0.4mV AC noise level
300B plate voltage: 0.0mV AC ripple
300B filament voltage: 0.04mV AC ripple with CLCR configuration
6SJ7 Driver Plate voltage: 0.05mV AC ripple

This amp is VERY quiet!  With my ear on the speaker driver, I can't hear any hum at all!

I also attached an updated voltage chart.  My AC mains tend to run 123 to 125 most of the year.

Time to start making holes in the top plate and assembling the wooden bases...