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

[EricS]

Ah - I've seen that formula before - I'm still learning how to wield each of these great tools!  I'll get there eventually  .

I was surprised by the low rectified voltage from the 6.3v winding as well.  I was expecting something north of 5.0vDC with a choke input.  I'm using 80SQ045NG diodes (https://www.mouser.com/ProductDetail/ON-Semiconductor/80SQ045NG?qs=%2Fha2pyFadujIGlmyYa9M8%252BWQ31K%252Bb%252BCtAaPPtZbg%252Bfg%3D) that have a 0.55v forward voltage drop.  I also tried using IXYS DSS 10-0045B diodes (https://www.mouser.com/ProductDetail/IXYS/DSS10-0045B?qs=%2Fha2pyFadug59d17CRjWjt%252BNDUp2lITVLL7Zh40I8PUxMJTDr5BNUA%3D%3D), but the final filament voltage was identical to the previous.  I'll add some 1N5820's to my next order - the curves on the data sheet look encouraging.

The EML mesh tube draws 1.5A for the filament, so this is dragging things down a bit.  There is a clear difference in voltage level between my PSVane tube and the EML tube.  All of my notes are at home right now, so some of this is from memory.

[Deke609]

The EML mesh tube draws 1.5A for the filament, so this is dragging things down a bit.

That may be your culprit. I use EML tubes in all my BH amps and the additional filament current draw always drops filament voltage below Jac's targets and requires tweaking (which is pretty easy in BH amps b/c they employ dropping resistors at the filament that can be decreased). I just did some really quick and rough PSUD sims of a 6.3V filament power supply and switching from normal 300B current draw of 1.2A to 1.5A dropped voltage by approx. 250mV. 


cheers, Derek

[EricS]

Yep, I can confirm that missing 0.25v!  I was at 4.98v on the filament and doing my happy dance.  Then I adjusted the plate voltage back to target and switched to the EML tube and was suddenly the party was over.

I'll just use a larger first cap for the CLC to reduce the ripple current to sane levels and then insert some resistors to hit my target.

Caps on the filament PSU definitely reduce the audible hum from the speaker, even when the hum pot is dialed in.  At first power up, I had 5mVAC on the speaker terminal.  I could get it down to 0.0mV by adjusting the pot, but I could still hear it (I have no idea what the sensitivity my old test speaker is).  Adding a cap cured this by reducing ripple on the filament from 1v5 to 80mV.  Adjusting the hum pot now produces a very subtle difference because the hum is so very low.  These tests need to wait until the kids are in bed so the house is quiet.  The CLC takes it down to something closer to 35mV at the filament.

[Paul Birkeland]

Yeah, 1.5A is no longer a 300B, and you'll lose a lot of filament voltage because of that.  The Hammond 156B might be a better choice, or more likely a different set of tubes.  If you intend to design and amp to use both, aim to be slightly low on the EMLs and slightly high on a 300B.

[Deke609]

How about one of these Hammond 8.5V CT @ 3A filament PT's for each EML 300B mesh? https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166M8?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGywL5FidNAeTk%3D

There's also a 2A version for about 4 bucks less.

You'd have some extra voltage to drop, but that would give you lots of wiggle room for filter stages to get ripple super low.

cheers, Derek

[Paul Birkeland]

It's easier to go down. The choke after the bridge just needs more DCR.

[EricS]

I did contemplate a separate PS for the filaments. 

Then I started thinking about the quantity of large parts that already need to go under the hood...  And that it would be one more part to spew an electromagnetic field...  and how that might just create a bigger problem than I already had on my hands...  For now, I think I'll stick with some additional caps and resistors.

Darn that "no free lunch" thing 

[EricS]

While the 1,000uF cap has an ESR of 0.016 ohms, it's capacitive reactance at 120Hz is 1.3 ohms.

Now that I have some time to think about this, does this mean that my previous calculation of ripple current was way off?  Is a more accurate calculation based on frequency specific reactance of 1v5/1R3 =  1.15A of ripple current?

[Paul Birkeland]

If I had concerns about ripple current, I would model the supply in PSUD, then insert an AC ammeter between ground and the negative lead of that cap and actually measure the current.  You'll need a meter with a current shunt that's got very low DCR though.  You could also put a 0.1 ohm resistor between ground and the negative terminal of the cap in the absence of a low resistance current shunt in your meter, then measure the AC voltage across this.  You can also try a 0.01 ohm resistor to get even closer to the actual value.

Doing this will eliminate any unpredictable behavior from your meter attempting to resolve AC voltage riding on DC voltage. 

[Deke609]

Just doing some "just in case" thinking here.


@PB - Could the Kaiju DCF handle a 1.5A draw, maybe with heavier duty heatsinks?


It looks like the board could be scored and split into two separate reg boards.  Since the DCF was designed to provide about 5V DCV after approx 1.2V filament resistor drop, it might offer a solution to the voltage sag issue with the EML tubes.
 
cheers, Derek

[Paul Birkeland]

You'd either have to ask PJ, or you'd have to buy one and try it out.  It's still not the most optimal solution, as it puts out a regulated 6.3V, which we drop with a couple of low value resistors to get to 5V. 

[EricS]

The mains voltages in my neighborhood seem to range from a low of 120v to a high of 125v, but seems to hang out most often in the 122-125 range.  So, I used my variac to see what happens to filament voltage levels with mains voltages of 120v, 122.5v, and 125v.  I can't seem to help myself, I'm a compulsive tweaker...   

I have a variety of datasheets and found lots of online discussions that talk about voltage tolerances for tube filaments for lots of tubes except the 5AR4 rectifier that I'm using.  All of the datasheets I have found merely state 5.0v but I'm measuring more than that in my build.  Does the "typical" window of +/- 10% apply here as well?  Will the tube last longer in the long run with a +/- 5% window?  My specific tubes are NOS Mullard f31 tubes.

I made a voltage chart and attached it below.  Should I leave the rectifier heater as is, or is it better for the longevity of the tube to insert a 0R1 resistor in series that will bleed off 0v19 and shift the heater range down to 4.97v to 5.19v?

Thanks for the perspectives!

[Deke609]

... insert a 0R1 resistor in series that will bleed off 0v19 and shift the heater range down to 4.97v to 5.19v?

That's what I would do, assuming your measurements were taken with all tubes in place (power supply loaded). Then you'd be within 4% of spec.

cheers, Derek

[Paul Birkeland]

+/- 10% is quite normal.  There's this image that's made the rounds, though its exact origin eludes me at the moment.

If you feel the need to bring that voltage down, I would do it with a wirewound resistor in parallel with the 5AR4 filament. 

[EricS]

Derek: yep, completed amp at full temperature, all tubes lit and properly biased

Paul: I presume you are recommending the resistor in parallel to place a more "even" load on the AC filament?  Does it achieve the same result if I put a smaller resistor in series with each side of the filament?

Guess I have a new round of experiments to try - I've been putting resistors in series with the filaments.  At least resistors in parallel don't require a power down/cool down cycle for each value I try...