Adjusting 300B filament voltage

[Deke609]

Many thanks PB. I think I understand what you're saying. I had wrongly assumed (without carefully looking at the schematic) that the fil reg was fed by a C4S, but I see that the C4S is used for the HV anode load. Apologies for that, I should have checked the schematic first.


So I think it follows from what you've written that the fil reg board is, subject to heat dissipation limits, not current limited, other than by the 3.5A ratiing of the 6.3V secondaries - which is plenty more than I need.


I will retry adjusting the Rset2 resistor. And it just occured to me that something else may be contributing to the hum issue.  On one channel I installed a pcb solder cup pin on one leg of the Rset2 (because I was testing only one side at first, and expected to make many resistor changes, and so feared ripping out the trace).  The pin diameter is a bit small so it's possible that during my previous Rset2 trial I unseated the pin, leading to a noisy contact. I will check and fix today.


Is there a max PosOut voltage that I should stay under? I ask b/c my first attempt at bumping up Rset2 resulted in about 10.45V out (IIRC) and I don't think I heard any hum at that setting - but did hear hum when I dropped PosOut to 10.1V. [Edit: I realize that in changing PosOut I am changing the bias, and am not sure how tight the acceptable bias range is]


Many thanks again for all your help,  Derek

[Paul Birkeland]

Regulator drop out should occur as PosOut voltage is increased, as the dropout occurs when the difference in voltage between PosIn and PosOut is not large enough.  As your bring the PosOut voltage up, the heat dissipated in the regulator will decrease.

I wouldn't worry too much about the bias voltage being off by a few tenths of a volt.  That will move the plate voltage around a little bit, but it shouldn't be very significant. 

[Deke609]

Ah, many thanks. So the reg board "reads" and regulates the difference between out and in? 


I must have been wrong about the 10.45V PosOut not humming, b/c I just tried 10.3V out and got instant hum on both channels - and both were dead silent before. 


I am going to try to hit exactly 10V out and then play with the cathode resistor value.


cheers and thanks, Derek

[Paul Birkeland]

Ah, many thanks. So the reg board "reads" and regulates the difference between out and in? 

The 1085 just regulates its output, provided the voltage at its input is high enough to do so.   

[Deke609]

Alright - so I'm guessing that the 1085 drops a minimum voltage doing so, and hence the need for a min difference between V-in and V-out. Increase V-out too high, and the regulator starves for voltage. 


No need to correct me if I'm wrong - fixing all my misunderstandings via forum posts could take a lifetime 


cheers and thanks again, Derek

[Paul Birkeland]

Alright - so I'm guessing that the 1085 drops a minimum voltage doing so, and hence the need for a min difference between V-in and V-out. Increase V-out too high, and the regulator starves for voltage. 

Sure, though when the regulator drops out, you'll still get decent voltage out of the regulator, but the DC regulation will become poor and the noise rejection abilities of the regulator will follow.

[Deke609]

Adding 18.2R in series with the 931R gets me 9.97V-out on one side and 10.02V-out on the other. PosIn is about 13.2V on both sides. Approx. 4.7 V dropped across the fil. No hum at start-up.  Going to bring the fil voltage up to approx. 4.85V or so -- just within 3% of target.  If that works and no hum after 2 hrs, I will call it good.

[Deke609]

Hmm. The right channel is humming - not much, but it's there intermittently. I am going to measure the difference between Pos In/Out on both boards to see if the right channel has the smallest difference - as I expect.  And then tweak Rset2 to get the same difference as the other channel.


Failing that, I will simply reduce Rset2 on the right channel to where there's no hum.

[Paul Birkeland]

Where you get no regulator hum today may be different than where you get no regulator hum tomorrow unless you're on a regenerator.  The big challenge is that if you lower the 8 ohm filament bias resistors, then you will both increase the voltage across the 300B filament (a good thing) and increase the filament current (not a great thing for proper circuit operation).  The extra current will drop more voltage across the active noise filter at the output of the regulator, can increase the thermal stress on the 1085 regulator, and can draw down the raw DC voltage available to the regulator in the first place.  It's a pretty delicate balance!

The BeePre is just not a particularly flexible circuit when it comes to altering it to work with a tube that isn't a 300B, and unfortunately the EML 300B doesn't meet the factor specs. 

[Deke609]

 It's a pretty delicate balance!

Yeah, you're not kidding there! 

I've swapped the 18.2R for a 10R in series with the 931R on the offending right channel, bringing Pos-Out down to about 9.93V from 9.97V which gives me almost exactly 4.8V across the filament (so just inside the 4% target, exactly where you recommended that I start over a weeks ago!)  with a 120R added in parallel with the stock 4R.  A tiny bit of hum of intermittent hum in the first 10 seconds after power-on, but no hum since then. I'm going to let it play for about 4 hrs and then check again.

If need be, I'm even willing to run the tubes with a little less than 4.8V across the filaments. This will undoubtedly hurt tube life, but for me it's worth it. To my ears the EML 300B has more energy and detail than the JJ 300Bs, and havign heard them in the Beepre, it would be hard to go back.

Well ... regardless of what ends up working, I can rest assured that I made some serious efforts to get the amp dialed in and that whatever "solution" I end up with is pretty much the best I can do. Plus, I learned a bit about Schottky diodes and the Beepre circuit - which is great.

The Kaiju fil reg circuit will be a little more forgiving, right?  Please say "yes".

cheers and thanks again, Derek

[Deke609]

Just to cap this off: it's day two and no hum. I ended up with only 10R in series with the 931R Rset2 on both boards. That brought PosOut down to 9.88V on one side and 9.93V on the other, @117V AC mains. That's still about 60 mV higher than where I started before installing the IXYS diodes and tweaking the Rset2 value. With a 100R in parallel with the 4R cathode resistance on one side, and 120R in parallel on the other, I get very slightly over 4.8V dropped across each filament.


My present mains voltage of 117V AC is the lowest I've ever seen it. We are having a hot and humid summer in Toronto this year and I suspect that everyone is running their air conditioning pretty hard and causing some voltage sag. I expect that by the end of September my mains voltage will be back up to 120V AC. I'm pretty sure I've seen it go as high as 124V AC in non-summer months.  So I expect the risk of shunt reg dropout to be limited to summer ... but so far it looks like I have a winning combination even for the low voltage months.  Very pleased. These tubes sound great to me, with more goodness to come as they are still breaking in.


cheers, Derek

[Deke609]

Well, I've only heard the regulator drop out once since finalizing the tweaks for the EML 300B. And I expect that with cooler fall temperatures, my mains AC V will bounce back up and it won't be a problem.


But ... I like playing with stuff, so I've been thinking. What about heating each filament with 2 x adjustable LM1085 reg boards, paralleled, each set for 5V out? I.e., split the work between them and stay well under any current and heat dissipation limits? Further, if the current used by two boards after voltage doubling and rectification, when added to the 1.3A filament draw, would exceed the current rating of the existing heater winding, the boards could be powered by a new separate heater PT.


AND ... if that's workable in principle, then would it not be possible, with the right heater PT, to use EML 300B mesh tubes with 1.5A 1.4A filament draw?


I have in mind a separate enclosure to house one large-ish heater PT with dual heater secondaries, rectification and regulation (four LM1085 boards).  I figure I could put together the LM1085 boards on protoboard.


Would this be workable in principle? 

[edited to correct misstated current draw of the EML 300B Mesh filament]

cheers and thanks, Derek

[Paul Birkeland]

Paralleling regulators doesn't do anything helpful in this scenario.  Regulators don't split work well.

Buying an AC regenerator would definitely be the way to go.

[Deke609]

Thanks PB.

Paralleling regulators doesn't do anything helpful in this scenario. Regulators don't split work well.

I was thinking this might help if the reg was hitting its thermal or current limits - not as a solution for my low summertime mains voltage (which just got worse! The city repplaced the line on my street today and now I only get approx 115V! Hoping it's just everyone's fridge and ac working overtime after being off for 8 hrs and that mains V will go back up to at least 117V tonight).

But I did some reading and see your point about regs not splitting current well. Plus, it looks like the current limit of the LM1085 isn't an issue anyway - since it's rated for min. 3A output. 

          Question: If the current limit of the LM1085 doesn't limit the output current available, what does (assuming, hypothetically, that one has more than enough VA from the
                           heater winding)?

Buying an AC regenerator would definitely be the way to go.

Yeah, it's looking that way. But they are crazy expensive (PS Audio, Monster Power, etc.). Tripplites with mains regulation are a lot less expensive, but they only kick in at 111V, and just pass 112V - 127V straight through -- so they're no help.  You'd think that a mains voltage regulator would be an ideal diy audio project, but so far my search for a schematic or build guide has turned up nothing.  But I will keep looking.  Maybe the fact that it involves mains power keeps people from posting about it? For liability reasons? But i find that hard to believe since there are lots of build threads/schematics for 211 and 845 amps that operate at really high B+.  Dunno.

In the interim, I'm using my variac to bump up the mains voltage - but it's only good for 6 hrs max continuous use and needs frequent monitoring. So I will need to find a better, permanent fix.

cheers, Derek

[Paul Birkeland]

You could also get a 115:120 autoformer or power transformer to sit ahead of your BeePre.

Dropout occurs when input voltage and output voltage get too close together.  That's why low line voltage is an issue.  The regulator will not overheat with some other major problems going on.