Beepre input voltage question

[grosen]

Hi all,

I built a Beepre a couple of years ago but pulled it from the system because of an intermittent hum I just couldn't track down. (I even sent it in to headquarters; they made some tweaks to stop an oscillation but couldn't reproduce the hum.)  Yesterday I had the brilliant idea that it might been a low input voltage problem — my wall voltage drops to 115 VAC on a normal day, and as low as 108VAC in the middle of the summer.  Sure enough, when I run it through a variac set at 120VAC, it's flawless. 

But here's the thing: I get the hum when I drop the input voltage to 117VAC.  That seems high to me.  Is there some component that might affect this that I should look at?  I can happily run it like this, using the variac to boost to 120V.  But if there's a tweak that might make that unnecessary i would be interested.

[Deke609]

I'm far from an authority about this (or about anything circuit-related), but I've spent a lot of time fiddling with my BeePre to lower the filament regulator dropout voltage. Here's a thread detailing my misadventures: http://forum.bottlehead.com/index.php?topic=11727.0

Based on my experiments and fuzzy sense of how the fil reg works, I don't think there's a way of preventing dropout and resulting hum if your wall voltage is dipping below, say, 115VAC, unless you're willing to  make some biggish circuit changes.

Subject to what PB or PJ say, I think there's a tiny bit of wiggle-room with the voltage divider that sets the POSout voltage. I believe that if you decrease the POSout voltage by tweaking the divider, the regulator will have more compliance (excess raw DC input voltage) and this will give you a slightly lower dropout point.  Downside: you change the bias/operating point of the 300Bs. And, just guessing here, this might give you maybe an extra 0.5V of wall voltage sag tolerance before dropout occurs -- i.e., not much. [Edit: I forgot to mention that if you lower POSout, you'll have to tweak the value of the cathode resistors to keep close to 5V dropping across the filament]

I suspect the easiest solution is to continue using the variac. Other options, in increasing order of fuss/difficulty:

(1) Add an autoformer in front of the BP that bumps your incoming mains voltage to around 120VAC.

(2) Add new filament trafos with a crc or clc filter so that there's less ripple at the input the fil reg.  Based on my experiments, more input ripple = less compliance = higher dropout voltage.  Less input ripple = lower dropout voltage. With my CLC filter, I can go as low as 108VAC wall voltage before dropout, and even with dropout the hum is fairly minor. The challenge will be making all this fit on the stock chassis. I rebuilt my BP in a much bigger chassis.

Just some thoughts. Hopefully not too erroneous.

cheers and good luck, Derek

[Alonzo]

A bucking transformer fixes it.  My voltages swing here in California as the wind blows, adding it fixed it completely.

[Deke609]

The buck/boost transformer is a good idea - it'll be a lot smaller than an autoformer.  But it will have the same limitation: once you choose a "boost" voltage you are stuck with that voltage, unless it has multiple secondary taps and you are prepared to actively monitor the mains voltage -- in which case you could add a switch for switching between different boost voltages and even bypass the buck/boost trafo if mains voltage approaches 120VAC.  Without such a switch, there won't be a one-size-fits-all fix for main voltage that varies 9+ VAC.  E.g., a 4V boost will work great for 114 to 117 VAC mains, but won;t be sufficient when mains voltage drops much lower.

Question for the OP: are you by any chance using EML tubes? I ask b/c the additional current draw of the EML filaments could conceivably be enough to cause a little bit of voltage sag on the low voltage secondaries -- perhaps enough to cause dropout at 117VAC mains. 

When I was first looking into to my mains voltage sag problem, I had a helluva time trying to make sense of how autoformers and buck/boost trafos work and need to wired. This article does a pretty good job of explaining all that, plus when and more importantly when not to use them: http://sound-au.com/articles/buck-xfmr.htm

cheers, Derek

[2wo]

The nice thing about the Varic is you can turn it up when need to, down when you don't. 

You can use a boost transformer as Alonso describes in a little box and tuck it out of the way.
 I build one with a 6V  filament transformer with a switch  to run line or line +6 ...John

[Alonzo]

Mine has a simple +3 and +6 switch.  When I hear that unique hum (it's unique in my system, no confusing it with other problems) then I try the +3 selection.  That usually clears it up immediately.    If it doesn't clear, I switch to +6 and check the meter on my power center, +6 is very abnormal for my area.  I can usually turn it "off" after 10 to 20 minutes.

[Paul Joppa]

Deviating a little from the thread ... several decades ago, I had an experiment going at the lab where I worked, and they hauled out this old General Radio voltage regulator - it must have been made in the forties. What looked like a half-horsepower electric motor controlled the rotation of a large variac through a chain drive. I never did find out how the controller worked. It may have been entirely electro-mechanical.

[johnsonad]

Do you live in central California?  It does the same to me here....  This year the low was 108v and a rare high of 117v. I use a 10 Amp variac at the wall and plug a PS Audio PPP into it. The PPP has a nice output voltage display and I adjust the variac before firing up the stereo. It amazes me how much voltage swing we can have in one day.

[grosen]

Thanks everyone. I live in Central NJ — not CA — but the swings in the summer are ridiculous: lows of 108V, with highs on the same day of 120V.  I'm happy to run the preamp through the variac, twiddling as necessary.  It's not a big deal.  I just wondered whether my BeePre was especially sensitive, since I get the bad noise at 116-17VAC, and if so, whether I should do anything about that. 

But to PJ's point:   Yes, that's exactly what I (and others in my situation) need:  A variac with a feedback circuit and a motor to keep the voltage pegged at 120V.   That should be a lot cheaper than a regenerator.  Why doesn't that exist?  Is there an engineering problem, or is it just that there's no market?

Happy listening! — Gideon

[grosen]

Anyone have experience with these Regvolt voltage stabilizers?  Ad copy says that they stabilize at 120V with a pure sine wave.  Some of them are suspiciously inexpensive.

https://www.amazon.com/gp/product/B01B8HSBW2/ref=ask_ql_qh_dp_hza

— Gideon 

[Paul Birkeland]

BeePre 1 is very sensitive, yes.  The new one is far less so.  Using a boost transformer is the easy solution. 

I would not buy that item on Amazon.  The photos show that the inside is mostly empty.  The few reviews that look real are not all that positive about it.

[pboser]

In the past I tried to address this issue (central NJ too) with a Sola constant voltage transformer.  Hummed like a son of a gun, had to retire it immediately.  I know they are not favored for this purpose for other reasons as well, but I don't remember why.

[Paul Joppa]

Motor-driven variacs seem to be available; they provide better than 1% control. The cheaper regulators are probably switched taps on a transformer or autoformer and usually have a +/-4% or similar spec.