Filaments: Is the 100R/100R center tap inherently better?

[R.G.]

I once built an amp that had the hum balance pot, and the pot position that gave minimum hum wasn't always the middle! If I changed the first preamp tube, I had to readjust it, and sometimes it would end up at 11 o'clock or 2 o'clock.

Classic! The "hum balance" control is one of those rare controls that do exactly what they say they do. The hum balance shifts the phase of any induced hum a little bit so it can offset hum from other sources and cancel it out.

What you describe is *exactly* what can be expected from a hum balance control.

In a way, the HB pot is an economy measure - it lets you (you, the designer and manufacturer) null out hum from other places rather than having to go hunt down and kill the other hum sources. Whatever hum you got, just diddle knob til it's a small as it gets and then box the unit and ship it.

[Redhat]

Forego the 6.3V from the mains altogether

Hey,

Or you might try what the hi-fi guys suggest and use an external switcher to produce the filament current.

Here's an interesting article that's got me thinking of a simple $30 100KHz 7.5 Vdc model to just replace the whole filament circuit off the mains.

Hi-Fi Tube Audio Nenad Filipovi?'s Homepage


rh

[Chuck H]

Not sure of the relevance, but...

Couldn't the false CT act as a sort of fuse in the event of a power tube short to heater? This could save the PT by popping the resistors where using the hardwired CT wouldn't.

Chuck

P.S. I've twisted my filament leads with both cordless AND corded drills and I can't hear a difference...

[Steve Conner]

Ugh, you said "switched mode"

Here are some serious reasons not to use one, though:

1. Any power transformer you buy for a tube amplifier will have a heater winding already.

2. AC is just fine for indirectly heated power tubes, it only causes problems in directly heated audiophile trinkets like the 300B.

3. AC usually works fine for preamp tubes too, and if not, you can rig up a small DC supply to power the preamp heaters only. Maybe only the first preamp stage will need it.

4. Many switchers can't start a filament load because of the high current draw when the filaments are cold. They just go into hiccup mode. But not all of them, I've seen some better-quality units that had real current limiting and would happily burn the tip off a screwdriver.

5. They're nasty complicated things full of silicon that spoil the Zen-like simplicity of tubes. I'm saying this somewhat tongue in cheek, as I've built a few hybrid amps infested with ICs and MOSFETs myself.

6. You're making life hard for anyone who has to repair the thing further down the line.

7. If you're going to introduce switchers, you might as well use one for the B+ supply too. This gets really tempting if you're building something the size of a SVT, it could cut the size and weight dramatically. But see 6.

@Chuck H: Yes, but that's not always a good thing. Say one of the tubes fails in some weird way (maybe the locating peg falls off and someone jams it in the wrong way?) and spits B+ back out of its heater pins. Then the 100 ohm resistors could fail open, and fry the heater-cathode insulation of every other tube in the amp. With a hardwired CT, the damage would be limited to the tube that originally failed. The odds of this happening are low, however...

[Redhat]

[QUOTE=Steve Conner;150248]Ugh, you said "switched mode"

Here are some serious reasons not to use one, though:

...
5. They're nasty complicated things full of silicon that spoil the Zen-like simplicity of tubes. I'm saying this somewhat tongue in cheek, as I've built a few hybrid amps infested with ICs and MOSFETs myself.

QUOTE]

So do you use silicon recifiers or tubes for your DC supplies?

I guess the one other problem I've encountered (with a Drake PT for a Marshall DSL201) was that the thing doesn't reglulate well enough to maintain even 5.5Vdc for the preamp tubes at high power. Just disconnecting the 100R hum blockers brought the filament voltage up to where you could actually hear the difference.

Plus any 60Hz flux you keep out of the PT will make it quieter,more linear and cooler, though for guitar amps (w/ adequate PTs) it may be overkill.

[Steve Conner]

Silicon all the way! The tube rectifier is possibly the most useless relic ever. It takes up space, gives off heat, wastes power, and doesn't affect the tone in any way that you couldn't mimic by adding a big resistor in series with your silicon diodes.

That DSL201 thing is an example of how a broken DC heater design could actually be worse than a properly implemented AC one.

[Redhat]

...
That DSL201 thing is an example of how a broken DC heater design could actually be worse than a properly implemented AC one.

No kidding! I originally found the problem when the bridge rectifier kept mysteriously dying. Solder joints checked out, but the thing would just up and stop working -temp didn't seem to matter. Until I would whack it with an old drumstick -- like some "loose" connection in the silicon was making an intermittent contact. Too weird. I replaced it and the intermittace is gone but the wimpy PT is now quite apparent.

I've already had to replace the OT because of a short in the primary. Maybe that's why Marshall discontinued the DSL201 so quickly.

[Albert Kreuzer]

I've twisted my filament leads with both cordless AND corded drills and I can't hear a difference...

Yes, but did you twist them clockwise or counterclockwise?
That's where the REAL difference is. IMO, clockwise sounds better, but only in the northern hemisphere

[Redhat]

Yes, but did you twist them clockwise or counterclockwise?
That's where the REAL difference is. IMO, clockwise sounds better, but only in the northern hemisphere

And when playing to the East

[Chuck H]

Yes, but did you twist them clockwise or counterclockwise?

That depends on wire color

I'm really gettin' this $h!t nailed down to a science, eh?

Chuck