100Rs for heater artificial CT ?

[Madison]

110Rs are okay to use?
I have to be sure about this.

[Dave]

Yes, that's fine.

Dave

[Madison]

Thanks.

[stingray_65]

Yes, that's fine.

Dave

+1

I have used 470R with no issue, but I wouldn't go much higher.

It is only a simple divider network to give a reference to ground. any closely matched pair will do.+-5 ohms

do not go lower than 100R, they tend to heat up.

110R is 10% difference from 100R and well within spec.

[phsyconoodler]

Bah humbug! Many amps used 47 ohm for decades.

[darryl]

It's a pretty simple calculation. The two hum balance resistors are effectively in series across the heater voltage.

Using 47 ohm resistors:   P = V^2 / R
                                     = 6.3 x 6.3 / 94 =  0.42 watts  or about 0.2 watts per resistor.

1-watt resistors would do fine - even half-watt resistors would have some safety margin.

With 100-ohm hum balance resistors, the power dissipation drops to a mere 0.1 watts per resistor.

[The_Gaz]

While you're at it, I recommend using a hum balance pot, where the pot goes across heater winding, and the wiper goes to ground. It's helped me get rid of some mysterious hum before. I thought of it because I use a 1k pot there, which would be equal to 2x470R. The adjustment isn't as fine, but that value is easy to ome by.

What I never understood was why people insisted on using a large wattage pot for a "hum balance" control . A 470ohm pot only dissipates milliwatts!

Some techs also love to yank out hum balance controls in in favor of the 110ohm resistors to ground. The only reasoning I could find was that the pots tend to sit in only place, never getting adjusted, and the contact becomes intermittent. I dunno. Perhaps that's why larger pots are recommended, because the are sealed.

[navdave]

Ok if you had a 800vac pt with no center tap and wanted to run it in full wave mode what would you use for an artificial center tap?

[darryl]

Ok if you had a 800vac pt with no center tap and wanted to run it in full wave mode what would you use for an artificial center tap?

From Merlin's website:



The output would be well over 1000 volts DC.  Enjoy! 

[navdave]

Trolling? That was a serious question. I ain't the sharpest tool in the shed.  

Darryl, that's awesome you posted that, but I think navdave is trolling. 

[eleventeen]

That's pretty interesting Darryl, I never saw nor thought of that config. I'm not quite sure why one would employ it. I assume your B+ out would be roughly the average of a 100% solid state bridge rectifier and a 5Y3 setup?

[navdave]

That's a FULL WAVE BR. From Merlin's site I meant "Two-Phase, Full-Wave Rectifier".
I've seen some very cheap toroidal transformers with 800vac but no center tap hence my question.
I have a 74 SUNN Model T with 400-0-400 @ 400mA in full wave mode that gives about 520VDC under load on the big tooobs.
I love BIG power, pipe wrench verses screw driver. Get a bigger hammer

Ok if you had a 800vac pt with no center tap and wanted to run it in full wave mode what would you use for an artificial center tap?

From Merlin's website:



The output would be well over 1000 volts DC.  Enjoy!  

[navdave]

Like this??

[darryl]

Like this??

Nope! You have 800 volts AC across 200 ohms.  

If the transformer secondary is 800 volts, without any other taps, the only way to reduce the DC output is by the very inefficient method of high power dropping resistors/zener diodes, or the slightly less inefficient method of a large bucking transformer.

Edit for the sake of completeness: The voltage could theoretically be reduced using a series pass transistor ( similar to a VVR circuit ), but it certainly isn't practical...   

[PRR]

> What I never understood was why people insisted on using a large wattage pot for a "hum balance"

History.

The actual part (which used to be common) may have originally been a filament voltage adjuster. It could dissipate a good part of a Watt. That far more than early carbon-track pots would handle (and they may not have been commercialized yet), so it had to be a wire-wound. Good-size wire is easier to wind than teensy wire, 1 Watt is a low-price wirewound.

Then pots were used in direct-heat filament stages as the "cathode" return. The value should be high enough not to load the heater power, yet low enough not to upset the "cathode" bias. The cathode-bias might be 500 ohms, so 100 ohms is a suitable size (in the cathode-path the two halves parallel for 25 ohms).

This was such a common size that when acceptable hum level dropped in heater-cathode gear, the same part was used for hum balance.

You would not question it at 1953 prices or with typical 1953 shop inventory. It only feels odd because the 100r 2W part vanished in the 1970s and you can hardly find one now.
> if you had a 800vac pt with no center tap

Then get another PT. That one is NOT suitable for normal work.

The CT on a heater winding is not CT-ing the heater power, just shifting both leads.

The CT on a full-wave rectifier handles the load current. If you try the resistor-split, the "best" you can do is with resistor waste larger than useful load power and huge sag.

If you can get a HEAVY choke (heavier than is found in guitar amps), you could get 1,100VDC no-load (start-up) and 720V DC when there is enough load to keep the choke conductiong through the cycle. Even 720V is unlikely large, and for what the choke costs you could go get a more-suitable PT.

Merlin's plan works great for 100V-300V windings, giving 110V DC to 400V DC "with bottle rectifier flavor".

It is NOT intended for a 800VAC single winding.

What do they sell that part for? (And is that why they are cheap?)

[The_Gaz]

Thanks a lot for the info on the hum balance pot, PRR.