Virtual Ground resistors - 100R or 220R there are differences ?

[kagliostro]

Is a bit of time that I've this question on my mind

there are more that one approach to obtain a Virtual Ground

many times 100R resistors are used, but also 220R is used, without mention the use of a pot

with 220R resistors, obiouvsly, there is less current on the circuit of the Virtual Ground respect the use of 100R

The question is .....

Using 100R instead of 220R gives a strongher Virtual Ground effect ? There are advantages of some sort ?

Thanks

Franco

[sluckey]

Talking about heater circuit???

The ideal virtual ground would be zero ohms, but that would not be good for the PT! So, compromise. Use the smallest value of resistance that will not strain the PT. The value is less important than using matched resistors. Best of all is a 100Ω or 200Ω pot.

[kagliostro]

Talking about heater circuit???

Yes, exuse me, I forgot to specify

--

OK for the pot that is the better and the use of selected resistors

About the value I've understand, if there is enough current, the lower usable value is better

Thanks Steve

Franco

[frankenxtein]

I have used 68ohm 1/2 watt , now 1 watt metal oxide power resistors  (see jpg)   

[kagliostro]

Ciao Phil

I knew that the pot sold for fender are 100 ohm (ideally 50R on each branch)

https://www.tube-town.net/ttstore/Potentiometer/Fender-Potentiometer/Fender-Potentiometer-Hum-Control-100R::5687.html

But I had never paid attention to those resistors on the Airline schematics, so 68R is a  not common value for me

if I can I use metal film resistors (2W)

Franco

[frankenxtein]

I've used 68's since I built one of those 1471's  .... I've used 100's when I didn't have 68's. Didn't notice a difference between the two.  Read somewhere between 50-100 ohm works. It's something I've been adding to every 6.3v.

Maybe it's not needed on the heater on power tubes?

That last one I built has 2 sets 6.3v lines and I'm using both I just made CT's for both. Mass produced equipment of yesterday ... they tried to save pennies , omitting parts. Notice the 1M grid stopper on the input on the 1471
 

[trobbins]

Good question K.

For many situations, a standard fixed humdinger or heater CT seems to do the job, with no gain from using a pot.  So I guess its the exceptions (valve heater-cathode resistance asymmetry, or heater terminal/wiring capacitance imbalance to a grid, or heater voltage harmonics/glitches from the mains or B+ rectifier) that benefit from a pot.

For the resistance effect, where hum current loops around a particular circuit path that includes the humdinger, I can't see any issue with making humdinger resistance significantly larger (ie. it doesn't matter what value is used).

Similar for the capacitance effect I think too.

I happily use a 220 ohm 10T 0.6W trimpot, cos I've got a bag of them 

[Merlin]

Using 100R instead of 220R gives a strongher Virtual Ground effect ? There are advantages of some sort ?

The lower the resistance, the greater the power dissipation, but the lower the susceptibility to HF hash leaking onto the heater supply. However, the latter can be fixed by adding a couple of capacitors to ground (even this is normally unecessary). I use a 500R pot because the dissipation is then only 80mW.

[pompeiisneaks]

I found a 200 ohm pot and used it on a bassman, and it really worked like a charm, but like Merlin said, many options will work, and the 500 ohm needs less wattage value than the lower ohm values. 

[kagliostro]

Thanks

..... HF hash leaking onto the heater supply

Can I translate that as problems at High Frequency on heaters ? (Sometime I lost in translation)

Franco

[PRR]

The Ampeg VT-40 "Distortion" amp had heater wiring on the PCB. Hum everywhere. Hum in the first stage could null hum in in the post-Volume stage, at some specific Vol setting. It had a humdinger pot. After extensive twiddling, I realized that I could optimize the humdinger for any ONE setting of Vol (also tone and reverb to lesser degree), but any other user-knob settings led to a different humdinger setting.

So I just set it up with user-pots where I thought played best, and set the humdinger.

On the VT40 Combo, this actually was not critical. It was mostly pure 60Hz, and the open-back cabinet could not throw 60Hz more than a few feet due to front-back cancellation. (Also the light cones were probably 90Hz resonance.) It was impressive with your ear close to the box, inaudible in large room. (Sadly it went to a small studio which might even have a 60Hz node.)

First lesson: don't do AC heaters on PCB if your speaker can do 60Hz well. Today we can score hefty DC supplies. Yesteryear we could route twisted-pair semi-symmetric socket to socket.

[HotBluePlates]

..... HF hash leaking onto the heater supply

Can I translate that as problems at High Frequency on heaters ?

Yes, High Frequency (possibly radio frequency) noise.

[kagliostro]

Many Thanks PRR & HotBluePlates

Franco

[kagliostro]

I give a search on my reserve and find some Allen Bradley pot



some has the same reference number and measuring it I selected a pair that are 468R, the others are around 460R - 470R
except one that is (strangely) 568R

I've a project where there are 6.3v heaters and 12.6v heaters, is better if I use the 568R on 12.6v heaters than one of the other pot ?

On the 12.6v is better to add a pair of resistors to the pot (one of those I've, also if I use the 568R) as to increase the total resistance ?

Thanks

Franco