Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on October 31, 2012, 03:44:12 am

Title: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: kagliostro on October 31, 2012, 03:44:12 am

Usually we find two resistors to have a virtual ground on heater circuit on which the transformer is missing of a CT

Common values for this resistors are 100R and 220R

Some old amp, in place of this pair of resistor, used a pot

the pot give the ability to balance perfectly the circuit and so is a better way to reduce hum

Old people to which I asked, told me to use a wire pot for that purpose, but is it really needed ?

Isn't a common 470R pot rated at 250mA adequate for that use ?

Thanks

K

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: The_Gaz on October 31, 2012, 04:45:37 am

Let's use the example of the 470R pot:

6.3v * 6.3v / 470R = .08W

So yes, any pot will do. I just think people suggest sealed pots to prevent dust/dirt build-up inside the pot since it is not moved much once adjusted for minimum hum. Sealed pots happen to be a higher wattage typically, which leads people to believe the pot must be a high wattage for some electrical reason. Fortunately, Ohm's Law can clear things up.

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: FYL on October 31, 2012, 04:53:23 am

470R and 6V3 gives 13 mA/82 mW max but balancing pots are usually 3- or 5w rated models, which happen to be mechanically more stable.

BTW, 470R is quite high for this app. Try a smaller pot or a R + pot network (say, 33R x 2 and a 100R pot).

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: stratele52 on October 31, 2012, 06:13:12 am

2 X 100 ohms it is better than a pot , it can move , cheaper eaier to install, forget the pot. Al Fender Blackface amp use it  and no amp is more quiet than a Blackface

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: sluckey on October 31, 2012, 07:10:18 am

Quote from: stratele52 on October 31, 2012, 06:13:12 am

2 X 100 ohms it is better than a pot , it can move , cheaper eaier to install, forget the pot.

The only advantage of using 2 x 100 ohm resistors is that it's cheap. Having an adjustable pot means you can easily balance the hum by ear, or precisely with an AC voltmeter. Hum balance pots don't have knobs. They are usually screwdriver adjustable and never located on a front panel.

Using fixed resistors is a cheap alternative that works well enough for most guitar amps and provides a 'good enough' hum reduction. But the hum may not be perfectly minimized if one resistor is actually 90 ohms and the other is actually 110 ohms. The expensive pot allows you to precisely set the 60hz filament hum to an exact minimum using a scope or meter, or maybe even your ears.

Most hum pots I've seen were wirewound 100 to 200 ohm 2 watt. I highly recommend using one in hifi equipment or boo teak guitar amps.

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: labb on October 31, 2012, 07:34:34 am

Humdinger--Try Bourns part no. 3309P-1-201. 200 ohm, 0.5 watt, single turn cermet trimming potentiometer. 85 cents from Mouser.

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: kagliostro on October 31, 2012, 09:24:34 am

Thanks to all for infos and opinions

I've just find this dedicated pot

(https://www.tube-town.net/ttstore/images/product_images/original_images/5687_0.jpg)

http://www.tube-town.net/ttstore/product_info.php/info/p5687_Fender-Potentiometer-Hum-Controll-100R.html (http://www.tube-town.net/ttstore/product_info.php/info/p5687_Fender-Potentiometer-Hum-Controll-100R.html)

It is disposable (available) at Tube Town a german online shop at which I can easily make orders

Sluckey

please, can you translate boo teak

I can imagine what you mean, but I'm not sure

Thanks again

K

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: sluckey on October 31, 2012, 09:55:49 am

Quote

please, can you translate boo teak

Boo, as in Halloween ghost. Rhymes with who.
Teak, as in "nice deck on your sailboat lovey."

Now say it quickly ten times

Boo teak
Boo teak
Boo teak
Boo teak
Boo teak
Boo teak
Boo teak
Boo teak
Boo teak





Boutique

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: phsyconoodler on October 31, 2012, 10:07:46 am

Funnin' with the furner's eh? :l2:

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: HotBluePlates on October 31, 2012, 06:02:26 pm

Don't make me break out the Engrish...

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: kagliostro on November 01, 2012, 03:54:09 am

Quote

Boo teak
Boo teak
Boo teak
Boo teak
Boo teak
Boo teak


Boutique

That is fun :laugh:

K

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: PRR on November 02, 2012, 12:08:43 am

> a common 470R pot rated at 250mA

250mA is 0.25A. 0.25A times 470r is 117V. 117V*0.25A is 29 Watts.

That does not sound like a common pot.

Maybe 250mW (milli-Watts)?

Title: Re: Virtual ground with 470R pot - Which is the correct rating in mA for the pot ?
Post by: kagliostro on November 02, 2012, 02:41:11 am

PRR you are right

I don't know what I was thinking when I wrote 250mA  :dontknow:

Thanks

K