Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jonas on August 05, 2018, 10:19:13 am

Title: Lifting the ground on AC Heaters
Post by: Jonas on August 05, 2018, 10:19:13 am: HI,

Regarding AC heaters and lifting the ground reference, I have a couple of questions and would like your advice.

1) What voltage would be ideal for ground reference?
2) Do you have any recommendations to add protection to this circuit in the event a tube shorts?

Thanks

Jonas
Title: Re: Lifting the ground on AC Heaters
Post by: John on August 05, 2018, 10:35:52 am: About 70 volts DC seems to be the preferred sweet spot. To make your voltage divider, typically a 470K over a 100K is used. I've used a 330K over 68K sometimes too. Mostly because I got a bunch of 330K cheap. :icon_biggrin: Don't forget a 10uf (or so, 22uf is fine) across the bottom resistor (that goes to ground).

Run a 100 ohm .5 watt resistor from each fil. wire to your elevated DC. In the event of a short, most times one or both of those resistors will burn open, saving your PT.
Title: Re: Lifting the ground on AC Heaters
Post by: Jonas on August 05, 2018, 10:46:47 am: Thank you John. Since there is no current draw through the voltage divider, what does the cap do?
Title: Re: Lifting the ground on AC Heaters
Post by: sluckey on August 05, 2018, 11:05:25 am: Quote from: Jonas on August 05, 2018, 10:46:47 am
Thank you John. Since there is no current draw through the voltage divider, what does the cap do?
The cap provides an AC ground for the elevated dc voltage reference. This will reduce the filament hum.
Title: Re: Lifting the ground on AC Heaters
Post by: terminalgs on August 05, 2018, 11:19:18 am: It depends on the amp. If it has a cathode bias power amp, a convenient place is the cathode of the power tubes.
Title: Re: Lifting the ground on AC Heaters
Post by: Jonas on August 11, 2018, 12:03:15 pm: Hi Everyone,

Could someone walk me through the benefit of adding 100 ohm filament resistors as mentioned above?

When elevating the heaters, the center tap is connected to the voltage divider so what happens when a tube shorts and how would the 100 ohm resistors provide protection to the power transformer?

I am familiar with grounding technique in amps without an elevated DC heater. The center tap is cap and stowed (not used) so the filaments get ground reference through the 100 ohm resistors. During a short, these resistors open up protecting the power transformer, essentially lifting the ground.

Thank you in advance!!!!

Jonas
Title: Re: Lifting the ground on AC Heaters
Post by: PRR on August 11, 2018, 07:53:34 pm: If coming from a typical resistor divider, or most typical cathode-bias points, the resistors add nothing. You can just use the CT if you have one.