Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on January 16, 2011, 04:40:03 am

Title: Which are the calculations to be done to determine the consumption of a tube ?
Post by: kagliostro on January 16, 2011, 04:40:03 am

I'm interested to know which are the calculations to be done to determine the consumption of a tube

assuming B+ is 315v

which is the consumption of V1 (ef86) in the Vox AC15 ??

How to calculate the consumption ?

Many thanks in advance

Kagliostro

Title: Re: Which are the calculations to be done to determine the consumption of a tube ?
Post by: DummyLoad on January 16, 2011, 06:02:28 am

calculate current thru anode resistor; IRa. given B+ and Va so then VRa = 315V - 80V = 235V  calculate current thru Ra = 235V/220K = .00107A or 1.07mA = IRa = Ibias

calculate Vg-k 2K2*1.07mA = 2.35V

calculate Vtube = Va - Vg-k = 80V-2.35V = 77.65V

calculate Pdiss = Vtube*Ibias = 77.65*1.07mA = 83mW

so then Pa = 83mW

what i do most times is ignore Vg-k calculation w/ small signal tubes since it's usually a couple of volts or less. you need to take it into account on cathode biased output stages though, as it can be a significant quantity. in fixed bias, Vtube is measured to Va to gnd.

Title: Re: Which are the calculations to be done to determine the consumption of a tube ?
Post by: kagliostro on January 16, 2011, 07:29:45 am

MANY THANKS ISOTone

Kagliostro

Title: Re: Which are the calculations to be done to determine the consumption of a tube ?
Post by: kagliostro on January 16, 2011, 11:41:24 am

I've read slowly and with attention the given explanations

one thing is out of my comprehension

Quote

given B+ and Va so then VRa = 315V - 80V = 235V

What Va means ?

from were comes the 80v value ?

Va (used as an abbreviation) isn't the max anode permitted B+ voltage ?

Sorry for the neavity but need further explanation

Thanks

Kagliostro

Title: Re: Which are the calculations to be done to determine the consumption of a tube ?
Post by: Geezer on January 16, 2011, 11:48:50 am

Quote

What Va means ?

from were comes the 80v value ?

I believe Va = Anode, or plate voltage

The 80v comes from your attached drawing, showing 80v present @ the anode/plate

Title: Re: Which are the calculations to be done to determine the consumption of a tube ?
Post by: kagliostro on January 16, 2011, 12:38:14 pm

MANY THANKS Geezer

Quote

The 80v comes from your attached drawing, showing 80v present @ the anode/plate

 :laugh: :laugh: :laugh:

Sometimes I act as a real stupid

 :lipsrsealed: :lipsrsealed: :lipsrsealed:

I've find the AC15 schematic, but was really not well readable

so at the last moment I posted another schematic that I've find to have a good readability

but didn't read it with attention

Sorry - I APOLOGIZE

This happens when I try to do more than one thing at the same time SORRY  :sad:

Kagliostro

Title: Re: Which are the calculations to be done to determine the consumption of a tube ?
Post by: PRR on January 17, 2011, 12:48:53 am

(http://i.imgur.com/5atcS.gif)

Title: Re: Which are the calculations to be done to determine the consumption of a tube ?
Post by: kagliostro on January 17, 2011, 03:57:31 am

MANY THANKS PRR

your schematic clarify the issue further

Kagliostro