Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Oddvar on September 22, 2018, 09:04:29 am

Title: Heater resistor burnout
Post by: Oddvar on September 22, 2018, 09:04:29 am

Hi.  Does anyone know what causes a heater resistor to burn?  It seems to be a 470R 3.75w Dale Vishay in this Mesa Boogie Mark IV.

And could I change it with a 470R 5w resistor?

Title: Re: Heater resistor burnout
Post by: shooter on September 22, 2018, 09:21:45 am

excess current more than likely burnt it
I only have the MKII schemo, it shows 100ohmers like most builds if that helps

Title: Re: Heater resistor burnout
Post by: Oddvar on September 22, 2018, 09:58:29 am

Here is a pic, clearly burnt...

Title: Re: Heater resistor burnout
Post by: sluckey on September 22, 2018, 10:06:32 am

That's not a heater resistor. It's a screen grid resistor. Excessive screen current burned it up. Most likely a bad tube.

Title: Re: Heater resistor burnout
Post by: Oddvar on September 22, 2018, 10:15:14 am

Quote from: shooter on September 22, 2018, 09:21:45 am

excess current more than likely burnt it
I only have the MKII schemo, it shows 100ohmers like most builds if that helps

It is 470R, 3.75w for some reason.  I was wondering if wattage could go up to five.  And if it could/would be the power tube that caused the failure?

Title: Re: Heater resistor burnout
Post by: PRR on September 22, 2018, 10:21:33 am

Quote from: Oddvar on September 22, 2018, 10:15:14 am

...if it could/would be the power tube that caused the failure?

Quote from: sluckey on September 22, 2018, 10:06:32 am

Most likely a bad tube.

Title: Re: Heater resistor burnout
Post by: Oddvar on September 22, 2018, 11:35:52 am

Quote from: sluckey on September 22, 2018, 10:06:32 am

That's not a heater resistor. It's a screen grid resistor. Excessive screen current burned it up. Most likely a bad tube.

You're right, I thought it was number 7, blame it on the bad eyesight...