Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on May 21, 2017, 02:41:18 pm

Title: weird power tube behaviour
Post by: Colas LeGrippa on May 21, 2017, 02:41:18 pm

Hola amigos que tal ?


I was modifying the preamp of one of my builds when I realised something was wrong in the power section. Right after the PI, after the coupling caps, one cap was showing 0 VDC while my meter was measuring .030VDC on the other one (orange drops 600V both). So I said to myself: ''Colas, this cap is leaking''. So I replaced it , took back the measurment : .030VDC.  Precisely at this moment, my wife called me because she was having a hard time trying to put on her dress.  ''One moment, babe, I'm coming''.  I took the time to swap the defective cap for a cheapo : still .030VDC.  Measured the other cap: .0000 VDC. 
So I took a deep breath, helped my wife with her dress (the attachment of her bra had torn her dress and she couldn't took them apart like if they were both welded, anyway....) I got back to my bench and swapped the power tubes. Surprise now: the cap that was showing .030VDC was all fine now while the other one that was good was leaking .030VDC now.....


How can a power tube leak dc voltage on it's control grid ? I presume that both plate and screen voltage of that tube are influencing G1 on that particular tube. Can we tolerate small dc voltage on G1 ? Is it the sign that this tube is becoming weak ?


Thanks guys (and gals too).


Colas LeGripppppppaaaaaaaaaa

Title: Re: weird power tube behaviour
Post by: sluckey on May 21, 2017, 02:52:27 pm

IMO, 30mV on a grid is the same as zero.

Title: Re: weird power tube behaviour
Post by: PRR on May 21, 2017, 03:35:53 pm

You want the "error" of bias to be much less than say 10% of the normal bias.

Power tubes run 15V-45V of bias. We want less than 1V-3V error.

> IMO, 30mV {0.030V} on a grid is the same as zero.

Agree.

Title: Re: weird power tube behaviour
Post by: ratgon on May 21, 2017, 10:44:32 pm

How'd the dress/bra situation turn out?

Title: Re: weird power tube behaviour
Post by: Colas LeGrippa on May 22, 2017, 10:24:16 am

finally, she got to remove everything to free the dress from the bra hook and I let her do her things while I got back to a more serious one: my amplifier.


Colas

Title: Re: weird power tube behaviour
Post by: Colas LeGrippa on May 22, 2017, 10:32:25 am

Talking about my amplifier, it was making a noise just like one tube was missing a supply cap, or if dc was present on a pot.  That's why I thought that yhe small dc on a power tube control grid was responsible for that noise. Thanks Sluckey and PRR. Ratgon, sorry for disappointing you.


Colas, lolo lo-lo loooo-la....