Elevated heaters: what not to do.

[Colas LeGrippa]

Hi boys and girls !


The first message is for Lisa. Darling please call me after work I have something for you.


The other one is for all hum hunters in this forum ( for-hum ).
If your power tranny has 6.3 V taps without a center tap, you have probably hooked up 100 ohm resistors from each 3.6V to ground to minimize hum. But if you still have a bit of hum which you want to get rid of, you probably ( like me today ) thought  to elevate the gnd referemce to 25-45V or so. But you felt lazy
and hoooked up both the 100 ohm resistors to the K of the self bias output tube, which has already the right voltage you're looking for.


DON'T DO THAT !!!    YOU WILL LOSE TIME TRYING TO FIGUREOUT WHY YOUR AMP DOESN T WORK AND YOU WILL LOSE LISA'S CALL AT THE SAME TIME !!!!!

[PRR]

It has worked for many-many amplifiers.

I think Lisa has your mind messed up.

[kagliostro]

Just asking

when you connected the junction of the two 100R resistors to the cathode of the power tube/tubes, did you disconnected it from ground ?

Ciao

Franco

[shooter]

is for all hum hunters in this forum

I have wired up like you, amp still worked, but the hum got worse, I finally killed it, but can't recall how, I do know you don't wanna eat the game, leave it for the armadillos 

[sluckey]

Keep it simple. Just don't let Colas touch your amp! 

[Colas LeGrippa]

Yeah PRR,


PRR OBABLY  Lisa's interference in my messy mind and/or point to point layout ! Kagliostro, you are turning on the light of my doubt. Even an invisible contact between the junction of the 2 x 100ohm and ground would cause the issue I have experimented. I will open up the amp once again and make more tests.

[Colas LeGrippa]

I have hooked up the CT of rhe filament wiring to the K of the power tubes thousand of times with any problems, never. As a mater of fact I do that all the time. It is prob a coincidence if for the very first time I hook up 100R something turned wrong.
Sluckey, no danger , I won't try to fix other amps than mine !  I only can understand my own circuits !!!!

[sluckey]

Sluckey, no danger , I won't try to fix other amps than mine !  I only can understand my own circuits !!!!

I like that thinking. No customers, no worries! It's all pure fun.

[Colas LeGrippa]

RESULT OF THE HUM GAME




Franco: 1
PRR, Sluckey, Shooter, Colas : 0


It was not normal that my meter was reading 0 ohm between the K and ground when a 390ohm resistor is the bias resistor. The only way it could have read 0 ohm is if the K was connected to gnd at   same time. And it was.
Both 100 R were connected to the grounded leg
of a terminal strip and I just could'nt see it....I ll use my fluke next time....




 

[shooter]

Franco: 1
PRR, Sluckey, Shooter, Colas : 0

PRR gets partial credit, the girl is AWAYS part of the problem/solution, me, I usually hover around -6, so 0 is an improvement 

[Colas LeGrippa]

Ok then PRR I give you half a point how's that ?


Just finished hooking up filaments the RIGHT manner (this always happens when you wanna finish before even having started... ) and the remaining small hum disapeared yeahhhh !
Now I have a small portable 8 inch alnico 4 stages of gain, plate driven 5881 that REALLY ROCKS  (with it s master vol), ideal to practice at low volume and lots of crunchy tone and overtones, with full of harmonics all along the neck, and of course, HUM FREE.






Next time I 'll tell you more about Lisa and her reaction.


Jack

[Colas LeGrippa]

Shooter, I know you tried to give your best shot....but sometimes we just miss it. Talk to Lisa she may have advices for you

[shooter]

Talk to Lisa she may have advices for you

before I got married for round 2 I told Pam I was NOT trainable! She laughed, 30yrs on that fact isn't as funny to her 

[ALBATROS1234]

Curiously I had a problem colas. When I worked on se amps and made a virtual c.t. from 2 100ohm resistors I grounded to cathode of power tube and all was fine. When I built a harmonica amp 6 months ago using a separate control transformer for high voltage and a separate 12v filament trans for heaters. I had bad hum so I decided to make the virtual c.t. and ground to power tube cathodes. It was push pull designed very close to 5e3 deluxe although it was 12AQ5 tubes and the pi was 1/2 of a 12SL7. It squealed and screamed at me. When I took the wire from the cathodes and grounded it to its own spot on the chassis it was fine. So I though maybe you cannot ground the virtual c.t. to the cathodes of certain push pull circuits because it causes problems. Perhaps because of the split load pi? Maybe because I used separate filament transformer? Idk I just know it would not work on that amp.

[Colas LeGrippa]

Hi buddy,


Is your amp k biased ?
If you are feedind the filaments with 12 V, it has to be regulated otherwuse it will hum loudly.You can hook up the artificial gnd to almost any sourrce of voltage since there is no electron current. It acts only as a reference. It wont work on fixed bias push pull because the k are grounded direct. In fact you could hook up the junction of the 100ohm resistors to that cathode but without any benefice.

[pdf64]

... I had bad hum so I decided to make the virtual c.t. and ground to power tube cathodes. It was push pull designed very close to 5e3 deluxe although it was 12AQ5 tubes and the pi was 1/2 of a 12SL7. It squealed and screamed at me. When I took the wire from the cathodes and grounded it to its own spot on the chassis it was fine...

Is the power tube cathode bias resistor fully bypassed, eg at least 22uF

[ALBATROS1234]

Yes kathode bias and it has a 25uf bypass cap.