Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: John on March 01, 2012, 08:33:16 am

Title: SE- screen higher than plate
Post by: John on March 01, 2012, 08:33:16 am

Hi fellas, on that SE I built, the plate is @315, screen is @330 on a 6V6. Question is, should I try swapping the OT leads to see if that makes a difference, or really not worry much about it? The amp sounds great,which I figure is the biggest indicator.

Updated schem attached- I did give the screen and OT their own separate supply node.

Title: Re: SE- screen higher than plate
Post by: sluckey on March 01, 2012, 09:33:20 am

Quote

Question is, should I try swapping the OT leads to see if that makes a difference, or really not worry much about it?

Swapping the OT leads wont change anything.

Quote

Updated schem attached- I did give the screen and OT their own separate supply node.

Hmmm, your schematic shows them both on the same node. Lot's of SE amps use the same B+ node.

Title: Re: SE- screen higher than plate
Post by: John on March 01, 2012, 09:38:43 am

Dang, wrong one, sorry!

right one attached.

Title: Re: SE- screen higher than plate
Post by: jjasilli on March 01, 2012, 10:08:49 am

Probably "normal".  At idle the plate is drawing current, but the screen is drawing closer to -0- current.  Per Ohm's law, current draw produces a voltage drop.  The plate voltage is also dropped by the resistance of the OT primary.  The screen is pulling it's negligible current through a choke.  The choke probably has only 500R of DC resistance or maybe much less.  Hence the B+ supply voltage to the screen won't drop much, but the plate's will drop more.  Note that the Fender Champ uses a 1K dropping resistor to the screen node, to equalize plate & screen voltage at 350.  Though in my limited experience the Champ's actual screen voltage is often a few volts higher than the plate's.

You might try adding a dropping resistor to the screen supply, so that the DC resistance of the choke + the resistor = 1K or less; and see how that works and sounds.

Title: Re: SE- screen higher than plate
Post by: HotBluePlates on March 01, 2012, 10:23:26 pm

It's normal. I wouldn't give it a second thought.

Even if you start with plate voltage = screen voltage, under signal, the instantaneous plate voltage is less than the screen voltage ~half the time.

Title: Re: SE- screen higher than plate
Post by: John on March 02, 2012, 09:04:32 am

"normal" is what I was hoping for! Buttoned it back up, banged together another little "open air" cab out of more scrap lumber, and I'm a happy man.

Thanks!