Cathode Biased 5881 am I in the Ball Park?

[stevehoover]

Hi Folks,
My build is a SE 5881.   I used a Silverface Champ PT, a 20W SE OT and a rusty Bronco chassis that the previous owner swiss-cheesed before me.
I'm using the 4k OT tap into a 8 ohm CN10 and an 8 ohm Weber Alnico 15" in parallel for for 4 Ohms.
1st Preamp 12AX7 is the BF champ circuit the second 12AX7 is my variation of a 5F2 preamp with lower voltage to the plates and shared cathode circuit.
Both pre-amps share a common input with their own volume and tone circuits
Both pre-amps tie together with a 250K master and then a 2.2k resistor to the 5881 grid.

I did the math that I was able to.. (a bit challenged here)... and determined that a Tung-sol Re-issue 5881 would not blow the PT or tax the 5Y3 regulator too much.
B+ string is 20uf/120ohm @10W/40uf/1H choke/Plate/40K/1K@2W/20K/Screen/20K/ 2.2K/BF Pre/ 200K/ 8uf/Tweed Pre
The amp is very quite and sounds Beautiful! 
My only concern is the high screen voltage on the 5881. 
I really like the way this one came out but I don't want to blow up my final.

Plate voltage is 380V
Screen is 375
Pin 3 to pin 8 was 344V
A 490ohm cathode resistor with a 28uf@50V cap yielded 29.5V across the 10W resistor.
Note: cathode is (I think the term is elevated) by tying it to my artificial center tap on my heater circuit.
The on-line Weber calculator gave me 20W @ 57ma plate dissipation when I plugged in these numbers.

Am I in the Ball Park? 
Do I need to drop the screen voltage?
Any input would be appreciated, I've searched a lot of forums and found very little on SE 5881's except hi-fi stuff.
Thanks for any input,
Steve

 

[silverfox]

Hi Steve,

I don't have the technical ability to assist you on this build but I'm interested in seeing the schematic if you have one or can point me in the direction of where to obtain it. I also save many of the technical posts here on calculations and troubleshooting for my reading/references.

Thanks,

Silverfox.

[tubenit]

I personally would be fine with those voltages in my amp on 5881's. If it sounds good, I'd just enjoy it.

With respect, Tubenit

[phsyconoodler]

You need to get your terminology correct.
 You have 20 watts plate dissipation and 57ma of current.That is pretty much perfect.

[tubeswell]

The other guys have said pretty much all that needs to be said for the bias point. You run a single-ended output tube at maximum if you have the right load (or slightly higher load). The formula for optimum centre-biased Class A load resistance is Zo = Va x Va/Pa, where Va is the plate voltage, and Pa is the plate dissipation.

So checking backwards to get the plate dissipation (Pa) - In your case you have (29.5V/490R =) 60.2mA tube current. If you assume about (say) 5mA for the screen current in a '5881', that leaves about 55mA plate current. Plate dissipation is 380V x .055A = 21W.

Then using our (above) formula for load resistance:

380 x 380/21 = 6,876R (or 6k9)

So if you go to a 7k (or 8k) load, you will be about right methinks. If your OT has an 2R secondary then you can run the two 8R||8R speakers on that, or just run 1 of the 8R speakers straight from the 4R tap to get the load resistance to 8k.

[HotBluePlates]

My only concern is the high screen voltage on the 5881. 
...

Plate voltage is 380V
Screen is 375

That's perfect, and is enough below published maximum ratings to ensure long tube life.

The original Tung Sol 5881 data sheet specifies 400v max for plate and screen, but Fender ran 5881's more like 430v plate and screen in the 5F6-A Bassman.

No worries at all! 

[alerich]

The screen voltage and all of your other measurements sound fine. I don't see any mention of a screen resistor. The Silverface champ style amps didn't have one but I added one to my 70s Vibro Champ when I rebuilt it on Lord Valve's recommendation. I want to say it was 1K 5W but I don't have the amp or schematic handy. I swap 6V6 and 6L6 tubes out in that amp.