Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Scsoul on February 21, 2017, 10:38:20 am

Title: 5881 biasing?
Post by: Scsoul on February 21, 2017, 10:38:20 am

Here's my voltage
Plate 371
Screens 363
Cathiode  28  280 ohm resistor 330 bypass
  I wanna make sure my TUBES are happy not very good with the math
This is my first build with 5881 they sound great

Title: Re: 5881 biasing?
Post by: shooter on February 21, 2017, 11:04:13 am

I get about 34W of plate dissipation

the math is straight forward;

28/280 = I
371 - 28 = Eff Plate Volts

Effective plate volts * I = plate disipation

Title: Re: 5881 biasing?
Post by: jecars on February 21, 2017, 12:41:06 pm

Quote from: Scsoul on February 21, 2017, 10:38:20 am

Cathode  28  280 ohm resistor 330 bypass
 

Using quoted B+ & screen voltage values and typical 5881 data sheet, I calculate an inconsistency between quoted cathode voltage of "28V" and quoted bias resistor value of "280 Ohm".

1) If 28V value is reliable, then bias resistor should be 470 Ohm. Idle power = 20 Watts.
2) If 280 Ohm value is reliable, then cathode should be 21V. Idle power = 34 Watts.

Acceptability of either 1) or 2) depends on amp configuration (Class A or AB), maker of 5881 and personal taste. Personally, 34 Watts at idle is too hot for most 5881 tubes, short life should be expected.

Title: Re: 5881 biasing?
Post by: John on February 21, 2017, 12:47:46 pm

If you're running a pair of the 5881's in push pull, that should be fine. Around 17 Pdiss per tube.

Title: Re: 5881 biasing?
Post by: PRR on February 21, 2017, 01:42:26 pm

Is a fine condition for two 5881 working into 6K or 8K load. Around 20 Watts output.

Title: Re: 5881 biasing?
Post by: Scsoul on February 21, 2017, 02:36:37 pm

Yes it's a pair,  and nos so I don't wanna abuse them,  There pretty impressive sounding