Some notes on tubes and biasing

 

Identification of ECC83 tubes

sometimes its hard to identify tubes that "lost" their label or relabeled tubes - this may help



Manufacturer of ECC83

~ Plate-Distance

in cm
Number of Holes*
smooth/ribbed
other
RFT
1,1
3-holes on each side of each plate
smooth
8 spikes
Siemens
1,65
4 holes on each side
smooth
Telefunken smooth
1,7
4 holes
smooth
RCA
1,65
no holes
ribbed
Mullard
1,45
1 big hole on each side
ribbed (3)
Valvo
1,5
1 big hole on each side
ribbed (3)
Electro-Harmonix
1,2
no holes
ribbed
EI
1,65
4 holes on one and 3 on the other side
smooth

* = look from the side thru the middle of the two triodes.

biasing a tube amp

 

I`m using a Weber Bias Rite VI Option - or simply a DMM and 1 Ohm resistors to do this,

 

The common equation to calculate the current for Class AB amps is:

PD/V x 0,6 = idle current *

PD = plate dissipation (see table below), V= plate voltage

Tube
Plate Dissipation
[Watts]
6550
35
EL34
25
5881
30
6L6GC
25 - 30 (30 for GC)
6V6GT
12-14
EL84
12
KT88
40
KT66
30

 

To calculate the current use this script:

(courtesy Weber VST www.webervst.com - I`ve dropped the factor in this script down to 60% for Push-Pull)

Bias Settings For Safe Plate Dissipation - Class AB 60%    Class A 90%%
1. select type of Class:    

 

 
2. select used tubes:      
3. fill in the Plate Voltage      
      mA (60%)  

* confused about 60%?

several amp-builders of today (Diezel, Peters, Roccaforte) seem to prefer

"colder" bias-settings closer to 60% of the max. plate dissipation,

But In all cases there`s a "safe area" for each tube between 450 and 500 Volts plate-current:

33 - 34 mA for EL34s

32 - 38 mA for 6L6s

44 - 45 mA for 6550s

 

 

some charts:

read your Plate Volate (y-axis)

(PIN 3 - with a DMM you have to set it to 1000 Volt DC, red to PIN 3, black to chassis)

and then look for your idle current (x-axis)

EL34s:

some more charts:

 

preamp - tubes and heater voltage :

how to do the heating?

(thanks to Peter for this !!!)

 

much more infos and articles about biasing, the different methods and details:
www.aikenamps.com
  
german website - my favourite seller of new tubes
www.tube-town.de
  
biasing Marshall DSL/TSL amps
here
  
biasing procedure for any other amp
www.duncanamps.com
  

 

 

 

some pics . . .

 

A bunch of NOS tubes: RFT EL34 and ECC83, Sylvania 6V6GT

gain-comparison chart (relative)

 

 

 

 

a sealed NOS Telefunken ECC 83

 

I`m a proud owner of a

Superior TV-11 Tube-Tester