Egnater Rebel-30 Bias Adjustment values

[opacheco]

Hi again!

I am in the repair process of one EGNATER REBEL-30 guitar amp and I am not sure about the bias values for this amp. The question is:

What will be and appropiated Bias Current in miliampres for the 6v6's and the EL 84's tubes in this amp in order to don't damage the tubes and output transformer???

What is the bias used for Egnater in this amp for each tube pair??


Thanks a lot.
Opacheco

 

[CJ]

depends on what kind of tubes,

this amp has both 6V6 and EL 84's?

you have a schematic?

to get a ball  park figure, just consult the tube data sheets and work from there,

if it is cathode bias (big pwr resistor connected to cathodes) then current will adjust by itself (self bias)

if fixed bias (neg voltage applied thru grid resistors) then try about 40 ma for both tube types and listen to the sound,

if too harsh, increase current, watch the plates to make sure they don't red spot,

you can also bias in the dark, turn out the lights and adjust till you see a faint blue glow, for some reason, this blue glow happens at the sweetest bias point,

watch out for high voltage when working around a hot chassis in the dark,  ;D

unless you want free electro-shock treatment, 

you have the schematic for a Tazer? i could use one around here,

 

[opacheco]

CJ,

Thanks a lot for your comments!!...you are on the right line always!!

Here the schematic
You will note that I draw a 1 Ohms near to the test bias voltage points! and I suppose that will be 20 miliamperes per tube in oder words 20 miliamperes for ech 6V6 and 20 miliamperes for each EL84 (I found that with the help of my friend "The Dude" in another forum) I think this 1 Ohm resistor is like a Voltage (in milivolts volts) TO Current (in miliamperes) CONVERTER but I need your consideration about you told me (cathode bias, fixed bias, circuit analyser....etc!)

I suppose that amp is fixed bias(correct to me if I wrong too)
Thanks a lot anticipated

 

[fazer]

I don't see the B+ voltage on the center tap of the output transformer but you can measure that voltage and calculate the power dissipation of the plates and screens  and figure current  to come up with the dissipation in Watts. This is given  in the tube manuals for the tube types.  You don't want to exceed the max plate/screen dissipation for each tube type. 

If 40 ma at 325 volts = 13 watts + add in Screen  approx  1.5 to 2 watts = 15 watts.  I believe that would exceed the 14 watts max per tube  so bias to 35 ma or less depending on the B+ .  I thought the more typical current would be about 25 ma for say 400volts  like a fender deluxe voltage.    It just depends on the B+ voltage. 

Personally I like CJ's turn it to red glow and back it down.  I used that a lot when I could afford NOS tubes and ran them hot.  It was a great sound in a blackface Fender.  You will also hear hum in the amp because your causing the power supply to produce 120 hum from running out of headroom current.  So back down the  bias current till it stops humming.    Crude but works.

Also Ken Fisher would do a current check across the tube  and center tap which sounds a little dangerous but there is a whole topic on it in books and on the web with a google search.    Typical tube power dissipation  is 70% of rated power to allow the tube to last which is more important today than it was back in the day of NOS RCA's and Sylvania Phillps Tubes.

 

[opacheco]

Thanks fazer, I will measure the CT DC voltage and let you know soon!

Thanks again
Opacheco