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Not your typical bias problem!

5/4/1998 10:31 AM
Derrick	Not your typical bias problem! Can anyone shed some light on a problem I am having with biasing tubes in older British amps? When I bias tubes in Marshall, Orange, Park, Hiwatt, etc., most times the plate(s) will get red hot to some degree, or I will see an incandescent light comming from the tinny screen windings (not the heater) wich will get brighter as I push the amp harder. These amps run higher voltages then the same models being made today, but I doubt this has anything to do with it as I have tryed many output tubes of different vintages and make in these amps (vintage-current). A typical example is: In my 50W Marshall Plexi, the B+ is 460 v, and I bias each EL-34 for about 27-28 current draw. Depending on the tubes I install, either the grid will be incandescent (I can also hear a short harsh sound that dissapears a split second after I stop playing. Arcing?) , or a plate will glow red. If anyone knows the solution to this problem, or has seen or heard of it before, I would LOVE to know. Thanks so much, -Derrick

5/4/1998 10:31 AM

Derrick

Not your typical bias problem!

Can anyone shed some light on a problem I am having with biasing tubes in older British amps? When I bias tubes in Marshall, Orange, Park, Hiwatt, etc., most times the plate(s) will get red hot to some degree, or I will see an incandescent light comming from the tinny screen windings (not the heater) wich will get brighter as I push the amp harder. These amps run higher voltages then the same models being made today, but I doubt this has anything to do with it as I have tryed many output tubes of different vintages and make in these amps (vintage-current).
A typical example is: In my 50W Marshall Plexi, the B+ is 460 v, and I bias each EL-34 for about 27-28 current draw. Depending on the tubes I install, either the grid will be incandescent (I can also hear a short harsh sound that dissapears a split second after I stop playing. Arcing?) , or a plate will glow red.
If anyone knows the solution to this problem, or has seen or heard of it before, I would LOVE to know. Thanks so much,
-Derrick

Replies:

Doc First of all, can you describe how ... -- 5/4/1998 2:49 PM