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Not your typical bias problem!

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5/4/1998 10:31 AM

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 2:49 PM

First of all, can you describe how you are measuring the current flow? Is the bias voltage stable?

At the operating voltages you've stated, that idling current is not excessive, unless the tubes are already bad.

The screen grid should never become incandescent. If it does, the wires will sag and dimensionally distort, usually causing permanent tube damage. The grid alignment & spacing will become screwed up, and you wiill get completely changed tube characteristics and possible short circuiting. Arcing is a tell-tale sign.

In a class AB circuit such as that marshall, the plates should not glow red.

5/4/1998 3:13 PM
Derrick	Yes, Doc. The bias voltage is stable. I am reading the current by the output transformer shunt method.

5/4/1998 9:49 PM
HM	Could it be the Z of your meter. I am new at this but remember reading that some meters will be too high resistance to use the transformer shunt measurment. H Meuse:

5/5/1998 3:36 AM

I've been having very similar problems with my Marshalls. The 50's seem to run fine on 36mA but my MV100 gets really hot on any more than 32mA. This is a real shame, because I want a better clean sound from the amp. I therefore contacted my friendly neighbourhood ex-Marshall tech. He tells me that this has been a known problem, and Marshall used to bias the amps to low currents to conserve valve life. This is evidently why the amps were renowned for only sounding good dirty. I'm not sure how seriously to take him!

Weirdly enough I had to modify the bias circuit to get less than 42.8 Volts bias on the MV100. Marshall obviously didn't intend for me to draw too much bias current. I'm going to get to the bottom of all this eventually, so I'd really appreciate any intelligence on the subject.

Another question: Why can't the bias current be checked by measuring the OT resistance, and then checking voltage drop across it? We're only talking about DC current after all. Surely this has got to be much more accurate than shunting current away from it.

Thanks for your help. I'm losing count of the number of people that have given me sound advice on this BB.

5/5/1998 12:30 PM

I was recently "tuning up" a 100w 1959 SLP reissue. I noticed that the tubes were biased way cool, like around 22ma (465v plate). I thought this was too low, so I raised it to about 32ma. Seemed reasonable. However, after the amp was warmed up with playing for about a half hour, the idle current was up above 36ma, so I set it back to ~28ma when hot. Maybe this drift-up is a characteristic of certain brand of tubes. I didn't fit new tubes, as the Marshall-branded ones still tested good, and sounded good.

I guess I'm sort of spoiled when it comes to measuring bias current. The meter I use is a clip-on probe type that reads DC current. As long as I can get the probe around the OPT's leads, I get an accurate reading. With amps that have cathode standoff resistors installed, the DC millivolt meter gets used. (The clip-on unit is an old tube powered unit that has a case as big as a car battery, and has to be warmed up some prior to using.) I've not had to rely on the OPT shunt method.

It's probably a good idea to install 1ohm or 10 ohm cathode standoff resistors on the tube sockets, in place of the ground wire coming off pin 8. This makes future biasing a breeze.

I haven't done it, but I don't see why a 1ohm resistor couldn't be placed between pin 3 and the transformer primary lead to read anode current, but I prefer to include the screen current on EL34s and therefore use the cathode resistor.

5/5/1998 2:01 PM
Derrick	I will check the Z of my meter tonight. It IS analog , and I have been told you want to use a DIGITAL meter instead, when checking bias. I don't know why when analog meters have been used for so long to do this. But hey, I am willing to try!!!! I doubt this is the solution, however. -Derrick

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