Tweet
Guys, I'm biasing some EL-34 tube heads, found the plate voltage at 470 volts. Did the math (25/470 times .7 = 37). But the bias control is maxxed out and it's only reading 7.9 mv on the bias probe test! What gives? Is there something that needs to be added or changed out to allow for a higher bias rating?
-
"Got a crazy feeling I don't understand,
Gotta get away from here.
Feelin' like I shoulda kept my feet on the ground
Waitin' for the sun to appear..." -
I'm definitely not an expert, but on my amp, the manual says to set it so the bias is .6V. Are you sure your meter is set to the right range? Is the amp turned on? 7.9mV is extremely small.Scott -
If the bias is really that low, the amp would sound weak and distorted.
If the bias isn't that low, and in reality is way too high, the plates might be glowing red ready to meltdown, and the tone would be clear and powerful.Comment
-
Weird, because the same bias probe is reading normal on some other amps. I'm trying to adjust a 100 watt JMP. The plate voltage is 470. So this means I want to set the bias around 37. Currently, it's reading about 7.9. Yes, the amp is on, and running for about 20 minutes, fully on, standby set like when you play through it. I also have a JMP 50 watt, and it's reading in the 30's with the bias probe and plenty of room to go in either direction. That's why I'm wondering if a mod was done or a bias pot was installed or a resistor or whatever to prevent setting higher. Amp sounds good as it is, just wanted to bring it to spec..."Got a crazy feeling I don't understand,
Gotta get away from here.
Feelin' like I shoulda kept my feet on the ground
Waitin' for the sun to appear..."Comment
-
Aren't you supposed to be measuring the plate current? hence mV=milliVolts would be wrong setting on your instrument...should be milliAmps
check again to see if you are doing everything correctly!Comment
-
did you try diffrent tubes? i ask this cause I bought some TunSol's 6550's for my ENGL Savage 120. Before i swapped them in, I checked with my Webber Bias Rite,, the Stock ENGL 6550's were at like 33, so i bumped them up to 41(IIRC) and still had some pot left to turn. i swapped in the Tung Sol's and they were reading 16,,so i turned the pot till it maxxed and they only read 22. so I took them out and put the stock ones back in and brought them back down to 41. so you might want to look at that maybe as a possibillity....
MODSHON
DAVE"It's because the speed of light is superior to the speed of sound that so many people look shiny before they actually sound stupid"
"All pleasure comes at someone Else's expense"
The internet is where, The men are men, the women are men, and the children are FBI agents.
Comment
-
In some case, when you run out of bias adjustment room, changing the bias resistor is in order. Such is the case with Laneys.I'm not Ron!
Comment
-
I think you have to do something special when you have twice as many tubes, or maybe that's just with the DSL100.Comment
-
He is reading current. The bias probe has a 1 ohm calibrated resistor. When you read the millivolts across the resistor it give you the current in milliAmps.Originally posted by axeman81 View PostComment
-
Most mods involving the bias at to use EL34's on a 6550 amp or vise versa.
To run 6550s, you need more negative bias on the grids to keep the tubes from runaway than you do with EL34's. So the mod to run 6550s gives you "more negative".
Putting EL34's in this amp, the extra negative bias on the grids would cause less milliamps to run through the tubes, hence what you are seeing.
The mods would be done close to the bias pot and involve changing some resistor values.Comment
Comment