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I think you're original post of biasing to 30-40 volts was correct. Driver/input tubes are usually biased somewhere around 6V so 300-400 mV is super-way low for power tubes.I think it's a better idea to monitor plate current and adjust bias to reach a particular reading (usually 30-40 mA for EL34) instead of setting the tube to a particular bias voltage, current be damned. You can get a tube scout form tubedepot.com...http://tubedepot.com/bt-bias-scout.html...and easily monitor current and adjust bias accordingly. There are also some offered by amp-head...http://www.amp-head.com/index.php?cPath=21...that are a little more expensive but test two tubes at a time plus tell you plate voltage. Either will work fine with you dmm. Your decision.If you still have a problem with the amp shutting down after using these bias tools then you've either got a bum tube or a problem with your amp.Best of luck and keep us informed!
I did read somewhere about the seemingly high value (400 vs 40) and it was mentioned that it had to do with how the test ports where wired ? I'm assuming tht these ports are only for measuring the voltage and not the current ? Sorry for the dumb questions but I guess that is how we learn The amp also has "EZ Bias" which is simply an LED for each power tube which comes on once the proper voltage is reached when adjusting the pot
I wrote this for a gentlemen about 5 years ago, might be of help.Jolida 302B biasing information.The Jolida manual suggests a value of 40mV as measured at the test points. I disagree.I did a bit of research and from what I read it's suggested to set the bias between 60 and 70% of the tubes maximum dissipation, not exceeding 70%. An EL34 has a max of 25W (so pushing this amp to 50W with EL34's is working them awfully hard) and 70% of that makes the maximum dissipation 17.5W (for bias purposes). I believe they are running the plates at about 490V (calculated, didn't measure it), which makes 70% bias 35.7mV and 60% bias 30.6mV. Obviously 40mV is going to be pushing the tubes pretty hard and may reduce their lifespan. For my tests and listening I set them to 32mA to put them in the middle of this range. From what I heard I had no desire to try and bias them hotter. You can if you like but tube life should be better if you bias them to about 32/33mV.Regarding the KT-88, the Jolida manual suggests a bias point of 50mV. This works out to a plate dissipation of 24.5W, which is just over 61%, so the tubes should last a long time at this setting. Interestingly they suggest biasing the EL34 too high and the KT-88 at the low end. A bias point of 53mV will put you at 65% of maximum plate dissipation which should be a very nice spot. When setting the bias use a screwdriver with an insulated handle. While the adjustment pots are isolated it’s always possible for them to fail. Using an insulated screwdriver will ensure safety. The setting procedure is well described in the Jolida manual but I suggest connecting the negative meter lead to one of the speaker negative terminals, it does not matter which one. This will leave your hands free for making the adjustment and removes the risk of shorting the probes together. Be sure to always have a load connected to the amp when it’s powered on. Either speakers or load resistors. It is possible to destroy the output transformers if they are unloaded.--- CAUTION ---When adjusting the bias pots use very small movements. They are very sensitive and it’s very easy to overshoot the adjustment. When changing tubes, turn the bias pots down slightly before powering the amp up with new tubes, this will ensure they are not running too hard as all tubes vary. Allow the amp to warm up for 5 minutes before setting the bias and then again after it has been on for 15 – 20 minutes. Once set I suggest checking it from time to time, perhaps monthly unless of course you change tubes. GroundingAlways, always, always use a power cord with a grounding pin and plug the amp into a grounded outlet. There is nearly 500 volts on tap inside the amplifier and if something fails and the chassis were to become energized there is easily enough voltage and current present to kill. There is a “ground lift” switch on the back of the amplifier between the fuse and power inlet. This allows the circuit ground to be connected or disconnected from the chassis (safety) ground. The circuit ground is disconnected (lifted) when the switch is in the up position. Be sure to try it both ways, the results will vary depending on system configuration. There is no right or wrong setting. Right is whatever is quieter and sounds the best. Please note the safety ground is never disconnected and this switch does not compromise safety in any way.
It's nearly certain your amp has 10R resistors for measuring the bias, so 400mV = 40mA, which is pretty high for an EL-34 IMO.
Sorry missed your post.My amp has Mundorf Supreme Caps, change in resistors, and it does have a ground lift switch.I use it in the bedroom so I'm not nearly as critical as the big rig. The amp is driving a decent load 88db @ 8ohm pair of Gallo 3.1's. It will put out the advertised rating. Pretty good amps that respond to minor changes.
Utter nonsence. You never heard of ohms law? I = V/RCurrent through a resistance is proportional to voltage across it. This kind of misinformation is why I get to repair stuff all the time.
Bias voltage is the voltage applied to the grid folks.
