![]() ![]() If you do this when you bias your tubes, if you bias your tubes at 35-ma, they should never drift past 40-ma. If you do this and you have a tube that drifts so much that that tube red plates, get another tube. This is not something to get overly worried or excited about. VHT Tube Tester + Amp Bias Meter | Ĭlick to expand.Bias should only drift a few milliamps. I use a nice biasing gadget from amp-head to bias my Deluxe reverb along with a DMM but I think they don't exist anymore but there are a lot of options you can buy amp does not provide external bias measuring points. It is important important not to exceed the maximum plate dissapation rating of the tube (Voltage x amps= Power) the power tubes but also not exceed the max bias voltage as this can cause internal breakdown and lead to "red platting" Here is a useful calculator Here is as a good short explaination of they types of biasing. ![]() For a single pot bias adjustment you should use matched tubes. If you can individually bias each tube you can get away with non matching tubes as you can dial in each tubes bias. Sometimes fixed biased amps is sometimes set and not adjustable or you may have one pot for all out put tubes, like my 1975 Fender Deluxe Reverb, or in some cases multiple pots so you can adjust each tube. Cathode is self biasing so no adjustment necessary, old Fender tweeds, while fixed bias should have some adjustment method through external adjustable pots. But in a given amp, when you swap one E元4 out for another, the bias point at which the tube is designed to operate is the same.Ĭan't speak for audio tube gear but for guitar amps there are two types of bias, fixed and cathode. What that bias point will be will depend on the class of amp operation (A, A/B, etc), the plate voltage, the cathode current, etc., so even for tubes of the same type, the bias point may be different from amp to amp. The circuit will be designed for a certain tube type operating at a certain bias point - typically in a range where at idle the tubes will be dissipating 60-75% of the maximum heat they can handle. (And some old production tubes could dissipate way more heat than spec, so some old amp circuits were designed around voltages and bias points that modern production tubes really can't handle.)īut the design point for the bias of E元4 tubes in an amp designed around E元4 tubes is the same regardless of what E元4 tubes you're using. Now, of course, not all tubes of the same type perform exactly the same - they're manufactured within tolerances and the variation can be pretty substantial, that's why with set the bias point manually when we change tubes (at least with fixed-bias amps). But in a given amp, when you swap one E元4 out for another, the bias point at which the tube is designed to operate is the same. ![]()
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