![]() My first probe was their low-end model, the "Eurotubes Octal Bias Probe," and it operates by measuring current and evidently the Pro One does as well see this brag quote from the EuroTubes product page: This definitely includes the various probes marketed under the EuroTube brand. Some operate differently: they measure current directly, not voltage, and are proud of it. In fact, all bias probes do not put a 1 ohm resistor in the cathode path. And I am not alone in my mistrust here is Lord Valve, who will give the closing argument for the prosecution - his biasing FAQ can be found at Duncan's Amp Pages at : I did not find this a satisfactory response. I wrote EuroTubes and they denied that customers had ever reported oscillation problems, let alone the slower instability I'd experienced. So I bought & assembled a Bias Scout probe kit courtesy of TubeDepot the Scout does use the 1 ohm resistors, and it works far better - with my DMM set up to read voltage, I get a stable reading from the probe as soon as the tubes are warmed up. The probe would settle down after maybe 10 minutes, but I didn't trust it. Something of the sort happened when I would use my Eurotube Octal Bias probe (plus my DMM set to read current) on my innocent little Laney Cub 10 amp - it was difficult to get a stable reading, because even after the tubes were warm, the readout on my meter would move up and down across a fairly wide range, as if at random, for many minutes - not quite what I'd call oscillation, since to me "oscillation" means something insanely rapid this was merely a constantly changing value, but even so it was unsettling. In the rare case where your amp is an "exception" we suggest you consult your manual or a tech to learn if any special service is required to change the tubes.This would be fine, except that probes which measure current directly in this fashion can weird readings, including oscillation. Generally speaking these amps are considered plug'n play when it comes to replacing the tubes.Īs with anything, there are always weird exceptions. These are still fixed/non-adjustable bias amps as there is no internal bias trimmer pot to fine tune.Ĭathode bias amplifiers shouldn't require any adjustments and will work with a wide range of tube plate currents, as the circuit is "self adjusting". Often it is a switch that can select bias for E元4 tubes, or 6L6 tubes depending which is installed. One more thing: some fixed/non-adjustable bias amps have a "bias switch" to select for a specific tube type. Fortunately, these amps are very forgiving and will work well with a pretty wide range of tubes.įor the best results, when you order be sure to specify the make and model of your amp in the Comments box on the checkout page (or simply order a Shop by Amp tube package) and we will make sure you get the best tubes for your amp. These amps (for example, all Mesa Boogie models) use a fixed resistor to bias the tubes, not a bias pot, and are designed to run with tubes that have a specific current draw range and should only use tubes that fall within that certain range. ![]() Non-adjustable fixed bias amplifiers without an adjustment pot cannot be bias adjusted. When ordering you should mention the rating you require in the “Comments” box on the checkout page. If you already have tubes with our Perfect Pair rating, or rated tubes from another seller such as Groove Tubes, you can replace them with a new set that has the same rating and safely avoid rebiasing. Until you learn how to do it correctly by yourself, it's best to pay a technician a few dollars to do it for you. ![]() Also, an unbiased amp may sound horrible or cause undue wear and tear on the tubes or power supply. It's not difficult and it doesn't take long but if you don't know what you're doing you may hurt yourself (amps can give lethal electric shocks even when unplugged, and NO, we're not just saying that to scare you). Adjustable "Fixed" Biasįor adjustable fixed bias amplifiers, the amp should be biased by a tech with experience with this procedure. Cathode Bias amplifiers (also commonly referred to as self-biasing)Įach of these types is unique.Adjustable Fixed Bias amplifiers (adjustment potentiometer to set the "fixed" bias point).Non-Adjustable Fixed Bias amplifiers (no bias adjustment potentiometer).The bias methods of amplifiers fall into three basic categories: Many tube amp owners are unaware of this simple regular service for their amp that will keep it sounding its absolute best. Some amps do this automatically, but many do not - particularly in the world of guitar amps. Just like the idle of your car may need adjusting when a major change is done to the engine, new power tubes need their idle (the "bias") adjusted to make sure the amp sounds as good as possible. Clearance - Bargains - Scratch and Dent. ![]()
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