What happens if you dont bias tubes

Don't worry I wasn't going to put them in without biasing them, I was just wondering what would happen. Tim speaks truth, which is why I'm going to sink some bucks into a new set of power tubes for my Classic Somebody on the forum has a quote from a well-known amp designer who states that tubes will have minimal effect on an amp's tone compared to the cabinets that the amp is run through.

Take that for what it's worth. I didn't notice much of a tonal difference when I replaced the original Chinese made 's in my Classic with the Sovtek WE's that I put in it.

But I did notice a big difference when I replaced the preamp tubes. I put Electro-Harmonix preamp tubes in it. I would agree with the comment above that the Chinese power tubes are ok, but they won't last as long as a better quality tube thats been made to a higher standard.

More than a couple old school techs stated the Sovtek KT88 to sound better than GE in most aplications. Though I have no issue with him and he is well regarded in tube circles Bob is a JJ importer, it's the only brand he sells. However, remember that tubes are hugely outdated technology only made in a few places in the world now and so they are HIGHLY inconsistent. Consequently, from tube to tube, each individual tube may require more or less voltage to get to its optimal operation point.

Consider a Corvette engine in a Corvette. It needs a certain amount of gas to get that Vette up to 70mph. Same engine, but it's gonna take a lot more gas to get the truck up to 70mph. Now, let's change it up to better understand tubes. Let's say Chevy had those engines made in, um, not a country known for its manufacturing skill, so each time they get one in, engine 1 puts out hp, and engine 2 puts out hp, and engine 3 puts out hp, etc.

Put them all in the same Vette and engine 1 requires less gas to get to 70mph than engine 2 and so on. Now, assume that it takes 4 of those engines per car. Engine 1 is hp. Engine 2 is hp. Engine 3 is hp. And Engine 4 is hp. When I put a single gas supply to those engines like the current in an amp , they'll all run differently and the car will be off balance. For those car nuts out there, consider this as a 4 barrel carb.

What happens when they're not synched up? When you set the "bias", what you're doing therefore is regulating the amount of DC current going to your tubes. As all tubes aren't created equal even same manufacturer same tube type , this is the importance of getting "matched" sets.

The more evenly your tubes are matched, the more evenly they'll run since I only get to send 1 DC current into the tubes. Basically, I need 4 of those Vette engines that perform about the same so when I supply a certain amount of gas the DC current , the tubes all run at that optimal spot. However, the tube really isn't that perfect, it just looks and sounds good from the seller's perspective. I just did a massive amount of research on this, some of which I posted on another thread.

Bottom line is that tubes do not have to be perfectly matched. For years I insisted that the phase inverter be matched. Now I find out it doesn't have to be. The author of this research paper did say that power tubes should be within 8 per cent of the highest to the lowest.

This has to do with proper bias adjustment. You wouldn't want a strong tube and a weak tube together and then try to bias it. It will bias, but the current flow will be way off on one tube and it will get hot.

Take a Peavey amp with cathode bias. Peavey is assuming that you will replace the tubes and that they are somewhat of a close match. Throwing in a wide set could cause some problems. Personally, when it comes to power tubes, I like a matched set, because it makes biasing so much easier to accomplish. Now as far as preamp tubes go, I'm not going to concern myself that much whether the tube is "close" or "wide. In closing, I studied this research paper and I have to agree with it. There are so many other variables in an amp that are not balanced, that to insist all the tubes be is just plain nonsense.

I think there is no harm nor do I think it is dumb behavior if you want to use matched tubes. I say, "Go for it! For safety's sake use a little common sense on the power tubes and make sure they are not real "wide. Not a problem. Also, I see a lot of post where people are giving advice about swapping power tubes. A lot of the older Marshall that used "non-EL34 tubes" usually had lower plate voltages.

This needs to be taken into consideration if EL34's are to be used. Another more dangerous scenario is taking EL34's out and going with a tube that operates on a lower voltage. Get professional advice before doing a tube swap. EL34's are usually set at around V and they can take a lot more voltage than that.

On the other hand a , 6L6 and KT66 cannot take that sort of punishment over a long period of time. A 6L6GC's maximum plate voltage is and you really wouldn't want to operate it like that, no manufacturer would.

So get some advice from your tech before swapping tubes. Find out what your actual plate voltage is and the bias range for the power tubes you plan on using.

This will ensure a quick and easy installation and bias adjustment. Hey Hamp, great follow-up by the way. MartyStrat54 , Apr 26, Thanks MartyStrat54 you are very wise indeed. Insert the black and red ends into the proper test points and note the reading on your multimeter. In fixed bias amps we adjust the grid voltage to set the bias. In cathode biased amps we must change the cathode resistor value to change the cathode voltage to adjust the bias. Usually You need to check the bias if new in that amp power amp tubes are installed.

A complete explanation of biasing a tube or SS amps can be found from the old amplifier textbooks. Unless your amp is cathode biased, yes, you need to have it biased when you change tubes and yes, you should periodically have that checked and adjusted as necessary. A self-biasing amplifier, also know as a cathode bias amplifier, has a cathode resistor that sets the bias current of the tube. Typically these are in the 30 watt and under power range, but not all lower wattage amplifiers are cathode biased amplifiers.

If your amp is a fixed bias amp, you will need to bias it. Retubing Guitar Amps. Your amp might need one or more new tubes if its volume, tone, or punch has gotten weaker. You might also need a new tube if you occasionally hear feedback from your amp for no reason. Most often, only a few tubes might need replacing. Cathode Biased amp do not require biasing to change power tubes.

Fixed biased amps require biasing to change power tubes as well as periodic bias checks to ensure they are working properly.