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While surfing around, trying to get info on different tubes i stumbled on http://www.eurotubes.com They have a video explaining how to Bias a Hot Rod Deluxe. The video shows a really high Bias of 91 mv. Fender recommends 60mv. I cranked mine up to approx 72 mv :banana: The Amp really starting to sound less solid state at 67 mv and more ballsy at 72mv. I wouldn't push the amp much more than that, after all these guys are in the business of selling tubes :D

FZ1
 

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While surfing around, trying to get info on different tubes i stumbled on http://www.eurotubes.com[/URL] They have a video explaining how to Bias a Hot Rod Deluxe. The video shows a really high Bias of 91 mv. Fender recommends 60mv. I cranked mine up to approx 72 mv The Amp really starting to sound less solid state at 67 mv and more ballsy at 72mv. I wouldn't push the amp much more than that, after all these guys are in the business of selling tubes :D

FZ1
You didn't pay close enough attention to the video!:smile:

First off, stop thinking in terms of mv. That's a completely arbitrary voltage used only as a test reference in your specific amp.

When we bias we are setting the idling CURRENT of the output tube(s)! There is a negative control voltage available that we can adjust to control the idle current level.This voltage also is not critical. Tubes vary in what specific bias voltage will set a specific idle current number of ma. or milliamps.

We couldn't care less about the actual amount of bias voltage, since it won't necessarily be the same when we change to different tubes. We also don't care about the number of "mv" at TP30, at least until we understand just how that MV figure was generated and what it actually is showing with the Deville's output tubes. Could mean something entirely different in another model of amp.

The voltage in mv at test point 30 happens to be from a 1 ohm resistor in series with both output tube cathodes to ground. This results in a conveniently low (and safe!) voltage that represents not just the plate current OF BOTH TUBES TOGETHER but also their screen currents, which are an extra 2-3 ma. at idle.

The formula for the maximum idle current was given in the video. You take the plate dissipation figure for the tube, which for a 6L6GC is 30 watts. In the video they used the value for the older, less robust 6L6 "no suffix" version, which is 25 watts. So right away you have a 5 watt safety margin!.

Anyhow, you divide that plate wattage of 25 by the plate voltage, which you were told in the video was 460 volts. This gives a figure of .054 amps, or 54 ma. The hottest safe current for this tube is generally figured at 70% of this figure. 70% of .054 amps is 37.8 or 38 ma. when rounded off.

So if you had a bias meter you would set the idle current in each tube to 38 ma. as a maximum. This would be the warmest and hottest tone you should run from the amp.

If you wanted to use TP30 and think in terms of mv you should understand 2 things. First off, TP30 shows a reading given by the idle current in BOTH tubes! If you work out things with Ohm's Law you see that a 1 ohm resistor will show a voltage drop of 1 mv with 1 ma. of current flowing through it. So you could take that mv reading to mean ma. If you wanted to run at 72 mv you would be seeing 72 ma. of idle current. This is the combined total of both tubes. So you would assume each tube if they are reasonably well matched would be running at 36 ma. of idle current.

However, as we had said using a small cathode resistor (1 ohm in the Deville but it could be any value if you didn't mind a bit more math to figure things out) gives a reading that includes the plate current AND the screen current. If you look at a tube data sheet you'll see that a 6L6 at typical idling current will also be drawing 2-3 ma. of screen current. That's 4-6 ma. with both tubes in the line.

So we have to subtract 4-6 ma., or 4-6 mv from that TP30 reading to see only the PLATE idle current! This is the only part we care about in setting the bias voltage to give our desired idling current. Therefore the max safe value of idle current would show as 76-78 mv!

Once this is understood you can see that 72 mv is a perfectly accepted as safe idle current limit in a Deville. Most techs if measuring with a 1 ohm cathode resistor will ignore those couple of extra ma. of screen current in the reading and treat the cathode current as the actual plate current. This gives a few percent of a "fudge factor", in that the tubes will actually be a little bit under that max of 70% figure. When you're biasing from 60-70% your ear can't tell any difference anyway.

The shop in the video probably used 25 watts as the plate dissipation figure to keep "newbies" out of trouble. If you used 6L6GC tubes (and who wouldn't, these days!) the real figure would be 30 watts. This would re-calculate into a value of 46 ma. per tube! (Same math: 30/460 times 70%). Go through the screen current stuff and the mv you'd see at TP30 gives an upper limit of nearly 100 mv! Nobody's going to get into trouble from following the shop's advice in the video.

And if the tech doesn't know enough to understand about the screen current in his readings he still won't get into trouble, 'cuz he'll still be setting things a wee bit on the lower side!:smile:

Just FYI!
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When I used to own that same amp I biased mine to 80 mA. It had a much richer tone and the plates never went orange/red. The OT wasn't hot either. Some people are afraid to increase bias from the factory setting. As long as you understand the entire bias concept, feel free to make sensible changes. I did not experience premature aging of the output tubes either.
 

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I have changed tubes in the jcm 800 I used to have. I have also changed them in my 4X10 Hot Rod DeVille, but I have never changed the bias. Is this really bad? Do you have to do it every time? I have always replaced with matched sets of groove tubes, and have not noticed much of a change in tone.
 

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I have changed tubes in the jcm 800 I used to have. I have also changed them in my 4X10 Hot Rod DeVille, but I have never changed the bias. Is this really bad? Do you have to do it every time? I have always replaced with matched sets of groove tubes, and have not noticed much of a change in tone.
Most of the time if you are using the same brand of tubes etc, you can get by without rebiasing. Talk to old time players, and alot of them have never had their amps rebiased, they never heard of "matched" pairs of tubes, you just replaced the bad one and they used whatever tubes were local and cheap.

Hey Wild Bill just out of curiousity, do you bias using a scope and setting the notch or by measuring and figuring out the power? I've done both and I use the latter method as it doesn't require as much set up and seems to work just as well.
 

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Most of the time if you are using the same brand of tubes etc, you can get by without rebiasing. Talk to old time players, and alot of them have never had their amps rebiased, they never heard of "matched" pairs of tubes, you just replaced the bad one and they used whatever tubes were local and cheap.

Hey Wild Bill just out of curiousity, do you bias using a scope and setting the notch or by measuring and figuring out the power? I've done both and I use the latter method as it doesn't require as much set up and seems to work just as well.
I only use the 'scope method if I get some anal retentive hifi audiophile who may not know an ohm from a hole in the ground but wants me to do it! Those guys just like to spend money...

I don't always put in 1 ohm resistors. Usually I will use the OT measurement method, where you measure the resistance of both plate sides to CT while cold, fire up the amp and the figure out the current from the voltage drop across each side of the OT. You do have to watch your 'gators don't short against something but it's not that big a deal. It's faster, especially with a pcb amp that is a PITA to put in the 1 ohm resistors.

As for rebiasing, I always rebias when changing tubes. First off, you get big differences even within the same brand. When I go to thetubestore.com Jon has a huge pile of every brand on the shelf and they come in a wide range of bias currents for a given bias voltage. If you're buying the same brand from the same store you can go with the same number they put on the tube. That is, if there's a pair of TubeStore 6L6s from ElectroHarmonix that have a "33" mark then just ask for another pair, same brand and also "33".

Those old players were correct but only for those old days! They're setting themselves up for some possible big disappointments today. I had to replace an OT in one old Rickenbacker amp last month. Two different brands of 6L6s, both widely mismatched. He was playing the amp on "11" with a boost pedal and the imbalance caused big current swings in the windings. It fried up big time!

That being said, what if someone had screwed around with the bias setting without you knowing? I always like to open up the amp, even with some of the newer amps that have the testpoints and adjustments brought outside for convenience. When your car is on a hoist for an oil change the mechanic can also take a quick look and make sure your wheels aren't falling off! It's the same thing with a tube amp. I like to take a fast look for soot or carbon, measure the critical voltages and also see if anyone has been monkeying inside.

I found a problem in a brand new Bogner Ubershall where the 4 screws that held one board in were never put in. The board was just hanging from the connecting wires and the 4 screws were lying in the bottom of the amp. Fortunately, with the amp normally upright the screws would be lying loose on the bottom plate but you can imagine what would have happened if one had bounced up during transport and had shorted against a high voltage point!

Also, in the "Golden Years" tubes were made to much tighter specs. There was a lot less variance between output tubes, even between brands. I learned this as a young pup working in the Canadian Westinghouse Tube Division, during its twilight days. If they were short on an order for a few thousand 6L6GC tubes they thought nothing of second sourcing from a competitor like RCA and rebranding them as their own. They were so confident the tubes would be close to spec that they never even thought about testing them!

Today those 6L6s would be all over the place. That's why dealers like thetubestore have to match them. Jon has "gangbanger" machines that test maybe several dozen of a tube at once, weeding out defectives and giving the match number for each tube.

What's more, that "match number" is no good between tube dealers. It's kinda unique to their own tester and no guarantees that another dealer would be even close. Guys like Jon learn by experience so if you've got "green dot MessyBooger" tubes he knows what to suggest.

Just my own POV, of course.
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