Biasing EL84

[deeaa]

EL84 tubes aren't usually very long-lasting, but under a year of light use is still too little.

I took my Blackstar HT-20r Mk2 apart and putting in new tubes; I was surprised to find out that there are actually trim pots for bias in the amp - I had thought it cathode biased.

Well, since I can't find any info on bias values or even the correct spot on the board to measure any values - Blackstar refuses to give them to mere mortals - I have to resort to temperature-based biasing.

That will take a while. It means I'll change the tubes and then spend half an hour with an infrared thermometer gun taking measurements and will adjust voltage based on that. My belief is it is set a little hot in the factory to make it sound better at lower volumes, but since I always crank the amp, I'm gonna run it a little colder to give the tubes some extra life. So I'll try to get the temps for both tubes to be 190C maximum - we'll see how the temps look at first.

It may seem like a crude way to set bias, but in the end, it's actually just about the most accurate way to get it good. Takes a while though, and lots of measuring to get reliable averages.

[mozz]

I wouldn't go by temperature. But if that's your only choice measure right now so you have some sort of baseline.
Use a bias calculator such as https://robrobinette.com/Tube_Bias_Calculator.htm If you have a decent meter,
If there are trim pots, or pot, see if there is a 1 ohm resistor on the cathode or any point to measure the bias. You also need to measure the plate voltage, chances are it's over 300v. Other than that you need a scope to watch the crossover distortion.

In a dark room you may be able to see them redplating.

[deeaa]

Yeah, it's not the perfect way to do it - but frankly I don't really see what's the point in measuring really carefully either...not in this kind of an amp. Whatever you measure, you can't really do anything but just tweak one potentiometer, and it's a pretty coarse adjustment anyway. I'd you read, say 11.7 V and it's supposed to be 12, well, chances are you won't get it any closer no matter what. And if some other voltage measurement point seems odd - can't do anything about it anyway so why bother.

I can see the importance when working on old amps where you may find and have to replace components gone bad, though, but with these PCB board things...nothing you can do about it much except check that it's in the ballpark. Most components are integrated on the board. It will work fine biased way cold, just sound worse, and it will work biased hot - just use up tubes quick. But I reckon the tolerances are pretty big, these being sort of semi-self biasing.

Back in the day I hardly ever biased tubes...just slammed in new ones if the sound started to go bad. Only after years and years of using tube amps I even heard about biasing, and started checking it, but the only time it needed any real adjusting was when I bought a Marshall JVM that had been set to use just two power tubes. When I added the other two the bias had to be set quite differently. I set that pretty dang hot, though.

I'm guessing I'll be turning the bias pot down a wee bit, those tubes look like they have been cooking.

The amp is - well was - under warranty too, but am not gonna be sending it away for tube chance....so I blew the warranty and opened it up meself.

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[mozz]

Maybe try other tubes, Russian 6p14p-ev are the best ones from them. American 7189a will handle 440v but are not close to being cheap.

Look up crossover distortion, el84 are known for it and makes them sound bad. Once you get rid of the crossover distortion and check the bias, they are running hot. It's a fine line and nature of the beast.

[mickey]

In my opinion, there is no need to bias EL84's.
They always sound worse than sh*t regardless.

[deeaa]

I've had 5-6 EL84 amps in as many years...I don't like the Russian m spec tubes. They sound cold to me. Tight.

Other than those...love me EL84 sounds.

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[deeaa]

Well... apparently, yeah, very much possible to set the balance. And it seems they're NOT running hot at all...the hottest spot I can find is about 130C which is nowhere too hot.

The sound, however, changes quite a bit...if I set one tube about 10C colder than the other....it sounds a lot better, go figure. Clears up a bit, and dynamics are better.

If I adjust them to as exactly same as I can, it sounds the _smoothest_ ie nice fat lows and such - but it also sounds a little more driven and more hairy, with less high note definition jumping out.

Damn it, is that the secret of the best sound - to set the tubes to work a little asymmetrically instead of just the same? Maybe that's one reason old tube amps sound so good sometimes, tubes are worn differently...

It's not like it's a huge change but it's clearly there.

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[mozz]

Asymmetrical clipping should give you more even harmonics, hence it would sound clearer.

[deeaa]

Asymmetrical clipping should give you more even harmonics, hence it would sound clearer.

That's actually an adjustment I'd like to see among normal amp controls!

Well, I'm happy how it sounds now....temps in the tubes are a wee bit different, at worst some 10C, but it depends on the side of tube.

If I set them to exactly same temp measured at like 5 different spots inside the tube, the sixth spot to measure at the very root of the tube is like C hotter on one. 115 in one and 138 in other. And it sounds smooth but a little 'hairy' too.

If, however, I set it so that one tube is like 0-5C different in those 5 spots, suddenly the sixth spot at the root is closer to being the same, 128 and 132 respectively. And it sounds better, clearer and punchier.

Even going through the maximum values in the pot there isn't any huge difference however. It's always between 110-140C no matter if it's maxed out or turned as low as it goes.

So I figure that's all I can really do to it. Now it's set to like 10 am or so, and that way I get the most constant temp values overall from both tubes.

I'll be amazed if the tubes will die as quickly as the original ones...but I guess only time will tell.

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[andrewsrea]

I saw this thread and thought I'd chime in (EL-84 pun intended), since I did a lot of cathode-bias experimentation over the last month.

I've looked at some of the 20w blackstar designs and they are cathode-biased push-pull with a small amount of fixed-bias and adjustment for fine-tuning. @mozz nailed a lot of the concepts you should cover to get your amp sounding its best, without destroying your tubes.

Here is a video which will get you in the ball park: . Less voltage across the cathode resistor = a lower idle wattage percentage and a longer tube life. However, too cold and your tone will suffer.

To really get the best results, I have provided my method. Safety reminder: high voltages which can kill you quick. My methods require you measure voltages of a live amp. I have an excel model I made, but you can use the Robinette models to good effect. You will need a shunt resistor (10 Ohms to 100 Ohms) connected in series to jumper wires with alligator clips on both ends and a multimeter which can measure ohms and DC voltage. An oscilloscope and 1kHZ toe generator would be a plus.

Note that cathode-biased push-pull are going to be at their best between 85% and 95% of max plate wattage (12w for most EL84, 14w for a few of the industrial variants). Note that some designs will be as much as 100%, which will work but eats tubes. Note that a fixed bias amp with a cathode resistor of 25 Ohms or less wants the idle wattage to be less than 70% per tube.

Play your amp to warm-up the components. Low ohm resistors drift meaningfully as they heat up and exact resistance will provide better results.

With the amp off, unplugged and chassis out of the cabinet: connect one end of the shunt wire to a circuit ground (metal chassis typically works) and discharge any stored voltage by touching the resistor end lead to Pin #7 of the EL84 socket for a few seconds. Next, measure the cathode resistor by connecting one lead of the Ohm Meter to ground and the other to Pin #3 and write down the result. Then, measure the Screen Grid resistor connected to pin #9 and the B+ (I suspect it is a 1K5 5w big, white wire-wound resistor - easy to spot). Record your measurement.

Make sure the speaker or an appropriate resistor load (probably 8 ohms) is attached, set all the knobs to '0', plug in the amp and let it warm up for 10 minutes. With the negative lead of your volt meter connected to chassis measure the Cathode voltage on Pin #3 (will be in the 10vDC to 35vDC range) and record it. Then carefully measure the Plate voltage on Pin#7 (will likely be in the 280vDC to 380vDC range) and record your result. Carefully measure the Screen Grid resistor voltage drop, by touching the voltmeter leads across the screen grid resistor. Note that the voltage in this circuit is very high, even though the reading across will be small (1vDC to 5vDC). A safer, but less accurate method is to skip this and use a 4% substitute that I'll highlight in the calculations below.

Once you've recorded your ohms and volts, here is how I calculate idle tube wattage for a single tube in a two tube circuit:
(((Cathode resistance X Cathode voltage)/2) -(Screen Grid resistance X Screen Grid voltage)) X Plate voltage = Idle Plate wattage.

Safer alternative: (((Cathode resistance X Cathode voltage)/2) * 0.96) X Plate voltage = Idle Plate wattage.

Idle percentage = Idle Plate wattage / Max Plate wattage. Example: 11w / 12w = 92%.

I use a 1kHZ sine-wave tone at 25mv and set the amp tones to: 2.5 bass, 7 mid & 7 treble, treble switch off. For master volume amps, master on '10'. With the oscilloscope probe connected to the (+) of the speaker or load, I turn up the gain (MV amps) or volume (non-MV) to the point the sine wave extremes ('tips') begin distorting (the line gets wide and begins losing shape) and then turn the gain or volume down a little to remove the distortions. This is very important, because even properly biased amps have some crossover distortion when they've exceeded their clean RMS.

At the baseline point (where the line is with no tone applied) is considered the 'crossover point.' I look for a disconnect close to this line. It looks like a few milliseconds of flat line. This is crossover distortion. If you see crossover distortion, decrease the bias to increase the Bias percentage to remove the crossover distortion. The volume will creep up and you will have to take a hair of volume off to get a good read, as you do this.

Once you have no crossover distortion, use your ears but don't exceed 95%. It won't sound better for long and you'll reduce your tube life.

This method can also be used on an amp with a cathode resistor only (no fixed bias addition) amp, by tweaking the cathode resistor value. Less resistance will increase Bias percentage and more resistance will increase it.