biasing/plate voltage

[dudiluty]

hi there.

i bought a 410H JVM head to learn my self how to bias. it's an easy to do job on these amps(they say) so i thought this should be a good amp to begin with.

What's the common millivolts reading for the bias for each side? 32millivolts ? and how about the plate voltage.. should i check/adjust this after biasing ?

thanks.

[AdrianSD]

If you're wanting to learn, as you said, please DO NOT play with the plate voltage, 480vdcc will fry your brains in an instant. For biasing see marshallforum or yt.

[dudiluty]

If you're wanting to learn, as you said, please DO NOT play with the plate voltage, 480vdcc will fry your brains in an instant. For biasing see marshallforum or yt.

Yeah i know how to work on amp but biasing is something new for me. That's why i'm asking if i have to check the plate voltage after it and what it should be

[Gtrjunior]

You set the mA according to the plate voltage reading. There is a mathematical formula that you must use.
It’s not just a one setting for all amps type of thing.
There are dozens of YouTube videos on this.

[Bogner]

Weber Bias-Rite is your friend. The website has the info for the proper calculations etc. Be very careful, you can easily hurt or kill yourself if you don't know what you are doing.

[dudiluty]

Ok tnx i’ll take a look at that.

[JamesPaul]

Weber Bias-Rite is your friend. The website has the info for the proper calculations etc. Be very careful, you can easily hurt or kill yourself if you don't know what you are doing.

I could be wrong, but didn't Weber discontinued the Bias Rite? I don't know why as it has a great reputation and if they did discontinue it hang on to the ones you have!

[Ewizard]

The plate voltage is used to figure out what your plate current is. As the plate current goes up, the plate voltage will drag down. it is a balancing act.

The basic approach to biasing is choosing a target dissipation number. For most fixed bias amps that number is roughly 70% of full dissipation. So if your 6L6's are capable of 30 watts of dissipation in fixed bias mode, you would shoot to bias them at about 21 watts of dissipation assuming you want 70%. The milliamps draw required to achieve that is not any particular number because it is dependent upon the plate voltage and other factors.

My prefered method for biasing an amp is the output transformer resistance method. It is pretty safe, very accurate and you can dial into a gnat's ass what you want with real numbers. This is a great link on biasing and leads with this method: https://robrobinette.com/How_to_Bias...%20see%20below.

[Demanic]

So, a properly biased 4 6L6 tube amp is putting out roughly 80 watts maxed? Is this at clean no breakup?

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[beaubrummels]

So, a properly biased 4 6L6 tube amp is putting out roughly 80 watts maxed? Is this at clean no breakup?

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I believe the output transformer has a say in the actual total (some 1960's 50-watt Marshalls have been known to put out 90 watts, while newer 100-watt Marshalls have been known to only put out 80 watts). But the 4x6L6 would be putting out about 80, if my understanding is correct. Except they are in A/B pairs, no? So I think that's disingenuous or my understanding is incorrect.

[Gtrjunior]

I believe the output transformer has a say in the actual total (some 1960's 50-watt Marshalls have been known to put out 90 watts, while newer 100-watt Marshalls have been known to only put out 80 watts). But the 4x6L6 would be putting out about 80, if my understanding is correct. Except they are in A/B pairs, no? So I think that's disingenuous or my understanding is incorrect.

I can’t speak to a 6L6 based amp since I’ve never owned one but all of my EL34 based amps do indeed run in pairs.

[Ewizard]

The wattage rating of tube amps is a little more complicated than just adding the total dissipation of all tubes and adding or subtracting a margin. Many tubes can be biased pretty hot safely and will tolerate it, some even dissipating more than what they are rated for! There is also the difference between tubes to consider. In most cases, we are only seeing the total dissipation of all 4 tubes or perhaps each pair of them, not very often are we actually seeing what each tube is contributing. In a quartet of tubes, there may be one tube that is actually operating very cold while another is operating rather hot. The only way to know is to measure each tube. Then there is the difference between clean watts ( no distortion ) and peak watts ( with considerable distortion ). A 100-watt amp may not be producing 100 watts of truly clean (non-distorted) output but may produce well over 100 watts when fully cranked, which would obviously have lots of distortion.

The class of operation and how a tube is biased will also be a factor in how it operates at performance level. A cathode-biased amp for instance can be biased closer to class A at idle ( no signal amplification ) because as you crank it, the bias will actually drop. A fixed bias amp needs a larger safety margin because as you crank it, the bias will actually increase. As you crank a fixed bias amp, it will cause a draw on the power supply which will drop the voltage supplied to the bias circuit. Reducing the bias circuit voltage increases the tube's bias. This is because there is now less voltage to create a negative voltage in relation to ground ( Ground potential in the amp doesn't change ), thus reducing the amount of negative voltage. Less negative voltage ( closer to 0 volts ) biases the tubes hotter. As you can see this is a factor for biasing your amp depending on how you play it. It is pretty wild to consider that one form of biasing allows really hot operation at low volume levels and is safe because it reduces its actual output as you crank the amp. And the other type of biasing requires you to bias cold at lower volumes because it will try and produce more power as you crank it.

As to how the amp works. Most amps are class A/B designs. If it has more than 1 output tube, it is 99% likely to be a class A/B amp. The only major difference will be whether it is cathode or fixed biased. In an amp with two output tubes, one tube is amplifying the positive side of the A/C signal, while the other is amplifying the negative side of the A/C signal. In an amp with four output tubes, the tubes will work in two pairs. Two will amplify the positive side of the A/C signal while the other pair amplifies the negative side of the A/C signal. Nothing more nothing less.

[Demanic]

So cathode biased low watt amps reduce output as they're cranked? That makes how my Picovalve performs make sense. Which should help me dial it in even better.

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[Ewizard]

I wouldn't say so much reduce output per-se, only because it isn't reducing by a significant amount wattage potential. Keep in mind that a 3db difference in SPL is either half or double the current wattage. I.E. if your amp is 50 watts and produces 100db at full bore, it will produce 97db when it is operating at 25 watts and it will produce 103db if it suddenly decides to create 100 watts. The bias shift in either bias circuit type isn't enough to significantly reduce potential output but is enough to affect the quality of life for the tube.

Biasing a cathode-biased amplifier to within a watt of the tubes rated peak spec is essentially running them in full Class - A operation. The tubes won't last very long. If you crank the amp when you play, it may pull down the bias enough to take them solidly into Class - A/B operation, but only so long as you are slamming the strings. It acts more like a compressor than anything. It is what gives cathode-biased amps that spongy, blooming, chewy, and round feeling when you play them at high volumes. The voltage drop on the HT supply also causes the rest of the amp to shift in operation too, further changing the feel of the amp.

A fixed bias amp tends to stay very firm, solid, and immediate when cranked. The HT supply still drops, so it does alter the operation for much of the preamp, but because of that HT drop, it also increases the bias for the power tubes helping to keep the sound firm and solid, adding to the amps KERAAAANG appeal.

A JTM-45 is a cathode-biased amp, even though its' circuit is similar in ways to a JCM-800, the bloom and chewiness of the JTM-45 vs. the KERANG of the JCM-800 is largely in part because of the bias circuit. But the bias shift isn't enough to appreciably increase or decrease the perceived volume level; only enough to change the feel.

[jeremy]

the jtm45 is cathode biased?