primosounds
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Like i said you need to follow the owner's manual. What the fixed bias setting for a power tube is not the same for every amp. It is a determination based on the circuit.
How And When To Tube Roll...
- Thread starter saggy
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RogerMod
Member
And yet, the designated setting for my VTL MB-450s is 270-300 mV using 6550Cs - which also use KT-88s as well.
Go figure.[/QI
And yet, the designated setting for my VTL MB-450s is 270-300 mV using 6550Cs - which also use KT-88s as well.
Go figure.
In that case the resistor is larger than 1 ohm or for multiple tubes. It is likely 10 ohms then running at 27-30 Ma. Call Luke and find out.
RogerMod
Member
Glad you mentioned the ST 70. Voltmeters were not very accurate at the time (no digitals) so David Hafler used that value to produce 1.56 volts across that resistor for 100 mA of cathode current for the pair. He then told you to calibrate your meter with a fresh flashlight cell which is also 1.56 volts and thus match whetever your meter read. Those were the days.. It also turned out that value actually reduced the distortion. I used 15 ohms in the RM-9 to light a LED
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Nixxuz
Super Member
Certainly doesn't have to. New production, while often looked down upon by purists, can get you as close as you want to the sound you are looking for. Unless you go Genalex KT77's for the EL34's, you are looking at under $100 for your preamp/front end tubes. I like the Genalex Gold Lions for both AU and AX. There seem to be lots of people who agree, and they are only about $30 on eBay and $40 on Amazon. You should change all 4 power tubes at the same time, if they are roughly the same age. While I didn't care for them, I just got a quad of Electro Harmonix EL34s (new production) for like $65. Unless you get a great deal, or have an amp that warrants the price, not much need to get into the expensive end of NOS, although there are cheap NOS tubes out there too.
No need to as the bias procedure and recommended values are covered in depth in the manual. I'm one of those folks who actually does read the instructions.
I've been biasing these amps for seventeen years. It is also the same setting used by the Wotan amps an audio journalist friend was reviewing at the time that triggered my purchase.
BTW, you can edit and *fix* your previous post. The initial quote of mine is not properly concluded with "[/quote]".
triode17
Super Member
No, sorry, that's overbiased in voltage, I am referring to current. If the current is too high, the tube is over biased.
KeninDC
Hazy Cosmic Jive
I roll e'm until I find something I like and then stick with it. This process took about 5 years. Then I purchase backups. My rectifier tube is NOS Mullard, so that gets expensive. All I need for my 5687 tubes is quiet, so inexpensive (relatively) NOS RCAs are the way to go.
primosounds
Powered with pure tube sounds.
Have you tried different 5687 types? I used to run MFA M75 into Magnepans and that was a good combo. The amp used the 5687 as driver/splitter tube. After rolling 7044, 6900, and 7119 i settled on the Amperex Holland 7119 as the best tube in that amp.
KeninDC
Hazy Cosmic Jive
My RCAs are 60s black plates. Tung-Sols were quite forward. Sylvania GBs were goopy and too "tubey." Have not tried the Amperex Holland 7119, but it is on my short list. I guess I'm not done rolling. FYI - I'm loading up a ModWright tube linestage.
primosounds
Powered with pure tube sounds.
My impression of the 7119 vs 5687 was that the Amperex was more "airy" or bloomy. While the 5687 had a darker sound . The difference was quite striking. But since the application is different it is hard to say without trying.
I used to own a RM9, if that is the one with 4 x el34 per channel. One of my buddies lusted after it so i sold it to him, for really cheap considering what they sell for nowadays.
RogerMod
Member
Let me know how it goes when you try that on a seasoned engineer or technician who is familiar with tubes. Bias always refers to the negative grid to cathode voltage. If the negative bias is too low the tube current will run hot. If it is overbiased it runs cold or is cut off in the extreme case. However in Solid State amps the bias is the idle current as there is no other way to express it. I would hope the readers of this site have the sophistication to make the distinction between tube and transistor amplifiers. When we get loose with terms they tend to lose their meaning.
Here is something to ponder. If a tube has no bias what is the idle (plate or cathode current). If a transistor has no bias (MOSFET or BJT) what is the idle current?
triode17
Super Member
Sag-Please help settle an argument, when you said this, did you mean bias voltage or bias current?
maxhifi
Addicted Member
Not to play the devil's advocate, but I have one amp which uses the 5687, and I tried a bunch of tubes, and measured distortion of each until I found the best one. The difference between the best and the worst was a factor of about double, it was way more variation than I expected.
saggy
These go to 11
BIAS CURRENT.
sorry for being AWOL on this thread...
https://hifipig.com/unison-research-triode-25-amplifier/
"For those that don’t own or know how to operate a multimeter, or have the confidence to go poking around within the internals of a valve amplifier to adjust the valve bias settings, Unison Research have thoughtfully provided a simple yet effective way of adjusting the bias current settings. "
RogerMod
Member
Hey Triode17, you are supposed to ask a person skilled in the art, not a newbee who is learning off the same misinformation on the internet. It's not your fault that the internet is not vetted.
The trouble starts when you used the term in your first post "bias current". There is no such thing in a tube amplifier, doesn't exist. Bias is a voltage. Idle current or plate current or cathode current are terms for current. Lets not put words together that do not belong. As I said, we adjust bias voltage to obtain an idle current. Knowing the resistor value across which you are measuring and dividing by one or ten (one in most cases but can be anything) you will get the current. When the MB-450 owner said he biased at 270-300 mV that implies 27-30 mA throught a 10 ohm resistor. If he biased to 300 mV over a 1ohm reistor his tubes would be in meltdown. The one ohm resistor is more popular but in this case Manley wanted more accuracy by giving the meter more voltage to work with. I use 10 ohms in the RM-10 for the same reason.
That's all I am trying to do here, not argue with you. There are some things you can learn from the internet and some things better to read in a book that was edited by peers in the field. Text books are good because they have been around and gone through many editions. The RCA tube manual is excellent, as it should be, as they made the tubes. They, and others, created the language which is now being misused. Unlike other companies, RCA devoted many pages to the topic in the front of the book. The section on amplifier classes of operation is virtually unchanged from 1930s to the last publication. The terms, bias, bias voltage and biased, appear some 5 times on every page. For example talking about the difference between AB1 and AB2 "..AB2 differs in that it is biased so that plate current flows for somewhat more than half the electrical cycle..."
Take a look at the data sheet for the venerable 45. https://frank.pocnet.net/sheets/021/4/45.pdf (hit the magnify a couple times to make it clear). Note the 3 columns of data each with a given negative bias value. Notice also that the bias values change a great deal and the plate current changes very little from left to right columns. In those days these types of tubes were so consistant that the bias voltage setting would give good relationships to the current. Nowadays the consistancy is not as good. RCA and others really had this down, yet they still recommend cathode bias which is far more forgiving. Why does the bias voltage change so much and the current change so little?
In most amplifiers there is some way to measure the idle current. Mike Sanders provided an actual current meter with a 1/4 inch plug to mate a socket in his first Quicksilver amp and actually interrupt the cathode circuit. Heathkit did this in the Williamsons because that was the common practice. The shorting jacks gave trouble over time and the resistor method we now use is preferable. However measuring idle current would not have been possible without modern digital meters that can accurately measure millivolts dropped across a 1 or 10 ohm resistor. You cant do that with a traditional VOM.
I feel like a guy in a canoe trying to change course of an ocean liner. The language deserves correctness and correctness not only demonstrates knowledge but aids in proper understanding of what is going on.
I will start a post on this and see what happens.
RogerMod
Member
If that is your amplifier then most likely that is a current meter. I presume you set it to the center between green and red. If so you are adjust bias voltage and measuring cathode current. We only have to change one word in your last sentence from "bias current" to "idle current" and then all is well. Of course you might want to know how many mA that meter is reading.
Nixxuz
Super Member
The Amperex Holland ECC83 I bought was "boomy" to the degree I feel like I may have gotten a microphonic tube.
saggy
These go to 11
It's been a real education, getting this thing. I'm so happy to have the resources of AK at my disposal.
triode17
Super Member
Yet your quote said, "Unfortunately the loose language of current audio..."
And, "I would hope the readers of this site have the sophistication..."
Plus, "There are some things you can learn from the internet and some things better to read in a book that was edited by peers in the field. "
So,let's review. My language in audio terms is loose. I don't have proper sophistication. I didn't learn electronics from a proper source.
But you're not trying to argue??? You didn't me much of a chance. Plus, this isn't your thread, but the OP is sidelined now.