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Author Topic: Overtone biasing  (Read 15491 times)
Fred.T
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« on: January 22, 2008, 12:14:51 PM »
Hi everyone!

I am new here so big hello to growing Ceriatone forum!
Right in the middle of assembling my Overtone, inhaling solder fumes, here's the question:

How do you bias the output tubes, what values do we looking for?

Thanks,

Fred
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mcinku
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« Reply #1 on: January 22, 2008, 07:18:57 PM »
Yep I want to know that too.


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« Reply #2 on: January 23, 2008, 03:41:38 AM »
I don't even have my kit yet, but after my build I will be biasing my 6L6s around 35ma.
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philooo
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« Reply #3 on: January 23, 2008, 07:25:06 AM »
So you can bias to your liking but of course in a limited range depending on the amp design, right ?

With 35 ma you expect a specific sound quality ?
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ic-racer
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« Reply #4 on: January 23, 2008, 04:56:10 PM »
More dependent on the tube design. The 6L6s have a certain heat that they can tolerate and beyond that they can fail or have a shortened lifespan. There are volumes written on biasing. The 'D' amp that the Overtone is based on had an output section like a 100W Fender twin (which Mr D copied) . So you can research 'biasing fender twin' on the net as I suspect the bias settings should not be different weather you have a 100w or 50w version.

Again, you can read pages and pages on biasing but it does come down to adjusting the trimmer pot to where it sounds best and measuring cathode current to make shure you are not pushing the tubes too hard.

The advantage of having a number to go by lets you set the amp to a reasonable bias setting (that has worked for you for other similar amps) while on the bench, without having to hook up a guitar and speaker.
« Last Edit: January 23, 2008, 05:01:10 PM by ic-racer » Logged
max
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« Reply #5 on: March 15, 2008, 06:21:06 PM »
Quote from: ic-racer on January 23, 2008, 04:56:10 PM
More dependent on the tube design. The 6L6s have a certain heat that they can tolerate and beyond that they can fail or have a shortened lifespan. There are volumes written on biasing. The 'D' amp that the Overtone is based on had an output section like a 100W Fender twin (which Mr D copied) . So you can research 'biasing fender twin' on the net as I suspect the bias settings should not be different weather you have a 100w or 50w version.

Again, you can read pages and pages on biasing but it does come down to adjusting the trimmer pot to where it sounds best and measuring cathode current to make shure you are not pushing the tubes too hard.

The advantage of having a number to go by lets you set the amp to a reasonable bias setting (that has worked for you for other similar amps) while on the bench, without having to hook up a guitar and speaker.
[/i]
Hi ic-racer
Another thing is to ask your tube supplier for the optimum bias for a particular tube, and within the range for that tube you can be subjective based on how it sounds to you biased either hot or cold or somewhere in between. If I understand it right you can also change a resister value to come into the the correct bias range of a type of tube, but you need to know the correct plate voltage to have a starting point-am I correct..my knowledge is limited so I will gladly stand corrected if wrong.

ic-racer do you know the plate voltage of the Overtone, Nik told me 440 to 450 volts and I want to know definitively what it is
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mcinku
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« Reply #6 on: March 15, 2008, 07:30:10 PM »
I measured mine and I have 450V.

By using the bias measure points I biased my amp to 42mV.

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« Reply #7 on: March 15, 2008, 07:44:14 PM »
Quote from: mcinku on March 15, 2008, 07:30:10 PM
I measured mine and I have 450V.

By using the bias measure points I biased my amp to 42mV.

 Wink

mcinku, what tubes are you using, if 6l6, that is a warm bias(hot?) just curious, still learning, how does it sound. I have some JJ 6l6 gc's and a set of el34(groove tube el34s) that I like. I will try the 6l6's first.
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« Reply #8 on: March 15, 2008, 08:01:24 PM »
Quote from: wbfree on March 15, 2008, 07:44:14 PM
Quote from: mcinku on March 15, 2008, 07:30:10 PM
I measured mine and I have 450V.

By using the bias measure points I biased my amp to 42mV.

 Wink

mcinku, what tubes are you using, if 6l6, that is a warm bias(hot?) just curious, still learning, how does it sound. I have some JJ 6l6 gc's and a set of el34(groove tube el34s) that I like. I will try the 6l6's first.

Nik sent Tung-sol 6L6GC-STR with my amp and that tube is rated for plate voltage up to 500V and up to 30 watts of power...
so if you go here http://www.webervst.com/tubes/calcbias.htm and enter the numbers. At my plate voltage (450V) and at 70% of its maximum allowable plate dissipation the bias can be 46.6mA.
So I guess my bias is not to hot.

 Wink
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« Reply #9 on: March 17, 2008, 04:52:53 PM »
Thanks mcinku, I didn't know about that site having that info-big help.
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« Reply #10 on: March 21, 2008, 06:29:42 AM »
Okay guys, I got it, it's sitting warming up. The amp I had that I could bias had two test points for each tube and a separate pot for each. This has only three, one ground two hot, and one pot. So I assume I adjust each one, I am just a little in the dark. I also am assuming 450 plate volts for now, because I don't fell comfortable measuring that yet. I will take it to my buddies house on Saturday and get him to show me the safe way. For now I just want to hear it Grin I won't run it too hot, my ear is still good even if I am old Shocked
Hope someone is up still, would be nice to have this info, what a noobz with teh toobz eh?
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« Reply #11 on: March 21, 2008, 07:27:55 AM »
Quote from: wbfree on March 21, 2008, 06:29:42 AM
Okay guys, I got it, it's sitting warming up. The amp I had that I could bias had two test points for each tube and a separate pot for each. This has only three, one ground two hot, and one pot. So I assume I adjust each one, I am just a little in the dark. I also am assuming 450 plate volts for now, because I don't fell comfortable measuring that yet. I will take it to my buddies house on Saturday and get him to show me the safe way. For now I just want to hear it Grin I won't run it too hot, my ear is still good even if I am old Shocked
Hope someone is up still, would be nice to have this info, what a noobz with teh toobz eh?

Congrats !!   Grin

Yes only one pot... which means you bias both tubes with one pot. With those measure points you can measure how even the tubes are. That's why you have to buy matched pair from now on.

 Wink
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« Reply #12 on: March 21, 2008, 04:03:15 PM »
Quote from: mcinku on March 15, 2008, 08:01:24 PM
so if you go here http://www.webervst.com/tubes/calcbias.htm and enter the numbers.

I had not seen that site before. Looks like a great on-line reference.
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« Reply #13 on: March 21, 2008, 05:49:10 PM »
Thanks for the replies. I haven't had a chance to really play it, other than to know this amp is going to take me awhile to dial in. I like it but don't want to gush until I have actually got the sound I want to hear, but that is for another category. I biased it hot last night. Interesting, the pot to adjust has no slot for a screwdriver so I had to use needle nose pliers..is there a tool I don't know about. Tonight I am going to put some EL34's in it..can't help it I am a tweaker. Mcinku where do you have your OD pot adjusted to, and which side do you read it from to know where say 10:00 is, this may seem like a stupid question but I have to ask. I am pretty sure I got it right just checking. I did not get the gain I wanted, but like I said I did not have a lot of time to play with.
Also have you change that resistor by the trimmer, just trying to get a feel for what makes it be all it can be.
It is definitely a good amp.
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marinblues
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« Reply #14 on: March 21, 2008, 06:16:25 PM »
I'm glad you like it! Cheesy


Marin
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