Author Topic: biasing power tubes  (Read 10537 times)

Offline mboldda1

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biasing power tubes
« on: November 12, 2011, 08:51:54 PM »
i had one tube amp manufacturer say bias the tubes with no signal going thru the amp and another say use low volumn music playing.  which is the correct way?
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Offline tmazz

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Re: biasing power tubes
« Reply #1 on: November 12, 2011, 10:23:09 PM »
I would venture to say that the "correct" way is to follow the instructions from the manufacturer of the amp you are trying to bias. With or without signal will give you a different reading at the test point and the manf. will give you a target reading to set the bias to. So that target reading will only be accurate if measured under the conditions that the manf set out in their instructions.
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Offline rollo

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Re: biasing power tubes
« Reply #2 on: November 13, 2011, 09:05:28 AM »
I would venture to say that the "correct" way is to follow the instructions from the manufacturer of the amp you are trying to bias. With or without signal will give you a different reading at the test point and the manf. will give you a target reading to set the bias to. So that target reading will only be accurate if measured under the conditions that the manf set out in their instructions.

 + one.  :thumb:

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Offline mboldda1

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Re: biasing power tubes
« Reply #3 on: November 13, 2011, 10:44:40 AM »
this is what the manufacturer says: " set the volume at lower level (less than one quarter). i guess you could take this as no signal present or with a signal.
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Offline tmazz

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Re: biasing power tubes
« Reply #4 on: November 13, 2011, 02:52:06 PM »
this is what the manufacturer says: " set the volume at lower level (less than one quarter). i guess you could take this as no signal present or with a signal.

I would take then exactly at their word. 1/4 volume is definitely not no signal.

Just by way of experience ARC in their instructions for setting the bias on my VT-200 says to set the bias after the amp has been idling for at least 15 minutes. When I first read this I took it to mean that it had been warmed up for at least 15 minutes.  One day I gad been playing the amp for about an hour and thought it would be OK to check the bias since it was by then well warmed up, I took one reading and before I had a chance to make any adjustments the phone rang and I was on a call for about 20 minutes. I returned to continue after the call (and the amp had been idling for the recommended amount of time, retook the same reading and the measurement had changed by about 20%. If the phone hadn't rung and called be away I would have made changes that would have been way out of spec. Moral to the story - follow the directions exactly.
Remember, it's all about the music........

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Offline jimbones

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Re: biasing power tubes
« Reply #5 on: November 15, 2011, 08:46:14 AM »
this is what the manufacturer says: " set the volume at lower level (less than one quarter). i guess you could take this as no signal present or with a signal.

I would take then exactly at their word. 1/4 volume is definitely not no signal.

Just by way of experience ARC in their instructions for setting the bias on my VT-200 says to set the bias after the amp has been idling for at least 15 minutes. When I first read this I took it to mean that it had been warmed up for at least 15 minutes.  One day I gad been playing the amp for about an hour and thought it would be OK to check the bias since it was by then well warmed up, I took one reading and before I had a chance to make any adjustments the phone rang and I was on a call for about 20 minutes. I returned to continue after the call (and the amp had been idling for the recommended amount of time, retook the same reading and the measurement had changed by about 20%. If the phone hadn't rung and called be away I would have made changes that would have been way out of spec. Moral to the story - follow the directions exactly.

And don't forget that it says to hop on on foot while adjusting!! :rofl:
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Offline tmazz

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Re: biasing power tubes
« Reply #6 on: November 15, 2011, 01:39:26 PM »
this is what the manufacturer says: " set the volume at lower level (less than one quarter). i guess you could take this as no signal present or with a signal.

I would take then exactly at their word. 1/4 volume is definitely not no signal.

Just by way of experience ARC in their instructions for setting the bias on my VT-200 says to set the bias after the amp has been idling for at least 15 minutes. When I first read this I took it to mean that it had been warmed up for at least 15 minutes.  One day I gad been playing the amp for about an hour and thought it would be OK to check the bias since it was by then well warmed up, I took one reading and before I had a chance to make any adjustments the phone rang and I was on a call for about 20 minutes. I returned to continue after the call (and the amp had been idling for the recommended amount of time, retook the same reading and the measurement had changed by about 20%. If the phone hadn't rung and called be away I would have made changes that would have been way out of spec. Moral to the story - follow the directions exactly.

And don't forget that it says to hop on on foot while adjusting!! :rofl:

If that's what it takes...... :)
Remember, it's all about the music........

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• McIntosh MC 275
• ARC SP-9
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Offline sleepyguy24

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Re: biasing power tubes
« Reply #7 on: March 29, 2012, 06:15:39 AM »
Sorry to bump an older topic but some questions I have may relate and I'm hoping they will help others when the tube gurus here chime in.

How often should one re-bias their power tubes? Do you do it quarterly, monthly or yearly? If you listen to your tube amp a lot do you re-bias the power tubes more frequently? How does one know when their power tubes need to be re-biased?

Thanks in advance.


Offline BobM

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Re: biasing power tubes
« Reply #8 on: March 29, 2012, 08:20:29 AM »
If you have to take the amp to the workbench, or turn it upside down, remove the bottom panel and take your life in your hands to get the meter on the test points = then once every 6 months/year is plenty.

If you have a meter built into the top plate and easy access to biasing screws right on top = then a once a month check is not too onerous.

The bottom line = tubes will change with age, but a well designed amp should not get out of bias all that quickly.
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Offline richidoo

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Re: biasing power tubes
« Reply #9 on: March 29, 2012, 08:51:20 AM »
How often should one re-bias their power tubes? Do you do it quarterly, monthly or yearly? If you listen to your tube amp a lot do you re-bias the power tubes more frequently? How does one know when their power tubes need to be re-biased?

It depends on the quality of the tube. You only know if it's stable by checking it over a period of time and getting to know that tube. Some like JJ require more frequent adjustment, some are rock steady the entire life. The only way to know is measure it. If it's way off it will sound less good, but that's not a reliable way to know when to check, but if something sounds off, bias is a good thing to check.

Offline JBryan

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Re: biasing power tubes
« Reply #10 on: March 30, 2012, 07:53:18 AM »
Really depends on the amp. I had a Music Reference RM9 that was easy to bias but needed adjustment every time I checked so I ended up checking it every week or so. OTOH, I have a Scott integrated that is a PITA to bias but once the tubes have settled in (about 200 hours), the bias is spot in each time I've check so now I'll check the bias once a year but have yet to make an adjustment (going on 5 years since i changed out the tubes). Another amp that's extremely stable is a Melody i2A3 which is relatively easy to check (requires a meter) and so far, hasn't needed adjustment (going on 2 years).

It comes down to the stability of the circuit as well as the quality and age of the tubes. Regardless of what volume level the instructions specify, don't play anything through the amp when making adjustments. I usually only have the amp on when biasing.

Offline sleepyguy24

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Re: biasing power tubes
« Reply #11 on: April 02, 2012, 09:23:33 AM »
Hi you guys.

Thanks so much for all the replies. It has helped me. I've made note of what you all have said. Now I'm beginning to see why the audio enthusiasts in my family have stayed away from tube amplification and just use tube preamps. There are lots of little things to take into account when it comes to tubes amplification. I'm going to keep using the Baldwin amp I have an take it to a competent tech soon to get more info about it. Hopefully they are willing to teach me how to bias the thing should I want to tube roll further. If it starts getting to costly to maintain this Baldwin amp I'll save my money for a quality tube amp that is easier to maintain.

For me I'm a fan of the NOS vintage tubes. I've purchased most of the tubes I have from a couple reputable sellers on eBay and Brent Jessee. I have nothing against the tubes being made today. I've just had more experience with the older tubes. I'm now thinking about replacing my current tube complement with a quad of Raytheon 5881 tubes. Hopefully the picture attachment makes it through.

Thanks again for the help.

Offline richidoo

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Re: biasing power tubes
« Reply #12 on: April 02, 2012, 10:32:07 AM »
Very nice! Vintage power tubes are awesome, if you can find good ones and afford them. You are wise to work with a reputable seller like Brent or Jim McShane.  The only time you don't get what you pay for is when you are getting ripped off.

In the last few years new production power tubes have improved dramatically. Russian tubes from New Sensor under the brands Genelex-Gold Lion and TungSol are very good, as are Chinese tubes from Shuguang sold under the brand name Black Treasure. Svetlana winged-C 6550 is excellent. But there are still plenty of new tubes that are just soso.

I hope your tech can show you how to bias the amp.

Offline sleepyguy24

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Re: biasing power tubes
« Reply #13 on: January 02, 2013, 02:06:14 PM »
I was researching biasing power tubes and found this thread I had asked questions in earlier. Man stuff changed in how many months. I ended up selling that Baldwin amp and got a Jolida JD-502P. It's got 4 Tung Sol KT-120 tubes, Mullard CV-4024 tubes and I've been rolling in various 12AX7 tubes.

One thing I noticed these past couple of weeks is that one power tube lost it's bias. I checked and it ready 492 millivolts when I had previously set it to about 525 millivolts. Would an instance like this be due to the power tubes or the 12AX7 tubes? When this occurred it was with a Tung Sol 12AX7 re-issue preamp tube in place. All the other tubes in the amp were the same. The bias slipping below 500 millivolts hasn't happened again but I am watching

Thanks again gents for the advice.

Offline richidoo

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Re: biasing power tubes
« Reply #14 on: January 02, 2013, 05:14:41 PM »
Bias of output tubes changes during break in, and again as the tubes age. There is a stable period in between. All New Sensor tubes like the TungSol are very reliable and stable.

I've never heard of the output tube bias changing due to input tube variance, but I guess anything is possible. Good to check bias after any tube change, even changing same tubes into different sockets.

Do you love the KT120s like everyone else seems to?