AudioNervosa

Systemic Development => Amplification and Preamplification => Tubes => Topic started by: mboldda1 on November 12, 2011, 08:51:54 PM

Title: biasing power tubes
Post by: mboldda1 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?
Title: Re: biasing power tubes
Post by: tmazz 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.
Title: Re: biasing power tubes
Post by: rollo 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:

charles
SMA
Title: Re: biasing power tubes
Post by: mboldda1 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.
Title: Re: biasing power tubes
Post by: tmazz 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.
Title: Re: biasing power tubes
Post by: jimbones 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:
Title: Re: biasing power tubes
Post by: tmazz 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...... :)
Title: Re: biasing power tubes
Post by: sleepyguy24 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.

Title: Re: biasing power tubes
Post by: BobM 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.
Title: Re: biasing power tubes
Post by: richidoo 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.
Title: Re: biasing power tubes
Post by: JBryan 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.
Title: Re: biasing power tubes
Post by: sleepyguy24 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.
Title: Re: biasing power tubes
Post by: richidoo 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.
Title: Re: biasing power tubes
Post by: sleepyguy24 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.
Title: Re: biasing power tubes
Post by: richidoo 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?
Title: Re: biasing power tubes
Post by: sleepyguy24 on January 04, 2013, 07:25:28 AM
Hi Rich

Thanks for the prompt response. I reviewed things with the Jolida and the Tung Sols I have in there and it turns out the set I have in there aren't fully broken in. The guy I bought the Tung Sol KT-120 tubes from bought them from TC Tubes with them partially broken in. He then told me he only put a couple of hours on the quad before he sold them to me. I believe from another thread here tubes need about 100 hours to stabilize. Also the CV-4024 tubes as well as most of the various 12AX7 tubes I've been rolling in are new and not fully broken in. Too many variables. I secured the tube cage for the Jolida in so this will help me to not roll in tubes.

To your question about the KT-120 tubes I do love them but my experience with the family of 6550 and KT-88 power tubes and tubes in general are limited. I have a quad of Penta Labs KT-88 tubes and when I compared the KT-120 tubes to those I found the bass to be better in my system with the KT-120s in place. I also have a quad of NOS Sylvania 6550 tubes but I haven't listened to them yet. These are expensive tubes to replace so I don't want to use them so much.

Thanks again for the help.
Title: Re: biasing power tubes
Post by: richidoo on January 04, 2013, 09:35:40 AM
That makes sense. When a new tube is first powered up it changes fast and big, you can watch the bias change drastically in real time on the meter for the first few minutes. Adjusting once a day is wise for the first week to prevent a runaway. You will need to adjust bias on them frequently in the beginning, then tapering off until they are stable. With New Sensor tubes this is pretty quick. As long as they are within 10% of mfg bias spec they are safe, in your case, within 50mV of spec.  You are probably already up to the point of checking once a week. If two weeks require no adjustment, then switch to once a month or two, depending on usage. When they start to change again, plan for a new set within 6-12 months, depending on usage. Does your Jolida bias individual tubes or by pair or quad?