Manual bias easy question to answer.

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Guy 13

Manual bias easy question to answer.
« on: 1 Nov 2012, 03:20 pm »
Hi Blair and all Audio Circle members.

The following question, should be easy for you to answer.

On your NS-SEP20 do you have two or four potentiometer to adjust the bias.
One potentiometer for each set/pair of tubes per channel
or four potentiometer, one for each tube.

I told you it was an easy to answer question.

Guy 13
 

cheap-Jack

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Re: Manual bias easy question to answer.
« Reply #1 on: 1 Nov 2012, 06:24 pm »
Hi.
Hi Blair and all Audio Circle members.

(1) The following question, should be easy for you to answer.

(2) On your NS-SEP20 do you have two or four potentiometer to adjust the bias.
One potentiometer for each set/pair of tubes per channel
or four potentiometer, one for each tube.

I told you it was an easy to answer question.

Guy 13


(1) No it is not that an easy Q to answer unless one did it before.

(2) No, I don't have any NS-SEP20. I only own a vintage 50-year-young Dynaco ST-70 power amp which comes with one bias setting pot for the 2 O/P power tubes for each channel. Still quite a unique feature even for today's standard though.

The problem of using one common bias setting pot for both PP O/P tubes is that those 2 O/P tubes are assumed matched at all time despite of aging. In reallity, no 2 tubes age at the same pace. period.
When one of the PP pair tubes ages much faster than the other due to whatever reasons, the common bias setting can go out of wack.

So years back, I've already changed the original common bias setting for both O/P tubes of the St-70 to
one bias setting pot for each individual O/P power tube by adding one more pot of the same value for the
other power tube. So 4 bias setting pots for 2 channels.

It works bigtime. No more worry about unbalanced PP O/P due to uneven tube performance due to aging.
Everytime after the power amp is switched on for warm up idling for an hour or so, I always measure the
bias voltage of each O/P tube. I always find a very minute bias voltage difference (0.01V or so) btween 2 tube, that can easily be adjusted to same bias voltage with the pot.

So no need of changing both power tubes of each channel even one tube is going down.

c-J

Niteshade

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Re: Manual bias easy question to answer.
« Reply #2 on: 1 Nov 2012, 09:51 pm »
All of my DC biased amplifiers have a single biasing control and work perfectly with matched quad tubes.

Tubes can age differently, but I have not found any issues so far. My amps do not push the tubes hard, prolonging the aging process.

I never liked individual bias controls: It's a pain for the customer. Fisher used a fixed bias control on many of their receivers and amps, it was not variable and operated all four power tubes. The only issue I had with them was they ran the tubes too hot.

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Re: Manual bias easy question to answer.
« Reply #3 on: 1 Nov 2012, 09:58 pm »
Very instructive topic, thanks for the replies.

Wayner

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Re: Manual bias easy question to answer.
« Reply #4 on: 1 Nov 2012, 10:30 pm »
All of my DC biased amplifiers have a single biasing control and work perfectly with matched quad tubes.

Tubes can age differently, but I have not found any issues so far. My amps do not push the tubes hard, prolonging the aging process.

I never liked individual bias controls: It's a pain for the customer. Fisher used a fixed bias control on many of their receivers and amps, it was not variable and operated all four power tubes. The only issue I had with them was they ran the tubes too hot.

As Nightshades has pointed out, bias settings is not a huge factor in amplifier performance. Since the bias is really line in voltage dependent, what would be set one day, is different the next day. It's like chasing your shadow. Set the bias to what ever voltage recommendation is when the voltage at your house is the usual average (some where between 115-121 volts) and set the bias accordingly. I think most manufacturers will all agree that the importance of bias setting is a little over the top. I was a bias compulsive guy myself until I was instructed by a learned one that I was basically wasting my time.

Listen to the music and don't worry about the bias. That's my advise.

Wayner

FullRangeMan

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Re: Manual bias easy question to answer.
« Reply #5 on: 2 Nov 2012, 05:04 am »
As Nightshades has pointed out, bias settings is not a huge factor in amplifier performance. Since the bias is really line in voltage dependent, what would be set one day, is different the next day. It's like chasing your shadow. Set the bias to what ever voltage recommendation is when the voltage at your house is the usual average (some where between 115-121 volts) and set the bias accordingly. I think most manufacturers will all agree that the importance of bias setting is a little over the top. I was a bias compulsive guy myself until I was instructed by a learned one that I was basically wasting my time.

Listen to the music and don't worry about the bias. That's my advise.

Wayner
Maybe is for this reason some hi-end pre-amps had batteries associated to the tube Bias.

Guy 13

Re: Manual bias easy question to answer.
« Reply #6 on: 2 Nov 2012, 10:27 am »
All of my DC biased amplifiers have a single biasing control and work perfectly with matched quad tubes.

Tubes can age differently, but I have not found any issues so far. My amps do not push the tubes hard, prolonging the aging process.

I never liked individual bias controls: It's a pain for the customer. Fisher used a fixed bias control on many of their receivers and amps, it was not variable and operated all four power tubes. The only issue I had with them was they ran the tubes too hot.

Hi Blair and all Audio Circle members.

If I get it right,
on your NS-SEP20 you only have on adjustment pot for the quad tubes.

If the input voltage varies,
the bias voltage will also vary, am I right ?

In Vietnam, they have auto transformer stabilizer
that can maintain the voltage (230/120V) to within 1%
that should be good for the bias?

For a 1Kva autotransformer, it cost only 60USD.
For a 3Kva autotransformer, it only cost 125 USD.

On your NS-SEP20, what is the bias range of voltage necessary
for good performance? (Just curious to know)

Guy 13

cheap-Jack

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Re: Manual bias easy question to answer.
« Reply #7 on: 2 Nov 2012, 01:46 pm »
Hi.
All of my DC biased amplifiers have a single biasing control and work perfectly with matched quad tubes.

Tubes can age differently, but I have not found any issues so far. My amps do not push the tubes hard, prolonging the aging process.

Sooner later, one or 2 of yr match quad tubes die & the new tubes work on slightly different bias voltage. To acheive MATCHED quad like before, tell us what are you going to do????  Leave it to unmatched PP or replace with a complete new set of matched quad???? Or you don't think yr ears can hear it?

If that's the case, why spent more money for MATCHED quad tubes to start with?

(1) Nightshades has pointed out, bias settings is not a huge factor in amplifier performance

(2) the bias is really line in voltage dependent, what would be set one day, is different the next day. It's like chasing your shadow. Set the bias to what ever voltage recommendation is when the voltage at your house is the usual average (some where between 115-121 volts) and set the bias accordingly.

(3) I think most manufacturers will all agree that the importance of bias setting is a little over the top. I was a bias compulsive guy myself until I was instructed by a learned one that I was basically wasting my time.

(4) Listen to the music and don't worry about the bias. That's my advise.

Wayner

(1) & (3) So why should the amp manufacturers install a bias setting pot on the amp to start with??
For quality control only before they shipped out the amps or let the service men to repair their amps more efficiently?

(2) I think you missed my point. What you just said is ASSUMING all the O/P tubes work equal at all time despite of aging or deteriating evenly. Biasing voltage & HV always change when the AC mains voltage fluctuates. Same to all tubes.

But I am talking about individual tube NOT working evenly due to aging or whatever reasons. They need to be adjusted individually for really balanced PP O/P function.

For SET power amps, such bias setting for individual O/P tube will be less critical.

That said, certain direct heat power tubes, eg. 300B which are notorious for their current fluctuating, individual bais voltage setting is imperial.

(4) If the PP power stage is not balanced, how can the amp deliver undistorted music signals for you to enjoy??????

c-J

cheap-Jack

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Re: Manual bias easy question to answer.
« Reply #8 on: 2 Nov 2012, 01:59 pm »
Hi.
Maybe is for this reason some hi-end pre-amps had batteries associated to the tube Bias.

High end or not, in my design-built 1-stage tube phonostage (using 1/2 50-year vintage Telefunken ECC83 per channel operating at 440VDC HV), which I've used a few years now, I used a rechargeable battery which is kept on being charged to a constant bias voltage 2.3VDC.

No replaceable dry battery, my friend. Changing bias battery is a pain beside affecting the sound when the battery drains out.

FYI, not all batteries sound same & sound good. I spent enough time to find the best sounding bias battery.

For phonostages, 100% DC bias provided by batttery is the best choice. Less hum/LF noise

c-J

Ericus Rex

Re: Manual bias easy question to answer.
« Reply #9 on: 2 Nov 2012, 04:16 pm »
Blair, slightly off-topic question;  why do you specify DC bias in your response below?  Isn't all grid biasing concerning DC voltage level?  Or is this an EE way of saying biasing without a source signal present?  Sorry for the tangent.  Thanks!

Niteshade

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Re: Manual bias easy question to answer.
« Reply #10 on: 3 Nov 2012, 01:35 pm »
The other kind of biasing is called cathode biasing. With cathode biasing, the grid has a leak resistor going to ground and no negative voltage is applied. The cathode is connected to a resistor which goes to ground. The potential across that resistor is what is considered the bias voltage.

With DC biasing, a negative voltage is applied to the grid through something like a 100K resistor and the cathode is grounded.

Blair, slightly off-topic question;  why do you specify DC bias in your response below?  Isn't all grid biasing concerning DC voltage level?  Or is this an EE way of saying biasing without a source signal present?  Sorry for the tangent.  Thanks!

Niteshade

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Re: Manual bias easy question to answer.
« Reply #11 on: 3 Nov 2012, 01:57 pm »
Power tubes should be balanced in a push-pull amplifier within a certain degree. They do not have to be 100% perfect. I like to replace my tubes, RF or AF when they get below 90%. You can have variations of up to 10% without issue. A properly ran set if power tubes in a NS-40 will run perfectly for several years. It depends on how many hours are on them too.

**I would rather see someone replace all their tubes every few years than tinkering and being concerned about the bias voltage.

Instructions for bias settings on my DC biased amplifiers: Adjust the bias for best sound with the least amount of heat output. A volt meter is unnecessary for adjustment.

That is all there is to it. Typically a tube is ran at no more than 25% of its maximum rating, usually much less. Given this information, even if a one tube were  off center from the neighboring tube, it would never be noticed.

**Issues with imbalances will arise at high power if a tube is about 10% to 15% off, either drawing too little or too much current. (Replace all tubes.)

NOTE: Most AF power tubes get gassy before emissions decline noticeably. A gassy tube will begin to run very hot. The bias should NOT EVER be adjusted to compensate, even with independent biasing.

A good question: If a tube(s) starts misbehaving relative to the others, how would you know if one or two tubes were gassy or if the others were drawing less current? In this case, it is best to replace them all at the same time.

A single bias control for a set of tubes is ideal. Abnormalities will be indicated typically with red plates, a hum (bad imbalance), or less volume and/or distortion. Don't worry about compensating for a soft or gassy tube, just replace the set.



Hi.
Sooner later, one or 2 of yr match quad tubes die & the new tubes work on slightly different bias voltage. To acheive MATCHED quad like before, tell us what are you going to do????  Leave it to unmatched PP or replace with a complete new set of matched quad???? Or you don't think yr ears can hear it?

If that's the case, why spent more money for MATCHED quad tubes to start with?

(1) & (3) So why should the amp manufacturers install a bias setting pot on the amp to start with??
For quality control only before they shipped out the amps or let the service men to repair their amps more efficiently?

(2) I think you missed my point. What you just said is ASSUMING all the O/P tubes work equal at all time despite of aging or deteriating evenly. Biasing voltage & HV always change when the AC mains voltage fluctuates. Same to all tubes.

But I am talking about individual tube NOT working evenly due to aging or whatever reasons. They need to be adjusted individually for really balanced PP O/P function.

For SET power amps, such bias setting for individual O/P tube will be less critical.

That said, certain direct heat power tubes, eg. 300B which are notorious for their current fluctuating, individual bais voltage setting is imperial.

(4) If the PP power stage is not balanced, how can the amp deliver undistorted music signals for you to enjoy??????

c-J

Ericus Rex

Re: Manual bias easy question to answer.
« Reply #12 on: 3 Nov 2012, 05:38 pm »
The other kind of biasing is called cathode biasing. With cathode biasing, the grid has a leak resistor going to ground and no negative voltage is applied. The cathode is connected to a resistor which goes to ground. The potential across that resistor is what is considered the bias voltage.

With DC biasing, a negative voltage is applied to the grid through something like a 100K resistor and the cathode is grounded.

Got it!  I've only ever heard that termed 'grid bias'.  Thanks!

Guy 13

Re: Manual bias easy question to answer.
« Reply #13 on: 20 Nov 2012, 09:54 am »
Hi Blair and all Audio Circle members.

Tube Depot offers a gizmo that can make manual bias adjustment simple and precise.

Have a look at this link:
http://tubedepot.com/bt-bias-scout.html

and tell me what you think.

Guy 13

cheap-Jack

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Re: Manual bias easy question to answer.
« Reply #14 on: 20 Nov 2012, 02:46 pm »
Hi.

Tube Depot offers a gizmo that can make manual bias adjustment simple and precise.

Have a look at this link:
http://tubedepot.com/bt-bias-scout.html

and tell me what you think.

Guy 13

Bingo! Even tube vendors offer tube bias adjustment kits for adjusting bias voltage of INDIVIDUAL power tubes in case the power amps which do not equipped with bias adjustments.

So who said bias adjustment for individual O/P power tubes is NOT necessary???
Unless one want to waste money to replace ALL power tubes when only individual tube goes down.


c-J


Ericus Rex

Re: Manual bias easy question to answer.
« Reply #15 on: 20 Nov 2012, 05:53 pm »
Hi Blair and all Audio Circle members.

Tube Depot offers a gizmo that can make manual bias adjustment simple and precise.

Have a look at this link:
http://tubedepot.com/bt-bias-scout.html

and tell me what you think.

Guy 13

Many versions here:

http://www.amp-head.com/index.php?cPath=21

I have the dual which measures both tube bias and plate voltage.  The dual aspect means you can measure a tube from each channel without powering down and refitting.  Very handy!

Guy 13

Re: Manual bias easy question to answer.
« Reply #16 on: 21 Nov 2012, 09:03 am »
Many versions here:

http://www.amp-head.com/index.php?cPath=21

I have the dual which measures both tube bias and plate voltage.  The dual aspect means you can measure a tube from each channel without powering down and refitting.  Very handy!

Hi Ericus and all Audio Circle members.

I had a look at the link you potsed and that's exactly
what I was looking for, it's even better than the kit Tube depot sells.

I will buy two units,
one for my 6L6 (Niteshade Audio NS-10)
and one for my EL84 (Decware SE84C+)

By the way, on Tube Depot website, they say that all their tubes have a small sticker on the base stating the ideal bias current for that tube.

That's what I call professional.

Thanks again for the link, today you are my hero,
tomorrow, well I will see...

Guy 13

Guy 13

Re: Manual bias easy question to answer.
« Reply #17 on: 27 Nov 2012, 09:55 am »
Hi all Audio Circle members.
This device I found on Tube Depot website might be more convenient,
but it is also much more expensive.

Have a look at the following links:

http://www.tubedepot.com/biastools.html

http://site.tubedepot.com/pdf/bias_king_manual.pdf

Have fun.

Guy 13


cheap-Jack

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Re: Manual bias easy question to answer.
« Reply #18 on: 27 Nov 2012, 02:48 pm »
Hi.
Hi all Audio Circle members.

(1) device I found on Tube Depot website might be more convenient,
but it is also much more expensive.Have a look at the following links:

(2) ://www.tubedepot.com/biastools.html

http://site.tubedepot.com/pdf/bias_king_manual.pd

(1) Yup, kinda costly to buy the kit & a pain to put it on & remove it after adjusting the correct bias voltage.
     If one's a DIYer like me, simple install a pot for each tube on the amp chassis & adjust it from time to
     time.

    Of course you don't have to be like me, so picky as to BALANCE all the bias voltages of the O/P
     power tubes each time I start my music session after warming up the amp for an hour or so.

(2) Quoting the Biastools text to which I agree 101% -
     "To keep yr tubes in good shape & yr sound consistent, periodically checking the bias of yr amp
      is extremely importamt."
     Mind you, I seldom read any vendors' sales pitches. But this one
      tells the very true fact that many many tube owners tend to take it easy.

c-J

Ericus Rex

Re: Manual bias easy question to answer.
« Reply #19 on: 27 Nov 2012, 05:22 pm »
Hi all Audio Circle members.
This device I found on Tube Depot website might be more convenient,
but it is also much more expensive.

Have a look at the following links:

http://www.tubedepot.com/biastools.html

http://site.tubedepot.com/pdf/bias_king_manual.pdf

Have fun.

Guy 13

It's convenient that the bias king has the meter built in but you can get the Amp-Head version and a cheapo digital multimeter (from ebay) for about 1/3 the price of the Bias King models.  And you'd then have the added flexibility of using the multimeter for other tasks and problem solving.  A no-brainer for me!