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Post subject: What determines bias reading?
Posted: Sat Jan 04, 2020 1:32 am
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Hello...

Re: my 1967 DUAL Showman, if I buy a "matched quad" of 6L6 tubes...matched within 2mA and I install them in the amp, if they are indeed matched will they read within 2mA of each other or can the amp determine the bias readings?

Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 2:26 am
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Google ; Amp bias, what is this ? :lol:

Very very short answer for newbies;
Tubes may have different internal resistance for same model like 6L6.
Different resistance = plus or less working current .
You need to control this current ; this is the bias. ****

Your amp will not do bias by itself
_______________________________________________________


You can never know in advance if the bias will be the right one for your new tubes before measuring it.
Only if you put the exact same make and model of tubes, the bias should be the right one.
It is not 100 % right but amp must be run safe.

It is a tech job unless you know what you are doing with right tool ; tubes amps have lethal voltage.

Match tubes ; mean your tubes will need ( will work fine with ) the same bias.

***If you don't check bias and adjust if needed ;
1- Amp may not sound at is better.
2- In worst case ; shorter life tubes ; weeks or months
3- Very very worst case ; tubes destroy in minutes or hours and you could destroy amp Power transformer.


Last edited by stratele52 on Sat Jan 04, 2020 2:34 am, edited 4 times in total.

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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 2:30 am
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Be sure this old Blackface amp use the right fuse.
Over years people may put a too powerful fuse wich may arm the amp

_________________________________

Bias by Spacemen Music.com


A correctly biased amp will run efficiently and maximizes the life of your tubes.

Under - biased (hot ) amps will lack of punch and tubes will run hotter , shorter tube's life .

Overbiased amps ( cold ) will sound thin and brittle. with tubes running too cool for proper performance .
A correctly biased amps will sound clean and tight a moderate volumes, than at higher volumes breakup and distort musically .


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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 6:39 am
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Each tube has its own circuit connection, with some shared resistors and capacitors. Using a matched set basically means any of the tubes should give the same result in any position in the output array.


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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 11:23 am
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So an "underbiased amp" will have a mA reading higher than it should be, therefore making the tubes glow brighter and an "overbiased amp" will have a mA reading lower than it should be making the tubes glow to a lesser degree...correct?

RB3988


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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 1:50 pm
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No
A tube is not a light bulb with a dimmer.

With an extreme "underbias" the tube will be red plating and its life will be very short.
As for the other bias range, we can't see it visually.

Search tubes bias with Google


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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 4:21 pm
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I understand...but my question relates to readings on the VOM:

As the VOM mA numbers increase as I turn the bias pot, the tubes are being increasingly "underbiased"...and therefore running "hotter"...yes?
Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 5:42 pm
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royb3988 wrote:
I understand...but my question relates to readings on the VOM:

As the VOM mA numbers increase as I turn the bias pot, the tubes are being increasingly "underbiased"...and therefore running "hotter"...yes?
Thanks,

RB3988


Yes if you read tubes Kathode current with your VOM


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Post subject: Re: What determines bias reading?
Posted: Sat Jan 04, 2020 7:00 pm
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Got it, thanks!

Royb3988


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Post subject: Re: What determines bias reading?
Posted: Sun Jan 05, 2020 2:22 am
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Best is to bias tubes at 60% to 70 % of their maximum power dissipation.
On some amps if you play loud or with Power attenuator I'll go for 60% . At 70% tubes may blow

6L6GC is a 30 watts tube, 60% is 18 watts.
Plate voltage X Kathode current = watts

Schematic show 405 plate voltage ; 405 X 45 ma ( bias ) = 18 watts for each tube.
Alway read Plate voltage when adjusting bias it move.
Less ma = higher voltage, more ma = lower voltage
Silverface Power transfomer are built to work at 115 volts ( ? ) house wall outlet. Tody we may have 120 to 125 AC volts.

Good luck and be carefull with voltage, it can hurt or kill you.
_________________________

Dual showman is a 1968 to 1981 amp;

http://ampwares.com/amplifiers/fender-s ... al-reverb/


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Post subject: Re: What determines bias reading?
Posted: Sun Jan 05, 2020 11:14 am
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stratele52 wrote:
Schematic show 405 plate voltage

My schematic for an AB763 Showman indicates 450 VDC at the plates.

????

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: What determines bias reading?
Posted: Sun Jan 05, 2020 11:22 am
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Stratele52

Thanks for the info..my amp is a '67 DUAL (in block letters) Showman...4 ohm output transformer. Image The amp shown is not mine...just a photo of one like it.
The tubes are Sylvania green lable STR387's. HOWEVER, the amp that ended up with the Sylvania Green Label STR387's is my 1966 Super Reverb...I bought new in '66...it reads 460 VDC and is biased at 40mA. The Super sounds terrific!


Royb3988


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Post subject: Re: What determines bias reading?
Posted: Sun Jan 05, 2020 2:12 pm
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460 volts X 40 ma - 18.4 watts Look great bias

Thank's for the picture.

Good information here, no pictures;

Dual Showman (63-67): 24½ x 36 x 11½, Dual Showman (67): 45½ x 30 x 11½

http://ampwares.com/amplifiers/fender-b ... e-showman/


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Post subject: Re: What determines bias reading?
Posted: Tue Jan 28, 2020 9:33 pm
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Well...

The DUAL Showman had tubes red plating this evening, not all but the two inside tubes...I adjusted the bias a day or two ago to be 40mA and the 3 other tubes read 35mA, 36mA and 33mA.

Question:

As I turn the bias pot and the mA number increases, when I switch the bias tool to read plate voltage, that number decreases as the mA is increased. Is this how it should be?

Which is more likely to cause the red plating...the plate voltage being too high (or too low) or the mA being too high (or too low)?

Could a single tube, if bad, cause other tubes to red plate?

Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 2:07 am
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royb3988 wrote:
Well...

The DUAL Showman had tubes red plating this evening, not all but the two inside tubes...I adjusted the bias a day or two ago to be 40mA and the 3 other tubes read 35mA, 36mA and 33mA.

Question:

As I turn the bias pot and the mA number increases, when I switch the bias tool to read plate voltage, that number decreases as the mA is increased. Is this how it should be?

Normal ; With more load ( ma) plate voltage decrease. You must re- adjust bias with this "new" plate voltage

Which is more likely to cause the red plating...the plate voltage being too high (or too low) or the mA being too high (or too low)?
With more load ( high ma) tubes work harder = red plating if exceed tubes specs


Could a single tube, if bad, cause other tubes to red plate?
No

Thanks,

RB3988


Red plating;
1- Bad tubes ( all in your case ? I don't think unless you damaged them before)
2- Bias too hot
3- Output transformer issue
4- Speaker load to low or bad ( short circuit ) wire to speaker

What is your plate voltage ?
Wich tubes do you use ? 6L6 or 6L6GC or .......
What voltmeter do you use ?
Wich bias probe ?


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