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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 5:11 am
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royb3988 wrote:
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


1.) Yes, the plate voltage tends to decrease, as the idle bias current is increased.

2.) Red plating can be caused by too much plate voltage and/or too much idle bias current (cathode current flow). It can also be caused by a shorting tube, which passes too much current, regardless of plate voltage or idle bias current.

3.) In my experience, red plating because of a faulty (shorted) tube, is about as frequent a cause as improper idle bias current or plate voltage.

If you move the questionable tube to another socket and it continues to red plate --- it's prolly a bad tube. If a good tube now red plates on that suspect socket, you have some issue with that socket. Like bad contact between socket tangs and the tube's pins. Or out-of-spec grid stopper and/or screen resistor, on that socket.


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 5:36 am
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Bias adjust the conduction of the tube which acts on the current.
Also short tube ( very low resistance ) will redplating.

Disconnect tube cathode from the ground, you can read voltage, you won't read any current.
Voltage is alway there, red plating or not . Current ( high) is not.

Red plating is too much current, not voltage .


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 9:16 am
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Stratele52,

What is your plate voltage ? 456
Wich tubes do you use ? 6L6 or 6L6GC or .......Svetlana Winged C 6L6GC Older from Russia St. Petersburg Plant
What voltmeter do you use ? Fluke 79
Wich bias probe ? AMP-HEAD DUAL BIASTESTER-MPD Cathode Current (1k), Output DC mV-1ohm Shunt, Vp= Plate Voltage/1000
https://reverb.com/item/302869-amp-head ... r-mpd-2014 (Like this one)


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 9:44 am
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I use same bais probe.

At 35 ma /460 volts ; bias is ok .

Where is the issue ?

Tubes are short ! All four ???
Check the four 470 ohms 1 watt on 6L6 socket
Intermittent contact on sockets ?
Cable between amp and speaker cab.
Wrong speaker impedance
_______________________________________

Test to do ;

Bias circuit is defective, intermittent contact ?
1-Remove all 6L6.
2- Power amp On and read voltage on each 6L6 pin 5 . You may read close to -30 volts DC ( minus )
3-Turn bias pot to see how more voltage you can read ( -35, -40 ? )
More negative voltage = colder are bias

Report


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 11:40 am
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Stratele52...will do.

I need to check out my Fluke...I set the decimal point to 3 places from the left, I plug the red lead from the Bias tester into the red input V/ohm (symbol), the black lead into the COM input and set the meter to V dc.
I switch the bias tester to the Ik position and read .038 on the meter, I switch the bias tester to Vp and read .475. I don't recall the plate voltage showing as a decimal yesterday...but I may be wrong.

From the instruction sheet for the bias tester:
"For amps with a bias balance control it is recommended that you install the Dual BiasTester on the middle two tubes.
For example, in a Silver Face Twin Reverb, the 6L6’s are number V7, V8, V9, and, V10. The Dual would be installed
on V8 and V9. As you adjust the bias balance control, current will increase in one tube and decrease in the other tube.
You should adjust this control so the current is equal in both tubes. Then check tube V7 and V10 to see how well they
are matched to V8 and V9."

This is NOT how my tester is working...as I lower the mA for one tube, the mA for the other tube also gets lower...there is no way to reach an equal number for both tubes.

More later.

RB3988


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 12:24 pm
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royb3988 wrote:
Stratele52...will do.

From the instruction sheet for the bias tester:
"For amps with a bias balance control it is recommended that you install the Dual BiasTester on the middle two tubes.
For example, in a Silver Face Twin Reverb, the 6L6’s are number V7, V8, V9, and, V10. The Dual would be installed
on V8 and V9. As you adjust the bias balance control, current will increase in one tube and decrease in the other tube.
You should adjust this control so the current is equal in both tubes. Then check tube V7 and V10 to see how well they
are matched to V8 and V9."



These instructions are wrong !


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 12:24 pm
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royb3988 wrote:

I switch the bias tester to the Ik position and read .038 on the meter, I switch the bias tester to Vp and read .475. I don't recall the plate voltage showing as a decimal yesterday...but I may be wrong.



This right.
38 ma and 475 volts


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 12:59 pm
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Stratele:

Pin 5 of each tube, amp on, tubes out, off standby (ready to play) volume all the way down.

Range is -60.2 - -41.8

Royb3988


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 1:14 pm
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royb3988 wrote:
Stratele:

Pin 5 of each tube, amp on, tubes out, off standby (ready to play) volume all the way down.

Range is -60.2 - -41.8

Royb3988


Good !
1- Set voltage a -60.2 or maximum negative.

2- Put 2 tubes in the amp ; one at each end of quad set
or both in the middle of the quad set .
Output transformer will see a pair in push pull.

3- Power amp ON and with bias probe read bias on these tubes.
At -60 volts the tubes must not redplate, unless tubes are bad or their socket bias contact are bad.

Report.

Remember you must always have a load / speakers at output speaker jack when amp is ON


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 3:40 pm
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Stratele,

Tubes are installed...inside pair, tubes have subtle orange glow at top and bottom. Amp has been on (Standby off...ready to play) no red plate.
Bias readings are 24 and 31, plate voltage is 470 for both tubes.
Amp has been on for almost 8 minutes.

RB3988


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 5:57 pm
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Silly question if you dont mind, what is your screen grid voltage compared to your plate voltage? Are they very close when you increase the current or decrease the plates voltage when biasing?


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 6:16 pm
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sfceric64,

Not a silly question at all... but here is a silly answer in the form of a question: how do I measure the grid screen voltage?

Thanks,

RB3988


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 6:31 pm
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With your Fluke of course is my silly answer.
BLk to chassis ground and Red to pin 4 while in voltage mode for DC.


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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 6:39 pm
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sfceric64,

Tubes in, tubes out? I measured all 4 tube sockets...inside pair has tubes in it, all 4 are 465 DC volts.

RB3988


Last edited by royb3988 on Wed Jan 29, 2020 6:44 pm, edited 1 time in total.

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Post subject: Re: What determines bias reading?
Posted: Wed Jan 29, 2020 6:42 pm
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Either way is fine.
So your screen is 465 and your plate is 470 on the mid pair.
When you adjust the bias, does the relationship stay the same @ 5 vdc? I dont mean during the sweep of the pot but at a fixed point, say -60 -53 or or -50.


Last edited by sfceric64 on Wed Jan 29, 2020 7:26 pm, edited 1 time in total.

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