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Post subject: Hot Rod Deville 212 bias puzzler...
Posted: Tue Apr 29, 2014 2:48 am
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I've read a fair amount of (sometimes contradictory) advice re a Deville's ideal bias setting, but understand that, over time, whichever setting chosen can alter, so periodically I check and adjust to my chosen setting.
This is (usually!) fairly easy to do with the Deville : make sure your multimeter is grounded, use the PCB test point to measure, and adjust with the blue trim pot.
Today?
No....
I'm getting zero reading from the test point, and turning the trim pot alters nothing - the multimeter still reads zero.
The meter is working fine : with the amp on standby, I tested the trim pot itself : turning the pot anti-clockwise, I got a reading of a negative 57/58v.
Power on, meter lead to test point, adust the trim pot clockwise, and I should see the value rising...but no!
A big fat zero!
Any ideas?
Anywhere else I can measure to see the bias?


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Tue Apr 29, 2014 5:15 am
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On my Deville schematic it is TP30 for bias . Meter on Millivolts scale DC, positive probe on TP30 and negative probe to ground .

TP30 reading is across a 1 ohs resistor , pin 8 on 6L6

1- Your meter is on the wrong scale
2- Wrong TP or/and not at ground
3- Dirty TP contact , oxide solder ......


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Tue Apr 29, 2014 6:08 am
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stratele52 wrote:
On my Deville schematic it is TP30 for bias . Meter on Millivolts scale DC, positive probe on TP30 and negative probe to ground .

TP30 reading is across a 1 ohs resistor , pin 8 on 6L6

1- Your meter is on the wrong scale
2- Wrong TP or/and not at ground
3- Dirty TP contact , oxide solder ......


Hmmmm...(great name, by the way..."Stratele52"....)

1) Multimeter set correctly, DC up to 200....
2) Test Point is the one I've always used, right on corner of PCB...
3) Hadn't thought of that!

Thank you!


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Tue Apr 29, 2014 7:01 am
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Jack Nelson wrote:
stratele52 wrote:
On my Deville schematic it is TP30 for bias . Meter on Millivolts scale DC, positive probe on TP30 and negative probe to ground .

TP30 reading is across a 1 ohs resistor , pin 8 on 6L6

1- Your meter is on the wrong scale
2- Wrong TP or/and not at ground
3- Dirty TP contact , oxide solder ......


Hmmmm...(great name, by the way..."Stratele52"....)

1) Multimeter set correctly, DC up to 200....
2) Test Point is the one I've always used, right on corner of PCB...
3) Hadn't thought of that!

Thank you!



+1


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Tue Apr 29, 2014 6:01 pm
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Jack Nelson wrote:
Multimeter set correctly, DC up to 200....


200 what? Volts?

Ah. No. As stratele52 said, you are measuring millivolts, somewhere around 60 to 80 mV to be exact. Well less than 1 volt. You won't see it on a 200 volt scale.

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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Wed Apr 30, 2014 1:33 am
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bluesky636 wrote:
Jack Nelson wrote:
Multimeter set correctly, DC up to 200....


200 what? Volts?

Ah. No. As stratele52 said, you are measuring millivolts, somewhere around 60 to 80 mV to be exact. Well less than 1 volt. You won't see it on a 200 volt scale.



You are not wrong Bluesky636, as we don't know the knowledge and skill of the people we try to help we need to read a complete answer .

Why is so hard to write 200 millivolts when the question is what scale ?


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Wed Apr 30, 2014 7:20 am
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No...no, sorry - my mistake!
I realise we're talking millivolts...I just didn't type my information in full...
Apologies for the confusion.


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Wed Apr 30, 2014 7:36 am
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Jack Nelson wrote:
No...no, sorry - my mistake!
I realise we're talking millivolts...I just didn't type my information in full...
Apologies for the confusion.



+1


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Wed Apr 30, 2014 7:45 am
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Are the 6L6GC lighting up? Maybe it's a bad heater line connection? You are switching the Standby to operating position?


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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Wed Apr 30, 2014 11:44 am
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Does the amp work? If so, then it is obviously passing current through the tubes, so nothing serious is wrong. Possibly a poor connection with your meter, maybe not connected well to the chassis?

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Post subject: Re: Hot Rod Deville 212 bias puzzler...
Posted: Wed Apr 30, 2014 11:51 am
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Jack Nelson wrote:
...Anywhere else I can measure to see the bias?


With amp off, unplugged from electricity, and caps discharged, read the resistance of each half of the OT primary. Then power on the amp, standby switch in the Play position, as if ready to use the amp, and measure the VDC across each half of the OT primary, somewhere around 4 VDC on each half. Then use ohms law to figure the current, I = E/R, current equals voltage divided by resistance. This will be the actual Plate current at idle, and more accurate than the test point reading. Note the resistance so you can reference in the future and you won't have to remeasure the resistance every time, just check the voltage.

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