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Post subject: Fender Bassman 59 LTD - HELP No Bias??
Posted: Thu Oct 01, 2015 2:16 pm
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So this beast decided to redplate and blowing fuses. So I swap tubes, pull out the Bias Master and Fluke Meter. With the Bias master I have 0 reading in Milliamps.

So I pull out my fluke and use it to test direct to the Bias Point grounded to chassis (Resistor R48) still no bias.

Tested R32 & R33 and those test fine.

Measured voltage between pin 3 and 8 = 324

HELP!


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Thu Oct 01, 2015 4:43 pm
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That is pins 2 and 8, but the bias supply does not come from the rectifier tube, it comes from diode CR2, fed from a transformer tap. Check for DC voltage from CR2, opposite end from the stripe, to ground. If you have DC voltage, approx -52 VDC, then follow the circuit, R40, etc, to see where you lose voltage, all the way to pin 5 of each output tube. If no DC at CR2, check for AC voltage from the striped end of CR2 to ground.

59 Bassman LTD schematics:

http://www.fmicassets.com/Damroot/Original/10001/217100_gamp_schematic.pdf

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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Fri Oct 02, 2015 6:37 am
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Thanks Shim -

Im reading -0.93 Striped side of CR2 and -078.4 on the other end (non striped)

R


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Fri Oct 02, 2015 7:24 am
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Ryan41018 wrote:
Thanks Shim -

Im reading -0.93 Striped side of CR2 and -078.4 on the other end (non striped)

R


I can't read schematic with my I Pad Mini, if CR2 is the bias circuit rectifier ,the strip side is a AC voltage . Minus 0.93 is DC , use your meter in AC scale for this side .

You must read more tan 40 AC volts.


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Fri Oct 02, 2015 3:22 pm
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Yup - she's reading 056.7 in AC Mode on the striped side of the diode with my Fluke.


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Fri Oct 02, 2015 9:36 pm
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Ryan41018 wrote:
Yup - she's reading 056.7 in AC Mode on the striped side of the diode with my Fluke.


56 AC volts at cathode side should give close to 60 DC volts other side ,
Diode look defective


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Fri Oct 02, 2015 10:48 pm
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Stratele - I checked it and the other side in DC mode is reading ( -77)

Does this reading mean it is fried?


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Sat Oct 03, 2015 7:09 am
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Ryan41018 wrote:
Stratele - I checked it and the other side in DC mode is reading ( -77)

Does this reading mean it is fried?
,

Diode is OK , I do not read carefully your 097 volts .
To me digit 0 is when is less than 1 volts.

Bias voltage supply is good .


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Sat Oct 03, 2015 9:28 am
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I've tested up & Down the circuit and all resistors test good.


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Post subject: Re: Fender Bassman 59 LTD - HELP No Bias??
Posted: Sun Oct 04, 2015 6:16 am
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Ryan41018 wrote:
I've tested up & Down the circuit and all resistors test good.


Shimmilou tell you all tests to do to find whybtubes are redplating


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