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Post subject: Fender 59 Bassman 5F6-a
Posted: Tue Dec 06, 2016 6:21 pm
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I have a weird issue, perhaps you can help. My 59 baseman has had the selenium rectifier changed with a 1N007 diode in its place. I get -55v on the PT side on the diode, I get .300 on the circuit side. The plate voltage on my 5881 tubes ( pin 3) is 453v , the resistance is between 43.4 and 46.9 on the 2 tubes. The Plate current is about 7.6 avg... and the voltage drop is 0.346 avg on both tubes. The Plate dissipation is 3.5 average and a working 5881 should be in the 16.1w range. The original 56K resistor is in the bias circuit.... any ideas what would cause the LOW- Voltage Drop in this amp? ( measured between pin 8 of the rectifer tube and pin 3 to the Output tube.)

thanks for your help in advance.


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Post subject: Re: Fender 59 Bassman 5F6-a
Posted: Sat Dec 10, 2016 3:53 am
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Location: Province de Québec, Canada
Voltage on Pt side is AC volts you can't read -55v or you don't use your meter right ?
Diode can't be a 1N007 maybe you mean 1N4007 ?

This is selenium diode;
Image

Give us the voltage at power supply test points;
Image


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Post subject: Re: Fender 59 Bassman 5F6-a
Posted: Sun Dec 18, 2016 10:57 am
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Stratle52 is right about your PT voltage measurement error.
Your low voltage drop appears to be measuring the DC resistance of the output transformer primaries. They will alway a little off. They need to be the same number of turns, but since one is wound over the other, the outer winding will always read a little higher.


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Post subject: Re: Fender 59 Bassman 5F6-a
Posted: Sun Dec 18, 2016 11:33 am
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Thanks for the replies. The diode is indeed a 1N4007. I will test this measurement again to see if I did something wrong. The transformer in there does have a 50V bias center tap so I assumed this was right. I will measure from this point to ground using AC scale and see what i get. thank you


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