'54 Bassman 5d6 Question

[abav]

Hi everyone. This unit just came in today with a slow increasing hum as it warmed up. Found a leaky astron .1 slowly pulling down the bias on one 6l6.Now its stable. the bias voltage seems low to me -30v. at 392v . The plates start glowing slightly red after a couple of minutes. This amp is all original and in perfect shape except for the power supply caps. They used a selenium diode for the bias supply, and is testing very high resistance in both directions. Am i correct in assuming the bias diode is faulty?? The owner is an elderly gentleman and wants to keep it as original as possible. BTW, the 100/25 cap in the bias circuit was also replaced.

Any help or info would be greatly appreciated.

Thank you for your time.

Art

[Enzo]

25v cap in a 30v circuit?

[MarkusBass]

If you look at the schematic, the cap is marked as 100uF/25V.

If you look at the layout, it is also marked as 100uF/25V.

But there is also -42V voltage on the cap (if I can read it correctly). So I would say that there is a mistake on the schematic and the voltage rating of the cap should be at least 50V, the bias voltage should be at least -40V. If you have only -30V, this is the reason why the valves start glowing red. I don't know whether the selenium diode is OK but you can simply replace it with 1N4007 (for a moment) and see if it helps.
Please note that the diode on the schematic is also marked incorrectly - it is reversed. It seems that it was the first amp in mass production and the guys from Fender were still learning at that time - they were making mistakes.

Mark

[txstrat]

Maby the draftsman got it wrong and it ought to be 25/100 for that cap. At that time 25uF was a common value for bias circuit caps. By the way, now that I've had a look at it, there are more Fender schematics which have those low voltage ratings for the bias caps. What the heck...?

[Enzo]

WOuldn't be the first time I saw a Fender amp with a 50v cap and -55v on the same schematic, and the part in the amp was indeed the 50v cap. If I had to take a guess, Leo decided that 50v caps had a 63v surge rating, so they'd work in the application, and he wouldn;t have to spend the extra $$$ on higher voltage parts.


Caps don't automatically self-destruct a volt over their specs, but they do suffere shorter life span when you run them too high. And an amp running way over voltage for 50 years before we come to it is a testament that the philosophy can work.

[MarkusBass]

Mayby the draftsman got it wrong and it ought to be 25/100 for that cap. At that time 25uF was a common value for bias circuit caps. By the way, now that I've had a look at it, there are more Fender schematics which have those low voltage ratings for the bias caps. What the heck...?

I agree with TXStrat. Bassman AA864 had 25uF/50V and -43V bias voltage. This is a mistake both on the schematic and on the layout. Make sure that your capacitor is rated at least 50V.
I was working lately on Dual Showman and there were many 25uF capacitors in it.

Mark