Sunn Sentura II - Cap PolarityQuestion

[earache]

Hello

I am fooling with a Sunn Sentura II that blows fuses. It is completely quiet when started up on the light bulb limiter; the bulb dims when power is put on but goes bright when switched off of standby.

I am trying to work my way thru the schematic while looking at the innards of the amp, as someone in the past has been working on it before me.

Per some previous posts on this forum (music-electronics-forum.com/t29751), there were some errors noted on this schematic - and corrected by fellow forum user G1.

At the bottom of this post is that schematic.

Here is the question I have for the populace: In the attached schematic, there is a 50uf X 150V capacitor that is connected to a 1N2070 diode, and shown just below the bias pot. Which end is the positive end of that cap? I know that bias caps have their + side to ground; immediately above and to the left of the cap I am questioning is another 50uf X 150V cap shown with it's "+" end to ground.


Someone has replaced both of these caps with Sprague 100uf X 100V caps. I am just wondering if the one in question is in circuit the right way around, polarity wise.

[g1]

Both those caps should have their + ends connected to ground.

[earache]

Both those caps should have their + ends connected to ground.

Thanks G1!

Is using (2) 100uf x 100v caps as opposed to (2) 50uf x 150v a bad idea, in regards to either capacitance or voltage rating?

[g1]

Increasing the capacitance up to 100uf is ok for this circuit, but it's never a good idea to decrease voltage rating. In this case you would probably get away with it, but you have a better safety margin sticking with 150V type here.

[earache]

If this amp blows fuses with all the tubes out, is it safe to say that the PT is toast?

[J M Fahey]

If this amp blows fuses with all the tubes out, is it safe to say that the PT is toast?

No.
There's other elements loading that PT downstream, you might have a shorted rectifier or large filter capacitor or OT or choke or filament string.

the bulb dims when power is put on but goes bright when switched off of standby.

It's reacting to the standby switch which is *after* the PT, which confirms my above guess.

Disconnect *all* transformer secondaries and recheck.

Even better, disconnect them pair by pair, retesting along the way; I suspect when you disconnect HV wires (always tape ends to avoid accidents) the PT will magically heal

[Enzo]

Agree^^^ If the transformer were bad, it wouldn't matter which position the standby switch was set to.

[lowell]

The rectifier tube is removed too and still blowing the fuse? Before testing the transformer I'd definitely check the filter caps first... per Enzo's suggestion. Also visibly give it a once over for any signs of burnt components or schmegma/junk/scraps shorting things.

[Enzo]

Smegma? Oh dear lord...

[nsubulysses]