• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Priboi (Surf :d) output transformer possible problem

So I just finished my second Priboi the 75watt per channel version , and it had one of it's two output transformers shorted primary to secondary due to old enamel and corrosion of the wire.
Now I rewound the transformer which consist of two identical bobbins , one of them was bad the other one was ok.
S I wind new wire on it and had to determine the secondary output windings , the starts of the windings etc, I think I did good and done everything correctly although when powering up there is a problem , the other channel having the good transformer sounds ok. the one which has the repaired transformer is extremely silent like almost no power as compared to the signal input level and the healthy channel and + to that small power it distorts heavily especially lower frequencies.

Now I wonder what could it be , could I have really missed something in the secondary windings since the primary anode winding is easy , or is it a bad solder joint or something else ?
I swapped the tubes between channels so they are ok nothing changed.I did a quick overview and found no problems , the bias current is set about right and all other things seem ok.
What do you think what would be an indication of a malfunctioning output transformer ?
I ask this because before I take apart the transformer again I jsut want to make definitely sure its nothing else because the transformer is quite a task to do work on.
 

Koonw

Member
2013-04-09 9:37 pm
So I just finished my second Priboi the 75watt per channel version , and it had one of it's two output transformers shorted primary to secondary due to old enamel and corrosion of the wire.
Now I rewound the transformer which consist of two identical bobbins , one of them was bad the other one was ok.
S I wind new wire on it and had to determine the secondary output windings , the starts of the windings etc, I think I did good and done everything correctly although when powering up there is a problem , the other channel having the good transformer sounds ok. the one which has the repaired transformer is extremely silent like almost no power as compared to the signal input level and the healthy channel and + to that small power it distorts heavily especially lower frequencies.

Now I wonder what could it be , could I have really missed something in the secondary windings since the primary anode winding is easy , or is it a bad solder joint or something else ?
I swapped the tubes between channels so they are ok nothing changed.I did a quick overview and found no problems , the bias current is set about right and all other things seem ok.
What do you think what would be an indication of a malfunctioning output transformer ?
I ask this because before I take apart the transformer again I jsut want to make definitely sure its nothing else because the transformer is quite a task to do work on.

Is it hot to touch? Since you have wound correctly and still no good response at bottom end shows it still lacks inductance. This could be due the shorting of lamination insulation, you need to check where that is. I have came across re-winded OP like that being burnt again with just a short of several lamination by some metal scrape, but at least it still sounds, maybe you have a lot shorted??
 

Sorento

Member
2008-03-12 9:59 pm
Now I rewound the transformer which consist of two identical bobbins , one of them was bad the other one was ok.

so you rewound the bad bobbin, but left the good one untouched, right ...
and there are two identical bobbins, both with half of primary and half of secondary, right ...
could it be that you wound the wire the other way round as compared to the original, so one is now wound clockwise, the other counter-clockwise ?
... or assembled the new bobbin reversed when putting it back on ?

! this would mean that ac signals cancel out !

if that's the case, you have to swap both, primary and secondary wires of the newly wound bobbin before you connect it with the old one, except for the B+ tap on primary and 4ohm tap on secondary (4ohm tap is a problem because it is not middle of winding and should be left unconnected on both bobbins - unusable)