Musicson 80+80

[chocopower]

Hi there.
I get this old stereo pa system.
Still in refurbishing state, but i found something really weird.
It uses a protection system in case you run the amp without a speaker connected.


Seems, to me, that in case of unloaded secondary,  there are a kind of relay that blow a fuse in the EL34 cathodes.
The relay is the 2 arrows you can see connected to el34 plates.
My question.
Is that arrangement safe in modern standards,  or should i remove it.


Thanks in advance.

[chocopower]

Pic.

[PRR]

> a kind of relay that blow a fuse in the EL34 cathodes.

That's a Spark Gap. If plate voltage goes high enough, it arcs to ground. (IMHO, this may cause more trouble than it cures.)

[chocopower]

Ok. I'll remove it.


Should i keep the cathode fuse?

[DummyLoad]

yes, keep cathode fuses.


--pete

[chocopower]

Thanks both!


Next, work in a readable schematic.

[chocopower]

Another question.


Main bias pot (there are 2 more balance bias pots), is locate in the AC side of the line, before the reverse diode.


Any problen here?


(I google-it without success)

[sluckey]

That's unusual but no reason to not work. Can you draw a schematic of that bias circuit?

[chocopower]

Schematic at first post

[sluckey]

That's fine. Where are the 2 more balance bias pots you mentioned?

[chocopower]

At "G" in EL34's grid leak.


Only one is draw. The other channel is identical.

[chocopower]

Is a bit confusing.
I know.
Both channels share the same power supply.
Just the screens have a separate node, the 2 voltage dividers in parallel marked as "B" and "500"

[sluckey]

Duh. I forgot this is a stereo amp.

[chocopower]

Time for this one...


Testing transformers.


One of the O.T. is shorted in the primary windings.


I have a O.T. from a another amp that could work.
Is a bit smaller, but according to schematic they worked in a very similar environment.


Both transformers are on the 4.2k  range for primary impedance


What do you think?
(Donnor amp schematic attached)

[chocopower]

The black one will be the replacement.

[chocopower]

Just in case the post get lost, i ask again. Will be a factible replacement?




Sorry for the insistence.....

[shooter]

Both transformers are on the 4.2k  range for primary impedance

[chocopower]

Ook. What i thought them.


With amps in the 400volt range, i dont use to be so undecided,  but when i see 500+ on the DMM i start to feel dumb as hell......




Thanks!