Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on March 17, 2019, 03:10:43 pm

Title: Musicson 80+80
Post by: chocopower on March 17, 2019, 03:10:43 pm

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.

Title: Re: Musicson 80+80
Post by: chocopower on March 17, 2019, 03:38:33 pm

Pic.

Title: Re: Musicson 80+80
Post by: PRR on March 17, 2019, 06:00:15 pm

> 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.)

Title: Re: Musicson 80+80
Post by: chocopower on March 17, 2019, 06:24:44 pm

Ok. I'll remove it.


Should i keep the cathode fuse?

Title: Re: Musicson 80+80
Post by: DummyLoad on March 17, 2019, 08:46:11 pm

yes, keep cathode fuses.


--pete

Title: Re: Musicson 80+80
Post by: chocopower on March 18, 2019, 01:38:42 am

Thanks both!


Next, work in a readable schematic.

Title: Re: Musicson 80+80
Post by: chocopower on March 20, 2019, 06:41:48 pm

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)

Title: Re: Musicson 80+80
Post by: sluckey on March 20, 2019, 07:21:56 pm

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

Title: Re: Musicson 80+80
Post by: chocopower on March 20, 2019, 07:47:48 pm

Schematic at first post

Title: Re: Musicson 80+80
Post by: sluckey on March 20, 2019, 08:03:17 pm

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

Title: Re: Musicson 80+80
Post by: chocopower on March 21, 2019, 12:19:24 am

At "G" in EL34's grid leak.


Only one is draw. The other channel is identical.

Title: Re: Musicson 80+80
Post by: chocopower on March 21, 2019, 12:24:45 am

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"

Title: Re: Musicson 80+80
Post by: sluckey on March 21, 2019, 08:21:41 am

Duh. I forgot this is a stereo amp.

Title: Re: Musicson 80+80
Post by: chocopower on January 04, 2021, 06:48:18 am

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)

Title: Re: Musicson 80+80
Post by: chocopower on January 04, 2021, 08:01:39 am

The black one will be the replacement.

Title: Re: Musicson 80+80
Post by: chocopower on January 06, 2021, 10:39:18 am

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




Sorry for the insistence..... :angel

Title: Re: Musicson 80+80
Post by: shooter on January 06, 2021, 11:22:15 am

Quote

Both transformers are on the 4.2k  range for primary impedance

:wink:

Title: Re: Musicson 80+80
Post by: chocopower on January 07, 2021, 04:08:08 am

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!