"tm"
"Phil Allison"
"Cydrome Leader"
Phil Allison


** Had an early 1960s guitar amp in this week, a "Maestro Viscount"
made by
Gibson, aka the GA-16T. All the valves were new, JJ types: 2 x ECC803,
2x
6V6S and a 5Y3.

The accompanying note contained no fault report and asked only for two
modest modifications:

1. Replace and rewire speaker wire with heavier wire.

2. Rewire to shorten power feed cable from tranny.

The speaker wires were soldered at both ends and just long enough to
let the
chassis rest on a bench. The AC cable referred to was internal and went
from
the chassis to a 230V to 115 V step-down fitted in the factory 50 years
ago.
The two wire cable had been wound neatly like a rope and rested on the
bottom of the case next to the step-down.

The 240V lead was modern, 3 core and there was a wire linking the frame
of
the step-down to the chassis.

I connected the amp to my Variac and gradually powered it up to about
00V - just as I expected it let out an almighty hum, mostly 50Hz,
through
the speaker. The hum mostly disappeared if the volume pot was turned
down.

Of course bad electros were the cause of the trouble, but the situation
was
not so simple since as someone had been there before me, decades ago
and
substituted two pigtail electros for ones inside a triple electro that
was
mounted off the chassis on a clamp.

It soon became clear that the third electro in the triple had now
expired -
but that should cause only 100Hz hum, not 50Hz.

Then came the *shocker*, one of the pigtail electros was grounded to
the AC
heater supply instead of the chassis - this imposed 3.15 volts AC at
50Hz on
a DC rail ( the screen supply) that should have had only a trace of
100Hz
ripple.

Removing the dead triple electro, adding a new pigtail one and rewiring
soon
put things straight leaving only a minor amount of 100Hz buzz at full
volume.

There was one pigtail electro hanging onto the 5Y3 socket, grounded to
a lug
on the frame. Soon as I shifted the ground point to the common chassis
ground, even that hum vanished.

With the amp now working nicely, I modified the speaker and internal AC
leads as requested in the customer's note.

Apparently the customer believed the AC lead tied like a rope was
causing
the hum.

Oh dear..........

It was still an honest repair, so everyone wins. I'm sure it took less
time to swap the wires than to explain it was something else.


** Sure - I did what the note asked to avoid an argument and since doing
it caused no harm, why not ?

The main point of my post was the grossly incompetent previous repair
that I found.

Can you figure out how the amp ever worked OK afterwards ?



Sure. They got tired of the loud hum and turned it off before any damage
occured. Then someone competent repaired it.


** No way.

The previous repair was done ( almost certainly by a TV tech) in about 1980
and the amp was usable afterwards.



.... Phil