[general disrepair]

All,

I recently purchased a pair of Model 2s, one of which has developed a short
in the power transformer from the B+ secondary to the bias winding. If I
disconnect the bias winding entirely, the amp (at idle, no tubes in) draws
no current and the B+ voltage appears spot on.

I realize I still have to do more testing, but I am trying to decide whether
to try adding a small transformer for the bias supply (neatly, no holes
drilled) or whether to have the transformer rewound, which is very costly.
However, given the value of the amplifier, it wouldn't be the end of the
world.

First things first. As long as I have the bias tap disconnected and
isolated, would adding a separate bias transformer be a safe approach? FWIW,
the transformer shows some signs of running hot. The wires are discolored
with potting compound residue, but there is no indication that anything ever
got hot enough to burn. Also, given that the B+ voltage is now appearing on
the bias winding, do I have to worry about the insulation breaking down
because of the high voltage (~400 volts)?

Assuming this is a valid approach, has anyone ever done anything like this
with good success? Am I just buying a little time, or could this be
considered a "permanent" fix?

TIA

BTW - I mistakenly posted this to alt.guitar.amps. They probably think I'm
crazy////

[Chris Hornbeck]

On Fri, 04 May 2007 01:34:51 GMT, "general disrepair"
wrote:

All,

I recently purchased a pair of Model 2s, one of which has developed a short
in the power transformer from the B+ secondary to the bias winding. If I
disconnect the bias winding entirely, the amp (at idle, no tubes in) draws
no current and the B+ voltage appears spot on.

I realize I still have to do more testing, but I am trying to decide whether
to try adding a small transformer for the bias supply (neatly, no holes
drilled) or whether to have the transformer rewound, which is very costly.
However, given the value of the amplifier, it wouldn't be the end of the
world.

Perhaps a bias voltage derived from the high voltage winding?
There are plenty of examples, and the required retrofit it
less invasive.

First things first. As long as I have the bias tap disconnected and
isolated, would adding a separate bias transformer be a safe approach? FWIW,
the transformer shows some signs of running hot. The wires are discolored
with potting compound residue, but there is no indication that anything ever
got hot enough to burn. Also, given that the B+ voltage is now appearing on
the bias winding, do I have to worry about the insulation breaking down
because of the high voltage (~400 volts)?

Assuming this is a valid approach, has anyone ever done anything like this
with good success? Am I just buying a little time, or could this be
considered a "permanent" fix?

This is an unanswerable question. We all spend our time and die.
Nothing further can be said, except that nothing is permanent.

All good fortune,

Chris Hornbeck
"If you're doing this as a volunteer, don't."
- Paul Stamler

[Phil Allison]

"general disrepair"

I recently purchased a pair of Model 2s, one of which has developed a
short
in the power transformer from the B+ secondary to the bias winding. If I
disconnect the bias winding entirely, the amp (at idle, no tubes in) draws
no current and the B+ voltage appears spot on.

I realize I still have to do more testing, but I am trying to decide
whether
to try adding a small transformer for the bias supply (neatly, no holes
drilled) or whether to have the transformer rewound, which is very costly.
However, given the value of the amplifier, it wouldn't be the end of the
world.

First things first. As long as I have the bias tap disconnected and
isolated, would adding a separate bias transformer be a safe approach?
FWIW,
the transformer shows some signs of running hot. The wires are discolored
with potting compound residue, but there is no indication that anything
ever
got hot enough to burn. Also, given that the B+ voltage is now appearing
on
the bias winding, do I have to worry about the insulation breaking down
because of the high voltage (~400 volts)?

Assuming this is a valid approach, has anyone ever done anything like this
with good success? Am I just buying a little time, or could this be
considered a "permanent" fix?


** This is the schem.

http://www.drtube.com/schematics/marantz/model2.gif

A short from the main HV secondary to the bias winding is a real worry -
it indicates three insulation layers have all failed. There are likely
already shorted turns in the bias and or HV secondary windings - or damn
soon will be.

That is why the tranny is running hot.

Bad news.




........ Phil