ST70 power up "kathump".

I get a loud, physical chassis "kathump" when powering up my VTA modified ST70.  I trigger the amp through a Xantech AC1 attached to my Parasound P5 preamp. The surge occurs intermittently, sometimes I get a totally quiet power up while other times I get the "kathump".

I have had to replace the quad cap and the rectifier tube (each less than 3 years old), which I am assuming were damaged by this repeated startup surge.  Is there a remedy to prevent this power up problem?

breading wrote:I get a loud, physical chassis "kathump" when powering up my VTA modified ST70.  I trigger the amp through a Xantech AC1 attached to my Parasound P5 preamp. The surge occurs intermittently, sometimes I get a totally quiet power up while other times I get the "kathump".

I have had to replace the quad cap and the rectifier tube (each less than 3 years old), which I am assuming were damaged by this repeated startup surge.  Is there a remedy to prevent this power up problem?

The moment you power up will be random in correlation to the AC voltage. If you happen to
close the switch when AC is at the maximum it will be a huge current draw.
If you want to avoud this you have 2 routes to follow :
1- install a NTC resistor in series with the main fuse. This will reduce peak current
2- install a SSR ( solid state relay) to supplement th emechanical power breaker. When
engaged these deviced will start conducting at the zero-crossing. This has the
additional benefit that a small DC ( 12V is ok) taken from the preamp may be used
to remote control the poweramp.

breading wrote:I get a loud, physical chassis "kathump" when powering up my VTA modified ST70.  I trigger the amp through a Xantech AC1 attached to my Parasound P5 preamp. The surge occurs intermittently, sometimes I get a totally quiet power up while other times I get the "kathump".

I have had to replace the quad cap and the rectifier tube (each less than 3 years old), which I am assuming were damaged by this repeated startup surge.  Is there a remedy to prevent this power up problem?

OK - first this is normal, and as you suggest a physical manifestation. Think of the power transformer as a massive electromagnet that comes on suddenly. All the internal laminates then 'clatter' as a result, and the magnet wants to grab anything magnetic nearby. As has been suggested elsewhere, the point on the cycle that power is applied may have an effect. But, more likely it is temperature related. When cold things are at their loosest and the thump will be loudest. When hot, clearances are less, the thump will be less.

Things you can do:
a) Tighten the bolts holding the transformer together. On my two very vintage 70s and 35 of similar age, I have done this and used a bit of Loctite on the nuts to prevent backing off - the vibrations are subtle, but constant.
b) Isolate the transformer using rubber or silicon grommets.

This will have nothing to do with the longevity of your tubes and/or capacitors. For that, look to your standing line-voltage. If it is excessive, that will be the cause. Bob uses 122 V as the highest safe/tolerable voltage as I remember on his very-well designed and built transformers. Ideally, somewhere around 115 - 117 V would be where I would be comfortable for my vintage units for tube life anyway.

This pretty much applies to all transformer fed equipment to one degree or another. Some of my more powerful SS power-amps do the same thing to a greater or lesser degree, but they are not switching 450 V.