inrush current

[kelly0995]

hi all....., i have a scott 222c , when i power on it comes up slowly , ... my question is ,.... is it a heat activated relay ?
im currently working on a fisher intergrated and would like to add this feature if i could.
what parts do i need & where available ?


happy 4'th ........don't let your projects become the fireworks...

[atmasphere]

Does it have a 5AR4? They have a controlled warm-up for this purpose.

[kelly0995]

Originally Posted by atmasphere

Does it have a 5AR4? They have a controlled warm-up for this purpose.

no , it's a x100c ,ss diode's.........when i turn it on i can really hear&feel the power tranny buzz for a couple a seconds.

[Eli Duttman]

The Scott 222C uses a 5AR4/GZ34 for B+ rectification. The 5AR4 has a cathode sleeve, which causes the B+ rail to rise slowly. OTOH, Fisher used SS B+ rectification in many models, which I suspect is your situation. Please post the Fisher model in question so the schematic can be examined. If, as I suspect, the B+ is SS rectified, you will not be able to get the same sort of delay as you get with the Scott. However, it is possible to install a negative temperature coefficient (NTC) inrush current limiting thermistor. Some "softening" of B+ rise is an improvement over none.

[kelly0995]

Originally Posted by Eli Duttman

The Scott 222C uses a 5AR4/GZ34 for B+ rectification. The 5AR4 has a cathode sleeve, which causes the B+ rail to rise slowly. OTOH, Fisher used SS B+ rectification in many models, which I suspect is your situation. Please post the Fisher model in question so the schematic can be examined. If, as I suspect, the B+ is SS rectified, you will not be able to get the same sort of delay as you get with the Scott. However, it is possible to install a negative temperature coefficient (NTC) inrush current limiting thermistor. Some "softening" of B+ rise is an improvement over none.

thanks eli , it's a x100c ,....ss diode's.....the power trans really buzzes for a few seconds on inital power
on....................im not used to this forum , left a post a bit ago ,havent seen it yet or i hit the wrong button

[Eli Duttman]

I took a look at a X100B schematic, which hopefully is close enough. The drawing shows a red wire from the power trafo's rectifier winding connecting to the junction of 2X 200 μF. 'lytics. Break into that line and install a GE/Thermometrics CL150 inrush current limiter.

While you are under the hood, replace the OEM B+ diodes with modern, low noise, parts. UF5408s are inexpensive and will do quite nicely.

There are other tweaks that can be done to improve the amp. Do you want to get heavily involved?

BTW, you posted as I was reading and typing my post.

[kelly0995]

thanks eli ,ill do that ,...................i just replaced the diodes,........everything else looks orignal, ,,,it sounds great ,dead quiet......is it worth messing with the orignal mylar,output stage coupling caps for new polypropylene type ,.........also ,....runs hot ,(the output tubes &power trans)...........the service manuel calls for bias set @ 39 volts at pin 5 ,(7868) ,could i go down a couple volts cooler ? its a combination fixed/cathode bias......the cathodes of the outputs are connected to the filaments of the 12ax7 preamp tubes& wired in series then to ground thru resistor......if i run a little less bias , it would reduce the heater/fila.voltage a bit on the 12ax7's.