Threshold SA/1 standard fuses replacement and use of Variac - diyAudio
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Old 24th November 2009, 10:52 AM   #1
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Default Threshold SA/1 standard fuses replacement and use of Variac

Blew a couple of Bussmann Series GBB Very Fast Acting GBB ceramic fuses when I repaired my Thresholds SA/1.
Can't find these fuses in Europe but a German manufacturer has ceramic alternatives only in slow blow version.
According to Thresholds specs the SA/1 needs two 20A railfuses and one 6A AC fuse with 220 Volts.
What would be the equivalent in the slow blow version?

Thse amps have no softstartup feature in them and one of the Heinemanns on/off magnetic circuitbreakers kept tripping so now and then on startup.
I inserted in every amp the biggest Inrush Current Limiting Thermistors, the MS35 10018 from Ametherm Inc. Now tripping on startup is over.

Before that I used to startup both amps with a 10A Variac with Volt- & ammometer and I have been continuiing doing so to avoid (I think) stressing the amp at startup.
When the dial is at 220 both amps take together approx. 3 to 4 amps continiously tapering of to 2,2 amps when all warmed up.

When driven hard into my Acoustats 2 + 2, the meters in to the right corner (red figures + 3dB), it never rises above 5 amps intake.

Is it wise or not to just keep on continuiing the use of this Variac to startup the amps?

My wife finds this procedure akward, so whenever she's alone I arrange to have the Variac already in the 220 Volt setting so she can switch them on us usual and the Inrush Current Limiting Thermistors come in handy now.

Last edited by RobertS61; 24th November 2009 at 10:55 AM. Reason: Misspelling
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Old 24th November 2009, 11:11 AM   #2
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if you must use slow ones , de-rate them in A at least 30% ;

leave inrush thermistors in , use variacs for something else

if needed - use thermistors with more ohms ..... some PA amps use even 100 ohms WW's in soft start
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Old 25th November 2009, 06:24 PM   #3
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Quote:
Originally Posted by Zen Mod View Post
leave inrush thermistors in , use variacs for something else
Thanks for the answer.
My friend has a Levinson HQD system with eight ML-2's and het powers them up with a 30 Amp Variac,. The ML-2's have a soft startup circuitry, a relais and a powerresistor, but he insists on using a Variac to minimize the stress at startup. I always thought it was silly but having had problems with the Thresholds that originally have no soft startup circuitry at all, I thougt why not.
I wonder what are the basic cons and pros for using a Variac every time.
I'm really curious and openminded about this issue.
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Old 27th November 2009, 01:52 PM   #4
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Wonder if you are the Brian Eno... nah, that would be too much of a coincidence.

Anyhow, I disagree with the former advice.

I would suggest that you build a circuit that consists of a large power relay, a time delay circuit (a variety of simple ways to do it), and a large power resistor.

The plan is simple enough.

When you turn on the amps, via this "box" the AC mains is run through a power resistor(s) that limit the inrush current to some resonable value. Turns out that the value of resistor needed is typically about 8 ohms, might be double for 220Vac mains, use ohms law to check.

Then after some seconds of time, controlled by the circuit, the relay pulls in shorting across the resistor, and returning your amp to a direct connection to the AC mains.

This is significantly better, imho, than keeping any sort of current limiting device in series with the power going to the amp.

Personally, I would prefer a very large variable autotransformer (Variac) to a Thermistor, if those were the only two choices.

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Old 28th November 2009, 03:03 PM   #5
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Thanks bear,

All my Rowland, Krell and Levinson amps had these provisions you describe. Threshold opposed to these circuits because the contacts of the relays became contaminated. If you examine the circuitboards of the later E series Thresholds (1988 and onwards, SA/12e etc..) on top of the capacitorbank you see 4 thermistors (believe CL-60's) to do that job of inrush current limiting. The Aleph series used them also, do not know about the current series of Passlabs.
Can anybody explain if there are any argumented objections against the use of Thermistors and/or Variacs for inrush current limiting purposes?

My Megasurge thermistors drops immediately from 8 Ohms to negligible values even when I use a 1.5 Volts battery.
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Old 28th November 2009, 03:21 PM   #6
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if thermistors are good enough for Papa , they're good enough for me .
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