Thermistor in Fender Deluxe reissue going to Iceland?

[Charlie Wilson]

Hello,
I have a customer that is taking is Fender Deluxe Reverb reissue to Iceland and I installed a Mercury Magnetics 230v power transformer in the amp. The only component I am concerned with is the C60-11 5A varistor. I studied the service manual for the amp and it looks like the one manual is the same for USA and Europe. It lists the changes between the 240v and 120v use but it does not state any changes for the varistor. I called Fender( and I work at a Fender authorized shop) and they REALLY don't want to help me with the conversion. They would rather the customer buy a new amp and spend three times the amount. So any techs out there know if the same varistor is used for the 120v and 240v amps?
CW

[Charlie Wilson]

I may have to correct myself on this one. I think the component is a thermistor not a varistor. However, I still wonder that if it is there to limit current at start up, then maybe it should have a different rating for the 240v.
CW

[martin manning]

That part is indeed a start-up current limiting thermistor, shown on the '65 DR RI schematic as TH1 (with the notation NTC, for negative temperature coefficient). The export transformer wiring is included, and there is no thermistor or note about its specifications for 240V operation shown there or on the PCB layout. Looks to me like they are using the same part for 120 and 240V. I don't have the service manual for that amp, but look at the parts list. I'll bet you won't find two parts listed. Of course you will need to reduce the amp rating of the mains fuse, and the value for 240V is on the schematic, F1A.

[Stevem]

Yes that is there to slow the start up surge and since it will be passing less current on 240 volts ac than 120 it will just take twice as long to warm up to zero ohms.
If you wanted to add a varistor across the 240 volt ac input for surge voltage protection you would need one rated at some 400 volts.

[Charlie Wilson]

Thanks guys. Yeah Martin the service manual only shows the one part and I looked at some schematics for other Fender amps and they show the same thermistor for 120v and 240v use. I also agree that since the current draw is half with 240v I would think the amps rating of the thermistor should be half but if that is what Fender uses for export amps then my customer will not have issues for use in Iceland. I am curious though, why would a varistor across the mains need a rating as high as 400v for 240v use? I ask this because I am also converting a Mesa Boogie and I found out the hard way that the domestic varistor is rated at 130v(it shorted when I put 240v on it). I was told by Mesa Boogie that for European use it should be 275v. Sorry I know this is not the Mesa Boogie forum.
CW

[martin manning]

In the case of a varistor the rated voltage is the voltage at which it is beginning to conduct current. It's like a Zener diode with the same breakdown voltage in either direction.

[Charlie Wilson]

Thanks Martin. I have read that it conducts very quickly once it hits that voltage. It just seems to me that a 400v varistor for an expected 240 line voltage is a bit high to protect against voltage spikes.
CW

[martin manning]

Maybe a difference in the way it is spec'd, peak vs. RMS?