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73 SVT on/off switch replacement

Discussion in 'Amps and Cabs [BG]' started by slap5string, Mar 27, 2013.


  1. My 73 SVT is having an issue with the on/off switch. Can anyone guide me through this repair? I have replaced the standby switch about a year ago and now its time to replace the on/off switch. I'm really not sure how to remove the faceplate to get at the switch or if the whole assembly with preamp tubes and all slides out? Any guidance would be greatly appreciated. Also - here are a couple pics from Fliptops - just wondering if anyone knows which of these I should get?
     

    Attached Files:

  2. Oh...in case anyone was wondering - Here's a pic of the amp.
     

    Attached Files:

  3. lowend1

    lowend1

    Feb 15, 2005
    I won't get into the electrical aspects of replacing the switch - that is better left to one of the more qualified members here. Suffice it to say that the SVT has enough voltage lurking around inside of it to drop you like a stone. To access The switch, however, you must remove the preamp section of the head, which is on a separate chassis bolted to the top of the inside of the head box. It is also tethered to the power amp section - which is bolted to the bottom of the case - with a wiring harness or two. I'm pretty sure the power amp will have to come out first. The faceplate should not have to come off of the preamp section in order to swap out the switch. Do not try and pull or pry it off. Faceplates on V-Series Ampegs usually slide into a channel on the front of the amp, which means that all knobs, jacks and switches would have to be loosened and pushed inside the preamp section from the front. You don't want to do that unless it's absolutely necessary.
     
  4. I've had the poweramp section out already (as I stated above, I've replaced the standby switch). I was just wondering the easiest way to access the on/off switch and which of the above switches would best do the job.
     
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    You will have to remove the power amp. There are several wood screws going into the top inside of the cabinet that hold the pre-amp chassis in place. Remove them and slide the chassis out from the front. There is a top plate on the chassis that will need to be removed to gain access to the inside of the chassis and the power switch.

    I don't expect that you will find one, but the original power switch's Ampeg part number is 160804-10.

    The power consumption of your amp is rated at 570 Watts at full power. P=VI. 570W/120VAC= 4.75A. This doesn't take into account the initial surge that hits the switch when the power is first turned on and the capacitors aren't charged. For that reason the switch is rated with a higher current capacity. The image of the switch on the right that you posted looks like a Carling 110-75 SPST. It is rated at 6A@125V. I would confirm the current and voltage ratings with them before making your purchase. This switch can take more current than it is rated for so there is a bit of a safety factor. This could be the same switch (or have the same specs) that Loud uses in the SVT-VR. You could PM Hodgy and ask him for a part number of spec of the one that they use if you want to be sure. A lot of people use this switch on the SVT so you should be fine with it. It has a small sized bat which matches the size of the original.
     
  6. Thanks for the info! I will try and get ahold of Hodgy today. By any chance is this the same switch used for the polarity toggle? My initial thought was just to swap those two out since the polarity switch is never used (my cord is now a 3 prong HD cord).
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The polarity switch is a different type. I call it a changeover switch. The top schematic in the image below is what is used for the power and standby switches; the lower schematic is what is used for the polarity switch. IF the voltage and current spec are up to what is required for the power switch, the polarity switch could be used as a power switch. Look on the switch for the specs.


    [​IMG]


    Here's an image of the switches on a vintage SVT. The power switch is the lower one. The polarity switch has three wires coming out of it. Part of the problem with these switches is that there is a limited amount of space in the area of the chassis where they are mounted. You need a small sized switch.


    [​IMG]


     
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    BTW, even though you have a three conductor power cord in place, you can still have the polarity circuit in place. This is done on the SVT-VR, if you look at the back, they have a three position polarity switch. In the center position it's like no polarity switch is present. Most people take the switch out when a three conductor cord is installed. There are cases where it can help make the amp quieter.
     
  9. Thank You for all the great info! You are the GURU!!!!
     
  10. anderbass

    anderbass

    Dec 20, 2005
    Phoenix. Az.
    slap5string, have you already tried cleaning/lubing your switch with Deoxit D5?

    Might be worth a quick try...
    You'd just need to lay your unplugged amp on its back,
    spray some in around your toggle so it could flow inside to the spring & contacts,
    then work the switch back and forth like 10 or 20 times, and then repeat if necessary.
     
  11. No I have not. Good idea though - Im going to give that a shot - Fliptops.com is on vacation or something until April 1 (posted on their website) so I have until then before I need to order the switch.
     

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