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Thread: Fender Dual Showman SR transformer issue

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    Fender Dual Showman SR transformer issue

    OK First off I feel bad enough so no comments on me please. I got a trade deal on a like new Dual Showman SR/ "The Twin" played great at location I brought it home and stupidly hurried and leaned over the head and plugged into the Ext speaker jack instead of the Main jack. Needless to say after 1 min. of play the amp started breaking up and shut completely down.xxxxx!!!!. Well turned out I burned out the thermo fuse inside the bell of transformer I am not a tech but have worked a little on Ham radio tube amps and know the dangers of high voltage caps. I replaced the fuse and the amp turned right on but the first of the 4- 6l6GC's started red plating and then showed a short on tube tester. The trans did not smell burned or show burned. My question is I am thinking of just getting one tube just to see if the rest of the amp is ok then buy a good matched set I would hate to buy a matched set and burn them up because something else is wrong due to my screwup. Can I leave all tubes out and turn on amp to check voltages out of trans or are there any other ideas out there. Some people like this amp and some don't but it is good for me so I would like to get this thing going. I just don't want to buy a set of tubes and do a hit or miss on $100 worth of tubes. Thanks for any ideas and again please no comments on me and my screwup

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    Re: Fender Dual Showman SR transformer issue

    Something has been modified, you should get no output with nothing plugged into the main jack.

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    Re: Fender Dual Showman SR transformer issue

    Thanks for the reply not sure about that I will have to see if anything looks tampered with. Do you think a shorted 6l6cg would cause that transformer thermo fuse to open because it definitely opened and power was restored after replacing the thermo fuse and the external and internal glass tubes never blew out

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    Re: Fender Dual Showman SR transformer issue

    Ok update to issue The Dual Showman Has 4 6l6CG's one of them was red plating quick with 4 tubes in it showed shorted on tester. The amp has two settings Hi and Low the manual says you can run on low if you remove the two center power tubes and change the switch on back from 8ohm to 4 ohm and use 8 ohm speaker. Well it is working good so far. I now have to wait until I get another 6l6GC and try it on Hi and see what it does. If all checks out I will get a set of new matching tubes

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    Forum Member phantomman's Avatar
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    Re: Fender Dual Showman SR transformer issue

    Fender was still building pretty sturdy stuff back then so you should be hokay. Just remember to double-check all your rear-panel switch positions before powering up.
    "When injustice becomes law then rebellion becomes duty."

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    Re: Fender Dual Showman SR transformer issue

    Ok Thanks again

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    Re: Fender Dual Showman SR transformer issue

    Tung-Sol's "STR" 6L6GC's are among the hardiest of tubes commercially available -- you might consider a matched quartet of those when you shop for output bottles. I installed a set in my ultra-linear Bassman 135 (plate voltage right at 500 VAC) several years back and thus far they're hanging in there. Plus they don't sound bad at all.
    "When injustice becomes law then rebellion becomes duty."

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    Re: Fender Dual Showman SR transformer issue

    I will check into those tubes. Does anyone think that when The first tube in line was red plating and shorted would cause the thermo fuse in the transformer to burn open

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    Re: Fender Dual Showman SR transformer issue

    Question about grid resistors I have noticed that the two empty inner number 2 and 3 of 4 sockets not being used for low power testing have burned grid resistors that go from socket pin 8 cathode to pin 4 grid they appear to be 1/4 watt 1.5k and I have 1/2 watt 1.5k on hand can I use them and if so should I change the other two 1/4 watts to keep all 4 the same ? Thanks for any help I am slowly learning some of the basics

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    Re: Fender Dual Showman SR transformer issue

    It looks like those 1.5KΩ screen-grid resistors should be tied from pin 1 to pin 5 and they should all be ˝-watt values.
    "When injustice becomes law then rebellion becomes duty."

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    Re: Fender Dual Showman SR transformer issue

    Thanks again, I am looking from underneath on solder side I guess I reading wrong but I can see how it would be 1 and 5. I will be attaching to same points that there on now. And 1/2 will be fine they are really small I was thinking they where 1/4 they do look original. If you read my original post do you think that is what burned them. As you can tell I am on a learning curve I have been messing with Ham radio amps and have some basic knowledge and do know the dangers also. I only mess with my own equipment thank goodness

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    Re: Fender Dual Showman SR transformer issue

    It's possible that screen-grid resistor failure caused the original issue. Regardless, those that look "toasty" need to be replaced. I would do all four as a prudent matter of course. You can use metal-oxide types there -- those resistors do not affect tone and modern MO resistors are more impervious to thermal extremes.
    "When injustice becomes law then rebellion becomes duty."

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    Re: Fender Dual Showman SR transformer issue

    You are a great help I am going to replace all four

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    Re: Fender Dual Showman SR transformer issue

    Glad I could help.

    "When injustice becomes law then rebellion becomes duty."

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    Re: Fender Dual Showman SR transformer issue

    That is a great message at he bottom of your reply more people need to see that

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    Re: Fender Dual Showman SR transformer issue

    Ok All here we go again I won't go into all that I have done to this amp but I did get it up and running on low power with the two of four good tubes. I am waiting on delivery of my 4 Tung-Sol STR 6l6GC's any day now and since one of my 6l6GC's was red plating probably from one of the other issues I was wondering if it would hurt anything to turn the amp on without any power tubes in it so I can check voltages going to power tube sockets before installing $120 worth of new tubes

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    Re: Fender Dual Showman SR transformer issue

    Yes you can turn on the amp with no tubes installed to check voltages.
    What happend?
    Who let the magic smoke out?

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    Re: Fender Dual Showman SR transformer issue

    Thank You new tubes should be here today

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    Re: Fender Dual Showman SR transformer issue

    Quote Originally Posted by johnnybegood View Post
    Thank You new tubes should be here today

    Which brand of tubes did you buy?

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    Re: Fender Dual Showman SR transformer issue

    Tung Sol 6l6GC STR

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    Re: Fender Dual Showman SR transformer issue

    Well installed new tubes today first on low power with two tubes and second with all fours. Tubes biased smoothly as per specs no red plating and sounds good no hum or crackling. Only issue is I built a short tester with a light bulb according to a diy video pretty simple 200 watt bulb bulb socket and outlet. Plugged in short tester and amp no light, turn on amp low power and low glow on bulb, turn on high power and bulb glows fully. Everything works fine sounds good even checked for voltage from chassis to ground on outlet no voltage. do you think the high current draw of a 100 watt amp would cause the 200 watt bulb to glow even without a short. Thanks for reading

  22. #22
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    Re: Fender Dual Showman SR transformer issue

    You are using a incandescent lamp, correct?
    What happend?
    Who let the magic smoke out?

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    Re: Fender Dual Showman SR transformer issue

    Yes I am and I might add that the bulb never lights bright like it would in a normal fixture lighting a room it just glows orange

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    Re: Fender Dual Showman SR transformer issue

    Well just went on another amp building forum and everyone says it is normal for a slight glow with a 100 watt amp plugged into it as long as it is not bright like a normal light bulb in a home . So my project is finally finished and all repaired. One good thing is I have learned a lot about tube amps here. Also for beginners there is a great book by David Hunter called "Tube amp Handbook " Thanks all for all the replies

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