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Master volume Twin Reverb blowing fuses

Discussion in 'Amp Tech Center' started by jondanger, Sep 24, 2016.

  1. Thin69

    Thin69 Friend of Leo's

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    I looked at a photo of my original SFTR in the same area and as best I can tell it matches your photo. I wish I had your amp in front of me as it's hard to trace circuits like this. I think you are OK here.

    You've got some really high current most likely caused by a short or defective component somewhere. Have you looked real close at the pin connections on your power tubes?
     
  2. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Okay, I see what you're looking at :) The missing resistor and cap wouldn't account for your strange d.c. voltage problem:

    [​IMG]

    That white cap to the right of your finger is another of those that should be changed. It's not related to this problem.
     
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  3. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    My amp is the one in post #78. Where your arrow is, mine lacks the .01 cap. I'm pretty sure this is related to the negative feedback, and probably not causing the high current. Still, I'm curious what's going on there.

    What do you think I should be looking for on the power tube pins? I tapped with a sharpie while the amp was on looking for cold joints, but I know there's other things that could be causing issues.
     
  4. Thin69

    Thin69 Friend of Leo's

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    Oh sorry, I should have said shorts on the solder connections between adjacent pins or wires.
     
  5. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Oh, yeah I had looked for that on the front end of the process. Just double checked with a flashlight, and didn't see anything obvious like a solder bridge.

    All this help is much appreciated. I'm going to turn in soon and pick this up again tomorrow night. Maybe one of y'all smart folks will solve it in my sleep. Thanks everyone!
     
  6. Andy B

    Andy B Friend of Leo's Silver Supporter

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    In response to my previous post, first I'd clean the pot with Deoxit, &a run the wiper back & forth a few times. Then I would plug the amp in & see if it works correctly. If so I'd replace the pot.
     
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  7. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Okay, I had time to do this before work, but no dice. Same issue.

    I've been wondering if the fact that the plate voltage starts higher (~285v) and then drops to around 240v after the amp has been on for a minute or so is informative.
     
  8. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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  9. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Well here's a weird update. I went home for lunch and thought I'd crack this thing open, check the choke, a couple other things. I got a wild hair and figured I'd plug it in without the limiter, and with a 2 amp fuse instead of the 3 amp that it calls for. The fuse doesn't blow. Now I'm confused. I check the voltages. 455 on the plates, heaters are at 6.63vac. The bias circuit voltages are in range too. Now I'm really confused. Then I plug in with the limiter, and have the low voltages again. What?

    So somehow the issue is with my current limiter, and not the amp. Guess I need to build a new one.

    I've got 455v on the plates, but the plate current is only around 28ma, so a dissipation of 12 watts per tube. I know these are 6L6WGBs, so I don't want to push them too hard, but I'd like to get them up to 17 watts (65% max dissipation). So I guess I'll add that trimmer to adjust overall bias.

    Pretty weird. I'm not blowing a 2 amp slow blo, or a 3 amp fast acting fuse, so I'm going to say that the overcurrent issue is solved? I'll keep a 2 amp fuse in it while I continue to tweak the bias section just for the additional protection.

    Thanks for all the help, and I'll continue to update this thread as I work on this thing.
     
    elmoscafeo likes this.
  10. Thin69

    Thin69 Friend of Leo's

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    Weird that your high current has healed itself. Unfortunately I suspect it will probably come back.

    The limiter will lower your voltages in the amp but you had some extreme low voltages. I suspect your voltages were dropping after start up as the tubes heated up and started to conduct.

    I keep my fingers crossed for you! :)
     
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  11. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    Well, I just buttoned this thing up and played it for about 30 minutes, and it sounds like a Twin! I'm still planning on heading down to the electronics store tomorrow morning and grabbing a trimmer to add the bias adjust, but it's looking like it's on it's way to being a player.

    This reverb is l-u-s-h. A lot more spacious than my Vibrolux. I haven't played a Twin in a while, and I forgot about that. Wow.

    I've got the ticking tremolo, so I need to figure that out. I know I've read about correcting that before.
     
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  12. Thin69

    Thin69 Friend of Leo's

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    You're doing great. Glad to hear it's not blowing fuses again!

    Geofex has a page on ticking tremolo that might help. From Google search it seems it is a common problem that has been corrected by a variety of methods.

    Good Luck with that vibrato!
     
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  13. elmoscafeo

    elmoscafeo Tele-Afflicted

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    I had to duck out of the conversation for a bit due to things happening around the house, and quite frankly, I had nothing else to contribute. After you checked the PT, I was out of ideas. I'm really happy that you've got it running! The cap job wasn't all for nothing, since they probably had to be replaced anyway. Like Thin69 said, really weird that the limiter was causing that much of a discrepancy.
     
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  14. jondanger

    jondanger Poster Extraordinaire Silver Supporter

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    I played the Twin for about an hour this morning and it was sounding great. I planned to go out and grab a trimmer for the bias adjust when the store opened at 9. After an hour, one half of the tube set started redplating. I shut it down, and swapped the tubes from one side to the other, and the same tube redplated (well, started to - I didn't let it get out of control). So I blew up one of these cool JAN Phillips tubes. Bummer. At least none of the resistors on the sockets seem to be burned up.

    I did get the bias adjust installed, and it's working with two tubes in and one speaker disconnected, so there's that.

    Nobody sells matched quads locally, so I'm waiting probably until at least Wednesday before I can really put everything together.

    Looking at the geofex ticking tremolo troubleshooting page, I'm going to try the .02uf cap across the 10M resistor that goes to ground off the trem bug.

    If I can get a quad for ~$70, I'll be at $310 + a tank of gas all in for the amp, which is pretty good. It sounds so good I'm thinking about selling my Vibrolux. Looking at them side by side, the Twin isn't that much bigger. Am I crazy? But it's fun to be crazy!
     
  15. deytookerjaabs

    deytookerjaabs Friend of Leo's

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    Definitely not crazy to sell the vibrolux, but having it might come in handy down the road. You get used to that bottom and all the air moving that switching from a twin down can feel like going to a lighter gauge of strings.
     
  16. Thin69

    Thin69 Friend of Leo's

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    Bummer, sorry to hear that but at least you know it followed a tube when you moved them to the other side. When you get ready to bias the new set you'll really appreciate having both the balance pot plus bias level. When I bias mine I put the two highest current tubes on the one side and the two lowest on the other side. That way after I balance the sides I can get the closest max plate dissipation possible on all four tubes.

    Good luck with the trem fix, if it doesn't work there seems to be a lot of different fixes that have worked for people.

    At <$400 in you're doing great!

    Edited for correction.
     
    Last edited: Sep 30, 2016
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