Yes, I disconnected the grid leads from the main board and connected them both to the bias board.
Maybe this pic will help and hopefully another set of eyes will tell me if they see something I don't.
I don't know why it's popping up sideways in a new window, sorry about that.
Blowing fuses
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gui_tarzan
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Re: Blowing fuses
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--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
Re: Blowing fuses
Cap on the bias pot is installed backwards.
Jerry
Jerry
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martin manning
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Re: Blowing fuses
Replace that cap for sure, and remeasure the voltages on the bias balance pot's lugs. Looks like that cap is still in the circuit since it is connected to the first bias filter by the 3k3 resistor? If so, I'm surprised that it isn't dragging the bias voltage up closer to ground, and that it didn't explode or take out the 3k3.
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gui_tarzan
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Re: Blowing fuses
Sorry, I noticed that and fixed it after taking the pic. Good catch though!
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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gui_tarzan
- Posts: 607
- Joined: Thu Jan 16, 2014 3:10 am
- Location: The 26th State
Re: Blowing fuses
I'm an idiot.
It dawned on me that I probably powered it up with that cap on backwards before I reversed it so I put a different one on it today and it seems to be running now.
I hooked the new PI caps up on the main board and put the grid supplies back on their eyelets as well, now I'm getting -58v at each of the tubes AND at the grid-side connections of the caps. That one PI cap must have been the culprit, I'm getting steady voltage on every connection and it's not humming, blowing the fuse or red-plating any more. Good grief, this was an exercise in stupidity at its best! One friggin' cap screwed up the whole mess.
I know the bias is cold, I can easily swap the bias resistor back to its original value and get it down (up?) to normal but doggone, it's been running about about a half hour now and it sounds good! I'll leave it on for a while just to make sure, but I think after all you guys did to help, I believe we have another success story. I can't thank you all enough, especially Martin for hanging in there with me. Remind me to buy you a beer someday if we ever get to meet in person.
It dawned on me that I probably powered it up with that cap on backwards before I reversed it so I put a different one on it today and it seems to be running now.
I hooked the new PI caps up on the main board and put the grid supplies back on their eyelets as well, now I'm getting -58v at each of the tubes AND at the grid-side connections of the caps. That one PI cap must have been the culprit, I'm getting steady voltage on every connection and it's not humming, blowing the fuse or red-plating any more. Good grief, this was an exercise in stupidity at its best! One friggin' cap screwed up the whole mess.
I know the bias is cold, I can easily swap the bias resistor back to its original value and get it down (up?) to normal but doggone, it's been running about about a half hour now and it sounds good! I'll leave it on for a while just to make sure, but I think after all you guys did to help, I believe we have another success story. I can't thank you all enough, especially Martin for hanging in there with me. Remind me to buy you a beer someday if we ever get to meet in person.
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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martin manning
- Posts: 14308
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: Blowing fuses
It's a good thing you didn't take a fresh pic with that cap the right way around, and kudos to Jerry for spotting it. I'm sure there is at least some conductive board syndrome going on in that amp, and that was distracting.
It might be a while until our paths cross, so in the mean time I'll raise a glass from here ;^)
It might be a while until our paths cross, so in the mean time I'll raise a glass from here ;^)
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gui_tarzan
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Re: Blowing fuses
Fair enough, I will do the same when I'm not on-call next week. I'll be in Toronto all that week for work training, hopefully there will be some close-by music stores and cigar shops and maybe some live music somewhere.
I am wary of that board too and will tell the shop when I take it back. It may last a long time, it may fail again sooner than later but once I've done my part it's out of my hands until it needs work again. It's just a huge relief that it's finally working again!
I am wary of that board too and will tell the shop when I take it back. It may last a long time, it may fail again sooner than later but once I've done my part it's out of my hands until it needs work again. It's just a huge relief that it's finally working again!
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Stevem
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Re: Blowing fuses
That's why I wanted you to try cathode biasing the output stage for even 5 minutes!
Any way glad to here it's up and running, now you need to see how much life you knocked out of those output tubes with all of that fuse blowing red plating!
If they do still currently work ( pun intended!) I can see them failing early by means of a screen grid snapping off and shorting out to the heater.
At this point I would install a inline 250 to 300 ma fast blow fuse on the OT center tap on that so that the PT does not get any more stress if/ when those outputs do go south !
This fuse mod is good practice and all amps with a OT that cost more than 50 bucks should have one in place!
Any way glad to here it's up and running, now you need to see how much life you knocked out of those output tubes with all of that fuse blowing red plating!
If they do still currently work ( pun intended!) I can see them failing early by means of a screen grid snapping off and shorting out to the heater.
At this point I would install a inline 250 to 300 ma fast blow fuse on the OT center tap on that so that the PT does not get any more stress if/ when those outputs do go south !
This fuse mod is good practice and all amps with a OT that cost more than 50 bucks should have one in place!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
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- Joined: Sun Jul 06, 2008 12:43 am
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Re: Blowing fuses
Not a good idea in amps with a dedicated bias tap on the PT!! If the fuse opens the bias supply becomes the ground return!!Stevem wrote:At this point I would install a inline 250 to 300 ma fast blow fuse on the OT center tap on that so that the PT does not get any more stress if/ when those outputs do go south !
This fuse mod is good practice and all amps with a OT that cost more than 50 bucks should have one in place!
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gui_tarzan
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- Joined: Thu Jan 16, 2014 3:10 am
- Location: The 26th State
Re: Blowing fuses
Steve, I was careful to shut the amp down as soon as I saw the plates starting to glow. They should be fine. They only did that a couple of times, it wasn't like every time I tested something and the fuse blew.Stevem wrote:Any way glad to here it's up and running, now you need to see how much life you knocked out of those output tubes with all of that fuse blowing red plating!
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Blowing fuses
You mean in regards to the screen ground path Martin, but his OT is UL so if you kill the B+ on feeding the center tap the tube has only heater and bias voltage on it?
If so the a guess a B+ fuse is the way to go.
If so the a guess a B+ fuse is the way to go.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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gui_tarzan
- Posts: 607
- Joined: Thu Jan 16, 2014 3:10 am
- Location: The 26th State
Re: Blowing fuses
It's not a UL, Steve, but thanks to all of you it's running good now. 
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Blowing fuses
Oh, sorry that was the amp you where doing before this one?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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gui_tarzan
- Posts: 607
- Joined: Thu Jan 16, 2014 3:10 am
- Location: The 26th State