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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Mon 12, 2019 3:36 am 
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i usually do that with precision 1 ohm 3 watt resistors and use the cathode resistor method and maths to calculate bias, idle current, and static dissapation.

:arrow: does anyone have an idea of the reasoning behind the two paralleled resistors in my post with pics on page 1 ?

also, if the tube goes south, will the 10 ohm resistor protect it or should i get a fused 10 ohm resistor (if they make such a thing) ?

yes, back biased.

steve


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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Mon 12, 2019 3:42 am 
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Nice find!

Dutch Rabbit wrote:
4. i don't like the ebay prices on the 5X4g
Steve, why are you buying them on E-Bay, instead of from Bob Dobush at findatube.com or other good suppliers?

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Mon 12, 2019 3:54 am 
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ebay was just a quick look ; i am just working up to the restoration that i plan to commence after labor day.

i plan to stock up on tubes after the restoration is done.

idk the status of this piece as i have not even powered it up yet w/o the rectifier to check for balanced HV on each leg of the power transformer.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Wed 14, 2019 2:06 am 
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I pulled the rectifier and brought it up to 117 vac on the variac.

I have balanced legs of HV and the transformer does not even feel like it was on after being powered up for an hour.

the interstage transformer that couples the 6N7 to the pair of 6L6 tubes measures out just fine and so does the output transformer.

I ordered some 6 position terminal strips that have a mounting hole on each end that will hold the big 10 uf film capacitors securely.

as of this information, I will proceed so far with the restoration for September.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Wed 14, 2019 2:22 am 
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Don Cavey wrote:
quite a few HH Scott HiFi amplifiers have the cathodes of the output tubes going directly to ground. For that matter, many HiFi manufactures did that. Often, it is suggested to lift that connection and put a ¼-watt 10Ω resistor in that location (especially if the amplifier has lots of watts). That way, should the output tube go south (short), the resistor will act as a fuse and protect the output transformer. The resistor can also be used to monitor/set the bias on the tube as well.


could I use a 1 ohm precision 1/4 watt instead of a 10 ohm one or will that not be a fast/good enough fuse ?

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Thu 15, 2019 3:52 am 
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Here's my Thordarson 31W25-A with grey knobs, flanked by a Bruno VS Ribbon Mic, a DuKane (Austrian made) rotary phone and a Triumph 420 tube tester.


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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Thu 15, 2019 8:37 pm 
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that is excellent.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Fri 23, 2019 12:03 am 
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well folks, i believe the project is over.

after tacking in all new filters and playing it for a while, there is that infamous hiss, crackle, pop, frying eggs sound of B+ arcing.

i pulled the 6N7 and it is still there.

i have heard this dispicable sound before a few times and it just makes me ill. i would say there is arcing in the HV winding of the power transformer rather than the output transformer.

so, that is a wrap. the condition of the amp isn't worth the cost to even ship a used transformer.

project aborted

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Fri 23, 2019 2:39 am 
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Steve, so sorry to hear that. I lost a fine Fisher amplifier to a fried output transformer.

-David


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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Fri 23, 2019 2:54 am 
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yep, thankfully, i only have $20 into it for the purchase price.

...can't win 'em all.

i knew bad luck would be knocking on my door after finding that fantastic and mint Bell which restored sooo perfectly.

viewtopic.php?f=1&t=352204

steve

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