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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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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Fri 23, 2019 8:22 am 
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Sorry to hear that Steve. Maybe someone on the forum has another one of these with a good transformer? I would hate to see you loose this project.

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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Fri 23, 2019 12:31 pm 
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Steve, one last thought. Is the sizzling present with no tubes in the amplifier? If so, that would probably (unfortunately) confirm your suspicion. You could take a piece os tubing, put it near the transformer and listen at the other end of the tubing. If you have a mechanic’s stethoscope, that would be easier!

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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Fri 23, 2019 2:29 pm 
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Pull the rectifier tube and see if it still sizzles. If OK, then it could be the output transformer; it would be worth a look inside. Over the years, I have seen a few of them short to ground, or winding to winding.

Don't overlook an arcing tube socket; that is not uncommon with wafer types.

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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Fri 23, 2019 8:40 pm 
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this amp uses the very thick and hard phenolic ?? ceramic-like plastic tube sockets. all look to be clean with no carbon tracks.

i disconnected all audio to the output tubes. there is nothing connected to them but HV and new filter capacitors tacked into the set. the spitter spatter is definitely in the B+ coming from the power transformer.

i've heard this gut wrenching sound a few times before and another transformer fixed it, but that isn't going to be the case this time.

although the amp is a weak 7/10 in condition, it is not worth the time and effort to locate another transformer. it isn't worth the shipping cost on a free transformer.

on top of all that, i just don't have the interest or desire to take the project to this level.

i have that Bell amplifier that is an absolute mint beauty that restired so perfectly.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Sat 24, 2019 3:28 am 
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sorry guys, i may have been unclear.

the hiss, crackle, pop, that is above an underlying frying eggs sputter is coming from the speakers.

it is an electronic noise from arcing B+ in the audio output, not a physical noise from the transformer.

it's all funny. i totally won big with the Bell amp and lost big on this thordarson.

however, i'd rather lose on a 20 dollar (spent on) amp than losing to that Bell amp for which i paid $200 and sank nother $100 or so to restore. now that would have been a nightmare with a cry towl.

i will gladly take this loss.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Sat 24, 2019 6:43 pm 
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Remove the transformer end bells and then apply B+ to see if you can see where the arcing is coming from.

If it is near the end it may be salvageable.

If not then the transformer is bad for sure.


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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Sun 25, 2019 1:43 am 
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the project is all over including the crying.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Sun 25, 2019 4:51 am 
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Totally understand.

I've gotten that way with projects before.


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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Sun 25, 2019 5:05 am 
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yes, some projects are just better off to be aborted as this is one of them.

if this were in a 9/10 condition like that big bell amp, i would proceed.

it is just not worth any further time, trouble, or even a shipping cost of a free part.

we all know that gut feeling when it is time to quit.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Sun 25, 2019 9:26 pm 
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Yes, we do, and it's a good thing that you were able to make that decision before you had invested a lot of $$$ or time into the amplifier.

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