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 Post subject: Thordarson T-31W25A Amplifier, Sad Ending
PostPosted: Aug Wed 07, 2019 3:10 am 
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well, just after i thought i was all done with that big Bell amplifier ; as i was in the shop and was packing things up, look what found me.

it is an early amp, a Thordarson T-31W25A amplifier.

a friend found it for me and gave me a call. i could not pass it up for $20.

for being 75 years old, it is in average to good shape. it has some marks, but it will be certainly displayable in the new room.

this thing is all stock, all original, and untouched. it uses a 5X4g, two 6L6g, three 6SJ7, and one 6N7. it is nearly a clone of lesser parts to that big bell that i completed this summer. however, this amp looks to be more primitive, simple, and much less populated.

i did not fire it up yet and i did not even do any preliminary checks, but i plan to get things going on it in September. it will be very simple and most likely just a day's restoration since it is simple and has much less parts.

it has some big 600 volt capacitors, a cardboard box capacitor, and some big adjustable tapped resistors.

before i begin the restoration in a few weeks, i have some questions/comments.

1. 5X4g--i don't want to rewire the socket for a 5U4. i don't want to move the nicely "stringed tied" wire bundles to reroute the wires around the socket. also, i just don't want to stress the old cloth and fiber insulation on the transformer wires. along with all that, i want to keep the design original.

2. I'm sure the plate voltages and B+ will hit a few volts over 500, so the first two filters will be great candidates for a big 10 mfd film capacitor. i have lots of room under the chassis.

3. those tapped resistors--shall i loosen the tap, wiggle it, and perhaps remove any oxidation that the tap may have on the wire wound element ? or shall i just leave them alone ? are they problematic making connection from the dimple in the metal band to the wire wound element on the ceramic ? again, should i just leave them alone ? I've encountered these resistors times before and never bothered them...

4. i don't like the ebay prices on the 5X4g, then again, 5U4 tubes are really no different. perhaps i can find some for cheaper here on the forum if someone has extras.

5. is the pin out safe on the 5X4g, as in are pins 7 and 8 too close for any flashover ? (yes, i know the engineers designed it to be ok..), but 75 years later, is there a good reason to rewire the 5X4g socket to accept the 5U4 ?

6. it is transformer coupled from the 6N7 to the 6L6g tubes.

so, this is it as seen below. i say it is a low 7/10 for condition, but for its age, i can deal with it. under the hood, it is completely untouched.

7. any 6N7 that I've had use as the phase inverter of an amplifier was weak. is the 6N7 one of those notorious tubes that are "always weak" in a barn find ? they are for me..

i do like the T for Thordarson on the chicken head knobs.

steve :(


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Last edited by Dutch Rabbit on Aug Fri 23, 2019 3:03 am, edited 2 times in total.
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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Wed 07, 2019 7:09 am 
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The Thordarson amplifier catalog....this brochure may have originally been on the sales counter of Terminal Radio Co.?
It's undated, but I've seen this particular T'dsn design as early as 1940. The late 30's versions used a similar color scheme and layout, but generally the corners are more rounded on the cabinets.


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Last edited by ampstamp88 on Aug Wed 07, 2019 6:13 pm, edited 1 time in total.
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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Wed 07, 2019 1:45 pm 
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Dutch Rabbit wrote:
well, just after i thought i was all done with that big Bell amplifier resistors.

1. 5X4g--i don't want to rewire the socket for a 5U4. i don't want to move the nicely "stringed tied" wire bundles to reroute the wires around the socket. also, i just don't want to stress the old cloth and fiber insulation on the transformer wires. along with all that, i want to keep the design original.
...
steve


Steve,

You might want to call these guys to see if they have a 5X4G in stock. $10.00 and should be NOS. I have no affiliation with them other than being very satisfied with what I have purchased over the years (good price, good tubes fast shipping)

http://www.esrcvacuumtubes.com/vacuumtu ... -5Z4G.html

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Wed 07, 2019 3:39 pm 
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The 6N7 should also be readily available. Nice find; definitely 1940s vintage.

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Wed 07, 2019 4:26 pm 
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Tim Tress wrote:
The 6N7 should also be readily available. Nice find; definitely 1940s vintage.

Steve, if you are looking for a 6N7G, the same people listed above have them for $14. Metal and GT are cheaper.

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Wed 07, 2019 6:04 pm 
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I have lots of 6N7 in various types for a lot less if Steve needs another 1 or 2 tubes, good to know about a reasonable 5X4 source however!...as those tubes are much more uncommon to come across...I believe they were only manufactured for a short time?
-Kent


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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Thu 08, 2019 4:03 am 
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thanks guys. i am still wondering if i should rewire the socket for 5U4, of which i have plenty.

is there any reason NOT to do so other than authenticity ?

i don't want to pay 30-50 bucks for a handful of 5X4 tubes when i have a pile of the 5U4.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Thu 08, 2019 1:50 pm 
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5X4 is a 5U4 (or 5Z3 - same) with base connections rotated. I suspect it was used to avoid buying from RCA, same as companies avoiding metal tubes. They're uncommon since they weren't used that much - 5Y4 with same base connections seems more common, I've seen those in GT so demand must have lasted longer...

Does the socket have all the pins for the 5X4? 5U4 sockets commonly omitted unused pins.

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Thu 08, 2019 8:14 pm 
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hi tom,

all pins are present on this socket, so if i do decide to make the change, all will be there.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Fri 09, 2019 3:56 am 
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what about those tapped dog bone resistors ?

is there any reason to loosen them to check for any corrosion between the dimple on the tap and the main wire element ?

I don't want to move them if I don't have to do so and risk breaking the main wire element.

anyone have any problematic experience with resistors of this type that I should know about ?

I have seen them before in many amps, but never touched or gave them a second thought.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Fri 09, 2019 1:33 pm 
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Steve, don't be in a hurry, check out the link above for ESRC tubes in Florida. They list the tubes that you are looking for. I have called them in the past and they responded to my question. Kinda unique to have that "different" rectifier. You could make it "All Original".

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Fri 09, 2019 8:37 pm 
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yes, i've been tossing the tube change issue over and over, but if i were to change it, the set won't be original.

i don't like change.

i kind of like having that oddball 5X4g in there.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Fri 09, 2019 8:46 pm 
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Dutch Rabbit wrote:
yes, i've been tossing the tube change issue over and over, but if i were to change it, the set won't be original.

i don't like change.

i kind of like having that oddball 5X4g in there.

steve

Go for it. For that matter, here is an offer to you. I will buy the "G" tubes and send them to you, gratis! How can you loose! You just gotta wait for about a week (going outta town) and I can order/send them to you. Sound good?

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Sat 10, 2019 3:39 am 
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oh heavens don, thanks much, but please don't.

i have not even powered the set up yet w/o the rectifier to see if both HV legs match in VAC.

that is quite the generous and the most kind offer, but you don't have to do it since i have nothing to offer back at this point.

again, thanks so very honestly much for that kind and unselfish offer...but hold on, i'm not even close to plugging it in yet...LOL.

we have big baseball homestands down in altoona. i've been basically living down there since the last week in july.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier Restoration--Some Question
PostPosted: Aug Sun 11, 2019 3:53 am 
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R43 and R44 are paralleled in the amplifier just as they are in the schematic.

doing the math quickly in my head, did they do this to achieve a 680 ohm resistor, to double the wattage., or both ?

if one or both of those thought processes are true, why did they not just use a single 680 at 1-2 watt resistor ?

I understand they coupled the cathodes together on the 6N7 and that would pull some wattage, but I don't understand the thought process on paralleling a 1000 and a 2200 ohm resistor to get the resistance and the wattage when a single 680 at 1-5 watts would have done the job.

as seen in my pics above, there was no shortage on using some big wattage resistors in this amplifier.

:arrow: :!: a big shout out and many thanks to ampstamp and tom bavis for the schematics and other interesting information on this amplifier.

it was received in the emails.

thanks guys,

steve


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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Sun 11, 2019 4:31 am 
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another observation: the cathodes of the 6L6g tubes go directly to ground.

this is an interesting setup.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Sun 11, 2019 12:17 pm 
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Dutch Rabbit wrote:
another observation: the cathodes of the 6L6g tubes go directly to ground.

this is an interesting setup.

steve

Heck Steve, 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.

Scott used the Westinghouse designed 7591 tube in push-pull in several of its amplifiers. Lots of watts from a very small tube.

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Sun 11, 2019 2:32 pm 
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thanks, that is an idea.

steve

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Sun 11, 2019 3:43 pm 
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This uses "back bias". All the current in the amp (not just the output tubes) is flowing at the transformer center tap. Putting a resistor there gives a negative voltage that is connected to the 6L6 grid resistors. It's still pretty much cathode bias, just rearranged.

A further refinement would be TWO 10 ohm resistors, one per tube. That would let you check cathode current and select matching tubes.

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 Post subject: Re: Thordarson T-31W25A Amplifier PreRestoration Questions
PostPosted: Aug Sun 11, 2019 4:31 pm 
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Tom Bavis wrote:
This uses "back bias". ...
A further refinement would be TWO 10 ohm resistors, one per tube. That would let you check cathode current and select matching tubes.


Absolutely. I have a Sansui 1000A which uses four (stereo) 7591 output tubes which were cathode grounded. I placed a 10Ω resistor from each cathode to ground. Makes the math easy to calculate each tubes idle current.

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