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 Post subject: 1947 Thordarson 20A26 Compressor Amplifier Circuit
PostPosted: Nov Thu 14, 2019 6:49 am 
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Joined: Apr Sat 04, 2015 6:15 am
Posts: 1645
Location: Dos Rios, CA. region
Here is a project that has taken some time to put together...
I originally received the T'dsen control transformer 20A26 (with instructions, schematic) a couple of years ago from fellow Nor Cal ARFer Norman B.
The project to put together this amp for my own use was a bit overwhelming at first, as I've not ever put anything together from scratch before.
For almost the past year I've studied many ancient compressor/expander circuits ...contemplating building, acquiring parts...
then realizing after learning a bunch, that this circuit doesn't seem as difficult to troubleshoot as the 6L7/ 6386 (expensive) and 6BC8 type circuits,
so here I am back at square one.


Again, this has become one of my "hybrid" repair projects, as the only similar T'dsen/Meissner "products or kits" from this era using 6B4G's in P/P are a expander only T-31W11 and the T-31W10AX, both of which call for different transformers and ratings than the ones used in this parts list.
This time I used a Scott Radio Labs 800B chassis for base of operations, a little speckled but not too bad after scrubbing with Brasso.
A potted A.D.C. power transformer with similar ratings as the inobtanium T'dsen 22R32, although I'm adding a Chicago potted filament transformer
to make up for an extra 6.3 volt winding not present in the ADC.
The 20C52 filter choke will mount underneath below ADC in it's own potted Scott IF can and the ouput transformer is pulled
from a Scott 16A Metropolitan ( this matches another inobtanium T'dsen 22S70).
Fortunately I've saved all those fancy dancy 5692's, 5693's and not sold them yet , whew!
Any suggestions ya'll might have to "mock up" pictured here, feel free to post,
I'll be posting the schematic soon.

PHOTO & PARTS UPDATE: 6/4/20-

I did away with the potted power transformer and Chicago potted 6.3v filament, as I finally found the original transformer used in the circuit papers,
the ubiqitous 22R32. I also put the OPT transformer (pulled from a otherwise intact Scott metropolitan 16A amp) back where I found it and replaced it with a T'dsen output (same specs, more impedance selection) kindly sold to me as parts (With faceplate and other parts) from Steve the presently masked Dutch rabbit off a 31W25 amplifier he parted out.
I'm getting ready to drill holes in the 800B chassis now (hence the odd photo), so trying to figure out final placement of major parts for least amount of noise or hum. Any suggestions welcomed...
The 20C52 filter choke will be mounted underneath the power transformer (shown in photo on top), facing farthest away from the "dangerously sensitive " 20A26 3 to 1 control transformer in front right corner. The input 1A70 will be mounted in place of the electrolytic can it's sitting on, hence my desire (in WTB) for another red 5693 to match the other, having the unbalanced line as short as possible also seems a good idea.
Attachment:
T'dsen 20A26 compressor amplifier 6 4 20 5.JPG
T'dsen 20A26 compressor amplifier 6 4 20 5.JPG [ 924.14 KiB | Viewed 375 times ]


Attachments:
Thordarson Compressor Expander 11 12 19 9.JPG
Thordarson Compressor Expander 11 12 19 9.JPG [ 919.12 KiB | Viewed 743 times ]
Thordarson 20A26 11.jpg
Thordarson 20A26 11.jpg [ 625.22 KiB | Viewed 375 times ]
Thordarson 20A26 5.jpg
Thordarson 20A26 5.jpg [ 558.53 KiB | Viewed 375 times ]
Thordarson 20A26 6.jpg
Thordarson 20A26 6.jpg [ 532.39 KiB | Viewed 375 times ]
Thordarson 20A26 12.jpg
Thordarson 20A26 12.jpg [ 443.14 KiB | Viewed 375 times ]
T'dsen 20A26 1947 Transformer catalog page 6 4 20.jpg
T'dsen 20A26 1947 Transformer catalog page 6 4 20.jpg [ 694.29 KiB | Viewed 375 times ]


Last edited by ampstamp88 on Jun Fri 05, 2020 6:51 am, edited 1 time in total.
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 Post subject: Re: 1947 Thordarson 20A26 Compressor Amplifier Circuit
PostPosted: Jun Fri 05, 2020 6:44 am 
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Joined: Apr Sat 04, 2015 6:15 am
Posts: 1645
Location: Dos Rios, CA. region
Here is the instruction manual for the Thordarson 20A26 control transformer with additional possible circuits using the unit. I posted earlier the schematic and parts list from this...


Attachments:
Thordarson 20A26 1.jpg
Thordarson 20A26 1.jpg [ 686.82 KiB | Viewed 375 times ]
Thordarson 20A26 2.jpg
Thordarson 20A26 2.jpg [ 707.67 KiB | Viewed 375 times ]
Thordarson 20A26 3.jpg
Thordarson 20A26 3.jpg [ 688.27 KiB | Viewed 375 times ]
Thordarson 20A26 4.jpg
Thordarson 20A26 4.jpg [ 677.73 KiB | Viewed 375 times ]
Thordarson 20A26 7.jpg
Thordarson 20A26 7.jpg [ 611.48 KiB | Viewed 375 times ]
Thordarson 20A26 8.jpg
Thordarson 20A26 8.jpg [ 523.84 KiB | Viewed 375 times ]
Thordarson 20A26 9.jpg
Thordarson 20A26 9.jpg [ 550.68 KiB | Viewed 375 times ]
Thordarson 20A26 10.jpg
Thordarson 20A26 10.jpg [ 523.78 KiB | Viewed 375 times ]
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 Post subject: Re: 1947 Thordarson 20A26 Compressor Amplifier Circuit
PostPosted: Jun Fri 19, 2020 5:53 pm 
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Joined: Apr Sun 23, 2017 11:22 pm
Posts: 850
Location: 44035 (Near Cleveland Ohio)
You certainly have gone to a lot of work and expense on this project. I've gotta ask - - - what is it's intended use once completed?

Dan


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 Post subject: Re: 1947 Thordarson 20A26 Compressor Amplifier Circuit
PostPosted: Jun Fri 19, 2020 6:54 pm 
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Joined: Apr Sat 04, 2015 6:15 am
Posts: 1645
Location: Dos Rios, CA. region
Thanks Dan for your question,
I've liked the sound of that T'dsen 1A70 input transformer for multi track recording in my restored Bogen LLO preamp (1947) for some time now,
so have wanted to set up another system/channel for recording original music direct to ADAT 16 track or 8 track ½" tape.
I plan to build the T'dsen circuit as a preamp/compressor/expander first (without the p/p 6B4 for now, add later perhaps) while "working out the kinks"
in the recording studio as a "plug-in" or "add on" to certain tracks individually.
Down the road I may add the 6B4 p/p output back to the 20A26 circuit , as I've since acquired another mono 6B4 triode amp I'm restoring with similar transformers and can possibly use both as a dual mono monitor system in my studio...
Then perhaps let go of some of my "name brand" vintage Hi-Fi amplifiers to recoup some of my costs invested.
Through the assistance of some fine folks on A.R.F. here due to this post, I've further improved my education on
vintage sound equipment as well.
I'll post some updated photos soon.


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