General Audio Research - REDD47 build thread

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HyprDrivr

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Hello Friends!

I am building a pair of these and could not find a build thread, so here it is.
Feel free to share your experiences, progress, questions, finished units and all the usual stuff here.

You can find the PCBs and build docs here:

I have no affiliation with General Audio Research, just trying to add to the community.

image-1.jpg
 

HyprDrivr

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I have just finished stuffing the boards.

I finally decided to use Vishay/Dale MetalFilm resistors going against the BOM recomendation of Carbon. It was just very difficult to find the correct values. I know Sammy from GAR uses Amtrans AMRG but those are unironically 100 times more expensive than standard resistors, and i have trouble finding them in the EU. 2W resistors is a mix of what i could find.

MKP caps is Vishay with the exception of 0,022uf where i could only find a fitting Nichicon. 0,47uf cap is oversized for the board due to faulty information when I ordered the parts. Cornell Dubilier MICA caps and Würth electrolythics because they are pretty :)

Currently i am looking at transfomer options as the recomended ones dont work for me for different reasons. Sowter is too expensive and have a long waiting period, The Jensen seems out of stock everywhere and difficult to get in the EU and the Vigotronics just seem a bit cheap to me and i dont like the mounting options. Leaning towards the UTM at the moment.
I also found seven vintage EF86 tubes (Telefunken, Philip and Osram) in the hoard so i am looking forward to giving those a go.

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HyprDrivr

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Cool that you start this.
I’ve had the PCBs thousands of times in my basket but never buy them.
Looking forward for your build.
What about Sowter transformers ?
Sowters would be my first choice, but they would set me back something like 650€ as I would have to import them from the UK that are no longer in the EU. I but too much for me right now.
 

Ricardus

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I want to build a pair of these also.

I did a tube build with Sowters 2 years ago, but their wait times are insane right now, so I am always looking for alternatives, also. If OEP makes something that would work, I'm totally fine with that.
 

HyprDrivr

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I want to build a pair of these also.

I did a tube build with Sowters 2 years ago, but their wait times are insane right now, so I am always looking for alternatives, also. If OEP makes something that would work, I'm totally fine with that.
As far as i can see the recommended outputTX Vigortronix VTX-101-003 has the same specs as OEP A262A3E

As input i have been looking at OEP X187B, looks fairly similar to

However, this would result in different winding ratio on the In and Out transformers.
I have no knowledge if this is ok or what effect there would be. I am trying to find out so if anyone cares to inform me this i would be happy for the info.
 

rock soderstrom

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These guys from Poland got a nice output transformer for the REDD47. I will test them with my next project.

 

rock soderstrom

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As far as i can see the recommended outputTX Vigortronix VTX-101-003 has the same specs as OEP A262A3E
The A262A3E is a not really the best choice for an output iron. In my book this is a mic to grid/gate transformer. Can it handle the high levels in this position??
 

HyprDrivr

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These guys from Poland got a nice output transformer for the REDD47. I will test them with my next project.

This and the UTM2555 is the hottest contenders for me at the moment. A pair of each is 150€ shipped in the EU, even with the insanely expensive FedEx shipment.
 

rock soderstrom

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This and the UTM2555 is the hottest contenders for me at the moment. A pair of each is 150€ shipped in the EU, even with the insanely expensive FedEx shipment.
The UTM2555 looks interesting, but no information if this guy is shielded or not?

 

HyprDrivr

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The A262A3E is a not really the best choice for an output iron. In my book this is a mic to grid/gate transformer. Can it handle the high levels in this position??
I cant say, but it seems to have the exact same specs as the recommended Vigortronix VTX-101-003.
But i might be missing something
The UTM2555 looks interesting, but no information if this guy is shielded or not?

@igs Perhaps you can let us know?
I assume its just as shielded as the similar looking Carnhills
 
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dmp

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You could also look at Edcor XSM for the output ($26). I used these in a LA2A output and was happy with the sound. The GAR data sheet says 10:1 but it really can be 8:1 or even 7:1 (like the sowter)
Input is going to dominate for the sound quality. You want it shielded 1:10.

The power transformer spec in the build doc is a little strange. 200 VAC but the B+ is 280V? The peak voltage of 200 Vrms is 281.
Antek makes affordable toroids that I have used several builds.
 

HyprDrivr

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The power transformer spec in the build doc is a little strange. 200 VAC but the B+ is 280V? The peak voltage of 200 Vrms is 281.
Antek makes affordable toroids that I have used several builds.

Yes, i had the same thought. I have a R-Core transformer with 270v and 6.3v secondaries on order. I belive i could make that work.
 

ruffrecords

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I cant say, but it seems to have the exact same specs as the recommended Vigortronix VTX-101-003.
But i might be missing something
Vigortronix was set up by ex employees of OEP and they make many very similar products. Neither the Vigortonix recommended or OEP equivalkent output transformers are really man enough for this job. They are both input transformers not output ones. Edcor XSM series would be much better.

Cheers

Ian
 

ruffrecords

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I have just finished stuffing the boards.

I finally decided to use Vishay/Dale MetalFilm resistors going against the BOM recomendation of Carbon. It was just very difficult to find the correct values. I know Sammy from GAR uses Amtrans AMRG but those are unironically 100 times more expensive than standard resistors, and i have trouble finding them in the EU. 2W resistors is a mix of what i could find.
There is zero benefit in using carbon resistors in this design unless you like additional noise.. There is zero benefit in using weird value resistors too. Closest E24 series will do.

Cheers

Ian
 

Ricardus

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There is zero benefit in using carbon resistors in this design unless you like additional noise.. There is zero benefit in using weird value resistors too. Closest E24 series will do.

There is one advantage to brown carbon resistors. You can read their color code values much easier.
 

HyprDrivr

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There is zero benefit in using carbon resistors in this design unless you like additional noise.. There is zero benefit in using weird value resistors too. Closest E24 series will do.

Cheers

Ian
I had nu issues finding the exact values called for with the Vishay/Dale resistors.
When you say "will do" do you mean it "will work" or "will make no measurable difference"?
I have already populated, but it could be good for future builders to have your opinion on this.
 

HyprDrivr

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Vigortronix was set up by ex employees of OEP and they make many very similar products. Neither the Vigortonix recommended or OEP equivalkent output transformers are really man enough for this job. They are both input transformers not output ones. Edcor XSM series would be much better.

Cheers

Ian
Yes, it i stated as the budget alternative in the BOM, and they are VERY cheap. I suspect its mostly to get it running but takes a lot out of the performance.
 

ruffrecords

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I had nu issues finding the exact values called for with the Vishay/Dale resistors.
When you say "will do" do you mean it "will work" or "will make no measurable difference"?
I have already populated, but it could be good for future builders to have your opinion on this.
I mean it will make no measurable difference. The original used E12 series values because that was all that was available back then.

Edit: For example the 8K25 listed in the BOM was 8K2 in the original. The only one that might be valid would be the 6K81 but then only if it was for phantom power feed resistors. The documentation fails to provide a schematic so it is not possible to say for certain. Here is a link to the original schematic:

Redd 47 Mic Preamp

Cheers

Ian
 
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