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Edcor OPT in S5 Electronics K-16LS kit?

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At the advice of on of the forum members I have ordered two Edcor CXPP25-8-5K output transformers and hope to use them in my kit.

I do need to ask question though and I ask it simply to assure myself. Given the specs of the Edcor transformer, which are:

Pri. Watts: 25W
Plate (Z): 5,000 ohms
Sec. (Z): 8 ohms
Screen Voltage: 40%

Can I simply swap out the OPTs or do I need to do some mods to adapt the amp to receive them? Likely I did not give enough data here; of so let me know what other data I need to give you. I can provide the kit schematic if that helps. My skills just are not up to the level that know this.

Many thanks for any help.
I see no reason why you can't use the Edcors without modifications as long as you do not use the screen taps. If the amp uses feedback form the speaker secondary, then you might have to adjust the feedback resistor since the Edcors are 8 ohm instead of 4 ohm like the originals. If you desire to use the screen taps an make the amp into an ultra-linear, then other modifications will be necesssary. Perhaps a schematic might help.
Thanks guys. Your reassurance is most appreciated.

I am pretty certian that this is not configured to provide feedback (that's ultra-linear mode, right?). I would be interested for understanding how I might do this for a future modification.

If you have suggestions as to the modifications necessary for the ultra-linear conversion, I would welcome those. Oherwise I plan to drop in the new Edcor OPTs for now.

So you can 1) validate my believe that feedback is not used and 2) provide some suggestions on the future conversion to ultra-linear mode I am providing the schematic.

My scanner is not working so I took a quick pic of the schematic. Hope it is readable.

Thanks Again!


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Feedback is used, take a look at the connection from the secondary of the output transformer back to the cathode of the input voltage amplifier - that is global feedback. Given the moderate loop gain and the high closed loop gain it appears that the amount of global feedback is quite limited. (Without a thorough analysis it is hard to guess, but I would be surprised if was over 6dB.)

Ultralinear connection would just connect the screens to taps on the output transformer primary, and is a form of feedback local to the output stage. It's worth trying, but expect to do a little work to make it stable, which may also be the case with the new Edcor transformers you've purchased.
Boris_The_Blade said:
That amp makes me cringe at what it will do to prices of 6AQ5s...

I'm not worried...I think that there were a hell of a lot more 6AQ5s made than the 11BM8, 11MS8, 10GV8 that S5 previously used. They're $4-6 at Antique Electronic supply (and the 5AQ5 and 12AQ5 are half that...)

I have about 250 used ones, so I've been considering building a PPP amp myself...
Tom Bavis said:
I'm not worried...I think that there were a hell of a lot more 6AQ5s made than the 11BM8, 11MS8, 10GV8 that S5 previously used.

But I never had any interest in those tubes :D :p

Tom Bavis said:
I have about 250 used ones, so I've been considering building a PPP amp myself...

As did I at one point (I have maybe 20 of them), but got quickly distracted on other projects. I think I may throw a small office amp together with simple PP pairs and some cheapie line transformers.
edcor pwr trans for k16

Greetings: If you ever wanted to upgrade the pwr. transformer, Edcor has it on file already; therefore, you will not have to pay the 20$ setup fee like I did.
The specs are:

Dual 120 Primary
Sec red= 180 V, 0.2 A
Sec yellow=12.6 V 4A White/yellow CT
K16 too!

So how did the output transformers sound? Mine are coming this Wednesday. Just wondering if you are happy with spending the money over the stock transformers.
The stock transformers are excellent in the mid and upper frequencies, but really lacking in the lower end of the spectrum. S5 claims that the response curve is from 20-20Khz, but I seriously doubt it with the stock setup.

Have you done any of the other mods on your kit?



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Got the transformers in.

The overall sound characteristics of the amp changed defintiely.

Here's where it gets hard for me.

I feel that the sound has more depth and that the bass was improvied. I have to admit that I am having trouble comparing the new sound to the old as I can't just switch back and forth. I'll say this....it is different that before and I like it.

I am working to install this all in a chasis and one of the things I am considering is having both sets of OPTs in the kit with the ability to switch betwen them. Maybe just connectorize them so I can switch back and forth relatively easy (with power off of course).
I have installed the Auricap capacitors replacing the provided caps. I put bigger electrolytic caps on the power side and put drain resistors across the leads.

I have but have not installed a ladder-type stepped attenuator for the provided volume pot. I just like the stepped click feel of the new pot.

I plan to do more but right now I am almost literally "monkey see monkey do" - I mean just straight copying from the more experienced guys on the forum. This one is my learning and for fun project. Later on I'll go for a more real audiophile type system.
Output transformer connections? Help!!!!

Hey, I got the Output Transformers in tonight and I am listening to them now. I immediately noticed that I have to turn the volume up louder to get the same level out as I had with the previous OPT's in. I was wondering if you noticed the same with yours? I attached the Edcor sticker to show you where I placed the wires in the location on the circuit board.

How did you wire your output transformers in? I noticed that there is also a brown wire and a white and brown striped wire as well. I just capped these off and did not connect them into the circuit.

Should I have? Is this why I have to crank up the volume to only get a minimal amount of sound?

If anyone can answer this, it would be really helpful.

Thanks, Andy


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OPT stock schem

I'm just really confused about where to hook each wire on the schematic for the OPT's. Especially since there are a total of 5 wires on the left side of Edcor's schematic and 3 on the left side of the K16 amp.

I also am attaching a blown up portion of the output section on the stock K16 amp, if this will help with telling me where to solder each wire!

Thanks in advance for your help!!




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DigitalJunkie said:
Move the BN wire off of the white/blue transformer wire,and move it to the brown transformer wire.

the white/blue and white/brown are the UL taps..insulate them well,since you won't be using them.

DigitalJunkie's got it right, and some point once you have sorted out the current issues and listened for a while you might just want to try UL connection. To do that you first have to disconnect the screen grids of your 6005 output tubes from the screensupply, then connect the white/blue wire to both screens of the tubes with the blue wire plate connection, and the white/brown wire to the screens of the tubes with the brown wire plate connection.

There is a small possibility of oscillation which should be easily resolved with just a minor change to the amplifier's existing compensation with this new transformer in either pentode or UL connection - if it does not show any signs of instability with pentode connection, it should be less likely in UL as the overall loop gain will be a few dB lower.

Screen voltage rating is same as the plate voltage rating so you should be ok there.
Yes, you don't use the striped wires. Cap them off. The Red, Blue and Brown wires ONLY are used in the "stock" transplant. On my kit it's like wire color for the original stock transformers and the new. And, the PCB labels the conections by color too.

I would like at some point to try the Ultra-linear connection which is discussed above. Right now I'm not educated enough to figure it out without literal step-by-step instructions. I understand the concept but would need lots of hand holding for sure.
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