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Old 28th June 2014, 09:23 PM   #1
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Default Need help to modify circuit to suit replacement power XFMR

Hi folks. I have picked up a used Norh SE9 Integrated Power Amp. The amp currently does not work. From a few simple tests, it appears that the power xfmr is damaged - the HV secondary is non-functional. I could not locate a schematic for this amp anywhere on line so I took a stab at drawing one (I am a newbie at this so hopefully it is accurate). Please note that, in my schematic, I have replaced the original power xfmr with a different one that I have on hand. I don't have enough circuit design knowledge so I need help modifying the circuit so that it will work correctly with the power xfmr that I have inserted into the schematic. Thanks for any help you can offer for correcting the values for the passive components to match the existing tubes.
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File Type: pdf Norh_SE9_Schematic.pdf (49.0 KB, 51 views)
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Old 28th June 2014, 10:08 PM   #2
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Apologies, I see that it is not clear from the schematic that the power xfmr supplies 800V CT at 200mA, 5V at 3 A and 6.3 V CT at 6 A.
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Old 28th June 2014, 10:40 PM   #3
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Your nOrh SE9 power transformer has 400AC (800VCT) before rectification and with cap input supply, your B+ will be quite high, possibly over 500VDC, and there's pretty much no voltage drop to the screen grids so plate and screen are the same high voltage. High screen voltage is bad for tube life. To be safe, I wouldn't want the B+ to be higher than, say, 420VDC, after choke. Do you know what the stock voltage point is? The stock filter caps' voltage rating will give you an indication of its limit, which is 450VDC, so your B+ should never exceed that limit.

In your schematic, the two halves of 12AX7 are parallel so pin 1 and pin 6 should be tied together. The output transformer's secondary looks peculiar. Are you sure it's not Common/ground and then 4Ω and then 8Ω in that order?

Click the image to open in full size.

Last edited by directdriver; 28th June 2014 at 10:45 PM.
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Old 29th June 2014, 12:01 AM   #4
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Thanks, DirectDriver. Unfortunately, I don't know what the stock voltage point should be as I don't have an original schematic and could not find one. The original power xfmr was potted.

As you stated, pins 1 & 6 of the 12AX7s should be tied together. Thank you for that correction -- I have amended the schematic to show that.

As for the output, unless the amp is incorrectly labelled, the schematic is as I have shown. On each output xfmr (also fully potted), what appears to be the 4Ω ( (a larger wire which is attached to a binding post that is labelled as 4 ) and the smaller Com leads are twined together and come off the xfmr in one sheath, while the second, separately sheathed, smaller lead from the xfmr is attached to the binding post labelled Com, which is connected to ground. I suppose it could be mislabeled, what do you think?

Given the power xfmr that I currently have inserted into the schematic, what would be appropriate values for the caps and resistors in order to bring the B+ voltage to a better level?

Thanks for your help.
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File Type: pdf Norh_SE9_Schematic.pdf (49.9 KB, 10 views)
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Old 29th June 2014, 02:46 AM   #5
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Quote:
Originally Posted by wcakey View Post
Thanks, DirectDriver. Unfortunately, I don't know what the stock voltage point should be as I don't have an original schematic and could not find one. The original power xfmr was potted.

As you stated, pins 1 & 6 of the 12AX7s should be tied together. Thank you for that correction -- I have amended the schematic to show that.

As for the output, unless the amp is incorrectly labelled, the schematic is as I have shown. On each output xfmr (also fully potted), what appears to be the 4Ω ( (a larger wire which is attached to a binding post that is labelled as 4 ) and the smaller Com leads are twined together and come off the xfmr in one sheath, while the second, separately sheathed, smaller lead from the xfmr is attached to the binding post labelled Com, which is connected to ground. I suppose it could be mislabeled, what do you think?

Given the power xfmr that I currently have inserted into the schematic, what would be appropriate values for the caps and resistors in order to bring the B+ voltage to a better level?

Thanks for your help.
Oops! Just noticed that I had one of the input channels going to the wrong pin on V3. Corrected now.
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File Type: pdf Norh_SE9_Schematic.pdf (50.0 KB, 5 views)
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Old 29th June 2014, 03:50 AM   #6
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Quote:
Originally Posted by wcakey View Post
As for the output, unless the amp is incorrectly labeled, the schematic is as I have shown. ..... I suppose it could be mislabeled, what do you think?
The below is the more likely scenario. You can measure the DC resistance of the secondary to determine the order. Assuming COMMON is zero ohm and ground and then the lower DCR tap is 4Ω and the highest should be 8Ω

Click the image to open in full size.

Quote:
Originally Posted by wcakey View Post
Given the power xfmr that I currently have inserted into the schematic, what would be appropriate values for the caps and resistors in order to bring the B+ voltage to a better level?
You can use 5R4 (5V 2A filament) or 5U4 (5V 3A filament) rectifier as they tend to put out lower voltage but the rising time is faster than 5AR4. C1 might be too high for tube rectifier, I would swap it with C3 so after tube rectifier, would be 33uf and then choke and then 100uf and then 68K and then 100uf. But I have a feeling it will still be too high or higher than 450VDC.

Make sure you bring the voltage up with a variac and keep an eye on the B+ voltage to not exceed 450. If ultimately the voltage is higher than 450, you either have to change higher rating caps or put series resistors but that wastes a lot of heat. Depending on the choke's (L1) value, you can eliminate C1 or reduce it to a really small value as if it's a choke input supply. L1 has to be high value inductance to be effective. Instead of cap-choke-cap B+, you will have choke-cap B+. If you decided to have a higher than 450 B+ voltage, you must change caps with higher rating and then add dropping resistor and cap to reduce screen voltage to about 400VDC. High screen voltage is one of the most common cause of short tube life.

Maybe the Hammond 260J can fit inside the potting case?

Again, use a variac and keep an eye on the B+ voltage to not exceed 450VDC!

Good luck!
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Old 29th June 2014, 09:43 AM   #7
Ketje is offline Ketje  Belgium
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If you need a lower HT, put C1 after L1 in parallel with C2.That way it's not the peak value but the mean voltage of the xfmr on C1//C2.
Mona
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Old 30th June 2014, 11:52 PM   #8
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Thanks for your suggestions, Ketje and directdriver.

WELL DANG, I now feel really stupid.. probably am :-). I thought that there was no fuse anywhere in the circuit and because I had never seen one before, I did not notice the fuse built into the IEC socket. And yes, that fuse was blown and that really helps to explain why I was getting some peculiar readings from the circuit board when it was plugged in (without tubes, of course).

Upon making this discovery, I put everything back together, including the original power xfmr and a 500mA slowblow fuse, hooked up some speakers and a CD deck, plugged it in to a variac and fired it up. At 110V, it sounded pretty fine (no hum, just music) and I thought I might be home free. NOT. After roughly a minute or so, the amp shut down -- blown fuse. A second trial with a new fuse resulted in a nasty loud hum before blowing the fuse and shutting down.

I realize that this behavior could probably have many possible causes, but I would interested in hearing what you folks would do next to try to find the cause of the problem.

Thanks for any help you can offer.

Last edited by wcakey; 1st July 2014 at 12:20 AM.
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Old 16th August 2014, 02:37 PM   #9
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Thanks again for the previous suggestions. Since I liked the sound of the amp (during the time when it was briefly working), I decided to completely rebuild it -- kept the PT and OTs, dumped the PC board for a homemade turret board and point-to-point wiring and replaced everything else with new, higher quality components. Plugged it in and it worked just fine... for about a minute and then the fuse (500 ma slow-blow) was blown.

I replaced the fuse and brought it up slowly on a variac -- worked beautifully and sounded great at 90 volts and it did not crap out again even after an hour of run time (I did not try a higher voltage).

What is causing this behavior?? I am pretty much a newbie at this stuff, but it seems like there must be a fundamental problem with the circuit design, given that I replaced all components (except the trannies). Is the fuse blowing a symptom of the B+ being too high? If so, what is the best way to reduce it, without changing out the PT?

I am attaching a copy of the final schematic for the original amp.
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File Type: pdf Norh_SE9_Schematic.pdf (55.1 KB, 7 views)
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Old 17th August 2014, 06:46 PM   #10
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A 500mA fuse is way too small. According to the ratings listed above, the transformer can deliver 212VA of power, which would equate to 1.77A from 120V.

Try a 3A slow-blow fuse.

Pete
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