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Old 8th May 2013, 12:20 AM   #11
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Location: SE PA
Quote:
Originally Posted by Eli Duttman View Post
The B+ can't come in too high. OTOH, if you lived in an area with out of spec., 130 VAC "HWM", mains 39 Ω dropping resistors would be needed.

Snubber caps. are used to suppress transients of various origins. The power switch situation you corrected is but 1 example. A "Death Cap." connects 1 side of the AC mains, or the equivalent, to the chassis. The unfortunate practice was common enough, before 3 wire (safety grounded) power cords became the norm. What's attractive about parts designated for snubber service is the high WVDC rating.
A Triad N-68X is very cost effective in the isolation transformer role. While I usually advise against reverse connecting the N-68's single secondary as the primary and its dual primaries as dual secondaries, it makes sense, in this situation. If a slight step down of the mains voltage occurs, it is of no consequence, but isolating the B+ PSU energy source from the heater string energy source is of considerable consequence. A superior and safe grounding of the 'X7 heater CT, via a 150 WVDC 'lytic, along with the noise suppression biasing off voltage divided B+ provides, becomes feasible.
I've read schematics that use this transformer in reverse, as you stated,,, however, my concerns are with an unbalanced load, with teh heaters on 1 side, and B+ on teh other, as it is teh same winding, I can't believe the B+ and filaments draw the same current....seems it could overheat the transformer, and/or cause distortion....

Now, if I'm incorrect, it seems like the way to go...
Regards,
John
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Old 8th May 2013, 12:32 AM   #12
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The 50 VA N-68X is plenty. Reverse connected, each of the dual windings is 25 VA. 25 VA is (sic) 208 mA. at 120 V. The series heater string draws 150 mA., which is obviously OK. Squeezing 115 mA. peak of B+ out of a 208 mA. RMS winding is OK too. NET heating of the N-68X should be well within the part's limit.

Order from Mouser. By and large, they offer the best overall deal. At least for now, Mouser, Allied, and DigiKey is the parts vendor "pecking order".
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Old 8th May 2013, 12:54 AM   #13
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I was thinking the three tubes draw 150mA each, times 3 = 450mA,, guess I'm forgetting my Ohms law!!
I didn't know how to figure teh B+,,, Ohms law again....
Glad to know you EE types!!!!!!!!
I'll order from Mouser, if they have them in stock....
Regards,
John

PS That was the problem,,,they are out of stock...

Last edited by knockbill; 8th May 2013 at 12:59 AM.
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Old 8th May 2013, 01:37 AM   #14
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Order the N-68X from Allied, who has 35 pieces available. Bundle any other parts you need into the order. Minor price differences between vendors are more than compensated for by holding shipping costs down. Shipping costs can be very painful.

Quote:
Glad to know you EE types!
Me an EE type? No sir! I have a 60 year long hobby interest in electronics. I was educated as a Chemist. Professionally, I'm an IBM mainframe computer System Programmer, who specializes in CICS. Yes, I'm still in the labor force. Damn Disney and his blasted dwarves! "I owe, I owe. Off to work I go."
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Old 8th May 2013, 02:22 AM   #15
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Originally Posted by Eli Duttman View Post
Order the N-68X from Allied, who has 35 pieces available. Bundle any other parts you need into the order. Minor price differences between vendors are more than compensated for by holding shipping costs down. Shipping costs can be very painful.



Me an EE type? No sir! I have a 60 year long hobby interest in electronics. I was educated as a Chemist. Professionally, I'm an IBM mainframe computer System Programmer, who specializes in CICS. Yes, I'm still in the labor force. Damn Disney and his blasted dwarves! "I owe, I owe. Off to work I go."
Yep,,, the shipping kills teh deal, most of teh time,,, problem I have is, I ordered everything I could think of this morning... Even some stuff taht I Might have a use for!!! Didn't think the trans would be the problem,,, There's a big antique radio show on this weekend,, If i can't find a trans there,, I'll bite teh bullet....

Well, you're way closer to an EE then I am, you know the math... I've been an electrician for 45 years,,, I fooled around with stereo equipment on and off, but never completely rebuilt any until I found that little Zenith amp in January,,, Then I recapped my Sansui 800, dug an old Heathkit AA1515 out of a barn, and got it cleaned up and running, and think I'm hooked...
A few months later, and I'm trying to redesign cheap, dangerous tube amps!!!
I have too many interests... not enough time to indulge them all!!!!

Thanks for talking me thru some of this stuff, I really enjoy the challenge of hearing them come to life...
Regards,
John
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Old 14th May 2013, 01:42 AM   #16
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[QUOTE=Eli Duttman;3482683]The 50 VA N-68X is plenty. Reverse connected, each of the dual windings is 25 VA. 25 VA is (sic) 208 mA. at 120 V. The series heater string draws 150 mA., which is obviously OK. Squeezing 115 mA. peak of B+ out of a 208 mA. RMS winding is OK too. NET heating of the N-68X should be well within the part's limit.

I got the transformer over the weekend, and did a couple of tests, to determine the windings and see what voltage I got from it, the house mains are 121V....
1. Fed the Red (secondary) wires and got 104.6V each out of each set of windings (primaries)...

2. Fed the Red (sec) wires and spliced the Red+Black wires and the Grn+Yel wires, and got 104.6V out (pri)

3. Fed the spliced Red+Black, and Grn+Yel (pri) wires, and got 138.5V out...

This being an Isolation transformer, I expected teh voltage to be teh same either direction, but there is a 17V difference... The 104V seems too low to feed the heaters,,, and probably also for teh B+.... I could put bigger resistors in teh heater string to protect teh tubes,, but what about teh B+?

I didn't expect this transformer to be a buck and boost...
Any thoughts or suggestions are welcome,,,
Regards,
John
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Old 14th May 2013, 05:05 AM   #17
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I expected a step down to occur, but not as large as that observed. Triad allowed for droop under load, when the part is wired "normally".

Actually, things are quite OK, with a reversed N-68X setup. Just remove the dropping resistors from the ends of the series wired heater string. 104.8 is well within 10% of 112.6 and it's on the low side. Expect the tubes to have long service lives. Wherever the B+ rail comes in, will be OK. The North American standard for house current is 105 to 125 V. You measured 104.8, which is just outside of "acceptable" and the forward drop in the OEM 1/2 wave vacuum rectified setup is considerably greater than that in a full wave SS bridge rectified setup.
Bottom line, ALL IS WELL.
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Last edited by Eli Duttman; 14th May 2013 at 05:05 AM. Reason: format
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Old 14th May 2013, 12:17 PM   #18
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Quote:
Originally Posted by Eli Duttman View Post
I expected a step down to occur, but not as large as that observed. Triad allowed for droop under load, when the part is wired "normally".

Actually, things are quite OK, with a reversed N-68X setup. Just remove the dropping resistors from the ends of the series wired heater string. 104.8 is well within 10% of 112.6 and it's on the low side. Expect the tubes to have long service lives. Wherever the B+ rail comes in, will be OK. The North American standard for house current is 105 to 125 V. You measured 104.8, which is just outside of "acceptable" and the forward drop in the OEM 1/2 wave vacuum rectified setup is considerably greater than that in a full wave SS bridge rectified setup.
Bottom line, ALL IS WELL.
Thanks for the reply,,, I will try reverse wired, both with teh heaters and B+ on separate windings, and also with teh heaters and B+ on teh same winding, as I originally wired it to get teh base readings...

In a previous post, you said "teh B+ can't be too high",,, so, would I be better off wiring it to get teh higher voltage, and leave teh resistors in teh heater string??

I cut out a few caps, and realize I need a higher voltage, so I'll report back when its fired up again...
Regards,
John
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Old 14th May 2013, 07:46 PM   #19
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Just remember that you can't bias the series heater string off B+, when the B+ PSU and the string share a winding.

The O/P voltage of a "normally" wired N-68X will be very close to that of the AC mains, when it's loaded down. The reason for a slight difference in turns count is to compensate for that drooping effect.
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Old 14th May 2013, 10:59 PM   #20
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Default Update,,,,

I tried running the amp, with the trans wired in reverse, with two separate windings and "normally",,, It sounds best wired normally,,, however, I can't separate teh B+ and heater string, with only one winding. Wired in reverse, I can only get 104V out, which is low for the heaters,,, and only about 110V B+... and it also has a lot of hum and distortion... Not gonna cut it, unfortunately...
Wired normally, gets rid of teh distortion, hum is still there, but its pushing teh outputs to about 144V,,, The heaters are a little high, 127V, but I think I can lower that with a pair of larger resistors,,, I still have 6-7 caps to replace, when they get here, and then I'll tackle the hum...
Thanks fro the help so far,,,
Regards,
John
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