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eskimo
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« on: April 08, 2009, 10:44:29 pm » |
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I'm building me a 438C and the closest match 230V power trafo I can find is the Hammond 369AX which have a 125V secondary instead of original 117V one. How would one go about altering the circuit to fit the hammond? SchematicLove /Sh*tforbrains |
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« Last Edit: April 20, 2009, 02:38:28 pm by eskimo »
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mad.ax
france
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« Reply #1 on: April 09, 2009, 03:56:41 am » |
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That's about 9%... Sometimes wall power gives bigger variations!
But if it's the case (say a 220V primary, and 240V wall power) that would lead to a bit high of an HV.
Find yourself a BIG adjustable resistor about 5 KOhms/10 Watts , put it in front of R20 and R21, and adjust.
(start with 5K maximum resistance, with every tube pluged in, let it warm up and lower the resistor value to get 255V)
Axel
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eskimo
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« Reply #2 on: April 09, 2009, 10:33:29 am » |
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Excellent, thanks!
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eskimo
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Posts: 379
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« Reply #3 on: April 20, 2009, 02:38:15 pm » |
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In my search for a suiting output transformer that's shielded I've come across the UTC A-26 which seems to fit, am I correct in asuming that? The original peerless was 28K CT:600 I believe.
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emrr
NC, USA
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« Reply #4 on: April 20, 2009, 02:47:49 pm » |
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Correct, disregarding output levels. I don't recall offhand what the 438/436 output is rated.
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Best,
Doug Williams
Electromagnetic Radiation Recorders
"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob..." - Crusty
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eskimo
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Posts: 379
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« Reply #5 on: April 20, 2009, 02:52:28 pm » |
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Swell, if anyone has any other contesters I'd be glad to know of them. The sowter and edcor are both unshielded and my case is sort of tight. As for the input(or perhaps interstage since it's the 438) the original is 15K : 30K CT right? I'd love to find alternatives to the sowter there as well. I rejoiced in finding the A-18 to be it until I discovered the "30K" was a scratched out "80K". Oh well...  |
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edanderson
atlanta
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« Reply #6 on: April 20, 2009, 03:31:43 pm » |
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for your power transformer, consider that the original takes the 117VAC secondary and doubles it to make 255VDC. since you aren't limited to the original transformer, you could just get a transformer with a ~230VAC secondary and rectify/filter it to give you 255VDC. the suggestion of a variable power resistor is still a good one, because your mains voltage may vary.
i do think the original output is bigger than a A-26, but that doesn't mean it wont work and sound fine.
the original input is a stepup, but consider that may folks end up with a pad on the input anyway. so a 1:1 input may be just fine.
for a small, well shielded transformer off the shelf, i'd suggest taking a look at lundahl, who should be familiar to you...
ed
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eskimo
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Posts: 379
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« Reply #7 on: April 20, 2009, 04:18:56 pm » |
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Yep, lundahl is only an hour drive away. Haven't found anything in their inventory though.
Good point about the power transformer, I'll scout my options.
Thanks!
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emrr
NC, USA
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« Reply #8 on: April 20, 2009, 04:31:41 pm » |
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CJ took the input apart and the data is here. I remember it being something like 20K:80K. Which ain't terribly different from 20K:20K in reality.
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Best,
Doug Williams
Electromagnetic Radiation Recorders
"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob..." - Crusty
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eskimo
Member
Posts: 379
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« Reply #9 on: April 20, 2009, 04:36:29 pm » |
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To a fool like me 80K sounds like a huge difference. Does that mean the A-18 could fit rather nicely?
edit: or did I wrongly assume it was center tapped?
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« Last Edit: April 20, 2009, 05:15:35 pm by eskimo »
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edanderson
atlanta
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« Reply #10 on: April 20, 2009, 05:51:14 pm » |
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the input is more like 15k : 90k , or 1:2.5 ratio, but again, i'd question whether or not you really need the stepup with modern hot studio levels. i'd think the ll1660 and ll1630 would probably work well enough in this circuit as input and output.
ed
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eskimo
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« Reply #11 on: April 20, 2009, 06:01:43 pm » |
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They're unshielded though right?
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eskimo
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Posts: 379
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« Reply #12 on: April 20, 2009, 08:51:50 pm » |
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If I want an Instrument input, can I simply connect a tele shorting jack between the input transformer and grid as done in the G9? |
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edanderson
atlanta
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« Reply #13 on: April 20, 2009, 10:02:37 pm » |
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They're unshielded though right?
that's true, but due to the dual bobbin construction, the noise rejection should still be pretty good. if you're really worried about shielding and size, you could take a ll7902 as the input and a ll7905 as the output. they're smaller (but probably comparable to the UTC A series core size inside) and shielded. there are certainly plenty of other options, old and new, but since you're in sweden, it would be a shame not to call or write lundahl and see what they'd recommend. and yes, injecting instrument level signal straight into the grid was quite common in older designs, so no reason you couldn't at least try it here. ed |
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eskimo
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Posts: 379
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« Reply #14 on: April 23, 2009, 10:30:46 pm » |
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emrr
NC, USA
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« Reply #15 on: April 24, 2009, 01:39:33 am » |
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Even less headroom than an A series UTC. I think maybe 8 dBm.
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Best,
Doug Williams
Electromagnetic Radiation Recorders
"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob..." - Crusty
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jensenmann
Karlsruhe, Germany
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Posts: 1603
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« Reply #16 on: April 24, 2009, 06:16:37 am » |
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I donīt think that you will run into trouble with the unshielded Sowters. In my DIY436 The Sowters are sitting a few cm next to the mains tranny (which is torroidal). No problems, neither audible nor measurable. I have a custom-wound mains tranny which was cheaper than the Hammond. You should check http://www.mueller-rondo.com/ in Germany. Finest quality and extremely good prices. With torroids you have way less magnetic strayfield than with an EI-core, hence less trouble. |
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Jens >The tubes of course don't care what frequency they distort |
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eskimo
Member
Posts: 379
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« Reply #17 on: April 24, 2009, 10:58:52 am » |
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Good ideas thanks! What did you pay for your torodial? Here's the enclosure I'll be using. Old geiger counter, hope it's not radioactive...   |
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« Last Edit: May 07, 2009, 07:35:32 pm by eskimo »
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eskimo
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Posts: 379
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« Reply #18 on: May 07, 2009, 07:34:48 pm » |
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A question about NYD's mods. In the schematic, R12 and R9 values are lowered, is that because of the addition of the attack pot and castling of P2 and R9, or is it because of the alteration of the PSU? (What does the PSU changes do btw?) Since I had not planned on altering the PSU in mine, should I leave the R12 and R9 values as in the original? http://eskimo.creotia.com/436Cmodified.jpghttp://eskimo.creotia.com/438C.jpg |
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eskimo
Member
Posts: 379
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« Reply #19 on: May 18, 2009, 10:39:04 am » |
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A wee bump for that last question, getting ready to order parts here. Anyone?
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