Pre-Flight Check - Funny Ohm-it-out

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Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Sat Oct 25, 2014 11:17 pm

Hey All,

I've got the board and chassis put together, just waiting for the Edcors. I obsessively checked component values while on the bench and while on the board, as much as I could of the latter. Checked for cold shoulders as well. Everything looked golden. I got close but incorrect readings while 'Ohming it Out.' At this point I don't have anything attached to the jumper blocks, except for the input signal jumpers, as they are only attached to the RCA jacks which aren't attached to anything. I don't have tubes in, I never installed a Potentiometer and all trim pots are mounted on the 'numbered' side of the board, (opposite side to the sockets.) THAT SAID; I still have alot to learn so maybe I've just put something in backwards. It could always happen. Maybe I needed to bias adjust first? Maybe putting all the trim pots on one one side of the board means some of them are backwards? And I assumed that one lead should be connecting with a ground plane, but maybe that's wrong too? Clearly I have no idea. I am hoping that some of you might...

Can anyone explain the discrepancies?

My Readings are as follows: [Expected values in these blocky sorta parenthesis]
J1
1. 3.4M [>2M]
2. 3.3M [>2M]
3. 1.4M [>200k]
4. 1.3M [>200k]
5. 1.1R [Ground]
6. 1.1R [Ground]
7. 100.1R [100R]
8. 99.6R [100R]
9. Out of bounds (above M-ohm range) [>10M]
10. Out of bounds (above M-ohm range) [>10M]

J2
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 1.1R [Ground]

J3
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 42.15k [28.3K]

J4
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 49.45K [28.3]

J5
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 1.2R [Ground]

J6
1. 520K [>10M]
2. 1.2R [Ground]

J7
1. 521K [>10M]
2. 1.1R [Ground]

Also, I am curious if anyone has expected values while *Edcors* are installed. The PA-774 is included, but Edcors are supposed to differ somewhat. It would be nice to know that; one more check before I plug things in...

Let me know if you need more info, and thanks for any help you can offer! In general, thanks for such a great forum. I've learned *loads* here. You folks are pretty great.

Cheers!
-Fishstix
Technics Sl-Q30 / RuneAudio -> Kenwood Basic C2 -> Parks Audio ST-35 -> KLF 10
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Shannon Parks » Sun Oct 26, 2014 6:08 am

Fishstix wrote:J1
1. 3.4M [>2M]
2. 3.3M [>2M]
3. 1.4M [>200k]
4. 1.3M [>200k]
5. 1.1R [Ground]
6. 1.1R [Ground]
7. 100.1R [100R]
8. 99.6R [100R]
9. Out of bounds (above M-ohm range) [>10M]
10. Out of bounds (above M-ohm range) [>10M]


All these are perfect. Note that expected values with a greater than symbol mean inexact measurement, and basically just high impedance as you record. 1.1 ohms is indeed ground.

Fishstix wrote:J2
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 1.1R [Ground]


Perfect.

Fishstix wrote:J3
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 42.15k [28.3K]

J4
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 49.45K [28.3]


The pin 3 resistance is set by the adjustable pots. See page 5 of the manual and adjust while you measure with your DMM.

Fishstix wrote:J5
1. Out of bounds (above M-ohm range) [>10M]
2. Out of bounds (above M-ohm range) [>10M]
3. 1.2R [Ground]


Perfect.

Fishstix wrote:J6
1. 520K [>10M]
2. 1.2R [Ground]

J7
1. 521K [>10M]
2. 1.1R [Ground]


These are correct. My old 10M measurement was with AC coupling capacitors installed. With a DC coupled connection, this is spot on.

Shannon
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Mon Oct 27, 2014 12:29 am

Sweet! Thanks very much, Shannon. My DMM has a 40M range, so I was worried out of limits was *way* out of limits. And thanks for putting my mind at ease about the other readings. I'll start playing with biasing tomorrow. And *hopefully* the Edcors get here this week. <fingers crossed>

Cheers!
-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Thu Nov 13, 2014 4:44 pm

Hey All,

Edcors arrived last night, so of course I dropped everything and got them installed. First off, I got the same rising squeal that MacL talked about in http://www.diytube.com/phpBB2/viewtopic.php?f=1&t=6254 and was able to fix it via Writer Frog's instructions. So thanks, guys!

I am still having some issues with hum, some of which I've been able to fix. If I don't have this rig attached to something, it produces a terribly loud ~120 hz hum, so I am going to guess ground loop. If I attach it to a battery powered mp3 player, the hum disappears, which furthers my guess that it is a ground loop. With lots of fiddling, the best I can get out of it is if I unhook the chassis ground altogether, and power it from a distant receptacle. I had hoped to power it from the switched power supply of a Kenwood Basic C2 (love it's phono stage.) The C2 doesn't have ground posts, so the ST-35 is only wired with hot and neutral. Interestingly, I jumped the ground pin to the phono ground on the C2, since it's an available chassis ground. It killed all the hum.... and all the music. When the St-35 ground shares the chassis ground on the C2, everything disappears, and then comes back once you unhook them. Any ideas? (In the attached photo, the EMI filter ties the ground to the chassis plate, but I have since unhooked that; less hum.)

DSC_1697b.JPG
DSC_1697b.JPG (749.76 KiB) Viewed 10094 times


When I built my first tube amp, an S-5, the instructions were clear that I needed to sand one of the feet down on all three transformers to create good chassis grounds. The difference was striking; that S-5 amp was incredibly quiet. (Well, noise free. I played it loud.) Is this something I should be doing with every amp build?

Speaking of loud, I am expecting more volume out of this puppy. It's equally loud, or slightly quieter than my 16 watt S-5 tube amp. Not that I really need to have my cheeks flap with every beat of the kick drum, but it isn't what I expected. Any ideas?

Other stuffs... the bias pots took 20-30 revolutions to bring them down to .350v, one of them pegging at .360V. I read from another thread of MacL's that voltage isn't hard on EL84s, and I shouldn't worry about it. But it still seems weird, so I thought I would mention it. Only five hours of burn-in time yet, so I am not worried.

I guess that's it. In general, this amp sounds better than the last, though I am having trouble really getting to the music through the hum. Whomp whomp.

Any ideas are welcome! (He says knowing full well that ground loop hums are an easy route to insanity.)

-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Thu Nov 13, 2014 5:27 pm

Oop. Also, the hum is not dependent on Volume level. It stays the same regardless of how high or low the pre-amp volume is set. I didn't put a volume pot on the ST-35.
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Sat Nov 15, 2014 3:56 pm

I've swapped out the leader tubes and shielded the tubes with no change in the hum.

The more I read, the more I hear that ground loops require a separate ground; lifting the ground often solved the problem. But nothing is grounded on my system... Maybe oscillation from the transformers? I am still using the EMI filter, I just pulled the chassis ground from it. That helped some, maybe if I pull the whole thing?

Scratching my head so hard I have grey matter under my fingernails.

-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Shannon Parks » Sun Nov 16, 2014 9:39 am

1) Make sure the RCAs are isolated from the chassis.
2) Run safety ground to one chassis screw (sand, grind off anodized aluminum for connection on inside)
3) Run a single wire from the analog ground on the PCB (J1-6) to the same chassis screw (or closeby - IEC says the safety ground should use a crimped eyelet and its own screw).

This should do it. But if this is how you've done it, email me with a few more close up inside shots.

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Tue Nov 18, 2014 9:21 pm

Hey Shannon,
Thanks for the reply!

RCA jacks are mounted on wood, so no problem with isolation there. The wire is mogami instrument wire, 90% sheild. I've run it over the top of the board to keep it in place, so I worried about induced hum from the board. But moving it around while turned on didn't change the hum level. I am considering swapping the mogami out for solid twisted pairs of 18ga, then I can run around the edge of the board. Any merit to that? The hum sounds far too loud to be induced, but if I knew, I wouldn't be asking... ;)

I ground down a patch for the chassis ground, and attached that; no change. In fact, I've pulled out a medical grade isolation transformer I use with my O-scope. I've put a switch on that so I could uncouple the ground when need be. No permutation of setup makes any change, lifted ground or otherwise. The preamp isolated, the ST-35 routed regularly, or vise versa. Anytime I add anything else to the system (media center, turntable) it gets progressivly noisier, but none of it is changed by the pre-amp volume pot, nor by it's location on the isolation transformer. If the hum is in the ST-35, then I suppose preamp volume wouldn't affect it; it will be static as the ST-35 amp volume is static. But if that's teh case, then why would the amp output more hum when more gear is attached?

I've pulled the EMI filter, in case it was a bum one, but still no change in hum. I tested the voltage at V points 4 and 5, in case I just wired up the heaters at 12 volts (I've had similar problems that way) and they are reasonably close to the expected values chart, at 2.95 VAC from ground. So no AC hum coming in on the heaters. Hopefully?

The one thing I haven't done is install a three-wire power cord with ground. Everything I've read is that ground poles tend to create rather than damp ground loops. I am still hoping I can use the pre-amp as a power center too, rather than permenantly devoting my testing iso transformer to the stack. I will likely try that tonight.

Lastly, the Power Transformer has a similar hum to it, though it's pretty quiet for a tranny, in my limited experience. Lamination hum doesn't usually make it into the audio circuit, does it? (I read a great article this morning on how to boil your transformer in shellac to stop lamination hum. Used a DIY vacuum bell, the whole thing. I *love* what people do to improve their system. (b) )

For now, here's a better quality photo of the same as above; it's hard getting too much info in under 1 meg. Let me know what section you could use a closer view of; I'll snap a shot and get it off to you all.

DSC_1697b.JPG
DSC_1697b.JPG (996.86 KiB) Viewed 10027 times


Thanks for brainstorming with me on this one. I do actually enjoy the problemsolving; it's a chance to learn something new. But I *would* like to knock the socks off the fam when they come over for Thanksgiving....

Cheers,
Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Tue Nov 18, 2014 11:17 pm

Update: Installed a proper 3 wire cord, hot/neutral/ground. No dice. Hum is still there....
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Shannon Parks » Wed Nov 19, 2014 2:54 pm

Upload a new pic with your 3-wire setup, please.

In that pic, the filter is grounded to the PCB. It needs to be grounded to the chassis. Run a wire from the filter to a chassis lug (same as your new safety ground lug). Then run a separate wire from J1-6 to the chassis.

Basically we don't want any leakage currents from the filter on the PCB ground. We'll get it.

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby 20to20 » Wed Nov 19, 2014 3:07 pm

Fishstix wrote:Update: Installed a proper 3 wire cord, hot/neutral/ground. No dice. Hum is still there....


Stix,

Is your top plate painted black or is it anodized black?

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Wed Nov 19, 2014 6:29 pm

Hey Gents,

As in the photo, I've tied the PCB ground to the chassis and then out the cord. Again, the EMI It made no difference on the hum, so I removed it permanently when I put in the 3-wire cord. If nothing else it makes for a cleaner upskirt. Hopefully this photo is a bit clearer than the other one. The pair of yellow wires connect underneath the pair of brown wires, I know it kind of looks like they disappeared. Any benefit of tying them together nearer to the PT and running a single wire to the board?

DSC_1709b.jpg
DSC_1709b.jpg (956.54 KiB) Viewed 9994 times


I was able to score a bit of a reduction in the hum by plugging everything into the iso transformer and keeping the grounds tied together. I had tried putting some components on one side of the transformer while 'ground-open' and it didn't help. Tied together at the Pre-amp was too close, tied at the box was too far.... still assuming this is a ground loop. While this maybe halved the hum, it's still loud enough that it cuts across most of the music, and especially during quiet sections. Le sigh. The upside? I rarely play my music quietly. (lol) To be fair, it *is* getting close to where I would like it to be.

20, the plate is anodized; I did it through Front Plate Express. VERY impressed. It took quite a bit of abuse to get down to bare aluminum again. I had to use a sharp woodworking chisel..... *very* impressed. My only complaint is that like slinky black dresses, it looks fantastic right until the cat sheds near it. (Not like I'll turn down Pretty Dresses *or* Pretty Amps. ;) ) I'll upload a photo of the topside here in a mo.

I also tied the output transformer common at the speaker jack, rather than running it to the board and back. Didn't change the sound at all, but it cleaned up the underside.

-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Wed Nov 19, 2014 6:40 pm

And a shot of the OT wiring...

DSC_1710b.jpg
DSC_1710b.jpg (960.2 KiB) Viewed 9993 times


Oh, I forgot to ask earlier. I was rolling my leader tubes just in case one had been dropped or otherwise damaged. I haven't found any difference in hum between them (both old and new stock) but I've been reading about spiral wound filaments in 12ax7 and 12au7 series; sounds like some have them, some don't. The spiral wound ones are supposed to reject AC hum better. Have you guys experienced a difference with those?

-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby 20to20 » Wed Nov 19, 2014 6:53 pm

Fishstix wrote:
20, the plate is anodized; I did it through Front Plate Express. VERY impressed. It took quite a bit of abuse to get down to bare aluminum again. I had to use a sharp woodworking chisel..... *very* impressed.
-Fishstix


I thought it looked anodized. Anodizing creates a thin non-conductive ceramic surface. Make sure all of your grounding points on the plate are well abraded through the black to the base metal. If you take an ohms reading between two points on the anodized surface you should see an open circuit.

Good Luck

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Wed Nov 19, 2014 6:58 pm

Oh, so much cat dander!

DSC_1707b.jpg
DSC_1707b.jpg (886.38 KiB) Viewed 9992 times


20, so what is your bead on grounding transformer bells? What would that accomplish? It was a night and day difference in my last amp, but I know grounding schemes are not all the same.

The leaders right now are a JJ ECC83s (new stock) and an old stock 12BH7 with lettering that's mostly worn away. Emerson? It stands a little taller than the 12au7, and sounds pretty nice. I am going to roll to old stock Westinghouse 12au7 and Sylvania 12ax7 and see how they do. I'll let you know...

Cheers!
-Fishstix

(And again, thanks for the help guys. I *really* appreciate it.)
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