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PostPosted: 10 Sep 2011, 21:51 
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Sarting my working life as a communications technician, advancing to a Principal Technical Office and later a network manager for 150 seats it often fell on me to fix things quick and get them going. I wanted to listen to the MKIV Universal now but didn't have the right pot to get the best out of the preamp. I wanted a fast fix. Matt's suggestion to put the 100K pot as the load resistor was just too hard to do on the completed cct. brd. The aim was to get the load to 250K at the preamps output.

My suggestion was to put the 100K pot on the input and 250K resistor on the output. My power amp has a 470K input impedance (DC) and no input caps. What I did was put a 500K resistor as the load resistor across the output RCAs. The preamp will see the 500K resistors in parallel with the power amp 470K which means a load of about 250K. I moved the pot to the input and placed the 500K resistors as mentioned. Now I have a constant hum. It does not vary with volume and stops the second the power at the preamp is turned off.

I always use unsheiled wire as hook-up for the RCAs and this time the input hook-ups run the entire length of the preamp, near the 24V AC internal hook-ups. I tried earthing the pot but no different. What I will try is shielded wire for the inputs. No other 4S preamp has hum using the same techniques. Any suggestions?
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MKIVCasedSS.jpg


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PostPosted: 10 Sep 2011, 23:15 
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A few thoughts with hum in mind. If the hum doesn't vary with rotating volume control it can't be being induced from the length of the hook-up wire to the input RCAs. Otherwise the hum would get louder and softer. Also now I have a 100K pot and a 470K input resistor in parallel depending how I wired the pot in. If the 100K pot has the input across it and the wiper position feeds the 470K/10K input resistor/blocking resistor then this maybe OK. I need to check how I wired the pot in.

Just looking at the preamp I have wired the input to the top of the pot with the bottom grounded. The wiper feeds the 470K/10K combo. One other thought. I soldered the two 500K resistors across the output RCAs to ground. There is about 6' to 8" of wire coming from the cct. brd. to the output RCAs/500K combo. Could this be the problem.

Should the 500K load resistors be on the cct. brd? They are 1/2 watt carbon types. Or could these be picking up the hum right where the 24VAC comes in? I may get a chance to look at this tonight.

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PostPosted: 11 Sep 2011, 16:23 
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mark, the placement of your RCAs right next to that little power trannie bothers me a bit.
can you pull the lid and move the whole shebang away from the trannie to see if that helps with the hum?

my wifes plans took care of any audio work i had planned for the weekend. not to worry, theres always after work :)
have to find a darn hole cutter as well

noted the stuff with the wire wrap. i've used a form of wire wrap (tinned cable 'wrapped' around the legs of the components from point to point) on the power supply cct but the actual wiring of the tube(s) i will do with the components on the legs of the valve socket(s) and then wired direct to the RCAs with only power coming (from the other end of the case) to supply heaters and +250v. i bought a little plastic jaycar box to house the whole shebang in

i'll ask my mate who works there to get some 250k double gangs mark. log or linear or both? :)

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PostPosted: 11 Sep 2011, 18:05 
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I ended up ordering 250K Apls blue velvet from the states plus a whole ot of other stuff. What I may do in the intrim is put the pot back on the output and add a 100K resistor from the output cap to the pot. I know I will be losing some of the gain put I don't need all of it anyhow. At least I know this works and the preamp will see a 200K load.

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PostPosted: 11 Sep 2011, 18:38 
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I was wondering if you could do that......but don't know enough about this field to suggest it.

Those alps blue velvet tackers certainly seem to be a good bit of kit that's for sure.

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PostPosted: 11 Sep 2011, 23:53 
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Some success: After trying out the pot on the input and 500K R as load off the 470nf cap I have swapped it back. The pot is back on the output and the input is feed directly to the 10K grid blocker. With the pot on the input I had a steady hum which I could not remove. Now the hum has completely gone.

Because Matt suggested better performance with a load of 250K and my output pot is 100K I have added 120K R from the output cap to the top of the pot. I am aware that I will lose half my gain but with the 12AT7 in place I have too much gain. I am listening to the MKIV Universal with a US made GE12AT7. A 2011 recording of Schubert no 9 with the Budapest Festival Orchestra on SACD is playing.

The sound is big and dynamic still bright but not unpleasant, very detailed, crisp! Like a water colour painting. From the GE 12AT7 I went back to the GD 12AU7; big rich sounding mids, deeper sound stage but narrower. Then a Phillips 12AT7, this fell some where between the GE 12AT7 and the rich sounding GD 12AU7. Now an Australian made Miniwatt 12AX7, same as I have in my SET power amp. Again this tube is not as rich in the mids as the 12AU7 but closer to the Phillips 12AT7 but with a little more sound stage. The 12AX7 seems to have a foot in two camps; good mids and wide deep sound stage.

Oh well there is some very early tube-roll'n notes. Beauty is the tubes can be pulled out hot and the next dropped straight in. I have some new Gold Lion 12AX7 and 12AU7s coming. I'll let you know how they compare.

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PostPosted: 12 Sep 2011, 06:14 
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mark where did you score your 12AU7's from?

bit of progress tonight, got the valve sockets mounted, heater wire is twisted and ready to go. outputs on the PSU PCB are done so I can hook them up easily. i've some pics which have disappeared on the interwebs somewhere which will hopefully appear soon and i'll put up here. need to also mount the RCAs, wall wart plug and pot in the case and we should be good.

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PostPosted: 12 Sep 2011, 06:42 
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Evatco in QLD Australia sell Gold Lion 12AU7s (about $24).

After some more listening to the MKIV Universal I have gone back to the 12AU7. To sum up a lot of listening and tube rolling this is what I have found so far:

1 - the best tube is the 12AU7 (GD at this point)
2 - the MKIII The Black is the preamp to beat.

I found the 12AT7 and 12AX7 a bit edgy in the MKIV Universal. But the 12AU7 in the MKIV Universal was really good. If I hadn't heard the MKIII The Black I would think this the very best. The MKIV/12AU7 is what I listened to for the rest of the afternoon. I'll hold final judgment until I get my Gold Lion 12AX7 and 12AU7s and listen to a lot more material. I found some more tubes to roll too.

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PostPosted: 12 Sep 2011, 16:28 
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Cool

Mark I also found http://lucasmiles.com.au/
There is also a nice fellow in Wallan (on ebay) who has quite a few bits and pieces too.

Teh internets also found one of my pictures the other is forever lost in the ether (attached) --> thats the PSU CCT.
I got the two valve sockets mounted I found a perfect sized holesaw compliments of the father out law which is 20mm and about spot on.

All being well tonight I will wire up the shebang and test my voltages :)

Tell me, why did the placement of the potentiometer cause so much hum - sorry as I'm new to this just trying to understand why its placement in the circuit caused so many issues - as I've seen pots on both the input and output sides of different schematics along my 'journey' thusfar?


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PostPosted: 12 Sep 2011, 16:53 
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Grega: Firstly thanks for the Lucas Miles link. Their shop is about 50mins drive for me and I must keep it in mind.

Jaycar have a stepper drill and so does Mitre 10. These are dead easy to use and drop through, Al, steel, wood and cct. brds with no problems or damage. You can drill holes for miniwatts and octals with it.

Like your images has vanished with no explanation there is NO reason why I got hum when moving the pot to the input. Somehow there was induction into the output cct. I tried a few different things to get rid of it but no luck. As soon as I moved the pot back to the output the hum went. It all works perfect now.

Make sure you ground the tranni frame. Also just taking another look at your PS put a 0.1uf brown Jaycar poly cap across either both the large filter caps or at least the last one. Reason is cheap big electros are noisy. This cap reduces that noise and helps with the overall sound of the preamp. My drawing has this snubber cap 0.01uf but that is wrong it should be 0.1uf but you need a cap which will take about 300V.

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