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Old 25th October 2011, 07:07 PM   #501
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The input grounds are connected elsewhere in the circuit - near the input valve grounds, and the output grounds kept separate and connected to a different star point near the output valve cathodes....

But other than the grounding, you have it right. But as you go actually building and figuring out the grounding as you go, it will become clearer.

I didn't ground the case of the 977 as in that pic....

Fran
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Old 25th October 2011, 07:12 PM   #502
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Thanks Fran.
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Old 26th October 2011, 08:39 AM   #503
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Default Help with my Itch

I've built the Valve Itch but so far with disappointing results! Music signals are coming through but they are drowned in weird, hum-like noises. I'm driving it from one of these ready-made HV regulated supplies on eBay. (The SSHV Mosfet shunt regulator will come later.) After checking my circuit and connections I took some voltage readings, some of which I don't understand and would like to check them out with subscribers to this thread.

A. Upper Valves:
1. 6SN7
Anodes: 286V / 288V
Cathodes: 186V / 187V

2. 6N2P-EV
Anodes: 216V / 217V
Cathodes: 3.9V / 4.0 V

B Lower Valves
6SN7 Anodes: 5.3V / 5.4V
6N2P Anodes: 1.68V / 1.69V

Then I checked the heaters and ground lift. The lower valve heaters to gnd are OK at 6.13V.
The upper valve heater readings are my concern. Across the heater valve pins it is 6.14V. However from valve heater pins to gnd there is only 0.6V.
I checked the ground lift network and across the two 470K resistors I measured 289V as it should be. However, across the individual resistors (R7 and R8//C12) I only get 0.6V.
All measurements taken with a DVM. I also checked C12 (1uF/250V) for short circuit and capacitance and it was OK.

Can you please throw some light on these readings. The upper section voltages seem OK to me. But the lower section anode voltages to me seem too low. I was expecting 50-100V, but I'm not technically informed about this topology. More confusing are the readings for the heaters of the upper section valves.

I enjoyed building this project and I'm sure here I'll get the help needed to get it going properly. Many thanks to Salas and a whole group of contributors that have made and continue making this thread so interesting.
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Old 26th October 2011, 12:25 PM   #504
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Thinking about it over lunch I swapped the heater regs and took readings with a different DVM. Now I have 139V from the -ve end of the upper section heaters, which is more credible. I also checked the anodes of the upper first stage (6N2P) and this time it is 268V and 270V. Previously I had a similar problem with widely different readings from different meters but I'm still not sure these are the correct voltages for the Valve Itch.

I still have to connect it to my system to see whether it is OK. Before doing this I'd still like to know what voltages to expect around each section valves.
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Old 26th October 2011, 01:10 PM   #505
Salas is offline Salas  Greece
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You look for about 80V across each 6N2P upper and lower, for about 90V across each 6SN7 upper and lower. With 300V B+ that is.
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Old 26th October 2011, 03:16 PM   #506
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Thanks Salas, prompt and helpful in a practical way, as always. I took several readings within a time span of 5-7 mins and between the two 6N2s I get 80-87V but between the 6SNs 115-135V, which is rather high. Any ideas as to why?
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Old 26th October 2011, 03:43 PM   #507
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Look across, not between. I.e. From plate to cathode on each. Don't know, some wrong connection? If it was alright the sound would be clean. Trace it against the schematic again.
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Old 27th October 2011, 08:34 AM   #508
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Thanks Salas, this is to let you know that my Itch build is working without the hum and buzz noises I reported earlier. Don't really know where the problem originated from but I suspect it had to do with my wiring of the upper valve heaters section.

It hasn't been playing for more than half an hour and at first I thought it didn't have the airiness of your NJ-Fet phono which I also built (with a lot of your help). Surface noise at the start of an LP is more pronounced and bassy. My Itch sounds rather closed-in, though it seems to be opening up as it runs in. It can also be the components I use.

My build is as per your Rev1.1 but I'm using 0.1uF Teflon for interstage coupling (C4) because it seemed the best I had available. I've ordered some 0.047uF K72P-6s hoping the sound gets a bit mellower.

For C2 (RIAA) I used SCR MKPs 5% and for C3 used 12nF//3.3nF 2.5% PS (polystyrenes), again because I thought they were the best I had. I'm waiting for Merlin's GB of these caps - Thanks again Merlin.

I still have to build your SSHV Mosfet so in the long term it should sound better. Apart from the enjoyment of building it, yes, I think it's opening up as I listen. I like its full and bodied character, just needs to be a bit mellower for my taste. I'm sure it will improve with the component changes planned above. Any guidance regarding specific and priority changes would be much appreciated.

Thanks a lot and Best Regards.
Joe A
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Old 27th October 2011, 08:47 AM   #509
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Subjectively, for me at least, the RIAA and coupling cap does have a pretty large effect on the sound. Its not profound, but definitely changes the tone of the build.

Like you, I am interested in hearing the comments back from the caps from the group buy, although I don't have a burning desire to change my build!!

More than one person has offered to pony up the cash for me to build one for them!!

Fran
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Old 27th October 2011, 09:40 AM   #510
Salas is offline Salas  Greece
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Quote:
Originally Posted by sonata149 View Post
Thanks Salas, this is to let you know that my Itch build is working without the hum and buzz noises I reported earlier. Don't really know where the problem originated from but I suspect it had to do with my wiring of the upper valve heaters section.

It hasn't been playing for more than half an hour and at first I thought it didn't have the airiness of your NJ-Fet phono which I also built (with a lot of your help). Surface noise at the start of an LP is more pronounced and bassy. My Itch sounds rather closed-in, though it seems to be opening up as it runs in. It can also be the components I use.

My build is as per your Rev1.1 but I'm using 0.1uF Teflon for interstage coupling (C4) because it seemed the best I had available. I've ordered some 0.047uF K72P-6s hoping the sound gets a bit mellower.

For C2 (RIAA) I used SCR MKPs 5% and for C3 used 12nF//3.3nF 2.5% PS (polystyrenes), again because I thought they were the best I had. I'm waiting for Merlin's GB of these caps - Thanks again Merlin.

I still have to build your SSHV Mosfet so in the long term it should sound better. Apart from the enjoyment of building it, yes, I think it's opening up as I listen. I like its full and bodied character, just needs to be a bit mellower for my taste. I'm sure it will improve with the component changes planned above. Any guidance regarding specific and priority changes would be much appreciated.

Thanks a lot and Best Regards.
Joe A
Try couple with 0.047u anything and make sure your actual Riaa HF cap value is ~15n. Maybe a straight 15n cap. C2 can turn the curve heavy if its closer to 50nF+ for a 5% cap, check. Are those polysterenes taking well the DC? There is ~200V at the point. The NJFET phono pre is a tough challenge to meet fast, but take your time with the Itch to balance it out first. Is your cartridge properly loaded on the Itch last but not least?
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