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Old 11th October 2008, 03:46 AM   #21
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Default Here is my 2 cents worth

I experimented with this design in which I subbed the D3a/E180f/E280f/C3G for the first stage. I tried the Ixys 10m45 current regulator but eventually settled on a 33k anode resistor feeding the c3G.The cathode resistor for the C3g was 330R and I used a 1k grid reistor instead of the 100 as depicted.
Like you I had a stock of 6SL7's so after experimentation I settled on an srpp design which for me sounded best.
I built a lot of phono stages including ccs/mu stage and Kimmel mu stage before I settled on this schematic.
All the grounds for this stage went to the ground of the input socket ( i have a bus bar arrangement there) which then went to the central star ground point.

Regards
Nick
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Old 17th November 2008, 07:43 AM   #22
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Over a month later, and I finally have something working. Here's where it is schematic wise. I'll post some pics once I slap a chunk of wood to the front.

I ended up using Jim Hagerman's riaa calculator with split stages. The first go with the combined stage just didn't sound right. I also used two stages of 6GK5 for gain as the 6gk5 to 6T4, as predicted by others, just didn't cut it. I also blew out the mosfets while tinkering, and of course had not bought more. So, the mosfet followers got replaces with a more wholesome cathode follower, and since the 6T4's used the spots drilled for the gas regulators, I resorted to a zener based shunt reg -- at 100V as that is what was in the drawer.

I think the next step is to drill some more holes to put the gas tubes back in as they are more glowey. Maybe a CCS in the cathode of the follower, too.

I think it sounds good -- my table and cartridge are really limiting factors here, so I'll need to find someone with a nicer turn table to really listen to it. But, I have at least had the opportunity to reacquaint myself with some of the more classic recordings in my collection.

Click the image to open in full size.
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Old 17th November 2008, 11:37 AM   #23
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The output CF is very problematic; you've got a super-low load and very little swing capability. Try biasing up the grid 40 or 50V, then resize the cathode resistor accordingly.
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Old 17th November 2008, 04:45 PM   #24
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Remove the gridleak from the 6T4 and the coupling cap from the presceding stage. The CF will then be biased from the 6GK5 and you could use the CCS as a current sink instead of the super-low value cathode resistor.

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Old 17th November 2008, 05:53 PM   #25
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Quote:
Originally posted by SY
The output CF is very problematic; you've got a super-low load and very little swing capability. Try biasing up the grid 40 or 50V, then resize the cathode resistor accordingly.
Good catch, I think. On hot recordings, there is a little crackling that I had not tracked down, so I think you are correct about the source.

Is the attached what you mean? This should bias the grid to ~32V, and I think run just under 20mA though the tube bringing the cathode to ~34 or 35V point. I probably need to check my math again, but I think this is close.


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Originally posted by revintage
Remove the gridleak from the 6T4 and the coupling cap from the presceding stage. The CF will then be biased from the 6GK5 and you could use the CCS as a current sink instead of the super-low value cathode resistor.

It's a good idea, but the plate of the 6GK5 is at close to 120V and this is too close to B+ for this to work here. Also, the PS is pretty noisy so it needs the shunt reg, or something, to keep it quiet. But, in the original version when I had a 6T4 as the second tube (w/ about 70V on the plate), this is essentially what I was doing which was why I wanted that tube there in the first place.
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Old 17th November 2008, 07:04 PM   #26
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Quote:
It's a good idea, but the plate of the 6GK5 is at close to 120V
The 6GK5s with its somewhat odd workingpoint will probably have as much problems as the CF .

This little spreadsheet could maybe be of some help:

www.revintage.se/triodecalc.xls
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Old 17th November 2008, 07:12 PM   #27
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Quote:
Originally posted by revintage
The 6GK5s with its somewhat odd workingpoint will probably have as much problems as the CF .
What's odd about the operating point of the 6GK5?
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Old 18th November 2008, 01:31 AM   #28
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That's better, but 1800 ohms is still a pretty low load. It'll probably work, but a CCS there would drop the distortion a notch or three.

You might also have some overload issues in the first stage, especially during mistracking events.
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Old 18th November 2008, 03:05 AM   #29
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Quote:
Originally posted by SY
That's better, but 1800 ohms is still a pretty low load. It'll probably work, but a CCS there would drop the distortion a notch or three.
It made a world of improvement. I ordered some transistors to add the cathode CCS.

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Originally posted by SY
You might also have some overload issues in the first stage, especially during mistracking events.
It seems OK, but looking at the filter calculations, I think that first Rk is supposed to be more like 60R anyhow which should take care of the possibility.

Butte, MT?
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Old 18th November 2008, 03:09 AM   #30
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Raisin' me up a crop of dental floss.
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