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Old 13th August 2007, 02:05 AM   #21
anatech is offline anatech  Canada
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Hi sorenj07,
I agree with Johan on your test equipment when he said .......
Quote:
Use those. The scope does not need to be calibrated, must just be able to show something for this exercise.
I am still fairly certain that your ground layout is causing your grief. You do not need designed in feedback to have positive feedback occurring in your system.

Keep high current ground currents isolated and direct to your last filter, your signal; grounds go there as well. Grounds or common points in a stage have your signal grounds for that stage connected closely together. Same for your B+ connections. Make sense?

-Chris
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Old 13th August 2007, 02:36 AM   #22
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got in a longish discussion about stuff over the phone with Eric Hayes from over at DiyTube. During the course of that discussion, I cut the wire connecting PSU ground to signal at the RCA jack and instead tapped off the main PSU cap and went straight up to the signal buss between signal ground and the first 10-ohm 6L6 cathode resistor. the problem disappeared. the tubes are still a bit microphonic but hey, that's life. i'll probably do a few more tweaks such as beefing up the 1W PSU resistors, and changing the forked last stages into a simple RCRC.

still, though, what kind of zener diode could I use to bypass the 10K bias resistor?
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Old 13th August 2007, 02:40 AM   #23
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Quote:
Originally posted by anatech
Hi sorenj07,
I agree with Johan on your test equipment when he said .......


I am still fairly certain that your ground layout is causing your grief. You do not need designed in feedback to have positive feedback occurring in your system.

Keep high current ground currents isolated and direct to your last filter, your signal; grounds go there as well. Grounds or common points in a stage have your signal grounds for that stage connected closely together. Same for your B+ connections. Make sense?

-Chris
I would use the scope but it just makes a dot under AC coupling so maybe this isn't the best situation. Turns out the grounding was pretty much the main problem.
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Old 13th August 2007, 03:35 AM   #24
anatech is offline anatech  Canada
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Hi sorenj07,
Great news!

Now, just go over it with an eye to eliminating small voltage drops in bad places.

-Chris
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Old 13th August 2007, 05:23 AM   #25
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yup. can't wait till the other channel is done!
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Old 13th August 2007, 07:33 PM   #26
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Ahhhhh!

You got it!
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Old 13th August 2007, 10:20 PM   #27
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yeah, I'm just happy that it turned out to be something pretty simple.
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Old 14th August 2007, 04:43 AM   #28
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Click the image to open in full size. Click the image to open in full size. Click the image to open in full size.

well, there they are. both work fine except for some substantial hum from the left one. bias is at 55mA each with a B+ of 500V. I don't exactly know what the bias voltage is, I just twiddled the pot until I got .55V across 10 ohms.

the amps seem extremely sensitive (they're a straight up williamson design). is there a way to skip a gainstage such as paralleling the first two 6SN7 sections to make one beefier phase splitter? I'm temped just to cut out the direct-coupled gainstage, because otherwise I have no volume control unless I build a unity-gain buffer driving a volume pot..
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Old 14th August 2007, 08:27 AM   #29
chrish is offline chrish  Australia
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I am definitely not a valve amp expert, but came across this today while doing some research:

http://www.google.com.au/url?sa=t&ct...zESQ2QXtLcreXA

In his paper he describes the four stage Williamson designs as not at all suitable for DIY as they are on the edge of stability. Might be worth a read.
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Old 14th August 2007, 02:01 PM   #30
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Quote:
Originally posted by sorenj07

the amps seem extremely sensitive (they're a straight up williamson design). is there a way to skip a gainstage such as paralleling the first two 6SN7 sections to make one beefier phase splitter? I'm temped just to cut out the direct-coupled gainstage, because otherwise I have no volume control unless I build a unity-gain buffer driving a volume pot..

Have you applied feedback yet? The standard Williamson has extra gain for all that global NFB it uses.... Also, have a square-wave response plot? That's a nice way to choose feedback cap. compensation values.

Nice work btw...
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