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Old 20th April 2010, 07:43 PM   #421
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chrish,
A Very nice build so far.
Your zeners should work nicely. Although I went the route of self protected, I have not had one fail since the change.

In answer to your question regarding the CCS under the FET source, I will say the jury is still out. I am just now getting geared up to do some serious measuring, my EAR in limited tests says the CCS sounded more detailed. There was a brief discussion on this (i couldn't find the thread) that had similar arguments as using a LED for bias. Keep in mind my EAR opinion is only one channel and I only had a resistor swapped in under the FET for a short time. Whether it stays or not will be based on tests and measurements and then finally whether I can listen to it. If I can't listen to it I will through the measurements out and set it so that I can ;-)

Keep Up the good work.
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Old 21st April 2010, 01:38 AM   #422
chrish is offline chrish  Australia
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Thanks for the comments!

I went through this thread and found some comments from Tubelab regarding the same issue. I think his opinion was that it did not produce much of a difference, so went for the simple approach. This thing is already complex enough, so I stuck with the original design. When it is up and working I may consider tweeks, but more components will probably mean more mounting hardware. I want to keep a clean top plate, and drilling blind holes will be difficult without a complete disassembly.

Hope to get some signal wiring done this weekend. Looking forward to seeing some results finally!

Thanks again,

Chris
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Old 22nd April 2010, 07:40 AM   #423
chrish is offline chrish  Australia
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Just a quick update...

Managed a little time this afternoon to do some wiring. Have wired up the input stage to the MOSFETs. All appears to be OK, with nice clean output from both diff amp stages and the MOSFET source followers. B+ regulating at 395V - close enough to the 400V I was aiming for. Too much hassle to change the set resistor. Hopefully I will finish wiring the output stage some time in the next day or two.

Cheers,

Chris
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Old 23rd April 2010, 03:34 AM   #424
chrish is offline chrish  Australia
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Finished the output tube wiring this morning. I wired it triode mode, strapped with a 470R resistor.

Brief problem with oscillation. Was oscillating at some combinations of volume and frequency. No problem with output tubes removed, so took a guess that it required some grid stoppers. First thing I tried was some 680R carbon comp right on the grid pins of the output tubes. It appears to have solved the problem.

Something else I noticed, when I drove the amp in to clipping, the 75v voltage regulator tube began to dim. I am guessing that the MOSFET source followers are drawing more current here than the VR circuit can handle. My initial impression is to leave it as-is, and use it as a visual clipping indicator. Any problems with this?

I tried playing music through it. Just iPod through a bookshelf speaker, but it worked.

I will tidy up the wiring this afternoon and post some shots.

Cheers,

Chris
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Old 23rd April 2010, 01:44 PM   #425
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Quote:
Originally Posted by chrish View Post
but it worked.
Good work!
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Old 23rd April 2010, 02:45 PM   #426
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Quote:
Originally Posted by chrish View Post
Something else I noticed, when I drove the amp in to clipping, the 75v voltage regulator tube began to dim. I am guessing that the MOSFET source followers are drawing more current here than the VR circuit can handle. My initial impression is to leave it as-is, and use it as a visual clipping indicator. Any problems with this?
I would say it's just right. Maybe put a scope probe on the rail to make sure it doesn't spike up and out of regulation during recovery, but it shouldn't as long as tube doesn't completely go out and need to re-strike. You aren't going to listen to sine waves much, I suspect.
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Old 23rd April 2010, 04:01 PM   #427
chrish is offline chrish  Australia
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Thanks!

I hooked it up to the main system, and it plays. Have a few minor issues to sort out, but isn't that half the fun? I have two versions of the 105v VR tube. Some RCA that have a nice pale purple glow and some Russian, with a neon orange. The Russian ones glow, with the glow flickering with the music. The RCAs, one will strike up, but end up going out after a minute, and one will not strike at all. Maybe have to tweak the current limit resistor. Had calculated 500R required. I suspect the power supply is sagging with the load and the limit resistor is dropping too many volts to allow the tube to strike. Fortunately I have a high wattage 1K variable in this position (the dark green one in the photo).

Worst problem is the amp has a high pitched "whistle" coming from the speaker. This whistle comes on and off according to the load on my Mac Mini computer that is doing duty as a home theatre PC. If I turn the Mac Mini off, no whistle. I suspect the problem is from its laptop-style external power supply. Switch mode? Anybody had any problem like this?

Also need to drill some vent holes in the bottom plate. There is quite a lot of dissipation going on, and I only have a slot above the top of the heat sink at the rear for ventilation.

Some photos. Tubes are Russian "test tubes", have Sylvania 6SN7 and Philips JAN 6L6WGB to replace. Yes, with the amp working that is a hint of 'purple glow' on the top of the 6P3S

Red and black probe jacks behind tubes for bias sense. The two holes to the right of these allow a small screwdriver to access multi-turn trim pots for the bias adjust.

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Last edited by chrish; 23rd April 2010 at 04:05 PM.
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Old 23rd April 2010, 04:12 PM   #428
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Again, very nice work. Will you be testing harmonics and such? Very curious to compare results.

I also hear a "whistle" from my HTPC periodically. Using a MAudio 192 card for that, so I came to a similar conclusion that it is related to the computer PS. Probably will build an external DAC one day to solve the problem.
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Old 23rd April 2010, 04:41 PM   #429
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The source is probably the power supply, but how it is getting into your amp may be more of a ground loop issue. PCs are generally grounded as well. Is the PC and amp plugged into the same outlet/power strip/etc?
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Old 23rd April 2010, 11:52 PM   #430
chrish is offline chrish  Australia
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Yes, there is a lack of power outlets so all tv and hifi are connected to the same outlet. Agree it is probably a ground loop issue. Have the signal ground connected to the chassis at the input end and have used star grounding. Apart from this sound, it is quiet. I have another home made tube amp plugged in to the same outlet (baby Huey EL84) that is dead quiet. I may try a signal ground lift circuit to see if that helps.

As for distortion analysis, I don't have any fancy test gear, just a second hand scope, a cheap audio oscillator that makes pretty good sine and square waves and a couple of reasonable DMMs. Square wave response looked pretty good at 1kHz, with just a hint of ringing at 10kHz. Top of the square slopes down ever so slightly, indicating a little HF rolloff? Will post picks of response later. BTW, not using any feedback at present.

Will keep posted!

Chris
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