New ST35 build

for the DIY ST35, the Dynakit and every other PP EL84

Biases etc.

Postby elbinster » Fri Feb 26, 2010 8:44 pm

I've been running this SC-35 for a month or so now, and have been quite pleased. I'm using it mostly for TV and music, through a pair of Dynaco A25s. My B+ is running comfortably at about 355V.

Now that things are settling down, I'm noticing some muddy sound in the ~300 Hz range, when running through my TV (Samsung 46a650). Sadly, my only real option for TV sound is to feed through a headphone port, which may be the cause of the problem, since my ipod sound a whole lot better. Even then, the problem really only shows up when I'm listening to music which is busy in that area (specifically, Air is tough to listen to).

I have plenty of headroom, since I rarely run it anywhere near the 100% volume setting, so I wouldn't mind cutting back the gain if it would clean up the sound.

Does anyone here have any ideas on why this one frequency would be bothersome? I've heard the ST35's downfall is it's tendency of favor one frequency. Could biasing be a problem? I've tried different tubes in all spots and haven't noticed a significant change.

I'm still new to tube amps, and my first guess would be that the headphone out on the TV is a bit too loud, pushing the amp into some kind of clipping, creating an abundance of harmonics.

I don't really have a good way to test the TV output, as I have no scope. If you all agree that the TV out is too loud, I may put together a stereo pad and see if that helps, otherwise, I'll just avoid the problem.

Thanks all for the help.
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Postby Thor61 » Mon Mar 08, 2010 7:09 pm

If you have a cable box or satellite type box most of them usually have rca outputs ! Dont use the yellow one thats for video, hooking to them should rule out the TV to aid in tracking down the problem. Good luck !
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Postby EWBrown » Mon Mar 08, 2010 9:09 pm

If you're using the headphone outputs for feeding the ST-35, it may help to resistively load the outputs with a value appropriate for the specified output impedance (could be 8, 32, 120, 300 or even 600 ohms, refer to the owners manual if you have it available.

The unloaded (or too lightly loaded) headphone outputs could be the culprit in your 300Hz "muddy" sound quality - I'd blame the TV, before the ST35.

Line level outputs are not so critical about having to be properly terminated / loaded.

HTH

/ed B
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Postby HiFiGuy » Wed Apr 28, 2010 6:20 pm

The photo you posted looks like you missed one of the 6.3 volt transformer jumper leads, and also if your using resistors instead of a choke it calls for a pair of 100 Ohm power resistors and I only see one installed. Also make sure that your sockets have plenty of solder, I had a problem with the cathode of my 12AX7 socket not having enough solder and there was only 40 volts measured on the plate!! Nice looking board otherwise.
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