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Noise floor of Tubelab SE

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If you're running out of poop even with a 100V tap, it sounds like there's still not quite enough voltage at the transformer outputs for the headroom you need. The 50 Hz line frequency adds insult to injury by increasing the output ripple for a given load/output filter capacitance. Using Schottky rectifiers and higher filter capacitance are both steps in the right direction to get a little more voltage to work with. Adding a little boost to the primary in the manner I described might also give you what you need. If it were me, I'd bitch at the guy who wound your custom XFMR.
 
I think the problem of running out of headroom for the filament regulators is solved. As you said schottky and extra cap did the job. You can see the big difference between the noise I posted few days ago, and the one from today, showing the 1kHz tone.

The thing I did not decide yet, is if I may replace the 100kOhm potentiometer with a lower one. I remember George saying that 100kOhm was too much for the 5842.

Thanks,

Davide
 
For those that didn't get it, here's a diagram. The buck/boost transformer has its primary hooked across the line voltage. Its secondary is placed in series with the primary of the transformer you want to trim. Depending on the connection of the buck/boost transformer, you can either buck or boost the primary voltage of the transformer you're trying to trim. The secondary current rating of the buck boost transformer must be at least capable of handling the primary current of the transformer you're trying to trim, meaning that in most situations, a modest filament transformer will work just fine.
 

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The thing I did not decide yet, is if I may replace the 100kOhm potentiometer with a lower one. I remember George saying that 100kOhm was too much for the 5842.

Sorry, I spent most of the last 3 days in a car driving 1200 miles, so I was away from this forum.

The resistance of the pot combined with the Miller capacitance of the 5842 can limit the high frequency response at mid volume levels. If you listen with the volume pot near minimum or near maximum with the typical low output impedance CD player, you will never see an issue.

Play some music with a lot of high frequency information and rotate the volume pot. See if you notice a loss of high frequencies near mid volume. If so reducing the value of the pot will fix the problem.
 
No problem George, no need to apologize.

I did some homework to improve my understanding:

I estimated the Miller capacitance as Cm=Cgk+Cgp*(A+1)

I ended up with a value of 120 pF for the 5842.

I then simulate the input of the tube lab SE to see what you mean. These are the results.

This is in the worst condition. I see what you say. Maybe 50k is a better choice, without going to 10k, as I have one source with small (1-5uF) output caps.

I hope I will have again the 300B running in few days.

Thanks,

Davide
 

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Hello all, could use a recommendation/advice here,

My 300B based TSE is humming a little and I'd like to get rid of the hum, so started looking at the heater voltages and their shapes. First I noticed that 5842 heaters are getting about 4V, with 100 Hz ripple, so I shorted the R3 and added 10000 uF cap in parallel with C1. That helped a little, but not enough. The 5842 heaters are now getting DC with ripple (Vmin=4.64V, Vmax=5.44V). Obviously, these voltages above are also found at the input of the regulator.
The output of the regulator shows (Vmin=4.48V, Vmax=4.96V, 100Hz ripple), AC voltage at the 6.3V transformer winding under load is 5.5V RMS. Looks like NOT enough to get a stable 5V DC out of the regulator ;-(
So here is the question - I am planning to add another 2x15000uF in parallel with C1 and implement the trick George suggested for Davide, i.e. replace R1 with a trimmer and decrease the regulator output voltage a little to get rid of the ripple. Since my Vmin at the regulator input is 4.64V, I will probably end up with around 4V DC on the 300B filaments.

Will this affect anything in a negative way? Any other suggestions (before I start looking at the transformer replacement)?

Really appreciate your help here guys,

Cheers,
-Victor
 
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