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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 11 Mar 2019, 17:43 
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This suggest that the 1k feedback makes the effective impedance accross 680ohms smaller and could be cutting off earier than 2.3hz
https://www.electronicproducts.com/RC_F ... lator.aspx

If you make the 100uF bypass to 220uF or 330uF that could solve the plorbem but is only a guess.


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PostPosted: 12 Mar 2019, 08:57 
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Hi, Unless you have really unusual speakers the NHB is sort of optional. All of these amps I have measured respond flat to about 8 HZ with it in place. Since it is there to stop potential resonance issues that could possibly occur in the range of 75KHZ with peculiar loads you can leave it disconnected. In the bigger amps I routinely (and in the kits) include a NFB defeat switch. Some folks like the sound better. You can increase the cap if you want. My data indicates that 100uf is more than adequate for flat response belows 20HZ . Try increasing it. If you like the result, great, that is what DIY is all about.

Good listening
Bruce

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PostPosted: 02 Apr 2019, 12:04 
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Guys,
what voltage rating should the capacitors in NFB have?


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PostPosted: 02 Apr 2019, 12:58 
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Hi, Anything really 25 is good enough.

Good listening
Bruce

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PostPosted: 02 Apr 2019, 18:45 
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Typically less than 5v maximum for cathode bypass capacitors for small valves, typically has arround 1-3v accross the capacitors.
As long as you have voltage rating of 10v or more should be fine.

EDIT: If you want to know the actuall voltage accross the bypass capacitor for a piece of mind, just measure it with a multi meter


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PostPosted: 03 Apr 2019, 12:33 
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Thank you both for the response, I should have measured before I asked.
BTW, today I connected multimeter to recheck balance in both pairs of EL84 and noticed, that in one channel it is 8mV vs. 75mV. Seeing that I turned balancing pot to the end, but still have 15mV vs. 68mV, so cannot achieve 42mV . I was wondering this may be something wrong with the other components than tubes, so I switched this pair to other channel, but still same bad.
Does it indicate that one tube in pair is poor?


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PostPosted: 03 Apr 2019, 17:59 
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If the one side balanced with the swapped tubes and the problem followed the tube pair that would not balance then yes it is a tube issue. If the problem stayed in the same side (did not change when the tubes were swapped and they would not balance either) then the circuit would have to be at fault. BTW the most common way a serious balance problem would occur (other than tube problems) is if one or the other grid is not "seeing" a good ground conenction. Soldering problems seem to be the most common way that occurs.

Good listening
Bruce

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PostPosted: 18 Jun 2019, 16:03 
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Hi Bruce,
The problem I mentioned in April was apparently tube issue, it completely disappeared once I used other tube set.
However last days I noticed by the chance another problem... :( Namely in one channel higher tones are slighly quieter than in another. It is not very obvious, but noticerable.
To eliminate speaker issue I swaped them, but the problem occured in the same channel. Also disconnected NFB, problem still occurs.
Which parts can be suspected?
I'm aware this is DIY, but loosing hope that this beauty amp will ever play properly :(
Any suggestions will be appreciated.


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PostPosted: 18 Jun 2019, 16:54 
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How did you connect the LM317.
You cannot rely on the tab to conduct the electric, you must use the pins on the legs.
Otherwise overtime the one will not work properly due to poor connection


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PostPosted: 18 Jun 2019, 16:58 
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LM317 pins are soldered to wires and those to rheostats.


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