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Old 8th April 2012, 10:19 PM   #11
DF96 is offline DF96  England
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I assume whoever swapped 2k for 28k was simply trying to drastically reduce the gain and didn't bother to think about other consequences (or maybe didn't even realise that there might be other consequences). A low triode anode load gives poor PSRR, and increased distortion. Maybe he was after 'tube sound'?

The 6111 has a mu of about 20, so that is the natural gain of the valve when used in a well-designed grounded cathode circuit. Far too high for a line stage, but there is no easy way to reduce it without serious knock-on effects:
1. reducing the anode load ruins PSRR and increases distortion,
2. removing cathode bypass (so adding cathode degeneration) increase output impedance so reduces load driving capability, and also ruins PSRR by raising anode impedance,
3. adding feedback by turning it into an anode follower seriously reduces input impedance so might not always be appropriate.

Possibly the best option is to use the original 28k, then follow the stage with an attenuator to throw away most of the gain. This may sound daft. It is daft, but it still may be the best option for someone who has set his heart on a grounded cathode line stage with very little gain. It retains PSRR and distortion (instead of making them worse), reduces output impedance, and allows for easy adjustment of gain. Or just ditch the line stage; in my opinion this is the best improvement.
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Old 8th April 2012, 10:53 PM   #12
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OK I'll try the 28k and report back - mu of 20 isn't too bad for a linestage (depending on the vrms of the source of course).

I might also increase the value of R2 & R3 and put them on the other side of R1 & R8 - as 'DGTA' suggested, they appear to be intended as grid stoppers, but the values are neither high enough nor are they in the correct position.
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Old 9th April 2012, 12:09 AM   #13
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Update - no luck, disconnected the connection between circuit ground and mains earth, also increased the anode load to 28k, but the buzz persists.

What is the next step - without access to a scope, all I can really suggest for a measurement of the noise is that it is about 440Hz - close to the 'middle c' sound in music. Does this help at all?

The other possibility is that it could be a pin misplacement? The filament is fine, the gain and sound is excellent - but given that there are no markings on the pins, could one of them be misplaced and result in this kind of noise?
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Old 15th April 2012, 01:05 AM   #14
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Another update - I notice an LED came with the kit which I have not used - could this be important for the bias (not on schematic)?
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Old 15th April 2012, 01:56 AM   #15
dgta is offline dgta  United States
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No. With 2 channels and 1 LED, it is improbable that the led is part of the circuit, perhaps it is a power indicator. On the other hand, a kit where the components don't match the schematic is a red flag of a poor quality supplier. Anyway, that circuit should work and not make noise regardless of led biasing or not.

First make sure the noise you hear is in the line stage and not the subsequent power amp.

Then put an ac voltmeter on the power supply rail and see how much ripple you have. The ripple should be 100Hz, not 440. Make sure your AC voltmeter can take the DC present on the rail, if not use a high voltage cap of a few uF in series with the leads. If you have significant ripple, you can test for that. Use a high voltage cap to connect that ripple to your power amp and see if it's the same noise.
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Old 15th April 2012, 06:36 AM   #16
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Hey,

Why do you call the thread with "6111/6N16B" which are two different miniature tubes when the thread is only about 6111?

Whereas 6111 is a really unlinear tube, 6N16B is acceptable in that respect.

If you have the chance to switch from the mediocre 6111 to the better 6N16B(but at descent anode-voltage), do it!

Quote:
I'll take out the 2k for R5/R9, what is the optimal value for these resistors?
As the whole circuit is suboptimal I suggest you start from the beginning using the Ua/Ia-curves to find the resitorvalues. Together with a realistic B+ of over 100V it might work, allthough with an all to high gain.

Another idea could be to use one pcb per channel and reconstruct to a GG stage with some NFB to get gain down to an acceptable level.

Last edited by revintage; 15th April 2012 at 06:52 AM.
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Old 15th April 2012, 09:03 AM   #17
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Quote:
Originally Posted by revintage View Post

If you have the chance to switch from the mediocre 6111 to the better 6N16B(but at descent anode-voltage), do it!
Another idea could be to use one pcb per channel and reconstruct to a GG stage with some NFB to get gain down to an acceptable level.
I have a whole bunch of 6n16b awaiting my attention, so that is easy. I also have two spare pcbs, so the idea to use one per channel is a distinct possibility. I also have a transformer which has secondaries of:

60v-0v
60v-0v
6.3v-0v
6.3v-0v

This will get us a B+ of 85v dc, could this be enough to work with?
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Old 15th April 2012, 09:36 AM   #18
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Double the 60+60V to get well over 200V B+ as the 6N16 probably will ask for up to 100V Ua to perform.

Last edited by revintage; 15th April 2012 at 09:43 AM.
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Old 15th April 2012, 09:55 AM   #19
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So you mean disregard the 0v windings of the two 60v secondaries and connect the 60v windings (= 120v AC) then use a voltage doubler?
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Old 15th April 2012, 10:35 AM   #20
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Yes, but you can also try rectifying the 120VAC and go for a lower Ua. Better to check the UaIa-diagram first.
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