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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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 Post subject: AudioNote Kit 1 - 300B
PostPosted: 18 Dec 2020, 09:54 
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Joined: 18 Dec 2020, 09:25
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Hello,

This is my first post on this forum so please be patient with me ;)
I've started to build tube amplifier based on AudioNote Kit 1 schematic - SE 300B. I've purchased ready PCBs on eBay and started to put everything together. I pretty much done but Ive faced an issue that Im not able to overcome myself.

Attachment:
Kit 1 Schematic.pdf


Voltage on 5687 tube on pins 2 and 9 is ~400V instead 200V, while pin 6 is 0V instead of 8.4V. Other voltages I've checked are more or less inline (~5% higher). Im pretty much stacked here so any comments or suggestions would be highly appreciated.

Thanks for help.

Cheers

Tomek


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PostPosted: 18 Dec 2020, 17:08 
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What this means is that there is no current flowing in the SRPP stage.

You said the pin 6 voltage is zero. The only way this can be is if there is no current through R17. So that's a clue. The second clue is that your grid voltage on the upper triode (pin 2) is essentially the same as the plate (pin 1) and I assume the cathode voltage (pin 3) as well.

There could be a few reasons for this. One is a bad tube. You were not clear as to whether this was happening in one or both channels. If only one channel, try swapping tubes. If it's happening on both channels there is a more systemic problem.

Now I'm going to suggest you check something and I don't want you to take offense. But a common problem with some circuit boards is that the tube socket gets soldered to the wrong side of the board. So check that.

There is also something on the schematic that is unclear. The heater is marked as pin 4 and pin 8 on the tube, but the note on the transformer winding say 4 and 5 on the 6.3v winding. For this to work properly pins 4 and 5 should be shorted together and the 6.3v winding connected to pins 4/5 and pin 8. Otherwise only one of the two triodes will be heating up.

A picture of the circuit boards, both top and bottom, would be helpful as well.

I hope this helps.

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PostPosted: 18 Dec 2020, 21:02 
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Hi, I would check the heater connections first (easiest too). Not heat on one triode=no conductivity. Pins 4 and 5 ddo need to be conencted together.

Good lisitening
Bruce

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PostPosted: 19 Dec 2020, 16:54 
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Joined: 18 Dec 2020, 09:25
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I've just checked and you were right. Pins 4 and 5 were not connected together so only one heater was working. Now the voltages are inline with the ones on the schematic. Thanks for your quick help on this one and Im astonished how quickly you got it right :)

I've done a sound check and Im disappointed. First thing there are cracking noise in the speakers when the music is playing especially around the bass. Speakers are quiet without music. The louder the music the louder are the cracking noises. Any ideas where to start looking?

Thanks
Tomek


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PostPosted: 20 Dec 2020, 14:16 
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tokl wrote:
First thing there are cracking noise in the speakers when the music is playing especially around the bass. Speakers are quiet without music. The louder the music the louder are the cracking noises.
Are the cracking noises in one or both channels?

If there are crackling noises in an amp, the first thing I look for is cold solder joints. So inspect all the joints to make sure they all look ok.

After that, in this design, look at the heater lift for the 5687. It should be around 85 volts according to the schematic, However, according to the Tung-Sol and Raytheon data sheets, the maximum heater to cathode differential is +/- 100v. With the upper cathode at ≈208v this puts the differential at 208-85=123v. The cracking you're hearing could be heater-to-cathode breakdown in the SRPP.

With the voltages in this circuit, you want that heater lift to be about 108v. This is right at the limit for both the upper and lower triodes. To get this lift, based on schematic values, R2 needs to be about 109kΩ, not 87kΩ. The closest 1% value is 110kΩ. This may or may not be your problem, but it is an error on the schematic.

The other thing to look for is overdrive. The 6SN7 stage has a gain of about 14.5. So depending on your input voltage and your volume setting, you could be overdriving the SRPP. You might try a lower input signal. Since the SRPP is biased to about 8.5v, I would suggest an input signal of not more than 8.5/14.5 ≈ 0.586 Vpeak or ≈0.415 Vrms.

I hope this helps.

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