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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: 19 Sep 2011, 10:22 
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Hi Everyone, yes the other thread did stir up the juices. :? I design both SE and PP amps. I think this is a bit like do you like chocolate or vanilla ice cream. I find advantages to both. SE is often easier to design, but places more severe limits on things like output trannies. PP can be difficult to design for low level performance (where much of the detail and sonic effects we all love hide) but is easier on the iron specs. Most of you have already long known that I prefer PP. But only U/L class A with the capability of precision balance at low levels (0-2 watts). I suggest that many PP designs, some quite famous, go for power output over low level detail. This IMHO makes them quite inferior to a good SE design. Another by product of PP U/L class A is extremely low distortion, I can easily get below 0.5% at power up to about 1/2 of the total output. Most SE designs have more, poorly designed ones much more. This does muddy the water a bit as the nature of the distortions may actually be "pleasant" to hear and be confused with accuracy and excellence. So we are back to the flavor of ice cream again. So build what your ears like and don't worry about what anyone says. :thumbsup:

Good listening
Bruce

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PostPosted: 19 Sep 2011, 10:28 
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Attachment:
PoddWatt with bias pot update March 2011.png
Hi, I missed the earlier question. In the Poddwatt a 50 ohm pot will be fine. In the larger ones (like KT88) it will fail. There is a modification to the Poddwatt only that eliminates this pot and uses positive bias (adjustable) on the grids to balance the tubes. I attached the schematic for the change. The sonic characteristics of the amp are unchanged and you get a bonus of about 1/2 watt more output. The down side is that the LM317 dissipates a bit more heat.

Good Listening
Bruce


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PostPosted: 26 Sep 2011, 13:10 
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thanks for the clarification and schematic bruce.

cheers,
edwin


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PostPosted: 05 Oct 2011, 17:12 
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Hi, Check out the recent posts on the Oddwatt main thread as there is additional information there about the Poddwatts and modifications.

Good listening
Bruce

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PostPosted: 25 Oct 2011, 17:53 
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Hi,
Can someone please confirm the resistor wattage ratings for the podwatt. I know the schematics show 0.5W resistors across the board but the photos on the link below show the pair of 100r resistors as 6.5W each (I think) and the 100k and 4k7 in the power supply look at be either 1w or 2w.

http://diyaudioprojects.com/Tubes/EL84- ... -Tube-Amp/

Thanks,

Paul


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PostPosted: 25 Oct 2011, 20:42 
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Hi Paul, All resistors can be 1/2 watt except those two. They can be as small as 2 watts. I prefer to use 1 watt resistors everywhere except there. Those two resistors must handle the full load of the B+ for the amp and actually are sized like fuses. If a serious B+ fault occurs and it doesn't blow the fuse, the resistors will normally fail opening the circuit. One then the other will fail as regardless of tolerance one will always carry slightly more load than the other. A single 5 watt resistor could be used, but it might not fail unless the fault was extreme. The 100K and 4.7K can be 1/2 watts. For my personal builds I now generally use 1 watt metal film resistors for nearly everything. BTW, on occasion the photos will show part sizes that were what I had on hand at the time and are not necessarily the size that is necessary (never smaller than needed - but might be larger).

Good listening
Bruce

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PostPosted: 29 Oct 2011, 18:48 
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Thanks for the answer about the resistor ratings. I have one final question (for now)... the mains fuse. If my calculations are correct, with a primary of 240V the mains fuse should be fine as a 500mA Slo-blo. Is this cutting it a bit fine? Is there any alternative recommendation?

Thanks again!

Paul


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PostPosted: 29 Oct 2011, 21:12 
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Hi, Your calculations are correct, however, knowing that fuses age and then tend to fail I would use a 1 amp SB or a 2 amp standard. The inrush current in the transformer is fairly high as the B+ supply is all solid state and the filter caps are moderately large. They look like a near short on start up. My experience with fuses is that a value between twice the normal current for slow blow to about 5 times the normal current for standard ones is fine. One thing about tube amplifiers is that if something fails, it is usually rather dramatic and will zap the fuse regardless of type quite quickly. I have 4 amp standard ones in my Poddwatt (120 volt version).

Good listening
Bruce

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PostPosted: 27 Nov 2011, 09:02 
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This is probably the wrong place to put completed poddwatt amps (please feel free to move this post) but I just wanted to say thank you for the support given while I was building this. My fathers Christmas present is complete and sounds wonderful. No hum, no buzz, almost totally true point-2-point... fantastic. :D

The power valves are Russian 6P14P-ER (which seem to bias equally at about 43mA). The preamp valves are Sovtek 5751 as recommended.

Image
Image

Thank you so much.

Best regards to everyone on the forum.

Paul


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PostPosted: 27 Nov 2011, 11:37 
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Hi Paul, This is as good a place to put it as any. Very nice build. :thumbsup: The chassis looks like one I have considered for other projects, but have not used in any projects yet. The only thing I would comment on is that I would have put the power transformer at right angles to the outputs. That is usually the best way to really minimize hum transfer between them. But since it is quite and hum free, not to worry. :) Just a tidbit for the next build. I'm sure it will make a great gift.

Good listening
Bruce

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