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Old 24th February 2011, 04:14 PM   #11
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Thanks guys. I will have this tube tested today and see what they say, I am not complaining at all about it as it sounds fantastic even in this little amp. It just behaves so different than my other tubes and was just looking for some insight on to the possible reasons.

Also my headphones are... 150ohm and the second set is 300ohm.
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Old 24th February 2011, 04:56 PM   #12
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The changes I have made are as follows...
- Changed the stock irf630's with irl530's
- Changed out the fixed 3K resistors at r13 and r14 with 5K pots
- Changed the stock 6800uF 25V at c7 to a 4800uF 35V as it did not make much sense to run a 25v cap on 24V input
- Changed out the two 1000uF 85C caps to 105C 2200uF at c4 and c5 since one of the units is right next to a scalding hot lm317
- Cut the traces between pin 4 and 5 as the filaments are connected which was the cause of a ugly crosstalk problem. Pic below shows the issue, cut traces in red and jumper wire in blue taken from rock grotto
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Old 24th February 2011, 05:24 PM   #13
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Wow, you've done all the right stuff. I would however add a small resistor in series with the CCS pots to avoid saturating the CCS.

Things to try would include increasing the value of R4 and R5 to make the grid current work for you, this could develop some real grid bias. Anything up to 1M might be worth trying, but I suspect the optimum resistance value would probably be a couple of hundred K ohms at most. Cathode bias would be another thing to try with small resistance values..

You could try a 6GM8 with a minor revision to the filament wiring, but I suspect based on your comments about cross talk a largish cap across the filament would be required as well. So maybe that isn't the brilliant idea I thought it was..
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Last edited by kevinkr; 24th February 2011 at 05:27 PM.
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Old 24th February 2011, 07:28 PM   #14
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Well the mods I cannot take credit for, the guys over at rock grotto came up with them. I just can sling solder quite well so followed their lead

They mentioned a 1K resistor across the pots but I did not know exactly what it was doing / they said it was optional so did not include it. You think it is a good idea then? I will add those in when I get a moment.

Only one major thing left, Need to isolate hum in the unit. It is present with any tube but goes away when I touch anything on the unit that is groun or if I touch any ground on the unit with a wire that is connected to earth. I assume it is being caused by the cheapy switching power supply they included.

Last edited by garage1217; 24th February 2011 at 07:39 PM.
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Old 24th February 2011, 07:48 PM   #15
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My little budget gem with the jj gold pin glowing away.

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Old 25th February 2011, 03:13 PM   #16
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Ok picked up another JJ 12au7 gold pin yesterday, same as the one I felt that biased strange / but sounded awesome! And the results? Same as the first! Cannot get it much below 16V but the right channel I had to raise the voltage a little to get it to match. Sounds identical to the first unit so I am very happy. Great little tube!
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