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Old 28th July 2013, 06:10 PM   #841
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I have just make Mortens idea, of fixed resistor instead of the trimpot in the CCS
volt is stable ..one channel 124 v ,,the other 120 v
much more stable than the trimpot
Bjarne
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Old 28th July 2013, 08:20 PM   #842
sjs is offline sjs  United Kingdom
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During my recent rebuild I changed the trip pots for 20 turn units and that has made adjustment much more sensible.

Fixed resistors are a great idea but they require a mu tube with matching sections and its likely the fixed resistors only match one specific mu tube which makes swapping tubes more interesting!

If you have noise on one channel it could be a noisey resistor
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Old 28th July 2013, 10:14 PM   #843
Desmo is offline Desmo  Denmark
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Quote:
Originally Posted by sjs View Post
Fixed resistors are a great idea but they require a mu tube with matching sections and its likely the fixed resistors only match one specific mu tube which makes swapping tubes more interesting!
Yes, that's also why I did not recommend to do what I did Certainly a 20 turn, or a small resistor and a smaller value trimmer in series is the more flexible solution. That being said, I have swapped mu tubes, and the anode voltage did not change more than a volt or two so no adjustments needed (might be different with other Mu tubes than the ones I have).

Cool to see, that you got the Rod Coleman DHT regs installed... It is indeed a seriously good upgrade for the sound of the Tram II, and I agree on your sonic impression: Much more natural, dynamic and lifelike. People who say ''they have no real noise issues so don't install them'' are totally missing the point. As said many, many times: These regs are the biggest sonic upgrade for this preamp, the much improved noise performance is just the bonus.

For the slight mechanical sound you still hear, then try different input caps. The teflon caps these preamps come with are really good, but IMHO all teflon caps are slightly mechanical sounding. Duelund (or Dueland as everyone from English speaking countries for some reason call the brand named after Steen Duelund) are not that expensive in these small values.
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Last edited by Desmo; 28th July 2013 at 10:20 PM.
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Old 28th July 2013, 10:37 PM   #844
sjs is offline sjs  United Kingdom
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I was planning to leave the Teflons in place but you have sparked my interest. I have old and new Hovlands, Audyn copper and Rel RT so I will try playing around with them.
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Old 29th July 2013, 04:30 PM   #845
Desmo is offline Desmo  Denmark
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Give it a try..! In my opinion replacing the input caps has less sonic impact on this one than the output caps, but if you are going for ''more natual'' or ''less mechanical'' then I think you will be pleased with some other caps - you have some nice ones to try
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Old 30th July 2013, 04:05 AM   #846
kazap is online now kazap  Australia
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Quote:
Originally Posted by beardman View Post
I have just make Mortens idea, of fixed resistor instead of the trimpot in the CCS
volt is stable ..one channel 124 v ,,the other 120 v
much more stable than the trimpot
Bjarne
Nice. I wonder if you are using 45's or 2A3's and what resistor values were needed? Thanks.
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Old 30th July 2013, 07:22 AM   #847
kazap is online now kazap  Australia
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How about using one 2K 25 turn trim pot in parallel with two 200ohm resistors to yield R=66-95 ohms =80 +/- 14 (using 200-2000ohm range on the potentiometer)? Will this reduce the impact of any trimmer drift with two fixed resistors dominating the load while still providing plenty of adjustment? This is my very first circuit design so be nice

Last edited by kazap; 30th July 2013 at 07:35 AM.
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Old 30th July 2013, 08:09 AM   #848
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Hello kazap
i use JJ2a3 tube,,,,and the value for the resistor is close to 80ohm...to get the 120v right i use a parallel resistor..2.7kohm.
it is not so difficult to do this...
i have not tried to use a trimpot to get the right resistor value...but i think it sounds like a good idea
the fixed resistor have give me less noise ..thats the best parts of it
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Old 30th July 2013, 04:14 PM   #849
kazap is online now kazap  Australia
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Thats great news about less noise.

It looks like 80ohms is about right for 2A3's.

So if using only2A3 tubes and wanting some very fine adjustment with minimal drift from the pot, these values might perhaps work.

All in parallel:
R1=150ohms
R2=180ohms
R3=Trim pot (25 turns) = 20k

Rtotal = 80 +/- 1.4 using the trim-pot from 2k-20k range
via ~20 turns gives +/- 0.14 V/turn
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Old 30th July 2013, 05:35 PM   #850
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these value looks fine...remember to let the pre warm up ..takes about 15min..before it is stable to measure
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