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Author Topic: Dropping String....a little help, please.  (Read 6138 times)
ChrisL
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« on: May 31, 2011, 07:27:44 AM »
After years of enjoying the clean channel on my original 50W OTS.....(one of the first few OTS amps) ... I'm finally addressing the OD channel to smooth and tighten it up a bit.  I've upped the first two PS filter caps to 200uf, slope to 100k, V2 plates to 220k and 150k, 330pF snubbers, 68pF master cap.  C'Lator is in the loop.....It's getting there....

  Right now V1 plate voltages are at 195/208v...(good)  V2 is 184/200v (low) and PI is a little high at 299/302.  As simple as Ohms Law is I'm a little unsure how to adjust the values of the 2k2 and 22k dropping resistors for V2 and V3 to bump them to optimal Dumble values with the higher V2 plates.  B+ at PS2....PI trimmer is 423v.  Voltages didn't change much with a 3k3 in place of the 2k2.  Any help would be much appreciated.  My D'Lite is up for similar fine tuning so I want to get a better handle on this subject.

  Thanks
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erwin_ve
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« Reply #1 on: May 31, 2011, 08:03:41 AM »
Measuring voltages is highly dependend on what kind of pre-amp tubes you use.
Fe. JJ 12ax7 tend to draw more current than Tad 12ax7. So voltages will be different, sometimes up to 15 volts(!). You can swap tube v1 and v2 and see what happens.


Btw if you use a Clator try lowering your snubbers to 270pF.


Quote from: ChrisL on May 31, 2011, 07:27:44 AM
After years of enjoying the clean channel on my original 50W OTS.....(one of the first few OTS amps) ... I'm finally addressing the OD channel to smooth and tighten it up a bit.  I've upped the first two PS filter caps to 200uf, slope to 100k, V2 plates to 220k and 150k, 330pF snubbers, 68pF master cap.  C'Lator is in the loop.....It's getting there....

  Right now V1 plate voltages are at 195/208v...(good)  V2 is 184/200v (low) and PI is a little high at 299/302.  As simple as Ohms Law is I'm a little unsure how to adjust the values of the 2k2 and 22k dropping resistors for V2 and V3 to bump them to optimal Dumble values with the higher V2 plates.  B+ at PS2....PI trimmer is 423v.  Voltages didn't change much with a 3k3 in place of the 2k2.  Any help would be much appreciated.  My D'Lite is up for similar fine tuning so I want to get a better handle on this subject.

  Thanks
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ChrisL
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« Reply #2 on: May 31, 2011, 09:11:18 AM »
Thanks Erwin.  Yes, I have been swapping tubes and will continue to do more tonight.  Right now I have Mullards in V3 and V1 with an old Baldwin in V2.  I have plenty of new and old pre's to mix and match however this combination gave the best results to get the voltages closest to optimum.   I actually have 270pf on half of V2 and 330pf on the other at the moment but will drop that down, too.   
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erwin_ve
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« Reply #3 on: May 31, 2011, 11:26:49 AM »
Hi Cris,

If that doesn't help: to lower v1 voltages you can make the last dropping string resistor a 3k3 or a 2k7(the one at at the side of the chassis near tube v1).
To lower the voltages for the PI: the 2k2 (you already swapped for a 3k3) can be lowered to a 3k9, but this will also lower the voltages at v1 and v2.
Make sure you check your wall voltage and B1 voltage; especcially if you're living in a 110/120v wall voltage area. 5v more or less (in a 110/120v wall voltage situation)does make a big difference in the B+ voltages.

Erwin

Quote from: ChrisL on May 31, 2011, 09:11:18 AM
Thanks Erwin.  Yes, I have been swapping tubes and will continue to do more tonight.  Right now I have Mullards in V3 and V1 with an old Baldwin in V2.  I have plenty of new and old pre's to mix and match however this combination gave the best results to get the voltages closest to optimum.   I actually have 270pf on half of V2 and 330pf on the other at the moment but will drop that down, too.   
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ChrisL
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« Reply #4 on: May 31, 2011, 01:02:28 PM »
Wall voltage is right at 100V....right where it should be for Japan.  I either use PT's with 100v taps on most of my builds or use step up transformers for my vintage amps.  Anyway, I have the D'lite on the bench right now and the voltages are all pretty good.  It's a surprisingly good sounding amp for the "little D bastard" that it is. 

  On the OTS.. B+ is 440v.  I did some more tube swapping and as before V1 is perfect at 190v,  V2 still low at 184v and PI at 307v.  Optimally I would like to have them at 200 for V2 and 290v for V3.  I may go back to the lower plates on V2.  I'm going to order a range of dropping resistors to see what I can do.  I would still like to better understand the science behind the calculation to come up with the best value of dropping resistor....(for this amp). 


 
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erwin_ve
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« Reply #5 on: May 31, 2011, 03:06:53 PM »
Hi Cris,

Are you comfortable making a loadline for a 12ax7 tube? The plate resistor/cathode resistor is 66:1. 
When you have ideal voltages there is a point where the amplification is non-linear when the grid signal increases.
When plate voltages are too high you move too far away from that point. When voltages are too low you don't have much headroom.
That's why I would go for changing your dropping string and not the anode resistor because anode resistor swapping is gonna change the characteristics of the the amplification.
Unfortunatly is a bit hard to calculate the dropping string because of the diffrences between tubes drawing current.

Quote from: ChrisL on May 31, 2011, 01:02:28 PM
Wall voltage is right at 100V....right where it should be for Japan.  I either use PT's with 100v taps on most of my builds or use step up transformers for my vintage amps.  Anyway, I have the D'lite on the bench right now and the voltages are all pretty good.  It's a surprisingly good sounding amp for the "little D bastard" that it is. 

  On the OTS.. B+ is 440v.  I did some more tube swapping and as before V1 is perfect at 190v,  V2 still low at 184v and PI at 307v.  Optimally I would like to have them at 200 for V2 and 290v for V3.  I may go back to the lower plates on V2.  I'm going to order a range of dropping resistors to see what I can do.  I would still like to better understand the science behind the calculation to come up with the best value of dropping resistor....(for this amp). 


 
« Last Edit: May 31, 2011, 03:08:29 PM by erwin_ve » Logged
ChrisL
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« Reply #6 on: June 01, 2011, 04:21:11 AM »
Hey Erwin,

   Well, lets not reinvent the wheel.. Wink I know all the tried and true values of the V1 and V2 load resistors of the various generations of Dumbles and pretty much understand how HAD and others established them.  I think I just have so many schematics and layouts floating around in my head that it's hard to make a commitment to any single one of them...lol.  Especailly when you play single coils.. Cry
  The second generation low plate Dumbles kind of intrigue me and I wouldn't mind building an amp to those specs. 

   Right now I think I'm about ready to throw in the towel and finally succumb to the Lerner line of thinking......Dumble tones can't exist in 50 watt amps.   I have a few more tricks to try with this one but I foresee building a 100watter out of it.
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