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Re: All Lundahl Transformers - 417a Preamp

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In Reply to: All Lundahl Transformers - 417a Preamp posted by Mr. Stone on December 03, 2003 at 06:20:46:

I presume that you mean to use the LL1660 in connection alternative "Q", meaning 4.5:1 stepdown? Be careful with the 417A. Being an RF triode designed to operate differently than we commonly use it in audio, care must be taken to prevent signal dependent oscillation and RF excitation. A "grid stopper" resistor is a really good idea. The value should be chosen by listening, but start at 100 ohms and work up. I would also advise ferrite beads on the heater wires right at the socket and a low inductance ceramic 0.1uF 50v or 63v bypass across the heater pins at the socket. You may also find that a lower value volume control (like 10K) is sonically beneficial, too.
Have fun,

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Re: All Lundahl Transformers - 417a Preamp - AnandR 09:30:08 12/03/03 (8)
I actually second Kevin's observations and warnings. I finally received my chassis from Front Panel Express as well as my wonderful looking wooden base for my version of Gary Dahl's Espressivo...definitely plan on using:
1) DACT 10K-2 stepped attenuator
2) Lundahl 1676 input transformer wired in 1:1 ratio
3) Minimum 50 ohm grid stopper and plate stopper resistors
4) Small 0.1 uf ceramic bypass caps over the heater rails on the 6C45pi's and if necessary,
5) Ferrite beads in series with the heater rails.
Best of luck!
Anand.
- Re: All Lundahl Transformers - 417a Preamp - Mr. Stone 17:02:57 12/03/03 (7)
  Hi Kevin & Anand,
  Thanks for your suggestions!
  If I want to use AC for the filament, do I need to add the ferrite beads & 0.1 uf ceramic bypass?
  Thanks so much!
  Cheers
  Mr. Stone
  - Re: All Lundahl Transformers - 417a Preamp - gdahl 17:21:11 12/03/03 (6)
    Hi Mr. Stone,
    I consider the bypass cap mandatory, whether the fils are powered by AC or DC. I connect the cap from one filament pin of each 417A to ground, and make the distance as short as possible. I haven't used ferrite beads on the filaments, but my K&K Phono Preamp has them. The cathode circuit should also be as short as you can make it.
    For a grid stopper resistor, carbon comp is preferable because of its minimal inductance. Use as close to zero lead length to the grid pin as possible. I make the connection to only one grid pin, and remove the remaining grid pin connectors from the tube socket. Metal film resistors are the worst choice in this position, because of their inductance.
    Gary Dahl
    
    "So many tubes, so little time..."
    - Re: All Lundahl Transformers - 417a Preamp - Mr. Stone 22:46:04 12/04/03 (5)
      Hi All,
      I never try ferrite beads before, is it something like this?
      Thanks!
      Mr. Stone
      - Re: All Lundahl Transformers - 417a Preamp - Mr. Stone 00:07:57 12/05/03 (4)
        
        In Reply to: Re: All Lundahl Transformers - 417a Preamp posted by Mr. Stone on December 04, 2003 at 22:46:04:
        
        Hi Folks,
        Do I need to add a load resistor at the LL1660 output? THanks so much!
        Cheers
        Mr. Stone
        
        Re: All Lundahl Transformers - 417a Preamp - KevinC 07:14:28 12/05/03 (3)
        
        In Reply to: Re: All Lundahl Transformers - 417a Preamp posted by Mr. Stone on December 05, 2003 at 00:07:57:
        
        You can experiment with load resistors. I find that they are useful for reducing high frequency resosnances (> 20kHz), but must be used gently, because they also tend to drain the sound of life when they are too low. It's best to adjust by listening.
        
        Please define too low ? - akltam 00:15:00 12/06/03 (2)
        
        In Reply to: Re: All Lundahl Transformers - 417a Preamp posted by KevinC on December 05, 2003 at 07:14:28:
        
        Kelvin,
        I am now experimenting LL1654 in a 5K:150ohm configuration. And I am using 470 Ohm. Is that too low ?
        Alan
        
        Re: Please define too low ? - Dave Davenprt 05:51:33 12/06/03 (1)
        
        In Reply to: Please define too low ? posted by akltam on December 06, 2003 at 00:15:00:
        
        Hello Alan,
        "Too low" is a subjective evaluation. Start with no resistor, then a large resistor, several K, and then a small resistor, hundreds, and compare the difference in sound. Which is better? Then try a third resistor that is closer to that chosen. Which sounds better? Keep this up until you find the value of resistance that gives the best sound.
        Now, there is another factor involved. Things will change as the components break in. So you will have to repeat your experiment after several hundred hours of music.
        Dave
        
        The breakin problem. - akltam 07:15:59 12/06/03 (0)
        
        In Reply to: Re: Please define too low ? posted by Dave Davenprt on December 06, 2003 at 05:51:33:
        
        Dave,
        In fact that is the most troublesome thing that can happen and the DIYer that have to deal with. After you try another value of resistor, and then how can you remember few hundred hours ago with another.
        I was using a 1660/25mA before I try to use the LL1654. Immediately I found that the LL1654/40mA does not do too well in the treble section. (without a terminating resistor) And I knew at that time I cannot expect a new output transformer to be at its best. It was such an agony that I have to wait for it to break in. Time move so slow while you wait.
        Then I try different configuration and also I don't need too much gain, I use the LL1654/40mA with the 5K:150 configuration plus using a 470 ohm resistor on the secondary. It doesn't sound good at first. (that is what I found). After some time, it seems that it does improve. I then use a signal generator to test the response of this 1654 with 6C45(running at around 36-37mA). It was indeed pretty flat from 22Hz to 20K. The interesting thing is as soons as I get it down to 21Hz, the gain dropped. (haven't got time to test the band beyond 20k)
        Too little time, too lazy to try some other values of resistors.
        Thanks for sharing your experience.
        Alan

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