Let's talk preamp kits

Discussion in 'Tube Audio' started by airtime, Oct 7, 2017.

  1. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    32,922
    Location:
    Southern NJ
    I have a Foreplay II sitting here that I did some work on. It had a nasty hum that I couldn't work out. Finally the C4S boards came out, and it started working right. Not sure what the problem was, but it did not seem happy with those installed. Works fine with two resistors though.

    One thing about the way they did the heater to ground reference is sort of problematic though. Its run off one end of the heater string instead of being balanced. I changed that with a couple of 100 ohm resistors to make a virtual center tap. I did that at the same time as the C4S boards came out so I don't know if it would have hummed without it. It did hum with that setup and the C4S, but it was less than the stock setup.

    I also just have general concerns about the construction of those things. The whole wood box not being shielded thing is a big one, but the fact that I can see noise on the scope if I get near the tubes is another. They really should have used shields on the tubes. A cap from circuit negative to the grounded metal plate wouldn't have been a bad idea either. I may have to experiment with that before it goes back home.
     
  2. sfox52

    sfox52 AK Subscriber Subscriber

    Messages:
    3,605
    Location:
    Maryland
    I have my original FP II with the C4S boards sitting on a shelf, one channel out. Maybe I should try pulling the C4S boards and do the cap to ground thing (what value?) & add some tube shields , then I could A/B the two units. I have the standard 100K Alps pots in the one I use now, I have considered going to a stepped attenuator. I had the sweetest whispers in the original, but I didn't like the 11 steps and clunky action. I guess if I end up with two working units I could put one on BT.
     
  3. kirk57

    kirk57 Some guy on the Internet Subscriber

    Messages:
    6,030
    Location:
    Chicago's North Shore
    I'm thinking an Aikido build will be in my future. Sounds interesting.
     
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    32,922
    Location:
    Southern NJ
    Probably not real critical. If you've got an 0.01 to 0.1uf around that should do it. Honestly I'd try it without the boards first and see what that gets for you.

    The one here also as the Sweet Whispers steppers, I'd also prefer more steps. I was running it with the Phase Linear, which has input level pots on it so between the two it was easy enough to match levels. I think the PL is a 20 step or something, its just a pot with a clicker though, not a real stepped attenuation arrangement.
     
    sfox52 likes this.

Share This Page