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  1. J

    Mark 3 simul-class for sale question

    I overpaid for a blue stripe on Reverb not too long ago, $4k. It was a mint condition HRG hardwood combo with 415 power tubes, a 12L, and a road case. Even had the original spare preamp tube and came with the footswitches. It must've been worth it to me at the time, but I don't think I'd do it...
  2. J

    New Member. Mark IV A. need help

    Thank you for sharing! I learned some good stuff reading this yesterday, and I tried the instant gratification lead setting last night and it was hard to put the axe down.
  3. J

    Mark iii Blue Stripe 60w gain and volume loss on lead channel.

    Correct that the output section is push pull, the difference between the inner and outer pairs in simul amps is that the inner pair is setup for pentode Class AB, and the outer is setup for triode Class A. In a push-pull Class A output section, tubes are biased at max plate dissipation at idle...
  4. J

    Mark iii Blue Stripe 60w gain and volume loss on lead channel.

    Interesting, why did you decide to drop the bias on the outers? For Class A, tubes are typically biased around 100% (or even slightly over 100%) of the plate dissipation and rely on the operation to drop below on average. Dropping the idle dissipation gets you closer to, or in, Class AB. The...
  5. J

    Mark iii Blue Stripe 60w gain and volume loss on lead channel.

    It's hard to help with the limited information that's been provided unfortunately, my best advice would be to go through the amp and check your DC voltages to make sure everything looks correct. If that's all good, fire up your function generator and give it a sine wave input and trace the...
  6. J

    Vintage Mark-style PCB and chassis kit

    I haven’t had as much time to work as I’d like but here we are Most of my time has been spent finding parts and figuring out what I need to complete the build so it’s been slow progress. I started wiring the power amp, will hit the rest of the power stuff next and get it ready for the...
  7. J

    Vintage Mark-style PCB and chassis kit

    I believe these chassis are going to work out. Apparently the powder coat is significantly thicker than the shop said it would be, I had to clean it up a fair amount to get everything ready to build. First step was to mount the bottom chassis mounted items and I installed the EQ board to test...
  8. J

    IIc transformer ID

    My JP-2C plate voltage is >500V, I don’t remember what year I got it but it has the magnetic components transformers. I will guess that they use the same transformer for domestic / export, so all things equal you’d need your PT core to be approx 10% larger for the same performance on 50 Hz power.
  9. J

    IIc transformer ID

    The JP-2C high voltage is the same as a 105, but the other windings are different. Plate supply is slightly over 500V at idle, it will vary based on your tubes and wall supply. It functions as the original but they are not interchangeable. The PT I had made runs the same as a 105 or JP-2C...
  10. J

    IIc transformer ID

    The transformers I designed are based on the 105 PT and the SC152019B simul OT. Also, the JP-2C has a “105” PT, so any tubes that run in his amp will run in an amp with a 105.
  11. J

    Vintage Mark-style PCB and chassis kit

    So these showed up today Next steps will be fit testing the chassis mated components and then building the DRG and HRG
  12. J

    Mark iii blue stripe 60w redplating

    It’s basic indeed, but not the same as a fender
  13. J

    Mark iii blue stripe 60w redplating

    I bet if you trace the circuit you’ll find the schematic to be incorrect
  14. J

    Vintage Mark-style PCB and chassis kit

    The output transformers arrived today and are looking handsome. Chassis are supposed to ship today, so hopefully I’ll have one of these up and running within the next week or two!
  15. J

    Mark iii blue stripe 60w redplating

    Use a larger value pot if you want to increase the bias voltage, which will reduce idle dissipation. Hopefully there is enough range with a 100k-250k for your Tung Sols. Looks like you’re starting to see that all power tubes aren’t created equally. You might also check min resistance case to...
  16. J

    Vintage Mark-style PCB and chassis kit

    I hope not but you may be right. I put large series resistors in place that are bypassed by the relays and fly back diodes around the coils so hopefully it’s minimal. I’ve done other switching schemes like that on different amps that weren’t audible
  17. J

    Vintage Mark-style PCB and chassis kit

    Quick update - I have been unable to identify any readily available LDRs that have acceptable performance with the stock IIC+ circuit. Due to this challenge, and the dwindling supply of NOS Vactec VTL5C1s and VTL5C4s, I would only suggest my initial preamp board to those that can source Vactecs...
  18. J

    Vintage Mark-style PCB and chassis kit

    First few transformers arrived today, here is a new one next to the no stripe with a 105. Cores are pretty much bang on. I’m happy with how they came out
  19. J

    Vintage Mark-style PCB and chassis kit

    This build is the guinea pig!
  20. J

    Vintage Mark-style PCB and chassis kit

    Some good news, just got the invoice for the power transformers so they should be getting ready to ship. Will post some pictures of when they arrive!
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