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  1. kward

    Gain stage ideas

    Not necessarily...depends on the internal characteristics of the output transformers.
  2. kward

    Gain stage ideas

    My guess is that 6.8k resistor was added to the tuning of the amp after that version of the schematic was drawn. I'd leave it, for now.
  3. kward

    Conrad Johnson MV75A ... revisiting a pretty old thread for some help !

    To verify if it's the tubes or the circuit, swap output tube locations. Put the "cold" tubes in the sockets that were measuring "hot", and vice versa. If the tubes that now measure "hot" change positions, you know it's the tubes that are the issue. If the tubes that now measure hot are the same...
  4. kward

    Conrad Johnson MV75A ... revisiting a pretty old thread for some help !

    Right, with the significantly higher currents involved in the B+ supply, it won't be easy to just "tune the B+ supply down" without compromising the performance of the amp. If you really wanted to tune down the B+ supply, you'd want to do it in the AC domain by adding a secondary bucking...
  5. kward

    Quick question for the masses re. power tube cathode bypass cap value.

    Hit the 50 uF target and overshoot the voltage rating if needed. Most electrolytic caps have + or - 20% tolerance, so anywhere in the 40 uF to 60 uF range I think would be fine. Avoid the idea that bigger is better. If the amp has a global feedback loop, making that cap too large could cause...
  6. kward

    UL vs Pentode & Tube Life?

    What does the author mean by “easier on the finals?” Maybe he was making a reference to the screen. Maybe he was making a reference to wearing out the cathode. Or maybe he was making a reference to total output power differences. All else being equal I think the idle current would have the...
  7. kward

    Voicing my KT120 monblocks

    I assume you mean 0.33 uF to 0.47 uF ;) But I think we get the idea. Glad things are sounding good. Larger coupling caps will start to introduce the potential for blocking distortion--a situation that will force the output stage into cutoff after the grids of the KT120/150 are over driven. Oh...
  8. kward

    Power supply ideas.

    Correction to my previous post. I think the Heathkit PT will work for supplying the 285 VDC source, certainly. But to obtain the 180 VDC source in the same manner as Fisher did (tapping it from the “half voltage” point off of the doubler), you may only be able to squeak out 165 VDC to 170 VDC...
  9. kward

    Power supply ideas.

    The Heathkit won’t _quite_ work since you need both 190 VDC at 66 mA and 300 VDC at 15 mA. You could shoe horn it but I think a better approach is to use a HV secondary voltage that produces 190 VDC at the half voltage tap point off the doubler and then obtain the 300 VDC off of the full voltage...
  10. kward

    Power supply ideas.

    According to the schematic, the 180V source needs to supply about 66 mA, while the 285V source needs to supply about 15 mA. If those current requirements were reversed it would make for a straight forward voltage dropping scenario off of one B+ rail, while still offering a fairly low impedance...
  11. kward

    Power supply ideas.

    I’m confused a bit by what you are trying to do. Do you need a 385V DC supply or a 180V DC supply, or both? If you need both, then you really need a 150V secondary and wire it up as a voltage doubler as shown in the original schematic. Even if you use one side of your proposed new transformer...
  12. kward

    Conrad Johnson MV75A ... revisiting a pretty old thread for some help !

    If the MV 75 uses a common bias supply for both channels, you will need to double the current load that the bias power supply sees, or just duplicate the other channel’s bias circuitry in the simulation. Hence I believe the resistor value in the bias power supply will need to be half of what the...
  13. kward

    Conrad Johnson MV75A ... revisiting a pretty old thread for some help !

    AC voltage sources in LTSpice are modeled as peak volts. If you measured 81.7 VAC RMS from your digital meter, you need to multiply that value by 1.4 to get the value of your voltage source amplitude that you’d enter in LTSpice. In your sim, you should measure about -112V DC at the first filter...
  14. kward

    Conrad Johnson MV75A ... revisiting a pretty old thread for some help !

    There is not a lot of current in the bias supply. I really believe a 47 uF and 22 uF will be fine, but there's no reason you can't go a little larger I guess. I would avoid the temptation to go too large on those filter caps because you want the bias supply to come up to voltage a lot quicker...
  15. kward

    Conrad Johnson MV75A ... revisiting a pretty old thread for some help !

    I'd probably use 47 uF and 22 uF 250V for first and second caps (left to right), respectively. To know what resistor value to use in the bias power supply, we need to know the secondary voltage on the bias winding. Can you measure it? I expect it would be somewhere in the 65V to 75V AC range...
  16. kward

    Voicing my KT120 monblocks

    Doesn’t quite matter. You could measure at the driver plate, the power tube plate, or the OPT secondary. I usually go for the OPT secondary as that gives you a view of the entire amp as a whole, unless you are trying to debug something specific in one of the stages. If you measure at the...
  17. kward

    Voicing my KT120 monblocks

    Keeping the input signal constant, measure the output signal amplitude with and without the FB loop connected. Divide those two amplitudes to obtain a ratio. Convert that ratio to dB. That will be the amount of FB applied. With a 1M FB resistor, I would assume there’s very little actual FB being...
  18. kward

    Voicing my KT120 monblocks

    @AudioFanMan There's something about the behavior of your amp that's been nagging at me. It didn't seem like I saw nearly this dramatic of a ring effect when I tested the plate-to-plate feedback topology on my amp with Edcor output transformers. (A feedback approach that I ended up not using, by...
  19. kward

    Voicing my KT120 monblocks

    What does a 10 KHz square wave at 1 watt output look like under these conditions? Ideally you want no more than about one cycle of damped ring while still retaining a "sharp" leading edge corner and having a "fast" rise time. The feedback cap limits overshoot on the leading edge, but at the...
  20. kward

    Voicing my KT120 monblocks

    Here’s some info on my amp: 3.5V RMS input volts to drive the output stage to full power. That is with 7.8 dB feedback. If I were to run mine open loop, it would require about 1.4V RMS. THD at full power (14 watts), with feedback loop closed: 2.3% at 50Hz, 0.82% at 1 KHz, 1.6% at 20 KHz.
  21. kward

    Voicing my KT120 monblocks

    I seriously doubt the overshoot is PS related but I guess it’s possible. Temporarily disconnect the feedback resistor and take some measurements in open loop conditions. That will set your base line and give you something to compare against as you start tweaking. Yes I know It won’t be ideal...
  22. kward

    Voicing my KT120 monblocks

    As a data point for you to consider when evaluating your amp's power bandwidth and frequency response, here are the numbers for what my KT120 SEUL amp will do. Configuration on mine is similar to yours (except I used a different feedback topology): a single 12AT7 driver into the KT120 0.22 uF...
  23. kward

    Voicing my KT120 monblocks

    The coupling cap size in tandem with the grid leak resistor on the output stage form a high pass filter—passing high frequencies and blocking low frequencies. So making that cap a little larger could increase low frequency pass through. But don’t make it too large or you introduce other...
  24. kward

    Help me verify this schematic- SE Pentode KT88 with half-section E80CC

    Not meaning to pile on, but to underscore Gadget's point for others who might be reading: it's not so much about "the amount" of feedback applied (but certainly feedback amount is a factor--you can have a great sounding amp with 6 dB or 9 dB or 12 dB, etc., of negative feedback). It's about...
  25. kward

    Help me verify this schematic- SE Pentode KT88 with half-section E80CC

    Agreed...before going back, you really need to adjust/tune the feedback loop or at least determine if the feedback loop is operating optimally...that is assuming it has feedback of some sort.
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