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  1. kward

    Bootstrapping in a cathode follower

    The subject of bootstrapped grid to cathode capacitance in a cathode follower was discussed on another thread recently. It was a side discussion relative to the core subject of that thread, and it was taking that thread on a tangent. I am starting this thread to discuss this concept in its full...
  2. kward

    Why is it so hard to wind good transformers?

    A recurring question in my mind is why the two big modern transformer manufacturers Hammond and Edcor seem totally inept in winding decent output transformers on par with those best units from the heyday era? If you don’t have the skill just copy a good one from that era such as the good Fisher...
  3. kward

    Roll your own EFB (Enhanced Fixed Bias)

    EFB was invented by Dave Gillespie and is his trademark. I have received permission from Dave to discuss the design process I use in adding EFB to a given amplifier. Please keep in mind the content presented here and the use of EFB is for private/individual use only. Any commercial use of EFB...
  4. kward

    Refreshing one of my old amps: A maple 6CA7 (now KT88)

    Back in 2008 I built this amp: I built it for my son; he used it for about 6 months and then returned it because it was too big (form factor) to fit in his small apartment. (it is about 24" x 20" x 8", made from solid maple, with a steel top). So it sat in my living room waiting for the day...
  5. kward

    EL34s with A430 vs. A470?

    Greetings. I know EL34's are normally paired with A470s, and KT88s/6550s are normally paired with A430s. But in my situation I've got a donor chassis pre-drilled for A430s, and yet I want to build an EL34 PP AB1 amp. The output stage will have approximately 440V quiescent plate voltage, will be...
  6. kward

    What's the proper way to measure output resistance of a tube power amp?

    Can someone explain the steps you'd follow to accurately measure the output resistance of a tube power amplifier? I have a rough idea, but I'd like to hear the specific procedure to obtain an accurate result.
  7. kward

    Theory question: turns ratio of a PP AB1 output transformer at full power output.

    Let's say I have a 5.5K to 16 Ohm PP output transformer on an amplifier, and the amp is configured for AB1 operation, and the amp is being driven to maximum output power it's capable of delivering, so that the output stage, at full power, is operating in class B mode. Under those conditions...
  8. kward

    Building a power tube tester?

    I'm thinking about building a power tube tester. Probably not serous about it yet, just mulling over the difficulty of it. The problem is power tubes I've purchased in the past from the big commercial vendors are never really matched even though they say they are matched. I purchase them, put...
  9. kward

    "Veruul" - A 6AV5 sweep tube amp

    Taking a queue from the late Kegger, I was inspired by his 6AV5 amp design, so thought I would give his idea my own spin. There's nothing really new in the frontend--a standard voltage gain stage coupled to a differential pair that drives the output stage. The output stage is triode strapped (a...
  10. kward

    Looking for a modern audio spectrum analyzer that won't break the bank

    An instrument missing from my test bench is a decent audio spectrum analyzer. I've done a bit of research and find that there aren't apparently a lot of choices on the market at budget prices. On the high end there's the Audio Precision line but they are super expensive like over $10k. On the...
  11. kward

    What to build with these Sansui output transformers?

    While I save some green for my 845 SET build, I thought I would whip together a Push Pull amp with these Sansui output transformers. I've probably got all the parts I need already in my spare parts pile to build it. These OPTs are 5.5K primary with a 16 ohm secondary and an 8 ohm tap. They do...
  12. kward

    Thinking of building an 845 SET. Have questions.

    I inherited some parts for an 845 SET—PT. OPTs, chokes, those huge 4 prong sockets. filament transformers, stuff like that. Looking into the requirements needed to drive the output stage. Not trivial. I need a driver that will deliver 400v peak to peak swing. Probably will need to drive the 845...
  13. kward

    Analysis of plate to plate feedback SE amp

    I've been curious about plate feedback topology for quite some time. I have wondered how it performs, and how it measures on the bench. I am quite curious about the HF and distortion characteristics of this approach, since these are the tell-tale signs of a good sounding amp that uses feedback...
  14. kward

    6L6 Class A PP amp, with Fisher TA-800 outputs

    While I wait for the SE OPTs from Edcor to be delivered for my SEUL build, I might as well get going on the design of another amp. I acquired these Fisher TA-800 OPTs a while back and I've been sitting on them for over a year now wondering what I can build with them. These are smallish OPTs...
  15. kward

    My take on a Single Ended amp

    It's crazy, I know...I'm finally biting the bullet and building a single ended amp. All the fuss you single ended advocates have been making over the years has piqued my interest and I'm going to try it! I'd appreciate any feedback or thoughts anyone may have on this design. I have only ~90 dB...
  16. kward

    KT120 Amp Mk. 3

    I've been mulling over a few performance enhancements for my KT120 Mk. 2 amp. The major addition I wanted to add was an Enhanced Fixed Bias (EFB) output stage regulator. While I was in there, might as well throw in a few power supply upgrades I've been contemplating also. The basic design of...
  17. kward

    Thoughts on feedback circuits

    Textbook feedback theory says that the closed loop gain ACL of a system is dependent only on the open loop gain of the system (AOL) and the feedback ratio Beta (B): Beta is the ratio of the output signal that is applied back to the input. In a typical tube amp, Beta is simply the voltage...
  18. kward

    How do I add EFB to a UL connected output stage?

    The example schematic below shows a "normally" wired EFB regulator for a PP output stages for a stereo chassis configuration. The 380V from the point labeled A comes from the center taps of the output transformers. The 326V from the point labeled D feeds the screens of the output tubes, and the...
  19. kward

    What is your favorite PCB design software?

    I've got a more complex project in the works that I think would be well served with a custom PCB. It's a mix of a few tubes and a lot of solid state circuitry. This board will need to be probably 6 x 8 inches in size. I'm looking for decent PCB design software that runs on the desktop, and that...
  20. kward

    Has anyone used the 6AR6 tube?

    Looks like a pair of 6AR6s need a similar load as 6L6s in PP class AB1 mode. Heater current and pinout are different of course. I notice the grid curves have a shallow knee more like an EL34 than a 6L6. Looks like it is a beam pentode though. Also the plate curves don't look as linear as a 6L6...
  21. kward

    EFB for an EL34 triode stage?

    This question is probably for @dcgillespie, or anyone else who has experience here. My daughter saw my recently refurbished and hot rodded Motorola console amp and she asked me if I could build her an amp. Looking through my closet of crap, I see I have a power transformer and two output...
  22. kward

    How I spent my summer modding my Moto

    (a.k.a. Malcontented Mr. of Modification) I just can’t leave well enough alone. This amp started out as mid-fi three channel 6V6 push pull Motorola console amp. Some initial analysis and fussing around with the thing are documented here. I wanted to convert this amp to a full frequency two...
  23. kward

    Thinking of building a low gain preamp

    I have always tried to steer away from preamps in general because I just can't find one that I think sounds good (meaning is neutral and doesn't suck the life out of the music). At least, with the preamps I've tried in the past---commercial units, kits, and some early attempts at home brew...
  24. kward

    The 1625 amp, Mk. II

    About 3 years ago I scratch built a 1625 PP amp. It used a seldom seen together tube lineup of 6DJ8, 6FQ7, and 1625. It sounded pretty good. It had a bit of a lush and slightly sweet character. It didn’t use much feedback (10 dB), hence 6DJ8 tube rolling provided obvious sonic differences. I...
  25. kward

    Why the shallow knee in EL34/6CA7 pentode curves?

    Why is it that the EL34/6CA7 has a significantly shallower knee in the pentode plate curves than does the 6550 or 6L6? Is it something to do with how they are constructed, or is this an optical illusion since the scales between the two graphs are not exactly the same? Example comparing 250V...
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