I also wanted to add the final schematic changes + BOM of parts needed, in case videos disappear in the future. The BOM xlsx spreadsheet is on my website Boyuurange A50 Modifications
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Stephe - great video series! I just finished re-watching and making some "how-to" build notes. I'm planning to do the full bunch of mods to an A50 this weekend. Between the notes and schematics, I'll have 2 checksheets to know the work is done. And if something isn't clear, I marked the video name and timestamp in the notes.
example: Advanced Video 2 - 13:00 - 680K from B1 to 6SN7 Pin 4.
Thank you for the clear side-by-side schematics, BOM, and videos. Absolutely super work!!!
example: Advanced Video 2 - 13:00 - 680K from B1 to 6SN7 Pin 4.
Thank you for the clear side-by-side schematics, BOM, and videos. Absolutely super work!!!
Done! Updated amp is sounding great! I used Elna silmic II 300b cathode bypass caps and CDE 940C series coupling and 6SN7 cathode bypass caps. Only comment on BOM is that I used M4 x 16mm screws for the transformer / 25w resistor heatsink. 5mm looked a bit big, so I tried M4 and they worked. I ended up tapping the heatsink / 90 degree aluminum for the 25W resistor.
Thanks once again Stephe for such a great analysis, this thread, and the super videos! The why, what, and how are all nailed - superb work!!
Thanks once again Stephe for such a great analysis, this thread, and the super videos! The why, what, and how are all nailed - superb work!!
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Happy people are finding this useful and are enjoying how much better this amp sounds! I'm doing the same sort of thing to a Reisong A12 and it's proving to be much more of a battle lol.
And thanks for the BOM note, I'll update it. That's one of the things I honestly hate dealing with and am not very good at.
And thanks for the BOM note, I'll update it. That's one of the things I honestly hate dealing with and am not very good at.
Stephe, have you tried this amp on horns? Did you have a hum problem?
I was testing on JBL 4410 monitors (91dB) and it sounded fine.
Then I put it into my active horn system (JBL Hartsfield, Fostex Supertweeters, Danley DTS-10 Sub). The 300B amp gets 500Hz and up duty. it was giving a whole lot of HUUUUMMMMMMM. I tried a cheater plug and it knocked the hum way down.
So I put a CL60 thermistor from chassis to AC Mains ground. Thus cuts hum down significantly. It's not totally gone, but it's much improved.
I think additional hum busting would probably have to involve more PSU filter work, maybe filament work, and maybe a ground lift resistor/thermistor between channel grounds. I've been playing with solid-state amps for the past few years, so that would be my thought process. Perhaps a SET like this requires other techniques.
I should note I didn't have the opportunity to hear the "before mods" state of the amp because the owner shipped it to me with 300B's only. I did the mods and used the new 5AR4 and new driver tubes for testing after modifying. So I really can't judge before/after hum. The amp's owner will bed the judge when it's on his horns next week.
I was testing on JBL 4410 monitors (91dB) and it sounded fine.
Then I put it into my active horn system (JBL Hartsfield, Fostex Supertweeters, Danley DTS-10 Sub). The 300B amp gets 500Hz and up duty. it was giving a whole lot of HUUUUMMMMMMM. I tried a cheater plug and it knocked the hum way down.
So I put a CL60 thermistor from chassis to AC Mains ground. Thus cuts hum down significantly. It's not totally gone, but it's much improved.
I think additional hum busting would probably have to involve more PSU filter work, maybe filament work, and maybe a ground lift resistor/thermistor between channel grounds. I've been playing with solid-state amps for the past few years, so that would be my thought process. Perhaps a SET like this requires other techniques.
I should note I didn't have the opportunity to hear the "before mods" state of the amp because the owner shipped it to me with 300B's only. I did the mods and used the new 5AR4 and new driver tubes for testing after modifying. So I really can't judge before/after hum. The amp's owner will bed the judge when it's on his horns next week.
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" The amp's owner will bed the judge when it's on his horns next week"
We try to not talk about these personal things on this dignified forum. Arf!
But seriously, hum issues like this are almost always something about the particular local hookup. Plug everything! into the same power distribution strip, etc. I cannot recommend a thermistor as a ground isolating device - they can't survive long enough to throw the breaker, so become a safety issue.
All good fortune,
Chris
We try to not talk about these personal things on this dignified forum. Arf!
But seriously, hum issues like this are almost always something about the particular local hookup. Plug everything! into the same power distribution strip, etc. I cannot recommend a thermistor as a ground isolating device - they can't survive long enough to throw the breaker, so become a safety issue.
All good fortune,
Chris
I don't have any of those super efficient speakers, I'm sure any DHT would be challenged 🙂 I can't imagine how anything about this mod would make it hum worse than it would unmodified. On insolating the ground, I've read using a large bridge rectifier + resistor/cap works and is safe. Some good info here to try:
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
None of your circuit recommendations would effect either hum or safety. Both of these are issues primarily in the primary circuit. Although written for an Ozzy/Kiwi audience, your reference seems true enough for Americans too. Safety is well addressed, and safety first and last for responsible adults.
The 35A bridge plus 10R resistor, maybe with the 100nF cap, is currently the best available option for isolating signal "ground" from safety earth / PE in American lingo. The resistor never sees more than about a Volt and a half, so doesn't need to be very big. Most folks don't even need to bother with this, if everything else is hooked up right, but it's *reasonably* safe.
Nothing's perfect. I spent yesterday afternoon replacing a GFI outlet and some fat copper wire in the box that had melted and partly vaporized (could smell copper) at my ladyfriend's house, where a cat had peed into it. Scary sparks and flame, the GFI outlet was melted and deformed, plastic soot everywhere, but the cats were all OK. Safety earth/ PE had also melted open before the breaker tripped. So...
Boys in the South learn not to pee on electric fences.
All good fortune,
Chris
The 35A bridge plus 10R resistor, maybe with the 100nF cap, is currently the best available option for isolating signal "ground" from safety earth / PE in American lingo. The resistor never sees more than about a Volt and a half, so doesn't need to be very big. Most folks don't even need to bother with this, if everything else is hooked up right, but it's *reasonably* safe.
Nothing's perfect. I spent yesterday afternoon replacing a GFI outlet and some fat copper wire in the box that had melted and partly vaporized (could smell copper) at my ladyfriend's house, where a cat had peed into it. Scary sparks and flame, the GFI outlet was melted and deformed, plastic soot everywhere, but the cats were all OK. Safety earth/ PE had also melted open before the breaker tripped. So...
Boys in the South learn not to pee on electric fences.
All good fortune,
Chris
I know that one. I fabbed boards for that based on the Nelson Pass F5 V3 turbo design.
https://www.diyaudio.com/forums/pass-labs/361593-rawson-repair-reflections-10.html#post6862203
I just modded another one. This time I added a new way to address the ground. 12R resistor with 2 diodes as a ground lift / hum breaker in between the IEC ground wire and a screw that holds the 25w resistor L bracket / heatsink to the chassis. It’s sounding really nice!
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One of the best video segments I have ever seen...great Job Steffe....are you still planning to offer a 300B Kit, would love to have your build cant seem to get the point to point build...am simple soldering guy
Stephe,
maybe I missed it, but was input sensitivity changed after your mods? Can you detail what input signal is need to reach rated output w/ this amp?
Thanks,
Jim
Yes the input sensitivity is lower but a normal CD deck output will still easily drive the modded amp to full power.
They sound similar as the schematic is almost the same on my design vs the mod A50. Honestly I basically just adapted my design to this amp. Mine still sounds a bit better, in most part I believe due to having better output transformers. I'm soon going to post a minor modification to my design but I need to test my idea first.
Haven't seen any post on this thread in a while. Enjoying the video series and have done the first mod.
I'm working up the nerve for that mod! Have all the parts, just not quite sure about the second tube socket. It looks like it's a little different. Gotta watch the video a bit more.