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Post by svart on Mar 16, 2021 13:35:15 GMT -6
So the thread about what we regret selling got me thinking about a tube preamp. Figured I'd go with something super simple and very "tubey" sounding that I have most of the parts I need already. The RCA BA-2C seems to be a favorite for those things. I have transformers that should fit the bill and my chinese switcher power supplies should handle the power requirements pretty easily. It took me all of an hour to draw up the schematic, make the octal footprints and draw the PCB. I'll send the PCBs off tomorrow.  |
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Post by EmRR on Mar 16, 2021 14:52:13 GMT -6
R3 220K versus 120K?
Your C4 will likely be a different value, depending on the transformers and their response shapes. Or strap out your C4/R5 entirely.
Looks like you're going parallel feed instead of DC coupled output? Using a choke instead hopefully? You lower max output if it's resistive parallel feed.....unless you're going Hi-Z out and it makes up the diff. It'll work, but it's a different amp. Resistive feed outputs in this era tend to be 47K-56K.
Have fun!
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Post by svart on Mar 16, 2021 16:06:39 GMT -6
R3 220K versus 120K? Your C4 will likely be a different value, depending on the transformers and their response shapes. Or strap out your C4/R5 entirely. Looks like you're going parallel feed instead of DC coupled output? Using a choke instead hopefully? You lower max output if it's resistive parallel feed.....unless you're going Hi-Z out and it makes up the diff. It'll work, but it's a different amp. Resistive feed outputs in this era tend to be 47K-56K. Have fun! Good points. Mostly came at this from the position of I'm not really sure what cores my transformers have, so I'm not sure how much DC I can put through the output, so I decided to AC couple the output. I could certainly add jumpers for a choke, or just try DC through what I have and see how it works. All in all, a fun project that has potential for all kinds of experiments. Also, so cheap. NOS tubes are just a few dollars each. Unbelievable. |
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Post by EmRR on Mar 16, 2021 17:11:51 GMT -6
Yep. Best to avoid the popular guitar/Neumann/Abbey Road tubes, then you’re usually under $10.
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Post by svart on Mar 16, 2021 18:01:12 GMT -6
Looks like I have a few 10kct:600 output transformers, but they're steel core. Unsure how they'll react to DC though. I have a few 600:10k or 600:15k transformers for inputs.
I'm not really trying to do a direct clone, just something with some vibe for the fun of it.
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Post by EmRR on Mar 16, 2021 18:28:25 GMT -6
It seems like Ed Anderson experimented with using Edcor 10K:10K as chokes, running the windings in series. Could be worth a try with something similar.
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Post by svart on Mar 16, 2021 18:46:41 GMT -6
It seems like Ed Anderson experimented with using Edcor 10K:10K as chokes, running the windings in series. Could be worth a try with something similar. I have a few 70v speaker autoformers, which are just tapped inductors. I could put one of those to use as well. |
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Post by srb on Mar 16, 2021 21:52:13 GMT -6
I so wish sometimes (like right now) that I had occasioned myself to know more, formally, of these things. I've learned a lot; built a few things. But I'm the boy in short pants over here.
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Post by EmRR on Mar 16, 2021 22:05:21 GMT -6
I so wish sometimes (like right now) that I had occasioned myself to know more, formally, of these things. I've learned a lot; built a few things. But I'm the boy in short pants over here. Gotta start somewhere. I'm just an autodidact who's stared at the same stuff long enough to figure it out. |
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Post by keymod on Mar 17, 2021 4:07:29 GMT -6
I so wish sometimes (like right now) that I had occasioned myself to know more, formally, of these things. I've learned a lot; built a few things. But I'm the boy in short pants over here. Same here |
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Post by svart on Mar 17, 2021 7:50:44 GMT -6
R3 220K versus 120K? Your C4 will likely be a different value, depending on the transformers and their response shapes. Or strap out your C4/R5 entirely. Looks like you're going parallel feed instead of DC coupled output? Using a choke instead hopefully? You lower max output if it's resistive parallel feed.....unless you're going Hi-Z out and it makes up the diff. It'll work, but it's a different amp. Resistive feed outputs in this era tend to be 47K-56K. Have fun! So one of my transformers, an Edcor, can handle 3W. I don't have a 2.5K or even a 5KCT primary transformer, so I might simply try the CT of the 10K:600 output transformer and see what happens if I put DC through it. What's the worst that can happen, lol. Otherwise, I see that the old Gates preamps used 56K resistors on their AC coupled outputs. |
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Post by svart on Mar 17, 2021 9:29:08 GMT -6
I so wish sometimes (like right now) that I had occasioned myself to know more, formally, of these things. I've learned a lot; built a few things. But I'm the boy in short pants over here. No need for formality. I started out as a kid when my grandparents went to radio shack and got me one of those experimenter's kits. I didn't understand it at the time, but it was fun to play with and make lights do things. A few years later I traded a beat up skateboard for a first-gen CD player that a friend had, but didn't work. I took it apart to find a burned doo-dad (resistor). Determined to have a CD player in the early 90's, I rifled through the electronics kit to find a resistor that looked (same color bands) as the ones in the CD player. I stole my dad's soldering iron and burned myself enough to get that resistor in place. The CD player started up and worked! It wasn't until many years later that I realized that it was pure luck that Japanese (Sony) manufacturers tend to use the same values for resistors for cost reasons and I had simply had good luck in replacing this resistor with a part that worked. Anyway, it sparked a desire to at least get into things and try to fix them. All you have to do is start somewhere! |
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Post by srb on Mar 17, 2021 12:11:46 GMT -6
I will continue to plod along, gentlemen. Thanks.
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Post by ragan on Mar 17, 2021 12:13:10 GMT -6
Hell, I started messing with my audio gear some years ago, as a total novice, and got jealous of reading forum posts by guys like svart and EmRR who knew what the hell was going on and so I went back to school for an EE degree (one quarter left). "Gotta start somewhere" indeed! And watch out, it could take you on quite an adventure.  |
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Post by srb on Mar 17, 2021 12:25:45 GMT -6
Hell, I started messing with my audio gear some years ago, as a total novice, and got jealous of reading forum posts by guys like svart and EmRR who knew what the hell was going on and so I went back to school for an EE degree (one quarter left). "Gotta start somewhere" indeed! And watch out, it could take you on quite an adventure. [img alt=" " class="smile" src="//storage.proboards.com/forum/images/smiley/nerd.png"] About 20 years ago, I repaired the tremolo in a late '50's Gibson GA-40 I had purchased...then did the same to a GA-20. I was hooked. I've done the majority of my amp maintenance since. I subsequently built a 5E3-style amp and then a 5E5A. Still have a lot to learn. Appears I'm in a very good place for that.  |
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Post by svart on Mar 26, 2021 11:26:52 GMT -6
Looks like my boards will be delivered today. May or may not have time to mock something up over the weekend.
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Post by svart on Mar 29, 2021 8:21:12 GMT -6
The tube board and my existing switcher power board:  The tube board mostly built. Need a few caps to complete:  |
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Post by craigmorris74 on Feb 17, 2022 10:59:44 GMT -6
Hey Svart,
Curious to see how these turned out.
Thanks,
Craig
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Post by svart on Feb 17, 2022 11:08:49 GMT -6
Hey Svart, Curious to see how these turned out. Thanks, Craig Honestly I never finished them. I got wrapped up in another project and never got back to these. I should though, it'd be a quick build and test. |
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