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In Reply to: RE: 65uF capacitor? posted by maxhifi on February 21, 2017 at 09:31:41
The 65uF cap is really connected as shown. This is connected in the UltraPath method. As I understand it, the UltraPath method eliminates the cathode bypass capacitor from the signal path.
Follow Ups:
If you connect the 65uF capacitor as shown, it's going to be effectively in parallel with the output transformer... this is BAD, it's going to kill the high frequency response of the amplifier. Old radios would put a much smaller value cap in this location to roll off the highs.. I can't honestly believe it even works with it connected as shown.If you connected it instead to the other end of the output transformer primary (the high voltage supply, as opposed to the plate of the 2A3), this would properly bypass the cathode bias resistor of the 2A3.
I assume the attached image is what you meant to do, right?
Edits: 02/21/17 02/21/17
I have it connected to the B+ that is going into the OPT. So, good call on your part as it needs to be redrawn. Thanks!
I thought it had to be a drawing error :)
I notice there's only 2V on the bias diode of the ECC99, this explains the high current. If you add a couple diodes and get this to about 6V, I think everything will balance out nicely and you should be ready to go.
Yesterday I added a 2nd diode and got the bias to 4V. I will give 6V a try as well. thanks again for the input.
Sounds like you're on top of things. I strongly suggest reading the front section of any RCA tube manual, where it is explains how to design resistance capacitance coupled amplifiers, and how to draw load lines onto tube curves. It's all very intuitive, and graphical.. no heavy math or theory involved, and you will have a way better idea how all this works.
This is a great suggestion... and something I have not done. Taking a quick a look through the RCA Radiotron Manual (version 10), pages 1-21 look like a great introduction. I will be reading this as a refresher over the next few days.
I find endeavours more rewarding when I have some knowledge of the fundamentals/ basics. From that point I can build, question, experiment... and can develop understanding.
My tube learning was messy and my knowledge remains gappy. When I started seriously considering building amps as a hobby, I read the NEETS training manual (just the first few modules related to electricity, not the tube part) then goodness knows how many posts and tube-specific articles of various types... then attempted an early edition of Morgan Jones, which was a challenge. I learned how to calculate various types of circuits (and even designed some) and worked through converting schemas to layouts... all before building/modifying my first amp. I'm certainly not suggesting other beginners take this approach(!) - there are more efficient ways.
I still have so much to learn, but time is limited and becoming more so.
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
You can downloaded RC-30 too, It's a bit newer than the '10. It really help to have the basics in your pocket. The rca tube manual isn't going to turn anyone into a prodigal amplifier designer, but it will give you an idea what's what and why.
"The rca tube manual isn't going to turn anyone into a prodigal amplifier designer..."
I'm still trying to get my brain around this one!!!! :) :) :)
Chris
Thanks for the info max!
I think I have most of the very basics sorted... or at least know when to look, where to look to find out, and hopefully how to ask questions. Actually, maybe not so great on that last point ;)
Still, a refresher is in order. I just found RDH4 & 10, RTM30 and a heap of others stored on my HD... and also have a few more recent texts in hardcopy. I won't be reading them all in their entirety, but the key sections of the RCA manuals would be a good start.
Setting up and using a basic test rig will be next big challenge...
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I think RDH4 is pretty great, I especially like the section about power supplies and hum mitigation. I believe it's all online now too. The stuff about hi-fi is very dated, but still useful.There's also all sorts of basic textbooks out there about tube electronics which were intended for vocational level radio repair courses, from the 1930s - 1960s. I have a variety of books of this nature - all of them describe how to draw AC and DC load lines, and find operating points, etc. Some are better written than others but all are sufficient for an introduction and the depth is a little better than the tube manual, it's just the RCA tube manual is so ubiquitous. Keeping eyes open in used bookstores is a good idea.
Negative feedback isn't covered very well in most such books, but understanding the subtleties of negative feedback design isn't totally necessary to troubleshoot and repair amplifiers.
Edits: 02/22/17
They show up on ebay and the used book sites for about $12 and have all the important stuff in a much easier to digest form that its big brother #4.
dave
.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
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