Building by section

[gui_tarzan]

After a few failed attempts I figured I'd build this amp by section and see where I'm going wrong. My goal is a Princeton 6G2 base model with a few tweaks after I get the darned thing running.

Using parts I have on hand, I paired the two 16/475 caps in series (pre-standby) which I know only gives me 8uf there, but this is a test so the capacitance value shouldn't be all that important right now. However, when I turned this on it immediately popped the 470 ohm sandstone resistor. I tried a second one thinking the first one might have been defective and it popped that one too.

Is 505v too much for that type of resistor? And why am I getting 505v (DC) instead of mid-high 400v with a 5Y3? I realize this is unloaded but my pre-rectifier voltage is only 373v per leg with the CT going to ground so it seems a bit high.

[Tom Phillips]

...when I turned this on it immediately popped the 470 ohm sandstone resistor. I tried a second one thinking the first one might have been defective and it popped that one too. Is 505v too much for that type of resistor?

It's not the voltage. The problem is probably high current flowing through the resistor causing more power to be dissipated than it can handle. Something is probably shorted or mis-wired. I.e. not wired according to the figure you posted. This could be a good time to build a light bulb limiter. If you have a hard short, you can also find it with an ohm meter with the circuit powered OFF and caps fully discharged. At least add a fuse to your test circuit.

...And why am I getting 505v (DC) instead of mid-high 400v with a 5Y3? I realize this is unloaded but my pre-rectifier voltage is only 373v per leg with the CT going to ground so it seems a bit high.

Seems about right to me. For an unloaded circuit the caps will charge up to the peak voltage which ideally would be 1.414 time the Vrms input. The math is 373*1.414=527 which is pretty close to what you measure. There are still some losses through the 5Y3 and you do have a little bit of a load so it all seems normal.

I think you should take some time to study up on basic electronics theory. It will help you with all your projects. Check out the sources discussed in the various book threads on MEF. There are many links to free sources of information. It would be a good opportunity to start learning to draw real schematic diagrams too. When you learn to draw and read schematics it helps to understand the flow of the signals and power much better than working with layout diagrams.

Cheers,
Tom

[Tom Phillips]

After a few failed attempts I figured I'd build this amp by section and see where I'm going wrong...

By the way, I think that's an excellent idea. If you have the time, patience and curiosity you will learn a lot and the lessons will stick. The process will speed up if you find good reference literature.

[gui_tarzan]

I pulled the 2nd cap in the trio (the one right after the 470 ohm) and it shows no resistance and no rise and fall when I swap the leads. I'm thinking it's shorted. I put cap #3 in its place and it didn't pop the resistor this time.

I have no problem reading schematics, it's the theory of some of the design I am struggling with. I learn far better by getting into the circuits than reading although I have read quite a bit of material lately. Unless I actually put the parts in place and see it in action the theory doesn't make a lot of sense. Once I get the parts working I tend to "get it" if you know what I mean.

[Tom Phillips]

...I have no problem reading schematics, it's the theory of some of the design I am struggling with. I learn far better by getting into the circuits than reading although I have read quite a bit of material lately. Unless I actually put the parts in place and see it in action the theory doesn't make a lot of sense. Once I get the parts working I tend to "get it" if you know what I mean.

Right. I agree that one must build actual circuits to best learn about them. About the schematics...Maybe it's just me but I think if you show the circuits you are working on in schematic form it clarifies what is going on much better than the parts picture format. The schematic is kinda the language of electronics where the symbols convey what the part is better than just a box with wires connected. Anyway...seems like you are making progress.
Cheers,
Tom

[gui_tarzan]

Here's the schem added to the layout. I typically have them both handy when I work on something and it does make it easier for me to understand what's going on.

[Enzo]

a wiring diagram - we call them layouts here - shows the physical relationship between the parts.

A schematic shows the electrical relationships between the parts.


They are very different things.


When a schematic puts up a transistor or a triode, it is assuming you already know how the part functions.

When a wiring diagram tells you the middle pin of a transistor is connected to some resistor, it is telling you nothing of the function of that part.

Building stuff makes you familiar with the parts as things, it teaches you how to handle the parts. It doesn't teach you about circuit relationships. When a layout tells you to install a cathode bypass cap "here", you know the circuit as designed expects one there. A schematic shows one, it is assuming you understand that will increase the gain of the stage and affect the tonal response. The whole point of those boring light bulbs and magnets units in electronics books is to teach you HOW components work in circuits. I can tell you that that 22uf 25v cap across the 1.5k resistor in a preamp triode circuit, will affect how the whole stage works, and you will maybe remember that. But will you know WHY? We have to study it all.

[g1]

I pulled the 2nd cap in the trio (the one right after the 470 ohm) and it shows no resistance and no rise and fall when I swap the leads.

Do you mean it reads 0 ohms? That is a short.
The opposite is open circuit, which is what it will read when the probes are not touching anything. The display will usually say "OL".
A shorted cap would explain the burnt resistor.

[gui_tarzan]

Yes, zero ohms. An open displays 1. on the far left of my screen.

[gui_tarzan]

We have to study it all.

I'm working on it. The more I learn the more I realize how much I don't know.

[The Dude]

I'll add that it really doesn't take that long to discover what basic components do, how they work, and how to check them. One should know basic things like how to check a transistor, capacitor, diode, etc. It seems more and more people just "replace things until it works". Not a speedy or sound strategy in my book.

Years ago I worked at a place that manufactured satellite equipment. Part of my job was to train new techs as we hired them. The city was right next to an active air force base, so we hired a lot of retired military "techs". THIS IS NOT A DIG ON THE MILITARY, but these guys were trained to swap boards and had limited component level troubleshooting knowledge. Practical when you're in the field and need to get a communications system running ASAP, but that's about as far is it goes. I found myself teaching guys much older and more experienced than myself basic electronics. Many did not know how to check a transistor. Some thought a cap was bad because it showed no resistance with a DVM, etc., etc. Some of these guys did not appreciate some young whippersnapper teaching them. However, many of them told me when I left the company that they had much appreciated me taking the time to show them why they were doing what they were doing. They actually appreciated being schooled.

[gui_tarzan]

I'll add that I also appreciate being "schooled" by you guys that know your stuff. Aside from electronics class in high school back in the late '70s I am self-taught. But as with my guitar playing I skimmed the theory parts because I learned better by ear (on guitar) and with this stuff it has always been with a schematic in my hand, reading voltages and looking for visually bad parts. I know what the parts do, but I will admit I don't always know "why". That's on me and that's why I have to do the hands-on and at the same time learn the "why", which seems second-nature to you guys because you learned it right from the outset. I don't always like getting my pride bruised but that's part of what learning is all about - losing that pride and actually learning.

I'm old enough and mature enough to know it's that you're actually helping and trying to teach me something, not trying to make me feel like an idiot and I appreciate that.

In the early days of my IT career we had to know more about what DIP switches and software actually did to the hardware, now it's just replace this board or that board or whatever. I understand the cost relationship but in a way it sucks because we're not actually fixing anything. Sometimes I really kick myself for not paying attention to why my early electronic projects back in the 70's did what they did instead of just building them from schematics and marveling at what they did.

[The Dude]

Do not misunderstand me. I don't even pretend to know it all. After 30+ years of teching, I still learn something every day; much of it from this forum. It's a wonderful place and a wealth of knowledge. I first came here with a question. Then, I thought I should return the favor and try to help others when I can. And so, the addiction began. Much of the time, I just sit back, read, and learn. But, I will chime in when I think I can be of assistance. It's when you think you know it all that you become brain dead, which does by the way apply in at least one instance on these boards.

[gui_tarzan]

I didn't mean to imply that, I just wanted to let you guys know I appreciate what you do for guys like me that are trying to learn. And I think I know who you're referring to.