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In Reply to: RE: Why aren't you using your scope for problems like this? (nt) posted by elblanco on October 20, 2016 at 13:59:33
Sorry, didn't see your post until just now. Pins 5 and 3 are tied together and then tied to the plate through a 200 ohm resistor.
If you mean the resistor shown in the op that is tied to the grid and ground, that is 470k.
Follow Ups:
I thought pin 5 is supposed to be grounded when triode connected, pin 3 through the resistor to plate is fine. maybe others will chime in on pin 5. To my knowledge, RCA never gave guidance for triode connection. 30,000 ohms is given as max grid resistance and cathode bias is not recommended. fixed bias only.
I found several recommended ways to connect in triode mode, this was just one of them. I am only using 430 volts on the plates, so I am hoping I can take a few liberties like the cathode biasing. May try fixed bias too just to hear what it sounds like, but it is a bit more trouble. I will try 30k for a grid resistor. Someone mentioned that earlier and I had meant to switch it but forgot. Thanks for the tips.
Went back and checked and the resistors are actually 68k, so I must have changed them the first time around. From several articles I found on the web, I think this will be ok at the low B+ I am using. I wish someone knew why RCA did not give parameters for an audio amp.
you've probably seen it
That is a cool book. I just found that this morning while looking for info on fixed vs cathode bias. As far as I could see though, it did not give parameters for using the 813 as an audio tube. Maybe in those days now one thought it was a reasonable use for the tube.
they give guidance for class ab1 , two tubes push pull. many notes to sift through. pay attention to symbols that reference notes down below.
I have not seen this one before.... lots more data. I notice in the notes they do reference a couple of cases where a cathode resistor can be used for bias, so apparently it wasn't completely forbidden. Going to have to spend some time going through this & try to figure out which parts apply to what I am doing. Thanks a bunch for the links, I appreciate it.
How and where to measure voltages in your gear, how to interpret what you're seeing on the o-scope, or how to use a O-scope period?
I see you have built some gear but I'm just puzzled you don't know how to use an O-scope. This is the basics in electronics and a must if you're repairing, building and troubleshooting any gear or electronic device including Tube and Solid State audio gear. Not trying to disrespect you.
Knowing to use a scope will make your project building alot more easy and fun. It very frustrating to be without or not know how to use a scope and need to see what is going on in a circuit that is not working correctly.
All of those! I have built a lot of gear over the last 15 years the same way a carpenter builds a house, by reading the plans. Designing an amp may be rocket science, but building one from a good schematic is not. Craftsmanship, paying attention to the schematic and being careful where you put your pinkies are the most important things.
I never had an opportunity to learn how to use a scope. A few years ago I bought one and decided I would learn how to use it. It's proving to be a bigger challenge than I thought for lack of good reference material on trouble shooting with a scope. Lots of info on things like measuring voltages and frequencies, etc, but so far I have found no step by step methods for trouble shooting.
I have been able to use it and the signal generator to trace a signal through an amp to find out where there was a problem, and that helps a lot but it has its limits. I want to get more proficient with it but I don't want to just fumble around blindly if there are tutorials available. The links you posted above look promising and I will be going through them. Thanks!
'I have been able to use it and the signal generator to trace a signal through an amp to find out where there was a problem'
then you are there already.
where is the gap?
Not quite. The problem that started all this was a small buzz I was hearing that I wanted to eliminate. Putting in a grid stop on the input tube helped a lot, but some is still there. I was pretty sure it is due to the bread board wiring being a bit of a rats nest, so I decided to not worry about it for the moment. By that time however the 'scope thread had taken on a life of it's own. That's ok though, because I do want to learn how to make better use of it, and some of the suggestions for videos I am getting could help with that. I hope so.
For all the input connectors. From that pic you posted those are just at the length of being too long and maybe picking garbage from within the amp. Just one more thing in the process of elimination. 🙂
I noticed less buzz after I moved a few wires around that I guessed were likely trouble makers, so I think when I get it built proper like it will clear up. Just to be sure, what are saying is maybe too long, the rca interconnects to the amp?
In one of your post about this same problem a few weeks back. Is that the same project you are having issues with and this thread is about? If so, then those wires that are coming from all those input connectors maybe too long without shielding. If the wires were shorter like 1-2 inches long shouldn't be an issue.
That was the 845 amp, this one is an 813 and I re-did the wiring somewhat. I have done bread boards lots of times and got away with wiring like that, but my luck may have run out. Not worried right now though as I think it will disappear when I do a final build and I have it low enough that I can listen to the amp at least.
Got through the first video awhile ago finally, and I think they will help. Thanks!
Edits: 10/23/16
It's hard to grab the concept without seeing how it's actually done. He even discusses how some signals that look good to the untrained eye, actually are showing issues. This should go a long way to help you out.
You should try doing what he is doing in those video. The one about charging a cap and viewing it as it's charging is a good one.
Have fun with the videos!
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