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In Reply to: RE: scratching my head on this one..... posted by vinnie2 on October 20, 2016 at 11:35:23
You could try a grid stopper. It could even be the layout of the wire. Breadboards are excellent for producing oscillations :-)
Follow Ups:
I was going to say the same thing until I saw your post.
Ok, a 100k grid stopper and a 1 meg grid leak resistor on the 26 significantly reduced the buzz, so I will make them permanent I think. Probably wouldn't hurt to put them on the 813's as well. The probe still stops it completely when touched to the plate of the 26, but I noticed something else too. I am using a variac to set the B+ voltage, and when I put my hand on the metal case of the variac the buzz increases. Unfortunately I can't yet take the variac out of the loop or the B+ will be way too high. I expect it to be about right when the other channel is hooked up to the psu, so I really don't want to mess with it right now. I also could have something else causing the buzz as well (like the bread board wiring) and I may not be able to get rid of it until I build a final version. At least it is low enough now that it doesn't bother me to listen to it.
Edits: 10/20/16
nada aqui
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Buy Chinese. Bury freedom.
I still don't have a book that tells me how to do it, and I don't have enough experience to do it on my own.
and be very careful when probing around inside the amp when it's on. Remember to keep one hand in your back pocket when probing!
Roger on that. Let's save the fireworks for the 4th!
Vinnie2,
Go on YouTube or do a google search for videos on scope operations.
ElPaso tube amps has some decent videos. Even some stuff on ARTA.
dt 667
Now that I have not tried yet, but I will. Thanks.
I recall seeing some tube amp basics instructional videos on ebay a while ago.
You might find them helpful.
Learning this essential skill should be your very next priority. It's silly to ask people to guess what's wrong with the gear you build when you have the tools you need right there.
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Buy Chinese. Bury freedom.
I have been trying to. I have 3 or 4 older books on using a scope but they all go into things other than what I want to do with audio. I have yet to find one that concentrates on trouble shooting. Same for the manual that came with the scope. Believe me, I would love to use it if I could find some documentation for beginners that went through trouble shooting procedures step by step.
Vinnie - and I mean no disrespect when I say this - it's difficult for me to understand why you haven't figured out how to touch a scope probe to a circuit and adjust it to look at the signals. My initial education in electronics took place when I was a kid. I knew how to use a scope before I could even afford to buy one. I'm sure I'm not the exception.
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Buy Chinese. Bury freedom.
I think you probably are TK. The one electronics course I took in college (I was a CE major so that is all that was required) never even had a lab, just basic equations and such. Since you have been working with them a long time you have come to think there's nothing to it, but that's not the case for someone who is just starting to learn how to use one. Lots of knobs and dials there Pardner!
Edits: 10/21/16 10/21/16
there are many parts to it so look at each one and have fun learning! :)
It is really the pits to have crappy internet service. I started looking at the first video and at 10 minutes in it crashed. Rebooted it and it did the same thing again. I will try later today but I may have a problem watching these. I hope not because what I have seen so far looks good.
Just had a quick look and book marked 3 episodes. Looks like it has promise. Thanks!
TK gave you a hint how to use a scope. Simply touch the scope probe to the circuit and adjust the dials until you see a signal. Use a 100x probe when working with HV. You really cant screw up using this method.
Do you realize how many combinations there are doing that that will give you nothing? All it takes is one setting wrong. I have been going through the manuals for the scope and my signal generator and have gotten a trace of a sine wave that way, so that is progress. However, it seems to me that a slap happy run through the unit probing here and there is a waste of time. Surely someone has written trouble shooting procedures for tube amps that would use a logical sequence for determining what the problem is. THAT'S what I want to find.
you pretty much know the voltage your testing so that setting is easy.
The other setting you need to adjust is time (frequency)
Other than that, use internal trigger and center the beam. You really don't need to use any advance features for what your doing. Also, get yourself a large 8 ohm resistor for a load.
Once you get the settings in the right place you won't have to change them other than the voltage scale. Start by using you preamp output with a 1khz sine wave, once you get that dialed in, use the voltage setting from then on for other measurements. You cant blow up your scope by being on the wrong setting, it will just be out of range of the screen.
I've read my share of books which never helped me much. Keep building and troubleshooting as there's no better substitute for experience. If you have any experienced friends nearby, have them show you some of the tricks of the trade.
You can then start measuring output, clipping, etc...
Edits: 10/23/16
That's the kind of info I am looking for. Just to check though, I think (hope) you meant you can't blow up your scope instead of can. : )
Don't have anyone to show me this stuff which makes it a bit tougher too, but I got one through one of Cougar's tutorials a little while ago and I think they will help too. I will keep plugging away at it until I at least get the basics down. I know it can be a big help just from learning how to use it to trace a signal when trouble shooting. That was a real yahoo! moment for me. Thanks for the explanation!
I meant can't, not can. My eyes struggle with the IPad sometimes.
https://www.parts-express.com/8-ohm-100w-non-inductive-dummy-load-resistor--019-020You'll get the hang of the Oscope just as quickly as you have learned tube amp building.
Just keep at it and don't be afraid to experiment.
Grab one of those 100:1 (1000v) scope probes on EBay for testing HIgh voltage DCYour scope setting will now show 100X the setting and you'll be able to see ripple, not just measure. Most probes are 10:1 so you are somewhat limited.
Edits: 10/23/16 10/23/16 10/23/16
ok. thanks.
Loading the amp if that's what the output transformers are set up for. Also it should be higher in wattage rating than the output of the amp. Parts Express sells them.
on the 813 tubes, how are pins 3 and 5 hooked up?
what size grid resistor on 813?
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.
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!
Aha, grid stopper. Yes that might be it as there is not one on there at the moment. Will give it a try. Thanks.
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