Help! I tried to recap my amp!

[Cornelius]

Yes, pin 3 would be the Cathode, and i still do suspect that you have reversed that electrolytic cap. Being at the cathode, it won't blow since the voltage there are around 2VDC, but could lead to a funny result.

Looking at your pictures in your first post, i do see that the cap and a resistor are connected to pin3 on the preamp tube.

That 'convenient' connection point on the socket are - i believe - originally designated as a grounding point to screen the pins from each other... I think. ( i have a couple of such sockets in my shelf... )

Edit:
Look at the last pictures you posted; the other two caps connected to the socket. They have some black around the base of the leg that are connected to the tube socket. (looking under the 'skirt' of the grey plastic encapsulating the caps.) Your blue cap should go the same way.

[docz]

Yes this is why I switched it around, I don't have a picture of that yet.

That may very well be correct, I just did it the way it was done before.

DocZ

[Cornelius]

Oh, you did switch it around?!

I found a bottle of Red Vermouth here, so my reading might be a bit out of focus...

You're still welcome to drop by (with your amp), if your way finds you around this neighbourhood...

[docz]

Ok, I did some more work. Changed the remaing electrolytic caps (30uF 6V and 10uF 15V. I also changed the 3k cathode resistor, the 100k resistors of V2, and the 100k resistors of V1.

Now the tremolo is working again, and hum is a little lower. Noise is still there though.

Reverb does not affect the noise, but tremolo does. The noise tremmors with the tremolo on. So it has to be generated somewhere between the two gain stages of V1 right? Since volume and tone does not affect the noise either. Or am I wrong about this?

DocZ

[ampgeek]

Well done DocZ! Nice to see that you are sticking with it and, obviously, getting the hang of it.

Yes. If the tone and volume controls don't effect the character of the noise, it is getting "injected" somewhere down stream of those areas. The trem circuit is varying the bias on the second gain stage (and changing the gain) and isn't really getting involved in the signal path.

The noise is the crackly, frying bacon sound right? I didn't listen to all of your sound clips unfortunately.

Well...if I had to make a guess now, it would be the second stage plate resistor (250K if I remember correctly) or the sockets. Forgive me if you have already replaced that resistor and I didn't catch it in the thread.

You can replace that resistor with anything close that you have handy. The lower the value, the lower the gain of that stage. 56K to 470K would be just fine for a test. 1/2 watt recommended but higher OK. You could also sneak in a quick test with a 1/4 watt if you are desparate.

You are probably staring to see how handy the stethoscope really is. Diagnosing failures down to the component level takes all of about 3 minutes!

Cheers,
Dave O.

[Structo]

The noise level may be a result of lead dress.
In a point to point amp this can be critical because of the proximity of the components.

Signal wires need to be away from heater wires or any AC carrying wires.

Unfortunately, with the PTP, all you can do is bend the components one way or the other.

Which might lead you to discover a component lead that is ready to break.

Glad you are sticking it out. That's the only way to learn.

[docz]

I just tried to change that 250k resistor, I didn't have a 250k, but since you said it was ok with up 470k so I used a 300k. It sounded promissing at first, the amp suddely at a little more spank, hum was reduced a bit, and I really thought I had fixed it, but after about 15 minutes the crackling came back - bummer!

There is a big resistor that if I give it a hard whack while it is on, I hear a pop and the noise goes away momentarily. Could it be this one? It is the 130 Ohm resistor on the power tube. Can I change that with a 1/2 watt? Or is that a bigger wattage resistor since it is so big?

DocZ

[docz]

Ok now I've changed just about every resistor in there. Except for the input resistors, the 1M resistors on V2, the 250k on the right lug of the tremolo pot, the 1k cathode resistor on V1, 500k grid resistor on the EL84, the 130R resistor on k/grid3 on the EL84, and the large filter resistors.

Can I use my 1/5W 100R for the big 130R resistor? That is the closest I have, next value I have is 270R 1/5W

I don't have 250k or 500k, I have 1M, 330k, 300k, 220k and 200k any of these work for those?

DocZ

[docz]

Ok, I changed the 1k cathode resistor on V1, still noise.

The tremolo didn't work after all, it has to be switched on while there is a signal, and not turn the knob to far, then it works. If I turn the knob to far, I have to switch it off and off again with a signal to get it working. If I turn over about 70% it kills the effect. Weird?

And I also learned that having the thing connected to the wall socket while working inside is a bad idea... I got shocked twice today! I accidentally put my hand over the power transformer while soldering - that stung like a b......!

And reaching over to turn the tremolo while testing it with a guitar my palm touched the positive side of the three filter caps, and my other hand was on the guitar bridge, now that really made me feel alive!

I will not do either of those again! for sure!

DocZ

DocZ

[mcrracer]

OK enough is enough! I have been watching this thread develop like a trainwreck happening right before my eye ( and I don't mean the amp!) How come NO ONE has told this guy to slow down and LEARN something about tube amps before attempting to repair one? No disrespect toward the OP but it is painfully obvious that he is in way over his head. By the questions he is asking he does not understand what is going on. He does not need to get a degree or have 30 years experience, but he does need to acquire SOME knowledge and experience before he kills himself. Docz if you will do some searching on this and other amp building forums you will find loads of reading material on safety and how these things work. Just throwing parts at an amp may fix it eventually but you have learned nothing and wasted lots of money and time. Amp building is a very rewarding hobby but like any thing else it does require a basic knowledge of what they do and how they do it. Maybe some of the guys have resources at hand , I will have to search through my hard drive to list some stuff for you to read. We want you to be able to visit with us for a long time and enjoy your amp

[docz]

Hi mcrracer, thank you for looking out for my safety. I really got a wake up call today when I got shocked. I have actually been real careful the whole time, but today I let my guard down and forgot to unplug the amp before working on it. This will not happen again! You are also completely right that I have no experience, a painful lack of knowledge and I may indeed be in way over my head. I take no offence from this, because I know this to be true, and I sense that you are only trying to be helpful.

I have actually used a lot of time doing research, asking people I know, reading forums and asking you guys. The thing is that I for some reason don't learn well from reading - hence all the questions I'm asking. I didn't read or do homework in school, but by listening to my teachers lectures I was able to maintain good grades. This is why I may ask a lot of anoying questions, that I might be able to figure out on my own. If this is anoying to you guys, I am truly sorry, for that is not my intention.

This project has like you said been a rewarding hobby. Right before christmas I suffered from a heart attack and having this little project have really helped me get through it all. I may have devoted to much time to this project and rushed it, but it was nice to have something you could get totally submerged in without having to think about other stuff - you know?

I have actually learned a lot from this, but maybe not the right things I need to know in order to do this properly. But I have learned how to read a schematic, how to solder properly, what all the pins on the different tubes are, how they work, and lots more. I know what some of the resistors and caps do, but not all of them, and what is puzzling to me is why you use a certain value component in a particular place.

But I will not give up, I will learn more and eventually I might even understand all this stuff. But until then, I hope you guys will still answer my silly little questions and help me reach my goal. I will promise not to do stupid things like jump starting my self again.

Again thank you for looking out for me.

DocZ

[Cornelius]

Considering your heart, you really should be more careful. A healthy person with a strong heart are usually no worse for the wear after a shock, except for the surprise, but you with a heart condition?!...

You was saying 1/5W for that 130ohm resistor... Do you mean one-fifth? (does it exist?!) If it is the cathode resistor for the output tube, you really need 2W++ You should use the right value, but for testing purpose, go up to the closest you have...

[mcrracer]

Hey Docz, I understand from the health viewpoint I have quite a few health challenges myself. I design and build amps to take my mind off of my health problems Your questions are not annoying. Learn by whatever means is best for you. Just be careful. I would suggest to you to put this project on hold for a little while and first build yourself a simple SE amp (single ended) Something like a Champ or Octal One or some other simple two tube amp. I am sure the other guys can help suggest other circuits also. But something simple on an eyelet or turret board with an easy to follow layout. To help you get used to following signal flow and power distribution. It is my belief that you will learn far more from building an amp from the ground up plus you will have the reward of seeing and hearing something that you built. Then go back to the one you are trying to fix. Just a suggestion.

[docz]

Ok, I decided to do some research before doing more work. I remember from school way back we had an introduction to electronics. We learned something called Ohm's Law. Which is something I've never used since we tried to dim the lights of a bulb hooked up to a 9V battery back then. Well anyways, using that law - and please arrest/correct me if I'm wrong, I may be able to calculate the necessary wattage for those resistors right?

So that 130 Ohm between the cathode/g3 of the EL84 and ground, when I probed everything for voltages, I measured 8V on the cathode of the EL84 before the resistor, then using the law I can calculate 8/130 = 0.0615, then 8 * 0.0615 = 0.49, so it is about half a watt right? So a 130 ohm 1/2W should theoretically work, but since in real life things aren't perfect, I should maybe use a 1W to have some room for variation? Or am I totally wrong?

What I am wondering about is why is 130 ohm and not 100, 1k or something else? Is that because that particular tube needs a 130 ohm cathode resistor? Or is it because the circuit demands it? Or both?

Also if someone could explain the shear purpose of that resistor, I would be really grateful - please don't be mad at me, but I've been googling all night trying to figure that out, all I got was lots of people blowing up their Epi valve jr's trying to get more gain. Is that what that resistor does regulate the output gain?

Believe it or not, but I am actually understanding some of the concepts involved, but not all, far from it.

Docz

[Structo]

That is your cathode bias resistor.
The reason it is the odd 130R is because that is what was decided on for the proper bias current.

They don't mention the wall supply voltage on the power transformer but if the amp is from the 60's then it is most likely 110vac.
So now with the common 120vac or higher, all your voltages will read a bit higher.
Have you measured the heater voltage?

I like to use at least a 5 watt cement power resistor (the white square kind).
This amp may only need a 1/2 watt resistor but it should be at least a 1 watt or larger will not hurt anything.
It will dissipate heat so bigger is better.

In this case you would place your red probe at the tube pin/ resistor junction and black to ground.
So you are measuring the voltage across that resistor.
Then you can use the E/R = I for you bias current.
And yes, E x I = P
That doesn't take into account the screen current but it is usually small and can be ignored, it also gives you a little safety zone in the cathode current.

Now, is this power tube really a 37189A?
Or is it a EL84?
Because I can't find a reference to a 37189A.