Help! I tried to recap my amp!

[docz]

The tube that sits in the amp is an EL84, the one I replaced was also an EL84. I don't know if this has been replaced or modified through the years though. I can't find what 37189A is either, but I'm guessing it is a similar tube like the EL84?

Ok so that resistor is to regulate ampere and not voltage right? So in this case it sets the bias current to 8/130 = 0.0615, or 6,15mA? Is that supposed to match up to something else, which is why they chose that particular resistor value? Or is it's size determined by something else?

DocZ

[Structo]

Well current is a function of voltage and resistance.

Current is defined as the movement of electrons through a conductor.
In this case there is a resistor inline with the conductor.
This creates a load that will draw a certain amount of current and since we know the resistance and the voltage we can determine the current being drawn.

If nobody has referred you to this little program I recommend you download and install it.
It has data for just about every tube out there.
It will really help you understand tube specs better.
http://www.duncanamps.com/tdslpe/

The EL84 dissipates about 5.7 watts in a Class A single ended amp.
That is with 250v on the plate.
That is with 135 ohms on the cathode.

Now I haven't messed around with a SE EL84 amp so you may have to do some more digging to learn about how this tube behaves in a SE amp.

[Structo]

Here is a handy chart I have up on my wall.
If you use these formulas enough you will begin to memorize them.

Remember all that math you learned in school?

Now you get to put it to work.

[Phil_S]

The tube that sits in the amp is an EL84, the one I replaced was also an EL84. I don't know if this has been replaced or modified through the years though. I can't find what 37189A is either, but I'm guessing it is a similar tube like the EL84?

Ok so that resistor is to regulate ampere and not voltage right? So in this case it sets the bias current to 8/130 = 0.0615, or 6,15mA? Is that supposed to match up to something else, which is why they chose that particular resistor value? Or is it's size determined by something else?

DocZ

Good questions. The 7189/7189A is a heavy duty EL84 that can withstand higher plate voltages. Otherwise, for purpose of this discussion, it is equivalent to EL84. Good, now you can move on without wondering. the cathode resistor does several things, but "bias" is all about current (milliamps) and that an essential function. It also has interaction with the grid and controls overall operation of the tube. This is not so in a fixed bias amp.

You want to know about tube bias. For a cathode biased amp (like the yours), the goal is to bias it to somewhere between 90% and 100% of maximum static plate dissipation, which is 12W for an EL84.

To calculate the bias, find the following data: plate voltage (Va), cathode voltage (Vk), and the cathode resistor in Ohms (Rk).

Vk/Rk = 8V / 130ohms = 61.5mA. This is total current. (Your decimal is in the wrong place! Math is OK otherwise.)

Va-Vk = 270 - 8 = 262V. This is true plate (anode) voltage.

262V * 61.5mA /1000 = 16.1W. Divide by 1000 to accomodate mA instead of A (amps).

16W might be OK for a 7189, but not for EL84. I would bias it colder. Try a 180 ohm resistor. If all were static, it would come in just below 12W. Remember this isn't signal power, it is static plate dissipation. This is still a 5W amp. What will happen when you change the value of Rk, is that Va and Vk will shift. It presents a bit of a moving target, but in a narrow range. The next highest standard resistor is 220 ohms and that will bias it too cold. I don't know if they make a 200 ohm resistor. I've seen 100 ohms 5W and you can use two of them in series.

Please use a 5W (or more) resistor. It gets quite hot in operation and needs to be overrated. You've got only about 1/2W on the cathode 8V * 61.5mA = ~0.5W. You could probably get by with a 2W or 3W resistor, but I've seen many pictures of this sort of single ended amp (EL84 or 6V6) and they nearly always have a sandblock wirewound 5W resistor. There must be a good reason for it. I wouldn't use less than a 3W rating.

Your progress is very good so far!

Phil

[Structo]

Thanks for that Phil, I get to learn too!

Just in case you want to ogle a massive vacuum tube collection and the associated information of each tube, this is a great site.

http://www.r-type.org/muse/aaa0000.htm

[KT66]

Ok, after 10 pages I thought I would look in on this thread. I have skimmed over it, so please forgive me if I inject something redundant into all of this.

First off, I have to agree with Mcracer- safety is priority 1. Most of us have probably shocked ourselves at one time or another, so many think it's no big deal. Here's a list of musicians that have died from electric shocks :

Leslie Harvey of the Scottish band Stone the Crows
John Rostill of The Shadows
former Yardbird Keith Relf

I assume the "death cap" was to blame in these cases, but don't know for sure - the point is that electricity is dangerous.

Here is some safety info you should read Doc : http://www.drifteramps.com/safety.html

I noticed from the pics and the schematic at the beginning of this thread that your amp has a 2 prong AC cord, you should change this to a 3-prong cord - the ground prong connection in the chassis should be tightly secured on it's own bolt to the chassis. This prevents a malfunction from making the chassis "hot", because the fuse will blow as long as the plug is plugged into an outlet with a properly connected ground,

I know you learned the don't-solder-things-when-the-amp-is-plugged-in lesson, but not only should you unplug it, you should always drain the filter caps before working on it. Here is how I do that :

1. Unplug the amp

2. Using a test lead with insulated alligator clips on each end, FIRST connect one end to the chassis, and THEN connect the other end to the plate side of the plate resistor that is furthest down the power supply rail ( in your amp this is pin 1 or 6 of V1). I NEVER connect this jumper without first verifying that the AC cord is unplugged, so I know that if the jumper is in the amp that it must be unplugged. If the amp has a standby switch, it needs to be in the on position usually for all of the caps in the rail to be connected to ground through the plate resistor.

3. Use your meter to verify that the charged caps are draining. I usually Let it drain for a minute then when the voltage on the first cap in the chain gets below 10V I move the jumper to this caps positive terminal and leave it there. I NEVER work on an amp without the jumper in it, so I always know it's safe - if it's in the way of what I am doing, I move it to another part of the rail. My last rule is that I NEVER plug an amp in without making sure that there is no jumper connected.

If you discipline yourself to have rules like these for yourself, you will minimize the chances of having a bad accident.

Now, about the amps performance ( forgive me if somewhere in this thread this has been mentioned ), it seems to me that all of the filter caps are grounded at the same spot and this is not the optimal way to do it and could be the reason for noise issues you are having. The first cap should be grounded at the same point as the power tube cathode near the power section and away from the preamp. The other 2 filter caps should be grounded in the preamp section at the cathodes of the tubes whose stages they feed. There is more to it than that to make it perfect, but I think that will help a lot. There is a link in this thread: https://tubeamparchive.com/viewtopic.php?t=10260 to an article by Merlin about grounding that lays it all out, but it is kind of technical. Understanding and implementing it though will greatly aid you in building and repairing amps for optimum performance.

[docz]

About the grounding, I just put the new filter caps where the old ones were, I did not do anything to alter the grounding scheme of the amp. I'm not quite sure how to do what you and that article says, do I unhook the negative lead and hook it up a cable where it says in your post? Or do I hook the negative lead up to the point and then cable the positive side to the resistors?

I also found this site http://www.hans-egebo.dk/Tutorial/amplifiers.htm that explains a lot on why one uses a certain resistor and capacitor value, it has helped me understand this a bit more.

DocZ

[Cornelius]

Tell me...

Are you still using the original tubes?

[docz]

No, I changed all the tubes. That was the first thing I did, then I changed the filter caps, then I changed resistors and the remaining electrolytes.

DocZ

[MBD115]

Just a though (again )

Corroded tube socket?

And, you didn't listen to me docz. You were supposed to unplug the amp and check for voltage before sticking your hands in there. Everytime!!!

Be safe man

[docz]

Yes, I always do now I was getting a bit ahead of myself, and that made me a bit careless, I have learned that lesson.

I checked the tube sockets they look ok, but I'll order a replacement pair to be on the safe side, if it doesn't help I can always use them for my next project.

DocZ

[martin manning]

No, I changed all the tubes. That was the first thing I did...

It is not unheard of these days for a new tube to be faulty. I think you said when you tapped the cathode resistor the noise changed? That resistor is mounted on the output tube socket, so maybe you were also tapping on the output tube. Does it still do that if you tap the tube?

MPM

[tubeswell]

Here is a handy chart I have up on my wall.
If you use these formulas enough you will begin to memorize them.

Remember all that math you learned in school?

Now you get to put it to work.

I like this one. It has a certain logical intuitive layout and having somewhat pyramidy-power (remember that? , it is really simple to remember.

[docz]

Ok, update, I decided to give everything a good clean, I have now cleaned the switch, input jacks, pots, tube sockets and ground points. I also decided to try another set of tubes - in case my new 12AX7s were bad, I allready had those Russian tubes (6N2P) so I rewired the amp to suit those tubes. I had to get a new set of wires, because the old one was too short for the new wiring. So I went to the store to buy wires. They only had 1.5m2 cable, which turned out to be a bit to thick to fit in there. So as a temporary solution I used wired from some old computer parts, I am a bit worried that this wire is not well suited, so I will change it ones I get a hold of some 0.5m2 solid core wire in numerous colors. It was hard to find here, if any of you buys know where I can get suitable wire here in norway, please let me know!

Any way the new tubes changed a lot of the sound in the amp, it is a bit louder, but it starts to crunch up a bit at 70% volume, the old tubes were clean all the way up to 10. I liked the sound - but if this is a bad sign I will change it back if you guys say so. I really thought the crackle/static had gone away, it took a whole 20 minutes now before it made it's manifested itself. It is considerably lower now, but it is still there. Do I need to clean more perhaps? Or is it something else perhaps?

Hum is worse now, I didn't wire pin 9 to ground, could that be why?
And tremolo still works sporadically. So I guess that may be a cold solder joint somewhere or something like that? Reverb is suddenly a lot more "effective" I guess the new tube changed it's characteristic aswell?

Oh I still don't have clue about what is wrong with my amp, but I did start to source parts for my next projects. I am going to build a microphone, and a mini practise FET based amp, at least these projects woun't potentially kill me.

I still hope I can ask you guys some questions.

DocZ

[Phil_S]

Hi DocZ: you are certainly taking the bull by the horns! Good initiative on rewiring for the 6n2P tubes. It looks from the pinout diagram that pin 9 is an internal shield that should ground to the chassis.

As for wire, computer wire is probably OK for filament supply. This is a low voltage, high current circuit. What you took should be good for 6.3V. I"m not sure if I get it with the metric wire sizing, but if .5mm sq is the same as 20AWG, you can leave it there. http://www.technick.net/public/code/cp_ ... _to_metric If it is smaller size, I suggest it needs to change.

Dirty pots and leaky caps can cause static. More cleaning? Don't know. Sometimes, you can't overcome damage done over time and new parts are needed.

Tubes will change the tone. This amp should give you some crunch. Don't be concerned about it. If you now have a good reverb driver where the old one was weak, this can explain the improvment in the reverb.

It is always diffcult to advise from far away. Based on what I see in the pictures and what I think you are saying, I'd try to address the hum. The first thing I'd do in that amp is ... rewire the filaments....so maybe you'll be doing it again. Consider this: get the correct wire for the filaments. Take about 2 meters and fold it in half. Clamp the loop end to a table or something fixed, and put the other ends in the drill chuck. Use the drill to give it a nice tight and even twist. Now you have good twisted pair for the filaments. My work is just so-so and this is one of my earlier projects, but look at the last picture, which shows twisted pair (green) up in the air. That is how to do it. http://home.comcast.net/~psymonds/AA764/AA764.htm

Then, you should make a proper artifiical center tap. Get two 100 ohm (not 100K ohm) resistors. Run a resistor from each filament supply lug at the transformer to the high voltage center tap. Remove the short black wire that goes from one of the 6.3V terminals to the center tap. This will do several things, including elevate the heaters and give them a proper ground reference.

The reasons for doing these things are to eliminate 50 Hz hum. I am guessing you have plenty from the filament wiring.

Next I think you should deal with the why the grounds are made in that amp. Someone else has already touched on this. There is no system and you may have ground loops. One thing at a time. You should be aware, however, that you can't just ground things at any convenient spot and expect no hum.