Help needed with capacitors.

[grinderdust]

I am replacing some swollen caps on an old bass amplifier. I orderded the caps from 2 different places because the one ran out. The two batches I recieved look way different from each other. The originals are about the size of a pencil eraser and say 50v 47uf on the side. The first batch I recieved was about half the size of the originals and I thought nothing of it and soldered them in. (newer is smaller right?) Then the next batch came in and I started to get confused. First of all they are about twice the size of the original. (thats four times the size of the first batch) So I started reading the numbers on the side and it turns out the first batch says 47uf 50wv. I ordererd 47uf 50v. Is this the same thing? Why are the sizes so different? I am pretty confused here. Are there other specs I need to be looking at besides voltage and farads. I thought I knew what these numbers meant, but now I am not sure. Please help turn my ignorance into knowledge.

[Al_The_Man]

Different technologies in construction mean different size, the main thing is they are suited to the purpose you are going to use them in, for e.g. a Tantalum Cap for the same value and voltage will look micro miniature, but they will not work in all applications.
The WV is working voltage. Same thing.
This is a fairly small value and voltage so I assume they are for de-coupling or for audio output?
Al.

[grinderdust]

Thanks, Al_The_Man.
They have some capacitor type function in the audio preamp. I don't know what exactly. All I know is the amp is burning up preamp tubes and some of the caps look swollen. I am just replacing them and hoping things will work (iffy but cheap!).

So, 47uf 50wv and 47uf 50v is the same thing?

[Al_The_Man]

Originally Posted by grinderdust All I know is the amp is burning up preamp tubes and some of the caps look swollen. I am just replacing them and hoping things will work (iffy but cheap!). So, 47uf 50wv and 47uf 50v is the same thing?

If they are cathode decoupling caps, they could be a problem.
The WV & V is means the same.
Al.

[grinderdust]

O.K. I read Al's post again 50wv does indeed = 50v.
They are all electrolytic type caps. Big difference in size though.

[grinderdust]

I don't know what a cathode decoupling cap is so I'll just have to pray. I'm assuming it has something to do with that red hot tube so I'm wearing some safety gear in case something gets decoupled!

[Al_The_Man]

That is not all that usual that a preamp tube overheats, unless there is something shorted in the grid circuit, they often have high impedance in the anode that prevents this.
It is usually the outputs that glow!
Al.

[gar]

080229-2029 EST USA

Unless the vacuum tube plate is tantalum you do not want it to show any glow. The heater or fillament in the center of the tube will have an orange glow.

If the tube is a gas discharge tube, thyratron, then when on you will see a gas dischage glow between the electrodes.

.

[vger]

You might want to check the resistors connected to the grids of that tube. I've seen the old carbon composition resistors go bad by cracking in two. The crack can be so small as not to be readily visible, but will be bad (open) just the same. When I was reparing tube type equipment in the mid 60's I would just wiggle the resistors in a malfunctioning circuit and if cracked they would seperate sending me to the resistor box for a replacement.

Steve

PS: What's the brand / year of your amp? Ampeg? Marshall? Epiphone? Fender? Just curious

[grinderdust]

Thanks guys for all your help. The amp is a Hartke model 7000. It is one of the old mosfet models. If I had to guess I'd say it's about 15 years old. It only has a tube preamp and is mostly solid state. I like it because it packs a clean 700 watts and fits nicely into a rack. Very nice for traveling. I would get a new one but they don't make them that powerful anymore....

Do you hear that ringing?...

I don't see alot of big power resistors near the tube but I'll follow some leads and see what I find. Why would a broken resistor offer less resistance instead of more? It seems logical to think that it would cause a break in the circut rather than a short that would blow a tube. Can I use a meter to check the resistors? I looked into checking caps with a meter and I gather you can't tell much about a cap without some special equipment. I replaced all the caps and powered it up without the tube last night and everything seems ok. No fires or magic smoke!(This amp also has a solid state preamp so it still works without the tube.) I am going to hook up a speaker tonight and see if the new caps are still noisy when I tap on them and if the E.Q. is still fuzzy. (another strange anomoly) I won't be able to do a full test until I aquire another 12ax7 preamp tube.
Sorry for hijacking the electronics discussion. I hope to get back to cnc very soon!

[Al_The_Man]

The 12ax76 is a dual triode tube, and the reason for it glowing bright or overheating is if it is biased on the wrong part of the curve, normally a pre-amp tube does not draw that much current.
If you look at this example the components to check are caps and resistors connected to the control elements, for a triode that is control grid, anode & cathode.
http://diyaudioprojects.com/Tubes/12AX7_Preamp/
You will notice that the anode and cathodes have fairly high resistances in them.
If you check the voltage with a DC meter, the grid should be negative WR to cathode.
Where were your 47uf caps using the link.
Al.