6CA4 (EZ81) questions from a first time poster.

[tribi9]

Hi everybody,
First post here.. I'm 34 have been playing guitar since my teens and I love Fender gear both amps and guitars. I recently started messing around with amps due to the lack of local inexpensive techs.

I have no electronic background but have been cramming in as much electronic theory as I can for the past few days. I have successfully done some basic maintenance to my ELK Stageman. (I replaced some caps), switched the speaker out to a Fender 1/4 inch style jack. All with help from a member of a Guitar forum I visit. Simple jobs but remember I'm that guy that dropped out of electronics class on the second day in high school... Wish I hadn't.

I somewhat understand how a dual triode works. From my understanding a 12AX7 works as follows... The transformer heater wires heat pins 4 and 5, which in turn heat up the cathode, blowing electrons that the rectified DC positive powered plate attracts. These signal leaves through the cathode and feeds the power tubes and that in turn feeds the output tranny which feeds the speaker. (Feel free to correct me please)

What I need to know is how does a dual anode rectifier like a 6CA4 work? What actually happens in the tube that rectifies the power?


And here are some pics of my ELK Stageman.

Thx for all your help in advance guys.




[IMG:800:600]http://i114.photobucket.com/albums/n271 ... 0629-1.jpg[/img]

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... C00630.jpg[/img]

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... C00631.jpg[/img]

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... 0632-1.jpg[/img]

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... 0633-1.jpg[/img]

[IMG:800:600]http://i114.photobucket.com/albums/n271 ... 0870-1.jpg[/img]

[UR12]

Welcome to the forum! Nice looking amp!!

In your description of how a 12ax7 works, you mentioned the signal exits the tube via the cathode. The output signal from the tube can exit at either the Cathode or, most commonly through the Anode in most triode stages used in guitar amps. In regards to the operation of the dual diode , it is used in a full wave rectifier circuit. It consists of 2 diodes sharing a common cathode. Basically it takes the 60 cycle high voltage AC coming from the HV secondary of the power tranny and changes it from AC to DC. The high voltage DC is then used to provide a positive voltage for all the other tubes in the amp. All of the rectifier tubes have different ratings as far as voltage and current. Some can supply more DC current than others but most of the common types all have similar voltage ratings. Some guys like using different rectifier tubes for different levels of "Sag". Sag occurs when the voltage out from the rectifier drops a little as the tube is asked to deliver more current. Depending on the amp, this "Sag" can provide a natural compression of the signal eminating from the amp. You don't get this with Solid State diodes.

I had a wise old instructer once give me some advice. He said that there is no way we are going to remember everything and that it was easier to remember where to find the info you seek than trying to remeber it in all of it's details. Rather than me try and explain, you can google it or just check out this one of many links on the internet that teaches how tubes work.

http://zebratubes.com/new278643.html

http://en.wikipedia.org/wiki/Vacuum_tube

http://www.google.com/search?q=How+does ... lz=1I7GGLG

[tribi9]

Thx Dana,
Actually I just started reading the NEETS mod 6 on tubes but decided to check back here see if there were any replies. I guess in the case of the 6CA4 the filament wires would go to the heater pins 4 & 5, the high voltage on the transformer would go to the Anodes Pin 1 and 7 and then the rectified AC power exits through the cathode as DC onto a filtering cap?

Is little things here and there that confuse me, simple things at times. Like why are pins 1-3 triode number 2 and pins 6-8 triode number 1? Or like the filtered dc, does it feed the cathode or the anode?

Im trying here tho' Im doing as much reading as possible...

[doctord02]

here's some more reading for ya - you'll be wanting to bookmark this site, btw... You can search for data sheets on any tube there.

http://tdsl.duncanamps.com/show.php?des=6CA4

[tribi9]

here's some more reading for ya - you'll be wanting to bookmark this site, btw... You can search for data sheets on any tube there.

http://tdsl.duncanamps.com/show.php?des=6CA4

I had looked up the pinout of a 6CA4 and had already seen the site tho but again only for the pinout. Im at the plate plate-voltage and plate-current on the NEETS tube and power supply module. Once I'm done that Im going back to the link you posted because hopefully by then Ill know a bit more what I'm reading.

Lots of cool info and knowledgeable people here on site.

Thx guys

Any more tips on what to read send them my way. Next year I plan on doing a Champ 5F1 build. Id like to know what goes on on a opposed to just following the layout and building it.

[UR12]

Thx Dana,
I guess in the case of the 6CA4 the filament wires would go to the heater pins 4 & 5, the high voltage on the transformer would go to the Anodes Pin 1 and 7 and then the rectified AC power exits through the cathode as DC onto a filtering cap?

Yes

Is little things here and there that confuse me, simple things at times. Like why are pins 1-3 triode number 2 and pins 6-8 triode number 1? Or like the filtered dc, does it feed the cathode or the anode?

Im trying here tho' Im doing as much reading as possible...

The pins configuration are sometimes done in strange ways to eliminate noise or it' is just the best way to bring all of the contacts out of the bottom. Why they labled them the way they did i , I really don't know but I am sure there is a reason that probably makes sense.

The filtered DC feeds the Anodes/Plates of the triodes. It is that positive potential that attracks negatively charged electrons away from the cathode and over to the plate/anode, allowing current to flow through the tube. The small signal on the grid can then control a larger amount of signal on the cathode or plate of the tube.

The best way for me to learn is to just jump in there and do it. It is good to read and learn all you can but you eventually need to put down the books and get your hands dirty A small project to start with will teach you a lot as far as soldering techniques and troubleshooting. If you get into trouble we have a great bunch of guys here who never shy away from helping out and they live for a good challenge .

[Structo]

And of course you do know there is high voltage inside an amp that can be lethal.

Familiarize yourself with how to check for voltage on the filter caps and how to safely discharge them.

Never assume that there is a bleeder resistor in an amp.

There is a sleeping giant in most tube amps plugged in or unplugged.

[tribi9]

The first thing I did before I opened up my amp for the first time was to learn how to discharge the caps. (Fear of getting shocked actually kept me from opening up the amp for a while)

I got a question tho'. I'm replacing some more caps on the amp. I removed a few of them from underneath the board (The amp in the pic) but one of the caps I removed, a .1 @ 400 VW, I'm not sure of the polarity of the cap. Is there a way of finding out?

Dana your explanation answered a couple of questions I had.

Thx guys and I'll definitely look forward to completing my first build. I wanted to do a BF PR but I think I might go with a 5F1 as a first project.

[jjman]

Most caps with a rating of 0.1 @ 400 VW, in a tube guitar amp, would not be polarized. If neither end goes to a grounded connection, in this particular case, it is probably not polarized. It is more likely a "coupling" cap. Yes one side might go to it's internal "shield" but it way too soon to worry about that. Got a pic of that one?

BTW, the large resistor on the large (blue) filter cap appears to be "hanging" and has an "extension" on it. It's big enough that you should probably affix it to the chassis. Double stick tape should be ok. Bend the leads to ensure it stays against the tape and don't let the leads get close to the chassis.

[Structo]

Sometimes the older non polarized caps had a line on one end of the cap.
This was the foil (shield) side of the cap and that generally would go to ground or the lower potential side of the circuit.

[tribi9]

BTW, the large resistor on the large (blue) filter cap appears to be "hanging" and has an "extension" on it. It's big enough that you should probably affix it to the chassis. Double stick tape should be ok. Bend the leads to ensure it stays against the tape and don't let the leads get close to the chassis.

Yes it is just hanging there but it doesn't really move. The leads were to short, so when I replaced the big cap I had to add an "extension"

Sometimes the older non polarized caps had a line on one end of the cap.
This was the foil (shield) side of the cap and that generally would go to ground or the lower potential side of the circuit.

The cap in question does have a line on one end of it. Will post a pic later, but its just one of those gray "Atlas" oil caps with a line on one end. Ill post a pic later on.

Cheers.

[tribi9]

So would said cap then not blow up if wired backwards?