Ceriatone Forum

Guys,

I'm about to replace the PT in my 5E3 - in another thread I explained how the original PT was on its way out, and have gone with a heyboer replacement (a MOJO branded one). I've never replaced a PT before, but am confident I can do this!

My play is to clip the wires, leaving a small amount of wire and insulation from the original, remove old PT, bolt in new one, and use the old PT as a schematic of sorts - matching "purpose" from old PT with new.

Anyhting wrong with my plan? The new PT is slightly hotter - about 370v unloaded. Any considerations? I'm also putting in a VvR kit at some stage - after the filtered B+, but will get it up and running before I think about that.

You skip a detail of what your going to do with that last pit of wire from the old transformer.

I was ready to berate you for cutting and splicing.  Then it occurred to me that you probably don't intend to do such a hack job but will be desoldering and removing that last bit of old wire as you prepare to solder in each wire of the new PT.  That is your intention, correct?   To do it the right, professional way?   ;D

This transformer?
http://www.mojotone.com/amp-parts/transformers-tweed-power/Tweed-Deluxe-Power-Transformer-EX-HV-lowered-to-330V

You're going to have some inconsistencies between the two transformers.  So, you may not want to use the old wires as a reference at all.  As a matter of fact, the Mojo and Ceriatone PT's use ENTIRELY different wire color codes, so leaving old wires may confuse you more than it helps.

Ceriatone's 6.3V heater supply wires are brown, Mojo's are green
Ceriatone's high voltage rail wires are brown, Mojo's are red
All the primary wall voltage wires are different
Only the 5V heater supply wires are the same color between the two.

But the biggest issue is the Mojo doesn't use a center tap for it's 6.3V heater supply.  You have to create an artificial center tap -- one 100-ohm resistor (corrected from 100K) to ground for each of the two heater wires -- for a ground reference or you will have hum. Typically, I do this at the pilot light, but I wire up heaters slightly different than Nik's layout (and use an older style lamp assembly).

Cheers Wyatt - that was my intention....honest!! ;)

My transformer isn't the lowered version, but shares the same wiring codes as this one. I guess I'd be fine going with schematic only, but wanted a belt'n'braces approach. I'd be using the old pt as a reference to check I'd wired stuff up.

Tell me more about the artificial centre tap...I don't entirely understand.


Oh - should i just heatshrink the unused primary power taps? Individually, or as a large collective?

*PLEASE KNOW THAT VOLTAGES STORED IN THE AMP'S FILTER CAPS CAN BE LETHAL.  THESE VOTLAGES CAN BE STORED LONG AFTER THE AMP IS TURNED OFF AND UNPLUGGED.  AND THESE CAPS ACTUALLY HAVE THE ABILITY TO REBUILD A CHARGE WITHOUT BEING POWERED FROM WALL CURRENT.  RESEARCH HOW TO SAFETY DISCHARGE AND GROUND THE CAPS BEFORE DOING ANY WORK INSIDE THE CHASSIS*


Then your math is wrong.  It will NOT be 370VDC.  The coefficient for a 5Y3 rectifier (allowing for average drop from rectifier and first filter cap) is 1.13.

Mojo756DEX (non lowered) is 384-0-384.

384*1.13=427VDC

Ceriatone is 325-0-325

325*1.13=367NDC

That's a 60VDC jump in B+ (16% jump), you'll have to check bias and dissipation\ and change the bias resistor value if necessary.


Some transformers have center taps on the heater/filament supply, some don't.  Most older transformers didn't and many modern repros of those do not as well.

Here is the spec sheet for the Mojo...note the heater/filament secondary...two green wires and nothing else. 
http://www.mojotone.com/core/media/media.nl?id=12524&c=923962&h=dc99d59effb8ceaa51b6&_xt=.pdf

Now compare that to the Magnetic Component 5E3 PT and the Weber...these both have center taps...the Green/Yellow wire inbetween the Green filament/heater wires; it is, quite literally, a tap run from the center of that secondary coil.
http://www.classictone.net/40-18021.pdf
http://taweber.powweb.com/store/025130sch.jpg

For reference, let's look at your Ceriatone...the heater supply are the brown-black-brown wires marked 3.15-0-3.15.  The Ceriatone brown wires are the 6.3V filament/heater supply.  The black wire inbetween is the center tap. 
http://img.photobucket.com/albums/1003/porge7/5e31.jpg

The filament/heater wiring has to have a "ground reference," it has "know" where ground level is.  With a center tapped heater/filament supply, it gets that ground reference by connecting the center tap to ground. 

Since the Mojo does not have a center tap, you have to create an artificial one.  This means connecting a 100-ohm resistor (corrected from 100K) to each wire, then connecting both of these resistors to *the same ground point* (many builders twist the leads of the resistors together to make sure)...

http://music-electronics-forum.com/attachments/4594d1235714146-artificial-center-tap.jpg
http://music-electronics-forum.com/attachments/4428d1234244837-2200c-center-tap2.jpg
http://www.clarkamplification.com/images/beaufort/4.jpg

All three of these add the resistors at the pilot light (I do it like Clark in the third pic).  You see Leo Fender use to run the filaments to the pilot light and then the rest of the tubes and those metal pilot light fixtures are right there for the ground, so i use it.

There is no reason, you can't run your resistors off the pilot light lugs to ground as well.  I suppose you could ground the resistors right there at that closest eyelet where the cathode-bias resistor/cap are grounded. 


They are all taps off the same coil, you don't want them to short out together.   Trim them back and seal them off individually, then ziptie them or something to be neat and tidy.

This is a lengthy post and I didn't even embed pics.  If any of it seems over your head, if you don't understand all of this, I suggest using a professional amp tech instead.  Ask him to teach you as he does the work.

*PLEASE KNOW THAT VOLTAGES STORED IN THE AMP'S FILTER CAPS CAN BE LETHAL.  THESE VOTLAGES CAN BE STORED LONG AFTER THE AMP IS TURNED OFF AND UNPLUGGED.  AND THESE CAPS ACTUALLY HAVE THE ABILITY TO REBUILD A CHARGE WITHOUT BEING POWERED FROM WALL CURRENT.  RESEARCH HOW TO SAFETY DISCHARGE AND GROUND THE CAPS BEFORE DOING ANY WORK INSIDE THE CHASSIS*

Hi Wyatt,

Thanks for the lengthy reply - I'm very, very thankful for it. I'm happy enough working on amps - I worked in IT tech for 12 or more year...back when CRT's were still common place. As a bloke that probably worked on 500 iMac's, I have great respect for voltage and it's ability to make your day worse...or life shorter! ;) Never got bit by one, but did cop 240v fixing a tape backup unit in the early 1990's. I have massive respect for voltage.

I'll admit to being an amateur with the amp stuff - even when I started teching in 1990, IT stuff was sub-assembly replacement - but I do work on my own stable of amps. Over the weekend I gave my '73 Deluxe Reverb a cap job, and replaced the grid resistors in the PI section to hunt down a noise (ended up being the PI socket). I can drain the caps, and am comfortable with a soldering iron.

Great explanation on the center tap. I noted that these go to ground on the CTPT What rating resistors would you use? I have some 1w'ers in my stash (grid resistor stash!) - will they be okay? I too like the neatness of the Clark example.

I've had to carefully bend the tag on the standby switch to clear the "bell" on the bottom of the new PT. Haven't started soldering yet - was Daddy Daycare yesterday, had my older boys from 3pm and a quick practice for a gig tomorrow night. Might have a crack tonight.

If I can ask a supplementary, I purchased a VvR kit to go into this one to see how it worked. I'm thinking of mounting (looking at photo) the unit between the 5Y3's base and the 6V6, having the B+ from the standby into the board, then out to the rectifier. Can you see any problems with my idea here? I don't mind that the control is on the back.

Good.  I just wanted to be sure.  you had mentioned you hadn't replaced a PT before.


FIRST AND FOREMOST, note my correction above...100-ohm resistors to ground...NOT 100K.  It was bedtime when I wrote those posts yesterday.

1/2 watt resistor is plenty, so the 1 watt resistors are fine.  Both seem commonly used.


You location sounds right.  Will it be on the back panel...between the power outlet and the speaker jack?  Lots of room for the pot there, good excess when in a head cab, and you have room to have the heat sink either straight out against the back of the chassis or bend it up and around to the top of the chassis.

BTW, once again...100-ohm resistors.

Btw, the 100-ohm* resistors will actually offer a better ground reference than a real center tap.  In theory, that means less 120-cycle hum (I guess that means 100-cycle gum for y'all).  In practice?  you probably won't notice.  Actually, the way they wired out my original 5E3 in 1960 didn't even use balanced heaters, and the amp was still hum-free (instead of running two wires, one to each side of the heater, Fender used to run a 6.3V wire to one side of the tube, ground the other side and then correct the other wire from the transformer to ground as well, Leo used the chassis to complete the circuit).

It can also protect the PT better, the resistor will often burn out before the transformer if there is ever a short in the heater supply.

* (note the value, I'll worry about that until I hear back from you!)

Cheers again - will be dropping by my local electronics shop to grab some resistors on the way home.

Yes - between the socket and the speaker out. The kit I purchased was everything mounted on an eyelet board, and the mosfet at a right angle, so thinking about here....picture telling a thousand words and all! ;)
 
(http://img.photobucket.com/albums/1003/porge7/5e3-2.jpg)

Simple 3 wire job to install (B+ in, b+ out and a ground). If I get brave, I'll try one in my AC30HH!

Oh - message received loud and clear on the resistors! 100ohm resistrrs coming my way. I think I'll be grounding them at the same point as the cathode bias resistor - looks like an easy spot.

C'est parfait!

The best spot to me, plenty of room without getting crammed, good excess to the power supply.

Don't you love it when work, kids, gigs, minor tragedy take control of your life and it takes you over A YEAR to do a 20 min job?

Sat down with thesoldering iron a few days ago (up at 5am for some reason), and have installed the new transformer. Have the artificial centre tap installed, and the amp wired as per schematic

Have sat the VVR kit in place, and have done nothing else there. Have tested voltages with the amp on standby, getting 398 on the rectifier with no tube installed, 390 with a fat old RCA installed. Heaters are clocking in at 3.2v a piece, same for the pilot.

Have some JJ 6V6's handy and am about to test when my 19 mth old awakes.