JCM 800 build, hum issue, hum solved, smoked PT, rookie mistake?

[dwp]

Hello all, been awhile. I have a recent JCM 800 clone build, 50W, had a noticible, un-wavering, low 60 cycle hum. Thinking the hum might have been at the rectifier, i switched from two-diode to four diode balancing bridge type rectifier. Tied ground to same point as first filter cap can.
this stopped the hum. But I neglected to take any voltage readings, played the amp for a couple minutes, great tone, no hum, but by the time i noticed an odor, the PT was in over temp, wisp of smoke, (no flames) probably dead now, haven't tried it back on, holding much heat.
Did I create the problem by grounding the rectifier? was I supposed to lift the PT ground and re-route? I didn't.
Or is it possible i had a bad PT as was with the hum, then finally fried it?
Was new Heyboer PT with no hours on it.
Am ordering a replacement, but any insight on whether or not the rectifier thing was the mistake or my re-wiring will be much appreciated.
Thanks in advance, Rookie still, dwp

[2deaf]

Did you ground the PT center tap AND the bridge rectifier?  If so, that will burn a PT up.

[dwp]

yup.  ooops. so what is the correct way to ground a brige rectifier then, or any SS recitifier? was thinking this was the root of my hum. Any drawing will be appreciated. I checked the 100W Marshall diagram, but I have cap can, don't see how to achieve that with a 50/50 can.

[eleventeen]

Yeah, that doesn't sound promising. You probably cooked the tranny, given the smoke & stink. Both the above are known as "full wave rectifier" circuits. You probably started with the second one. We expect a DC output of .707 times the FULL secondary of the PT. So if the PT puts out 340-0-340, we would expect .7 * 680 or 476 volts. Load it down, smooth it out, we have 450 volts which is nominal for an amplifier B+.


First diagram produces 1.4 * the FULL secondary of the PT, so a 340-0-340 would produce 1.4 * 680 = 952. You might have scorched more than merely your PT, EXCEPT: Note that there is no center tap ground on the diamond-shaped 4-diode bridge. (That the thing is often drawn in a diamond shape means nothing, it's just a visual clue. Because you kept that center tap in place, the bridge rectifier alternatively shorted out one phase, then the other phase of the PT output. So the volts will never have gotten anywhere near 900+ volts. But it *was* quite the short on the PT secondary, hence the smoke & stench. This is possibly good news in the sense that you could have cooked a heck of a lot more stuff with 900+ volts. Unfortunately, the thing you *did* cook is the most expensive part.


No-center-tap-ground on a bridge (4-diode) rectifier. And double (approx) the voltage of a FW 2-diode rectifier.


Really....the last thing that would produce a hum would be the diodes in such a rectifier....now the electrolytic caps after the diodes....those can definitely age, deteriorate, and cause hum. Or, if they are really bad, short out and blow fuses, etc.

[eleventeen]

The above is a 2-diode full wave rectifier, the one that we expect to produce .707 * the full output of the OT.


That is probably what you started with. NOTE THAT many many amp makers place 1-2-3 diodes in series in this application. This is sort of a tradition left over Fender amps and from when people didn't trust diodes quite as much. Diodes in series are thought to resist big turn-on surges....and they are very cheap parts so it's not a bad idea. But circuit-wise, 1-2-3 diodes in series are for our HV rectifier purposes only a single diode.


You asked: " what is the correct way to ground a bridge rectifier then, or any SS recitifier?"


There IS NO way nor need to ground a bridge, UNLESS you are making a + and = volt supply, like an op-amp supply, say +15-0-15 volts. For tube amp purposes, you connect the CT of the tranny to NOTHING. Tape it off. BUT YOU CAN'T DO THIS!!  You'd be doubling your supply volts and then you'd be up in the 900+ volt zone. Let's just say that there would be smoke & stench, though it would be smoke & stench of a different character.

What are you thinking, that the rectifier up and rewired itself the wrong way?

Stick with the factory config. There, you DO have the grounded CT on the tranny, and two diodes instead of four.

[dwp]

thanks for the replies and information. this is my build, and was chasing down a hum.....
I read another post about the possible cause of 60 cycle hum could have been incorrect grounding of rectifier. then brilliance prevailed. i installed a "grounded" bridge rectifier. oops.


so when I get my second trannie in the mail, I will begin agsin.


You are correct eleventeen, I began with the second drawing, two diode full wave, with hum.


I do have more than several plexi style builds under my belt, I utilize Hoffman stlye turret boards, this build however I copied the 2204 JCM 800, and ended up with the low hum. not really bad, but noticeable, nothing I encountered in other builds. the hum was directly proportional to the master volume control, and I didn't have any luck moving wires around, couldn't get it to waver or vary except by reducing the MV pot.......


Thanks again, will continue with replacement.

[2deaf]

I checked the 100W Marshall diagram, but I have cap can, don't see how to achieve that with a 50/50 can.

I have a JMP 100W, 800 100W, and a 900 100W and they all use 50/50 cans.  Which 100W are you looking at?

[dwp]

looking at the power wiring diagram on this sight for 100 W marshall.


Hey would it being any indicator as to where is my hum issue, since it did completly stop once installing the bridge rectifier? (though briefly)

[2deaf]

The schematic is probably the one I have attached.  Marshall frequently hooked the two 50's in a can in parallel to get 100uf.  But, as eleventeen pointed out, you cannot use a bridge rectifier on a transformer meant for a 2-diode full wave rectifier.

HT power supply hum is usually 120 Hz, but it is possible to get 60 Hz if only half of your rectifier is working which makes it half wave instead of full wave.  As long as you're installing a new PT, you might want to install new diodes and maybe even three in series like eleventeen mentioned.

You also might consider a smaller mains fuse since the one you have allows the PT to smoke.

[dwp]

for the sake of full disclosure, even as embarrassing as it is, Hum resolved. was a preamp tube. I fired this thing up with a known good set, (yea right) finally pulled V-1 and swapped it with V-3, hum was greatly reduced. Pitched V-3, hum gone.

     I had never experienced this type of preamp tube symptom. I had good tone, no pop or scratch, just persistant hum. no way that's a preamp tube right?
Wrong.
     Is why I couldn't locate a grounding issue. Although I did get the opportunity to re-work the whole power end and grounds..
thanks for your input all. embarrassing alittle yes.


More later, dwp