RECTIFIER VOLTAGE PROBLEM on a JTM45 RI with HW board ?

electricskychurch · Post by **electricskychurch** » Wed Nov 15, 2006 11:07 am

i finished a 50w 74' i rebuilt with with georges board's 'mustard caps and AB'S instead of the pcb and it works fine, but i just rebuilt my 95' jtm45 ri , marstran 103 OT and 7H choke from MM and i have some voltage problems !

PT , RECTIFIER, FUSES, SWITCHES AND POWER TUBES WERE WIRED BY MARSHALL AS IT'S ORIGINALY A 95' JTM45 RI , SO I KEPT THIS ORIGINAL WIRING UNTOUCHED.

power on and stand by off, ALL TUBES OUT :

220v ac in (european voltage)
pin 6 & 4 rectifier 300vac
pin 2 & 8 80vac
shouldn't i read 5vac instead ?

power and stand by on
pin 6 & 4 300vac
pin 2 4,8 vac
pin 8 0vac

HT fuse is plugged on one side on pin 8 of the rectifier and other side to std by switch
other side of the stb switch goes to one + of the 32 uf cap next to ac plug (tied with one choke wire and OT center tap)

other + of the 32 uf goes to 8k2 1w , choke and V4 PIN 6

COULD IT BE A PROBLEM WITH THE 32UF CAP ?
any info would be welcome

electricskychurch · Post by **electricskychurch** » Wed Nov 15, 2006 11:09 am

all the other wiring and components SEEM to be ok by the way

Flames1950 · Post by **Flames1950** » Wed Nov 15, 2006 4:23 pm

This is the same thing everyone here seems to do.

You can't read the 5VAC on the rectifier with one lead of the meter referenced to ground like most of the other voltages. You have to put one lead on pin 2 and one on pin 8 to read the 5VAC.

electricskychurch · Post by **electricskychurch** » Thu Nov 16, 2006 6:48 pm

ok, i have the good voltage now on the rectifier :300vac and 5vac (for 220vac in)
the voltages are good on the preamp tubes, and i measure 415vdc on pin 3,4,6 of the output tube sockets (power tubes out , preamp tubes and rectifier in) and -43 on pin 5 of output tubes.

the problem is that i can't get under 100mv with my bias prob, whereas it should indicate about 38mv !!
when i turn the 25k bias trimmer, it only lowers about 4mv max !!
i tried with a 220k bias (george's guide) resistor instead of the 150k (george's layout) or 180k found on some schems but it only lowered the grid voltage of the power tubes to -30v which is not what i look for (so, back to 150k).

the voltages i measure with all tubes in are about 300v on pin3,4,6 of power tubes (they should be about 400v)

any idea ?

electricskychurch · Post by **electricskychurch** » Thu Nov 16, 2006 7:22 pm

i just blew the HT FUSE !
i'm not sure but i think i read some negative voltage was measured by the bias probe before the fuse has blown up.

electricskychurch · Post by **electricskychurch** » Thu Nov 16, 2006 10:52 pm

BTW, the test guide of georges says that the vdm's black lead has to be on the chassis and the red lead on pin 2 & 8 of the rectifier tube to measure the 5v and pin 4 & 6 for 350v

Flames1950 · Post by **Flames1950** » Fri Nov 17, 2006 7:25 am

I think I always have the 220K as the first resistor in the JTM45 or 50 type circuit. Putting that value back will help cut down your current.
Then you will need to address what value resistor is inline with your bias trimmer to fine-tune your bias current adjustment. What value are you using now? (I've had the two I've built end up at 68K IIRC.)
Once your bias is in line your voltages should come back where they belong, if you're really idling at 100mA then that is dragging your voltages way down.

electricskychurch · Post by **electricskychurch** » Fri Nov 17, 2006 8:21 am

i tried with 220k instead of the 150k, before the diode but it didn't change anything and i was still reading about 100mv on the VDM with groove tube's bias prob in a power tube socket (power tubes in) .
i have the 15k between the two 10uf caps and the 25k bias trimmer with 56k resistor .
i tried to put another 27k resistor between the trimmer and the 56k resistor (on the wire bridge) to add more resistence to the bias but it didn't change anything.
what's weird is that when i turn the bias trimmer full on to the right or left, it only change the bias of 4mv through the bias prob (from 101 to 97mv ), not more.
as you say, my voltage are low in the output tubes (300v) but i just have 300vac on the rectifier (not 350 as on george's guide).
when i take the power tubes out , i think i read 415v on the plates but with power tubes on, it falls to 300v and my HT FUSE finaly blew up.

i compared the original PCB with schem and george's layout
and it seems the 32 uf / choke /pin 6 tubes 4 & 5 just go through one 10k 1w resistor before going to tube 3 pin 6 whereas on the schem , georges layout and my new HW board , it goes through both 10k resistors !
the other part of the schem and original 95' jtm45 ri seem to be the same except the HT fuse is between the rectifier pin 8 and the std by switch (main fuse is directly on the ac plug connector .

novosibir · Post by **novosibir** » Fri Nov 17, 2006 8:42 am

electricskychurch wrote:what's weird is that when i turn the bias trimmer full on to the right or left, it only change the bias of 4mv through the bias prob (from 101 to 97mv ), not more.

mV or mA ?

Sorry, I can't follow your description completely, because it's a bit confusing for my understanding, but...

... is your bias circuit grounded at all?

Larry

electricskychurch · Post by **electricskychurch** » Fri Nov 17, 2006 8:46 am

if i put 100k instead of the 56k + bias trimmer , i have a too high grid voltage (-60v or so) .
down to a 68k, i have -48v . all that with a 150k resistor at the diode.
if i use a 220k at the diode with the 25k bias trimmer + 56k, i have -30 on the grid which is too low.

electricskychurch · Post by **electricskychurch** » Fri Nov 17, 2006 8:51 am

all this measurements in the above reply are made with all tubes in except the power tubes.
i can only measure the grid voltage with power tubes out, no other high voltage are present.

yes, my bias circuit is grounded to the pots ( that's were marshall have grounded in this 95' jtm45 ri .
no ground coming from the board is going to a filter cap ground lug, only to the pots ground bus.

electricskychurch · Post by **electricskychurch** » Fri Nov 17, 2006 8:59 am

yes, mv, cause the bias tool ( to put between tube socket and power tube) has an internal voltage divider, to produce a voltmeter readout of 1 mv for each mA of current flow (1mV/mA)

electricskychurch · Post by **electricskychurch** » Fri Nov 17, 2006 9:54 am

i can't find any problem in my wiring or components values, and the amp was working correctly before i start to replace the pcb, OT,choke and filter caps, so i suppose it's not a problem in the original wiring.

is there a way for the choke to be plugged, cause the MM7H i bought has two black wires (i think i read it doesn't matter ) ?

i wonder about the 32+32 uf cap health as well !
i suppose i made a mistake but as i can't find it more and more i search, so i wonder about some components.

my HT FUSE keeps blowing up as soon as i power with power tubes on.
maybe it could be them but they were working correctly as well and are not old.

novosibir · Post by **novosibir** » Fri Nov 17, 2006 10:01 am

The choke doesn't care, which wire where.

Larry

electricskychurch · Post by **electricskychurch** » Fri Nov 17, 2006 10:03 am

is the start of the primary OT ( black on marstran 103) the wire going to the power tube next to V3 (PI) and the finish (red on the marstran 103 ) going on power tube closest to rectifier ?
originaly on the drake OT they were RED to V4 and WHITE to V5 .
CENTER TAP wax BLACK on drake OT and is brown on marstran 103.

anyway i'm not sure a reverse secondary OT would blow muy HT FUSE and lower my high voltages on power tubes .