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all voltages are the same on each tubes pins.does anyone have a ledgible schematic showing what winding are what for the power transformer.just thinking it could be shorted internaly sending 483vdc to the grid.sorry i just corrected one of the above voltages.i wrote pin 7 but meant pin 8 which is the cathode.-.36vdc

all voltages were taken with the tubes removed.also i notice there is a resistor that i believe doesn't belong. it is on the green grid wire going to the tube closest to the input jack. it has green,blue,red,gold for bands.all the other tubes the green grid wire is dirrectly soldered.if my calculations are right it is a 5.6k resistor(5600 ohm).why it is there i don't know as it would make the grid voltage different from the other output tubes.don't believe it is causing my problem thow.

just looking at the 3 shows 5881 power tubes not el34's.i'm sure the schematic must be simular thow.this is still the best schematis i have found yet...

hi roly! in regard to your question.the problem is pin 5 is the grid of the power tubes and the voltages there are supposed to be about -50vdc for the tube to function properly. i seem to have a problem as i am getting almost 500vdc @ pin 5.the tube will not function as the grid has to be more negative than the plate.i am getting plate like voltages at the grid.the grid is usually the lowest voltage in the tube usually a negative voltage. the cathode is next which should be slightly higher than the grid.the grid of an tube is always more negative than the cathode.the highest voltage on a tube is the plate. the second highest voltage is the screen grid which is only used on the power tubes as your normal preamp tubes dont have a screen grid (12ax7,12at7,12au7,ect)

i looked at all the sockets and don't see any carbon tracking.i measured the resistance between pin 4 and 5 of all the power tube sockets and i get ol (overload) no reading on my fluke.all 4 sockets have the voltages listed  as above.

i am counting the pins just like the diagram above my post.i am starting at the pin key and #1 is to the left of the key and i am counting the pins in a clockwise direction from the bottom of the tube socket.all the voltages seem normal on the preamp tubes.the power tubes(el34's) are another story i have the following voltages which are all uniform between all power tubes. i have 491vdc on pin 3   481vdc on pin 4    483vdc on pin 5   -.364 vdc on pin 8    .when i bought the amp it wasn't working . when the guy in the store hooked an extention speaker along with the reguilar speaker jack it wouldn't work with just one speaker cabinet. when i got it home it was dead in the water again. when i said i got 500+ volts it was measured on my friends meter which is a cheaper fluke than mine. mine is a fluke 87 iii.even so the voltages are way out of the ball park.

Amplifier Discussion / marshall 6100 100 watt anniversary amp head
« on: June 04, 2013, 01:04:41 AM »
i have a marshall 6100 30th anniversary amp head it is purple. i believe it is a 1992 model. i can't find a good shematic for it that is ledgible and clear for what i need to repair.i have 500+ vols dc on pin 5 of the power tubes.any ideas? i believe it is supposed to have -50vdc on this pin for the bias.500+ volts on the grid is really bad. any thought or ideas appreciated.i am guessing it might have a bad coupling cap between do i test it as all the connections are under the main board of the power tubes. i also get .010 ohms in both directions on the 2 large diodes on the power supply board.i replaced the diodes with new ones and i still get 0.010 ohms across them in both directions testing on the diode scale of my meter.the 2 diodes were good out of this low reading in both directions an indication that there is a short farther down the circuit....thanks...ed!

thanks to all the solid state guys on this site that helped me out. i found the problem! bad electro harmonics i said earlier the electro harmonics 6v6's are junk.i have 427vdc at the plate and the bias is rock steady with a new set of jj 6v6's.wish there was a way to test tubes buddy tested one of the old power tubes that blew the fuse on a b&k 707 tester and one was shorted. i brought the same tube home and tested it on my b&k tester(600 series) and my sencore mighty mite tester and couldn't find the the way the 2 electro harmonics that made the bias climb were tested on his tester and they tested good.they were brand new tubes.guess the tube testers are very unreliable.thanks guys rock when it comes to repairs,building and time spent to help others.....thanks to everyone that help solve this probleml!!!!.....ed!

saying for today.wisdom comes from many at solid state forum!!!!    thanks!!

i get -43vdc on pin 5 of the power tubes and the voltage stays steady.the only problem i have with the coupling cap thought is that the coupling caps are separate one for each tube,so for the amp to be doing what it is, they both would have to be leaky.what are the odds.i am waiting for new tubes.the tubes are both connected  and if one had a short to the cathode it would affect both tubes (which is what i have)there must be something we are missing as i have tried another set of tubes,but it is possible i have a bad one. i hate electro harmonics tubes from Russia. they seem ok with their preamp tubes but over 400volts the 6v6's seem to go bad. i have had alot of bad ones.the jj tubes seem a lot better.

hey Phil! thanks for jumping in.i had the same idea.the only other thing might be a leaky 1n4003 is the first part in the bias circuit. i think we are on the right track as both of these will affect the bias on both tubes.i just fixed an amp just like this and it also had bias trouble.the 22 ohm resistor right after the diode read 22k in circuit and was open. i jumper ed a 22 ohm resistor across it and the bias went back up to normal voltage...anyone with any more ideas welcome.thanks....ed!

hi i know this is a solid state site,but i also know there are some good techs here.i have a fender deluxe reverb reissue that blew a fuse. one or the power tubes was shorted.i put new tubes in it and tried to rebias it using a weber bias rite meter.the amp will bias then when you wait about a minute the bias starts going up a tenth at a time.then you keep turning the bias down till there is no more adjustment.there are 2 dcv test points on the print-34vdc(reads -35vdc) and -49vdc (reads 50vdc).the voltages are perfect and they stay rock solid on there readings. the bias probe is good as i just did a couple amps with it with no problem.also to prove it i let the amp run for about 3-4 minutes and finally the tube with the higher bias red plated,so i shut it down right away.the bias is affecting both tubes as the current keeps counting up a tenth at a time on them.all the components on the circuit test good.i also changed the 2 electrolytic s with no difference.the voltages do show no problem with the bias circuit.there are no cathode caps on pin 8 of the 6v6 power tubes.both cathodes are tied to ground.this one is a hair puller.lets see who has the answer to this tough problem?....thanks....ed i will try to attach schematic...thanks...ed! page 5 is the right schematic...

Amplifier Discussion / shorted kustom 400 watt bass head
« on: October 15, 2011, 05:21:12 AM »
i have a kustom 400 watt bass amp one of the newer china blew the 8amp fuse and a npn output chip.i replaced the bridge rectifier also.i replaced the parts and plugged it in with a homeade current limiter with a 100 watt bulb.the bulb is staying bright so i have a short in the amp. what is the best way to find it.if it is with an ohmeter what readings should i look for.are there any other ways to test for a short.variac? oscilloscope do you do it .....thanks...ed

Amplifier Discussion / Re: johnson j150 millennium pico fuses blowing
« on: September 21, 2011, 11:24:41 PM »
the bridge rectifier tests good and has been was good,but replaced it as a precaution.when testing it gets a barrier voltage on the ends have to check diagnally for the other 2 voltages.all the ic's look normal no burn marks anywhere on the board while viewing it with good light and a head magnifier. the ic's don't appear to be shorted between the 12v+/- terminals.the fuses blow before the ic's get hot.if i check from ground to the pins on the ic i get no short or low ohm readings.i have no +12v or -12v to the ist ic in the preamp right after the input jack due to the pico fuses blowing.i also don't have any 32 volt supply which is not layed out on the print where it goes other than connector diagrams.i looked everwhere for a low ohms or dead short figuring that a short is causing both fuses to blow.was hoping c11 was shorted as it is used by both the 32v and +/-12v luck though. i traced the 32v supply from the power board with a continuity test and it goes from the connector on the power board to the main board, then it goes along a trace to another connector(ribbon connector this time) to the jack board which has a connector for a pedal control board. i am guessing the pedal board is powered by this 32v supply.

Amplifier Discussion / johnson j150 millennium pico fuses blowing
« on: September 21, 2011, 09:53:13 AM »
i have a johnson 150 millenniium 2x12 combo amp and it keeps blowing the 2 pico fuses(f1 and f2).i have no 32v supply and no +12 or -12 supply voltage coming off the power board. everthing checks out good on the supply board and there appears to be no shorted output chips,or shorted capacitors. i also checked the main board too and found nothing.any help appreciated. i have the + / - 5v supply and the b+ supply.thanks...ed

sounds like you might have a shorted will cause parts to burn. had the same problem on a peavey classic 30 tube amp...should be able to find it with an ohm meter or cap tester.when voltage goes to ground it's not start burning parts........

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