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 stages.how 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 circuit.is this low reading in both directions an indication that there is a short farther down the circuit....thanks...ed!
Even if poor, it's better than nothing.
Post the schematic you have.
Otherwise it's the guessing game.
If you have 500v on pin 5, do you have that on ALL the power tubes? Is it on pin 6 too? If so, check pin 4 and see if your -50 is there. If it is there, then I fear you might be counting pins backwards.
A leaky cap wouldn't put 500v on a grid, a shorted one could only do it if the tubes were removed. And while leaky/shorted coupling caps do happen, it would be extremely rare that it happened at the same time to both sides of the power amp.
And your diodes? Not sure what two diodes you mean, but I suspect they are you chassis ground diodes, and it is normal for them to measure short because both ends are grounded. I can't read the schematic well on that page either, but LOOKS like D401,402, and if so I bet there is a cap and a 10 ohm resistor next to them.
Oh, and...
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/30aniv_6100_100w.pdf
Firstly: what is the nature of the fault/problem you have?
Secondly: the pins on an Octal base are counted clockwise starting from the keyway, looking at the underside of the valve (internal chassis view).
(http://www.ozvalveamps.org/generic/7ac.png)
Bottom view, looking at the pins
Assuming that you have been counting in the wrong direction this would give 500V on the screen connection which is actually pin 4 and is reasonable.
If you have 500 volts DC anywhere it suggests that the power supply is working okay; but to help you troubleshoot we need to know what the problem is that you are trying to trace.
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 worked.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.
OK, then apologies, that is a VERY unusual circumstance. is it that way on all four sockets?
Look for arcs on the sockets between pins 4 and 5.
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.
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)
just looking at the schematics.page 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...
EL34 or 6L6 (5881) difference is adjusting the bias voltage maybe a couple resistor values. Same circuit, same board.
Thanks, Roly and I are fully aware of how the tubes work, it is just an incredible thing to have 500v on not one grid pin, but all of the grid pins. 99% of the time if someone reports that, it is because the pins were being counted backwards. If you actually have that, fine, I believe you, but it is still hard to imagine. Like your dog suddenly speaking French or something.
If it is on all four sockets, then it may not be a problem AT a socket. it could still be an arc somewhere else. Turning off all the room lights and looking at the board sometimes makes a tiny spark visible.
Let's use the impedance of your meter to help. Ground your meter at one point and leave it there. Now note the EXACT voltage at pin 5 of each. If they are all exactly 500 (or 498.6 or 503..2, whatever) that tells us something, but if one of them is 500v, another is 498, another is 497, and the last 495, that also tells us something. The lower the voltage drops, the farther that point is from its source.
And go down to the bias supply. Is there 500v on the bias adjust control?
If all the sockets have the same voltage on pin 5 then it seems the voltage may be coming up the bias feed. But if it is highest on one socket and lower on the others, that points to a problem at that socket.
I don't recall, but have we removed all the power tubes and are now taking measurements on empty sockets? That is the first thing you should do - remove the tubes. If you have EL34s, a melted screen grid could easily short to the nearby control grid. That would go away when you pull the tube, since the short was inside the tube.
I don't think the schematic will help you much. There is no schematic related failure that can cause this. If something is touching both a pin 5 line and a 500v line, the schematic won't show us that.
Quote from: EDWARDEFFECT1when the guy in the store hooked an extention speaker along with the reguilar speaker jack it worked.it wouldn't work with just one speaker cabinet.
This is a very serious concern. A valve/tube amp operated without a load is very likely to flash over the output transformer and total it. That this apparently hasn't happened to you suggests that you either haven't tried driving it very hard at all, or you have a short in the internal speaker circuit, which while not good is much better than an open.
The circuit is marked 5881's which are the same as 6L6's, but there is a difference in the pin connections to the EL34/6CA7 in that the suppressor grid is brought to pin 1 on the 6CA7 and internally connected to the cathode on the 6L6/5881.
"Anode like" voltages on the grids is almost unheard of, by me anyway, can't recall ever encountering this personally or second hand in 50-odd years of valve amp building/servicing.
HOWEVER (as
Enzo says) 6CA7/EL34's are particularly prone to screen grid collapse and it is possible that one (or more) of the output valves has suffered this failure. I have also seen them flash over between anode and heater on the valve base itself.
So, pull all the output valves and re-measure all the voltages on the valve sockets. This will hopefully tell us if the problem is related to the chassis or the valves. You could also check all the valves for loose bits of wire floating about inside and for any signs of continuity between the control grids and any other connection.
ALSO I note fuses in the cathode circuits of each pair, F102 and F202 500mA, and you haven't given us the voltage on the pin 8's, the cathodes. If these fuses are blown it's quite possible you would see high voltages on the control grid circuit, cathodes, etc, and if you have a shorted internal speaker it would explain
why they are blown in the first place.
In particular I'd like to know the voltage on the grid resistors R234 and R235 where they join and go to the bias supply - this should be -50V; and the voltages on pin 8's, the cathodes, and are LED201 and/or LED202 lit?
It also must be said that Marshall are very naughty boys opening the cathodes with fuses and switches. This is specifically advised against by valve manufacturers because of the stress it places on the heater-to-cathode insulation.
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.
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
What do you measure on pin5 of power tubes if you set your meter to AC volts?
How about DC volts at output of DB201 (across C210).
Also, resistance from DB201 + pin to ground, and C210 + end to ground.
Voltages at the transformer will be AC, so 500V DC would not be likely to be coming from the power transformer.
The 5.6K resistor is a grid stopper, there are probably 3 more for the other tubes but mounted on the circuit board somewhere.
It's not really important now but may be helpful for finding correct schematic to know whether you have the 5881 or EL34 version. Are the screen resistors 470ohm or 2.2K ? (R251 thru R254)
As far as the speaker jacks, from what I recall there are 2 parallel jacks, no internal or external, both just regular. So it sounds like one works and one does not. Resistance checks will tell you which one is bad, it may just need resoldering.
If you don't carefully note our questions and
answer them we can't help you.
Quote from: EDWARDEFFECT1all voltages were taken with the tubes removed
{Le chien parle en français!}SPECIFICALLY: have you got ~+500V on pin(s) 5, the control grid(s), WITH THE TUBES REMOVED??? YES/NO?
AND:
@Enzo asked -
"Is there 500v on the bias adjust control?" YES/NO
I asked the same question -
"In particular I'd like to know the voltage on the grid resistors R234 and R235 where they join and go to the bias supply - this should be -50V"This voltage is
what?{grid stopper resistors should be fitted right at the valve socket control grid connection}
Quote from: g1What do you measure on pin5 of power tubes if you set your meter to AC volts?
You are aware that multimeters respond to
both AC and DC on their AC ranges unless a blocking cap is used?
i have 4 el34's.it is the 100 watt model.i have the board and tube socket out of the amp right now.i'll check the items for you when i put it back together.i tested all diodes with my meter on diode test and they are all testing good.back to the drawing board on this one.thought about the transformer shorting into the bias supply winding it can't be as it is acv...
there are 4 5.6 k grid resistors on the board and the all test good.i am going to remove the stray resistor from the last tube and connect the green grid wire dirrect just like the other 3 tubes.i did find a bad connection on the grid resistor of the power tube farthest from the input jack. i don't get continuity on it all the time. if i look close with my head magnifier i see the solder joint move slightly.still this is not my problem. i am going to fix the 2 problems mentioned and continue on. all the componets seem good.diodes, caps, resistors ect...
Maaaaaate! I am feeling so frustrated here - it's as if you aren't actually reading our replies/questions (and I'm really curious about this one).
The symptoms you have posted (e.g. +500V on the control grids with the valves out) are quite astonishing, and frankly, diddling with the grid stoppers isn't going to get you anywhere.
You have the attention of over a hundred years of amp repair experience here, so help us to help you.
Post voltage measurements:
- of each pin of one of the output valves (that is representative of them all, and any that significantly differ).
- the voltage found at the bias setting pot.
i am sorry for any frustration i might have created. i didn't notice that we were on page 2 of the replys.i have to put the circuit board back in the amp to take the readings that i am being asked for,i will reply with answers to voltage checks when i get it back in.possibly today.i have been busy getting my band ready for playing out.we had a girl singer that moved to california ,so we threw out 27 songs and have replaced her and our other guitar player with a male singer guitar player and a keyboard player.we are playing out next week and have put alot of new songs on the table. i appoligize.
If you have 500v on the grids, it won't be because some resistor is the wrong value or even disconnected. If you have +500 where -50 ought to be it is because something is creating a connection between those two things.
Also note that the extra 5.6K resistor at the socket was put there by the factory. They put it there at that one socket because it cured an oscillation problem.
So it would be best to leave it alone, otherwise you may cause a problem that could be very difficult to fix for someone else later.
Quote from: EDWARDEFFECT1 on June 04, 2013, 06:46:54 PM
when the guy in the store hooked an extention speaker along with the reguilar speaker jack it worked.it wouldn't work with just one speaker cabinet. when i got it home it was dead in the water again.
No way in hell it would have worked at all if it had the fault stated.I'd check and double check your testing procedure before replacing anything.
Might be as simple as a dodgy speaker socket and or lead.
I once replaced the whole cooling system in my car only to find a $2 radiator cap was the problem. :duh
With unknown fault type repairs you are wise to start with the cheapest replacement first.
I'm sure there is a Murphy law hiding in that somewhere but I just can't think of it right now.
;)
Phil.
Re: the grid stoppers; looking at the circuit it seems to me that (for reasons only known to Marshall, and not that I have a lot of faith in modern Marshall, but) they fitted three of the stoppers on the PCB and the other flying to that valve. Given that's more than a little off beat I agree with @g1 not to mess with it; and I repeat, the grid stoppers haven't got anything to do with the symptoms so far given.
I also agree with @phatt - simply changing speaker enclosures doesn't put +500V on the control grids. Your problem is most likely somewhere in the speaker circuit, the connectors being the most obvious suspects, and that the unlikely voltage readings around the valves are a distracting red herring.
{And there is a special case of Murphy's Law to cover every eventuality. Some that come to mind relating to your cooling system; "A million dollar project will fail due to a five cent part"; "A tool dropped will always fall where it will do the most damage (http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?entryID=2543)"; "The part to fail in any assembly will be the most expensive and/or the least replaceable" - and there are hundreds more being written daily.}
the phantom 500 i believe is fixed.i found a bad trace going to the grid wire of the power tube closest to the input jack.when i checked continuity with my meter fromthe 5.6k resistor to the green wire i would get of and on continuity.as i probed where the green wire was attached to the circuit board i could see it move(thank goes for magnifiers)i used my radio shack desoldering tool and after i removed the solder i noticed a crack in the trace. i cut the trace and jumperd a >018" bus wire from the green grid wire to the 5.6k grid resistor bingo good continuity. i also removed the 5.6k resistor someone soldered to the grid of the power tube farthest away from the input.i soldered the green wire dirrect to pin 5 of that tube,so it was just like the other 3 tubes.i put the 15v power supply board and the power tube board back in the amp.grounded my meter to the chassis.fired the amp up threw my light bulb short tester i made and continued on with no shorts to full power.i tested all 4 tube grid connections(pin5) and got -40.9vdc.i haven't looked a print to figure why this phenominon happened yet.i will try the amp out to see if it works later today and will reply with the results. i think the phantom 500vdc is gone. my apolagies to all that helped with this nightmare.i couldn't measure some of the voltages you wanted until i got the boards put back in the amp.wires from the tube sockets upon removal were touching the chassis as to not allow me to fire up the amp for testing.all you guys are the best on the net. i give you all a 12 on a 1-10 rating.thanks for your patience.maybe one of you can figure out what happened. i still don't have a reason for it to work just doing the 2 items i mentioned....thaks all....ed
:o :lmao: :duh :crazy2:
Houston we have ignition.just plugged it in and it is working.now that i have some time i'll probably study the schematic and find out why i had the french talking dog.hope you don't mind if i use this one! love it Enzo.i'm wondering if the trace being open caused a ground loss in the circuit creating the 500vdc problem.hope i never get another amp like this one.i don't think the removal of the resistor helped as it wasn't bridging any of the other tube pins or touching anything creating a short which could of caused the voltage to spike up.i had 481 volts on my weber bias rite on pin3 (the plate).i rebiased the amp @ 70% and shes up and running. 62 bluesbreaker,plexi,and jcm 900 all in one amp. nice sounding amp...thanks all for your help. dont know if anyone will figure out where the 500vdc came from.if i get some time to play i'll dig into the schematics that i received.....everyone have a great fathers day this weekend! drive safe and get a designated driver if you are tipping any drinks.stay safe! your friend ed!!
All we can do is suggest methods for tracking down the cause, which we tried to do. But ultimately you would have to find the actual problem yourself. The schematic won't help us. For example, if a 500v trace is next to a -50v trace on the board, and something shorts them together, the schematic won't help. The schematic says nothing about WHERE each part is.
If your bias is disconnected from the grid, the grid is not going to float up to +500v really. You may not have spotted it, but I think when you were soldering and moving wires and resistors, you cleared the short, wherever it may have been.
At this point, I wouldn;'t suggest you going back into it. It's working. But if I just HAD to know where the short was, I'd clip my meter to the pin 5 of one of the tubes and with the other probe, I'd check any solder point to either side of any connections to that grid wire to see if 500v was there. If you can find +500 next to a -50v line then somewhere along that parallel path a short probably existed.
Quote from: EDWARDEFFECT1i still don't have a reason for it to work just doing the 2 items i mentioned
And you won't, simply because
none of it makes any sense; the stated symptoms have no relationship to a broken control grid track, and a broken control grid track would have produced quite different and dramatic symptoms - gross red plating in the related output valve. But since you didn't report such a symptom it's a fair guess that you actually caused the broken track during your diddling with the grid stoppers and all of us are still in the pitch black dark on this one.
Quote from: EDWARDEFFECT1i'm wondering if the trace being open caused a ground loss in the circuit creating the 500vdc problem.
NO! "caused a ground loss" WUT? This is a science, not random black magic.
While I agree with
@Enzo that things can be wrong that the circuit diagramme won't show, it's also possible looking at the circuit to see that your idea that +500V is somehow coming through the bias supply due to "shorting windings" is
impossible; or that a single grid being open would silence the amp on one speaker cab and not another is
impossible - as
@phabb implied,
these ideas simply don't make logical electronic sense. I'm not even going to ask how you think a single broken grid track could cause "a ground loss" on the other valves, but that's
impossible too.
The bias supply actually comes from the
same winding as the HT, dropped by a pair of capacitive droppers. If these had failed the result would have been excessive
minus bias, and possibly exploding caps in the bias supply. If 500V+ had shorted to the bias supply then
there most certainly would have been fireworks in the bias supply. Assuming that you would have noticed explosions, smoke, and flames I think we can rule this out as a possibility.
Now apply a little theory to your broken track - what do you know about a power output bottle that loses its control grid bias? It
red plates and melts down, right? As
@Enzo said, short of it becoming a molten ruin inside, and again assuming you would have noticed one of the tubes doing a 4th of July inside, we can rule out +500V on the control grid from that source as well. If one of the output valves isn't a smoking ruin then the open grid wasn't there during earlier tests.
So what does that leave us with?
That the "500V on the control grid" reading was some kind of momentary brain fart.
That what you actually have is an intermittent fault in the speaker circuit, most likely a speaker
connector, and if it happens to go
open circuit under drive
you can kiss your output transformer goodbye.
Just because it's now working again doesn't mean you have fixed it. In fact I think you have managed to well and truly distract yourself from the real cause of your problem, and it's only sleeping, waiting to come out and bite your arse when you least expect it.
Quote from: EDWARDEFFECT1hope i never get another amp like this one.
The problem here isn't the amp, it's YOU. You have a huge amount of experience on tap here, yet you haven't answered one single question that was put to you, a critical one three times. It's almost as if you have been posting without reading, and I've even been wondering if you are a troll having fun at our expense.
The process here works as a
closed loop, we
ask questions, you make measurements and observations and
report back, then on the basis of what you report we ask more questions, and so on. It works like that so we can keep you to an orderly faultfinding progression and not jumping all over the place with wild random guesses - so the actual cause of the problem is correctly identified and repaired.
i was not trying to be a troll. i told everyone that i had 500vdc on the grid of all the output tubes and i was questioned if i was taking the readings off the wrong pins. i answered back that i took them correctly.the reason i couldn't take measurements as the board was out of the amp with the tube sockets touching metal of the chassis.when i found the trace that was bad and removed the resistor from the one output tube and replaced the board and everthing is working fine.i have put the amp back in my basement again as of now.i guess when i get more time i will check it out further. i sprayed all the pots with deoxit and cleaned the speaker jacks with crc qd electronic cleaner.sorry for any hard feeling this amp created because i was straight with everyone and was not trying to be a creepy troll.i don't play those games.as you can see from one of my earlier post. i rated this site a 12 out of a 10.i have the highest regard for you guys.sorry for any missunderstandings that have been created..everyone that helped i give my sincere appologies to anyone that this amp caused any hardship..thanks!!....ed
Hi Edward. I don't think you were a troll. The reason an experienced tech like myself or others might question if you knew the pin numbers is that we always look first for the simplest explanation for something. (That is known as "Occam's Razor") So if someone counts pins the wrong direction, then what they think it pin 5 is really pin 4. And all of a sudden, it makes sense to see 500v there. Add to that that despite that it happened to you, finding +500 on the grid pins is EXTREMELY unusual, and hard to imagine a cause for. SO we go out of our way to make sure it indeed is really what was going on.
Move forward to your explanation that you do know pin count, and then we have to start looking for ways it could happen.
We know from experience that an open trace from the bias to the grid will cause the tube to red plate and draw current, but we also know that will not likely cause +500 to be there. A lack of -50, yes, but no way to expect +500 there. I believe you that moving a resistor or replacing it or whatever made the problem go away, but also from experience, it SEEMS at least more likely that the resistor itself wasn;t the problem, but in the process of working with it, you also managed to move whatever was causing a short circuit and thus remove the problem.
SO hopefully that helps understand our reactions. I am glad you got your amp working.
thanks Enzo! i always respect your comments on this site.sometime when i go to my sisters house in michigan to visit i would love to stop in to your shop and shoot the bull if it was possible it would be an honnor to meet you. i am sort of self taught and have been lucky to have a good friend that has 50 + years of television repair.he is pretty knowledgable with testing and troubleshooting ect.he went to alot of classes on tv repair thru the years.thanks for being such a good friend Big Jim!!..had to give him a plug also as he deserves it.
Enzo thanks for all the help you always give.you have a very good outlook on troubleshooting with a very methodical approach.love your humor also.also my many thanks to all of the people who have helped take their time to answer my questions. there are to many of you to be able to list.....thanks to everyone that has helped!!...ed!!