marshall 6100 100 watt anniversary amp head

[Roly]

If you don't carefully note our questions and answer them we can't help you.

all 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}

What 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?

[EDWARDEFFECT1]

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...

[EDWARDEFFECT1]

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...

[Roly]

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.

[EDWARDEFFECT1]

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.

[Enzo]

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.

[g1]

  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.

[phatt]

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.

[Roly]

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"; "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.}

[EDWARDEFFECT1]

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

[Roly]

  :lmao: :duh :crazy2:

[EDWARDEFFECT1]

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!!

[Enzo]

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.

[Roly]

i 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.

i'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.

hope 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.

[EDWARDEFFECT1]

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