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Author Topic: marshall 6100 100 watt anniversary amp head  (Read 10326 times)

EDWARDEFFECT1

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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 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!

J M Fahey

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Re: marshall 6100 100 watt anniversary amp head
« Reply #1 on: June 04, 2013, 02:29:36 AM »
Even if poor, it's better than nothing.
Post the schematic you have.
Otherwise it's the guessing game.

Enzo

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Re: marshall 6100 100 watt anniversary amp head
« Reply #2 on: June 04, 2013, 03:49:18 AM »
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.

Enzo

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Roly

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Re: marshall 6100 100 watt anniversary amp head
« Reply #4 on: June 04, 2013, 10:57:53 AM »
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).


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.
If you say theory and practice don't agree you haven't applied enough theory.

EDWARDEFFECT1

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Re: marshall 6100 100 watt anniversary amp head
« Reply #5 on: June 04, 2013, 06:46:54 PM »
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.
« Last Edit: June 05, 2013, 11:46:32 AM by EDWARDEFFECT1 »

Enzo

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Re: marshall 6100 100 watt anniversary amp head
« Reply #6 on: June 04, 2013, 07:03:52 PM »
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.

EDWARDEFFECT1

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Re: marshall 6100 100 watt anniversary amp head
« Reply #7 on: June 04, 2013, 11:31:31 PM »
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.
« Last Edit: June 04, 2013, 11:37:04 PM by EDWARDEFFECT1 »

EDWARDEFFECT1

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Re: marshall 6100 100 watt anniversary amp head
« Reply #8 on: June 04, 2013, 11:50:42 PM »
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)

EDWARDEFFECT1

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Re: marshall 6100 100 watt anniversary amp head
« Reply #9 on: June 05, 2013, 12:05:32 AM »
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...

Enzo

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Re: marshall 6100 100 watt anniversary amp head
« Reply #10 on: June 05, 2013, 03:04:24 AM »
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.

Roly

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Re: marshall 6100 100 watt anniversary amp head
« Reply #11 on: June 05, 2013, 09:51:04 AM »
Quote from: EDWARDEFFECT1
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.

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.
If you say theory and practice don't agree you haven't applied enough theory.

EDWARDEFFECT1

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Re: marshall 6100 100 watt anniversary amp head
« Reply #12 on: June 05, 2013, 10:36:10 AM »
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.

EDWARDEFFECT1

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Re: marshall 6100 100 watt anniversary amp head
« Reply #13 on: June 05, 2013, 11:42:40 AM »
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
« Last Edit: June 05, 2013, 11:46:13 AM by EDWARDEFFECT1 »

g1

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Re: marshall 6100 100 watt anniversary amp head
« Reply #14 on: June 05, 2013, 01:27:16 PM »
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.
« Last Edit: June 05, 2013, 01:33:31 PM by g1 »