Marshall 1959SLP Problems

Peter Nerves · 09-16-2006, 12:37 AM

Hi All,

I am new to this forum so hello to you all.

I have a Marshall 1959SLP 35th anniversary re-issue. I have had a minor problem recently with no sound coming out of the amp when I flicked on the standby switch, which was solved by flicking the power switch off and on quickly and it worked fine for a few hours until it would cut out again.

I had a look around a few sites and decided that it was best to start with replacing the valves and see what would happen. I put the new valves in and I could only get the amp to work intermittently, cutting in and out all of the time. When I get a sound out of it I have to be really careful not to whack the strings, as this causes the amp to cut out again. I put the old valves back in and the problem stayed.

I tested the valve seat voltages and some of them seem far from what was indicated on the Marshall 100w voltage chart (although this chart may be the wrong one for the SLP) I was getting 380Vdc at the grid of valve 2 when the chart said 170Vdc.

I have been through all of the resistors in turn and tested their resistance to the values marked on them and came up with some supprising results. The 4 resistors that couple the 4 guitar inputs to the 2 grids of valve 1 were half of their marked values 34k (marked value 68k).

Could using effects infront of the guitar input cause this? I use an ebow and the output of this screams into the amp!

I have been testing the amp with a clean guitar signal straight in, all of the leads and connections are fine. It is definatley an amp problem!

I pray on your expert knowlege to give me some more advice on whether these results would cause the problems above, or if you have any further suggestions I would love to hear them.

Thanks for reading!

Pete.

Ray Ivers · 09-16-2006, 02:55 AM

Pete,

This could possibly be a case of the valves making intermittent contact with their sockets - which can be one of the easiest problems to fix (and it really makes sense to try the easy things first). There's a product called Caig ProGold:

http://www.tubesandmore.com/new/scri...?item=S-CG5S-6

which has saved my bacon many times. You'd pull all the valves, carefully spray the pins, then insert & reinsert them a few times to spread the ProGold around. Even if this doesn't solve this problem this time, IME ProGold is an excellent item to have around to clean plugs, jacks, AC connectors, etc. It's not cheap, but the best things never are.

Caig also makes DeOxit and PreservIt, which are similar (but not identical) products.

If this doesn't fix you up, I'm afraid it may be a bad component and/or connection inside the amp, which may or may not be 'user serviceable'.

Ray

Peter Nerves · 09-16-2006, 03:15 AM

Hi Ray, thanks for your reply.

The Pro Gold product looks really good and I would like to get hold of some. I had problems with the above link and I could not add the product to my basket. I am in the UK and will try and search for an outlet that I can order it from.

I understand what you say about user serviceable parts, but I have prior experience in electronics and it has always been a hobby of mine, and I will not give up and put the amp into a service shop as I really want to track down the problem and repair it myself. I hope I can rely on the help of the members of this board to help guide me through this.

Did the bad resistor values listed above concern you?

Thanks,

Pete.

Ray Ivers · 09-16-2006, 03:38 AM

Pete,

Hopefully you can find some (or all) of the Caig products on your side of the pond - they really are exceptional.

Quote:

Did the bad resistor values listed above concern you?

If you disconnected one end of each resistor and then measured it, then yes, each resistor is either incorrectly marked, or identically bad (FWIW, I've never seen the second condition in 28 years of working on amps). If, on the other hand, you measured them in-circuit, then your readings are completely normal, as each 68K pair are normally paralleled (for a reading of 34K). Do you have a schematic for the amp? If not, the one below may (or may not) be close enough:

http://www.schematicheaven.com/marsh..._100w_1959.pdf

The 380V reading concerns me more (you're absolutely positive this was at a grid socket pin, either 2 or 7?), but as this could be caused by a bad socket connection if the reading was actually at a pin 1 or 6, I figured that socket cleaning/treatment was the best course of action, at least to start.

Ray

Peter Nerves · 09-16-2006, 04:07 AM

I tested the resistors in circuit so that explains the half readings. I thought there was no good explanation for all 4 resistors to go faulty!!

Thanks for the schematics, those are the ones I have been working from all day.

The 380Vdc is present at pin 7 on the 2nd 12ax7, I have tested all of the valve bases for correct voltage, and they seem to differ either a lot, or slightly from the 100w marshall voltage chart that I have. All of the 12ax7 pin 2's have no voltage which is correct, and the only 12ax7 which has voltage at pin 7 is the 2nd valve. I am testing the voltages from the valve socket to chassis, and I have not confused the pin numbers. Pin 1 on the 12ax7's is at 240Vdc on all sockets. Pin 3 on the EL34's is measuring 505Vdc on all sockets.

Thanks for your help, does this sound like anything that you have come across?

The fact that I am in the UK (230v outlet) should not cause any difference, as the voltage selector is set to 230v, so I expect that the amp's internal voltages would be the same as in the US?

Thanks for your help!

Pete.

Ray Ivers · 09-16-2006, 04:18 AM

Pete,

Try checking R9 for an open-circuit condition, and/or trying another new tube in the V2 socket. It seems like V2a is not conducting any current, causing its plate voltage (and V2b's grid voltage, as they are wired together) to skyrocket.

Ray

Peter Nerves · 09-16-2006, 04:27 AM

Thanks Ray,

I must add that the voltage readings that I took were without any of the valves in place. Sorry if this has caused a red herring. Should I have tested with the valves in situ?

Thanks,

Pete.

Ray Ivers · 09-16-2006, 04:35 AM

Pete,

Quote:

I must add that the voltage readings that I took were without any of the valves in place. Sorry if this has caused a red herring. Should I have tested with the valves in situ?.

Yes - valve-out readings on preamp tubes, while not exactly meaningless, aren't really all that helpful when troubleshooting (at least IME).

Ray

Peter Nerves · 09-16-2006, 04:45 AM

Thanks again Ray, I will test the voltages again with the valves in place. I will search for some progold at the same time. I will post again soon with an update. Its a good job I have a 1972 Hiwatt Custom 100 to fill in for the 1959! I will try all I can, but I may have to put it in to a repair shop, only on the condition that I can watch & learn though!

Thanks again for your help,

Pete.

tim · 09-16-2006, 05:26 AM

peter, do a small experiment and take a short shielded guitar cord and jump your fx loop with it (plug one end into the send and the other into the return) and see if problem goes away...if it does, then you have either dirty contacts (usually on the return jack) or cracked solder joints under the jacks if they are soldered to a circuit board. I can't remember if they are on the reissue.

Enzo · 09-16-2006, 05:48 AM

Operating tubes have among their attributes a virtual resistance. The current flows through them, and voltage is dropped across them. They are part if the curcuit. With no tubes in place, most voltages in the amp will either be very high, or absent altogether. I have never seen a schematic or test point chart that referred to a no-tubes condition.

More than a red herring, that is a whole fishing boat full of mis-direction. A RED fishing boat.

Being a Marshall, I would ask if the first few preamp tube heaters were glowing when the amp is silent? If not, there's the problem. If this is the case, ther is a square bridge rectifier for the preamp heater current. It is in the middle of the voard somewhere. Try removing it, scraping the legs clean and resoldering it to the board. Or just replace it.

Otherwise, next time it happens, don't flick the switch to restore it, the symptom is troubleshooting gold. IN THE FAILED CONDITION, check for missing or very wrong voltages at the tubes.

Peter Nerves · 09-16-2006, 05:50 AM

Hi Tim, thanks for your advice, but my model has no FX loop on it, they came a couple of years later. Mine is the 1959WSP (white super plexi??) made in 1997.

Thanks,

Pete.

tim · 09-16-2006, 05:54 AM

OK then...carry on. I was looking at the schematic I had and it showed one but it must've been a later model add on as you said.

Peter Nerves · 09-16-2006, 06:01 AM

Hi Enzo, thanks for your reply. I now know not to test without the valves present! The flicking of the power switch was only something I needed to do a few times in the last couple of weeks, before then the amp was fine. I changed the valves and now it just cuts in and out (mostly out) all of the time.

Any further sugesstions?

Thanks,

Pete.

Peter Nerves · 09-16-2006, 06:02 AM

Hi Tim, Thanks for looking for options for me, I really appreciate it!!

Thanks,

Pete.

Enzo · 09-16-2006, 08:30 AM

I am still trying to find out which one you have exactly. But if it lacks a loop, OK. If there are no heater rectifiers, OK. The thing to do is have it ready on the bench, and when it fails, start checking voltages THEN. Note if the heaters are on in all tubes. If the symptom doesn't fade in and fade out, I guess the heaters would be OK. If the problem comes and goes as fast as flipping a switch, it ain't the heaters.

Verify high voltage is present on all plates and screens in the output tubes.

AN intermittent choke would kill screen voltage, and all preamp tube voltages, so missing or present screen voltage at pins 4 of power tubes tells that tale.

Use an insulated probe - fancy words for a "stick" - a chopstick works great - push on each little component on th thing to see if any respond to touch. Looking for a loose connection.

If plate voltages are present through the preamp, check the cathodes - pins 3 and 8 of the 12AX7s. Expect a volt or two each except for the cathode followers.

Go to Schematic Heaven and in teh Marshall listings under "90s reissues" open the 1959SLP (2nd). Is that pretty close? The second page of it shows the 1959SLP-01 which adds the FX loop, but if you just tie W3 and W4 together, it is the same as no loop. Note that pin 2 grid is tied directly to pin 6 plate. That would explain the high voltage on the grid you found. The right side of that tube is a cathode follower, so the high voltage is OK on that grid.

If all the DC levels seem OK, then apply a fairly strong signal to the input, and trace it stage to stage with a scope. No scope? Set your meter to AC volts and trace with that.

Satamax · 09-16-2006, 10:42 AM

My usual reply!

http://www.geofex.com/ampdbug/ampdebug.htm Once you've checked all that, then your amp has a more complex problem worth going to a tech.

Bye.

Max.

Peter Nerves · 09-16-2006, 04:28 PM

Thanks guys for your help.

I have all of the info from amp debug first, and I have been working through it before I started posting.

Heres the deal now.

All of the heaters are constant, no dipping out there.

I have retested the amp with the tubes in place and found some strange results -

12ax7 V1 - 0v at pins 3 & 8, It should be 1 - 1.5v
12ax7 V2 - 0v at pin 3, it should be 1.1v & 156v at pin 8 it should be 175v
12ax7 V3 - 24v at pins 2 & 7, it should be 0v & 36v at pins 3 & 8 it should be 24v

Voltages are around 50v lower than specced on pins 3 & 4 of the EL34's
pin 5 has a -39v reading on all EL34's

I have tried testing when it cuts out, but applying my test leads to the pins seems to bring it back to life!! especially on pin 3 of 12ax7 V1!

Any suggestions?

Again, thanks for your input.

Pete.

Satamax · 09-16-2006, 04:39 PM

Quote:

Originally Posted by Peter Nerves

Thanks guys for your help.

I have all of the info from amp debug first, and I have been working through it before I started posting.

Heres the deal now.

All of the heaters are constant, no dipping out there.

I have retested the amp with the tubes in place and found some strange results -

12ax7 V1 - 0v at pins 3 & 8, It should be 1 - 1.5v
12ax7 V2 - 0v at pin 3, it should be 1.1v & 156v at pin 8 it should be 175v
12ax7 V3 - 24v at pins 2 & 7, it should be 0v & 36v at pins 3 & 8 it should be 24v

Voltages are around 50v lower than specced on pins 3 & 4 of the EL34's
pin 5 has a -39v reading on all EL34's

I have tried testing when it cuts out, but applying my test leads to the pins seems to bring it back to life!! especially on pin 3 of 12ax7 V1!

Any suggestions?

Again, thanks for your input.

Pete.

Well, Ok pete, sorry to have bothered you with ampdebug. What you describe realy makes me think about a bad valve. And i would also check R15. The cathode followers are rarely at the voltage they should be, imhe. But if you have no voltage on the 3 first triodes don't have enough voltage on their plates, the voltage drop on the cathode resistors will be so low that it can't be measured. And which resistor can afect the three valves? R15, it could be also influenced by the 50/50µ and the two volumes too if the coupling caps have shorted or something.

Bye.

Max.

Peter Nerves · 09-16-2006, 04:44 PM

Cheers Satamax,

Ampdbug seems to make me think it is something to do with the coupling caps?

The problem only started this bad when I put new valves in, I have removed the new ones and put the old ones back in while I test so as not to blow the new ones up.

Cheers,

Pete.

Enzo · 09-16-2006, 04:51 PM

Well we ought to know what the plate voltages are as well. With zero on the cathodes, that means the tube is not conducting. If there is no B+ on the plate, then of course it won't conduct. If B+ is present and the tube is not working, then the B+ will be very high, in fact it will climb up to the supply voltage instead of the typical plate voltage. No current means no voltage drop across the load resistor.

If there is HV present, then the tube is not conducting because it is bad, or because the socket is not making contact with the tube pins, or the heaters are not lit.

SO next time it screws up, check the plates first.

If touching pin 3 of V1 restores sound consistently, perhaps the physical motion of touching the meter probe to it moves it enough to restore contact. Pull V1 and look down the socket holes closely and see if any of the female socket pins have spread.

Another idea would be to flick V1 with your fingernail, like flicking a bug off your sleeve. If that wakes it up, that is further evidence of a loose socket or bad tube. Or at least a loose connection somewhere nearby.

Flipping the standby off and on sends a little mechanical joly through the chassis. SO this theory is still consistent.

Ray Ivers · 09-16-2006, 04:55 PM

Pete,

I think your problem may be at the point labeled 'BK1' (below R1) on the SLP schematic, which grounds V1a and V1b cathodes. Your amp may have a different arrangement, though.

Ray

Peter Nerves · 09-16-2006, 05:10 PM

Thanks again gents!

The plate voltages are
V1 pin1 = 139v pin 6 = 145v
V2 pin1 = 156v pin 6 = 276v
V3 pin1 = 221v pin 6 = 211v

I have tried trpping all of the valves while I have sound and it makes no difference (V1 you can hear the taps). It seems like when I connect my meter I am creating a circuit which brings the amp back for a bit.

I need to check the valve seats a bit more I think now, but the heaters are on all of the time.

Cheers,

Pete.

Satamax · 09-16-2006, 05:58 PM

Quote:

Originally Posted by Enzo

If touching pin 3 of V1 restores sound consistently, perhaps the physical motion of touching the meter probe to it moves it enough to restore contact.

Hi guys!

Enzo, he's testing to teh chassis, so may be the metter, despite high impedence is enough to act as a cathode resistor?

Huh pete, what do you use as a metter? Uncalibrated cheapo, or something better? Do you have a mV seting? It reads an absolute zero when checking or does it go up a litle? I think Ray might be onto something, but if teh cathode res are lifted from ground, the B+ on the preamp should skyrocket!

Bye.

Max.

tim · 09-16-2006, 07:10 PM

Have you verified that every wire going to every pin on the tube sockets are actually soldered to the pin? I have seen many times when the wire was simply never soldered (wire wrapped around the pin but no solder) or was soldered and looks like it's soldered but the solder never wetted to the socket pin....I had a Fender Custom shop amp here a few weeks back and there were two "no solders" on one of the preamp tube sockets...right from the factory folks. Wire was wrapped around the pin and was making contact most of the time but rap the amp with your fist and it would have spasms. Just got done refurbishing a Fender Twin silverface last week and two! cathode bypass caps were installed backwards from the factory...I had to do a doubletake on that one.

Peter Nerves · 09-17-2006, 02:58 AM

Thanks Tim, I am going to check for high resistance from the the wires connecting the valve sockets to the PCB, I recon I may just give all of the valve connections a bit of new solder to make sure and treat the valve pins with progold as Ray mentioned. If it still plays up, I will then look at taking it into a tech to help me, but I really want to fix this myself!!

Thanks for your help!!

Pete.

Peter Nerves · 09-17-2006, 03:57 AM

Hi Max,

I use a fairly good Isotech meter, which does have a mv setting. I saw a slight fluctuation on the meter when I touched the cathode pins, and I will re-test in the morning with a fresh head (and mv setting!).

I will post the results here, but you have all given me food for thought, it may take me a while to test and try everything I have been told.

Keep posting, I will try all of the advice that is listed!! This is my learning curve!

Thanks again guys for your support, and even if I dont solve this problem myself (with your help of course) I will let you know what the local tech finds to be the problem (last resort!!)

Cheers,

Pete.

Satamax · 09-17-2006, 12:35 PM

Don't forget to retension the sockets!

Nother thing which could cause zero conduction on the preamp is if you had a very negative voltage on their grids!

Bie.

Max.

Peter Nerves · 09-17-2006, 03:57 PM

Hi all,

Continuity is good from the valve sockets to the PCB components

I have re-teststed the cathode voltages on V1 (pins 3 & 8) and they are 0.89v, which I think seems acceptable. V2 (pin 3) is at 1v which seems good. The thing that is really bugging me at the moment is the 24vdc on the 2 grids (pins 2 & 7) of V3 (the PI), my voltage chart shows that these should voltage free. The voltage goes when I test without the valve in place. I have swapped the valve positions, so it is not a bad valve.

Any advice? could this be a leaky cap (C9 or C8)?

Thanks,

Pete.

Ray Ivers · 09-17-2006, 04:17 PM

Pete,

Your +24V reading on the PI grids is most probably correct; it should actually be a bit higher, but this is a super-high-impedance circuit node and it's likely your meter is loading the reading down a bit.

If your voltage chart is like mine (the "genuine Marshall" item), you've noticed that certain tube pin positions have a horizontal line drawn through them. This does not necessarily indicate a 'voltage free' or 0V condition, but rather a 'no-read' (i.e., don't bother taking a reading here) recommendation. The 50W MK2 chart actually specifies 0V for pin 8 of the output tubes, although the others don't.

Ray

Peter Nerves · 09-17-2006, 04:44 PM

Thanks Ray, that helps.

This is what has just happened - I could not get a sound out of the amp, not even intermittent, so I put the new power tubes in. Still no sound, until I plug into input 1 high, and turned the vol up to about 5, back comes the sound. I plugged into input 2 high and turned the vol to about 2 and backed off the high gain vol. All is good, I am giving little thuds on the srings to see if the amp is going to cut out but it never. Then a very loud bang bang popping noise coming out of the cab and when I looked at the valves the 4th power valve (V7) had huge arcs and sparks inside it. I have switched off the mains to the amp now.

Any ideas on what has just happened? Will my brand new output tube be fried now?

Cheers,

Pete.

Ray Ivers · 09-17-2006, 05:13 PM

Pete,

As Max mentioned, it may be time to take your amp to the shop.

In the hands of a seasoned tech with years of troubleshooting experience, a DMM can be a consummate revealer of problems both big and small; I've seen this with my own eyes, more than once. OTOH, it can be very misleading; for example, the 0V reading at the V1 cathodes - which seems to come and go (does that sound like a problem to you? - it sure does to me!); is that caused by an open cathode resistor - which I mentioned earlier, but was evidently never checked as "the meter reading was OK" that time - or a shorted cap? An oscilloscope would tell you in a heartbeat, but a meter... well, I surely don't have to tell you, obviously you know the shortcomings of DC-only troubleshooting as well as anyone right about now.

Is V7 toast? I don't know... but the real question IMO is, "did you damage your output transformer - or something else - during the 'big bang'?"

If you're intent on continuing using just DC meter readings, I would pull the power tubes - no sense in repeating a high-power failure event. FWIW, I still believe the problem is somewhere in the cathode circuits of the first 2 or 3 gain stages, and the output tube arcing could have been caused by a huge preamp-generated transient (or a number of other things).

Ray

Peter Nerves · 09-17-2006, 05:57 PM

Thanks Ray,
I think the time has come to get this into a repair shop as you have said, I would have loved to have solved this, but I dont have the experience or equipment of a seasoned tech. I will get it in tomorrow, and ask the tech to give me the full lowdown on what the problem is and post their findings here.

The mains fuse blew when V7 started arching, I replaced the fuse put all of the old tubes back in, and it has gone back to the way it was when I started, just cutting in and out when it feels like it (I can make it come back by turning the vol to above 5, and it stays in when I turn down to 1, it stays in while I play softly, but whack a chord and it cuts out, repeat the above to get sound back). I wish I had hold of some Progold, as the problem was not anywhere near as bad as this before I pulled the original tubes, so it screams out to me that disturbing the tube seats has made the problem worse. Tapping the tubes does not cause the sound to cut in or out, and the heaters are lit all of the time.

Thanks for you help Ray and everybody else who has given me advice.

Its is going into a shop before I cause some major damage

Cheers,

Pete.

09-16-2006, 12:37 AM	#1
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Marshall 1959SLP Problems - Please Help Hi All, I am new to this forum so hello to you all. I have a Marshall 1959SLP 35th anniversary re-issue. I have had a minor problem recently with no sound coming out of the amp when I flicked on the standby switch, which was solved by flicking the power switch off and on quickly and it worked fine for a few hours until it would cut out again. I had a look around a few sites and decided that it was best to start with replacing the valves and see what would happen. I put the new valves in and I could only get the amp to work intermittently, cutting in and out all of the time. When I get a sound out of it I have to be really careful not to whack the strings, as this causes the amp to cut out again. I put the old valves back in and the problem stayed. I tested the valve seat voltages and some of them seem far from what was indicated on the Marshall 100w voltage chart (although this chart may be the wrong one for the SLP) I was getting 380Vdc at the grid of valve 2 when the chart said 170Vdc. I have been through all of the resistors in turn and tested their resistance to the values marked on them and came up with some supprising results. The 4 resistors that couple the 4 guitar inputs to the 2 grids of valve 1 were half of their marked values 34k (marked value 68k). Could using effects infront of the guitar input cause this? I use an ebow and the output of this screams into the amp! I have been testing the amp with a clean guitar signal straight in, all of the leads and connections are fine. It is definatley an amp problem! I pray on your expert knowlege to give me some more advice on whether these results would cause the problems above, or if you have any further suggestions I would love to hear them. Thanks for reading! Pete.

09-16-2006, 03:15 AM	#3
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks Ray Hi Ray, thanks for your reply. The Pro Gold product looks really good and I would like to get hold of some. I had problems with the above link and I could not add the product to my basket. I am in the UK and will try and search for an outlet that I can order it from. I understand what you say about user serviceable parts, but I have prior experience in electronics and it has always been a hobby of mine, and I will not give up and put the amp into a service shop as I really want to track down the problem and repair it myself. I hope I can rely on the help of the members of this board to help guide me through this. Did the bad resistor values listed above concern you? Thanks, Pete.

09-16-2006, 04:07 AM	#5
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks Ray I tested the resistors in circuit so that explains the half readings. I thought there was no good explanation for all 4 resistors to go faulty!! Thanks for the schematics, those are the ones I have been working from all day. The 380Vdc is present at pin 7 on the 2nd 12ax7, I have tested all of the valve bases for correct voltage, and they seem to differ either a lot, or slightly from the 100w marshall voltage chart that I have. All of the 12ax7 pin 2's have no voltage which is correct, and the only 12ax7 which has voltage at pin 7 is the 2nd valve. I am testing the voltages from the valve socket to chassis, and I have not confused the pin numbers. Pin 1 on the 12ax7's is at 240Vdc on all sockets. Pin 3 on the EL34's is measuring 505Vdc on all sockets. Thanks for your help, does this sound like anything that you have come across? The fact that I am in the UK (230v outlet) should not cause any difference, as the voltage selector is set to 230v, so I expect that the amp's internal voltages would be the same as in the US? Thanks for your help! Pete.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
"Promiscuity" problems - and I love this crew	Rob Mercure	Lobby	10	09-07-2006 08:54 PM
66 Bassman noises/problems?	Steve B	Maintenance, Troubleshooting & Repair	15	08-17-2006 07:27 PM
Pot Problems	escyrs2000	Guitar Tech	4	08-17-2006 03:10 PM
5E3 Voltage problems	Watto	Maintenance, Troubleshooting & Repair	11	07-21-2006 01:50 AM
Mesa Single Rec problems	iamtheparty	Maintenance, Troubleshooting & Repair	2	07-11-2006 07:23 PM

09-16-2006, 02:55 AM	#2
Ray Ivers Lifetime Member Join Date: Jun 2006 Location: Connecticut Posts: 442	Pete, This could possibly be a case of the valves making intermittent contact with their sockets - which can be one of the easiest problems to fix (and it really makes sense to try the easy things first). There's a product called Caig ProGold: http://www.tubesandmore.com/new/scri...?item=S-CG5S-6 which has saved my bacon many times. You'd pull all the valves, carefully spray the pins, then insert & reinsert them a few times to spread the ProGold around. Even if this doesn't solve this problem this time, IME ProGold is an excellent item to have around to clean plugs, jacks, AC connectors, etc. It's not cheap, but the best things never are. Caig also makes DeOxit and PreservIt, which are similar (but not identical) products. If this doesn't fix you up, I'm afraid it may be a bad component and/or connection inside the amp, which may or may not be 'user serviceable'. Ray

09-16-2006, 04:18 AM	#6
Ray Ivers Lifetime Member Join Date: Jun 2006 Location: Connecticut Posts: 442	Pete, Try checking R9 for an open-circuit condition, and/or trying another new tube in the V2 socket. It seems like V2a is not conducting any current, causing its plate voltage (and V2b's grid voltage, as they are wired together) to skyrocket. Ray

09-16-2006, 04:27 AM	#7
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks Ray, I must add that the voltage readings that I took were without any of the valves in place. Sorry if this has caused a red herring. Should I have tested with the valves in situ? Thanks, Pete.

09-16-2006, 04:45 AM	#9
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks again Ray, I will test the voltages again with the valves in place. I will search for some progold at the same time. I will post again soon with an update. Its a good job I have a 1972 Hiwatt Custom 100 to fill in for the 1959! I will try all I can, but I may have to put it in to a repair shop, only on the condition that I can watch & learn though! Thanks again for your help, Pete.

09-16-2006, 05:26 AM	#10
tim Senior Member Join Date: Sep 2006 Location: Boscawen, NH Posts: 206	peter, do a small experiment and take a short shielded guitar cord and jump your fx loop with it (plug one end into the send and the other into the return) and see if problem goes away...if it does, then you have either dirty contacts (usually on the return jack) or cracked solder joints under the jacks if they are soldered to a circuit board. I can't remember if they are on the reissue.

09-16-2006, 05:48 AM	#11
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,263	Operating tubes have among their attributes a virtual resistance. The current flows through them, and voltage is dropped across them. They are part if the curcuit. With no tubes in place, most voltages in the amp will either be very high, or absent altogether. I have never seen a schematic or test point chart that referred to a no-tubes condition. More than a red herring, that is a whole fishing boat full of mis-direction. A RED fishing boat. Being a Marshall, I would ask if the first few preamp tube heaters were glowing when the amp is silent? If not, there's the problem. If this is the case, ther is a square bridge rectifier for the preamp heater current. It is in the middle of the voard somewhere. Try removing it, scraping the legs clean and resoldering it to the board. Or just replace it. Otherwise, next time it happens, don't flick the switch to restore it, the symptom is troubleshooting gold. IN THE FAILED CONDITION, check for missing or very wrong voltages at the tubes.

09-16-2006, 05:50 AM	#12
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Hi Tim, thanks for your advice, but my model has no FX loop on it, they came a couple of years later. Mine is the 1959WSP (white super plexi??) made in 1997. Thanks, Pete.

09-16-2006, 05:54 AM	#13
tim Senior Member Join Date: Sep 2006 Location: Boscawen, NH Posts: 206	OK then...carry on. I was looking at the schematic I had and it showed one but it must've been a later model add on as you said.

09-16-2006, 06:01 AM	#14
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Hi Enzo, thanks for your reply. I now know not to test without the valves present! The flicking of the power switch was only something I needed to do a few times in the last couple of weeks, before then the amp was fine. I changed the valves and now it just cuts in and out (mostly out) all of the time. Any further sugesstions? Thanks, Pete.

09-16-2006, 06:02 AM	#15
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Hi Tim, Thanks for looking for options for me, I really appreciate it!! Thanks, Pete.

09-16-2006, 08:30 AM	#16
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,263	I am still trying to find out which one you have exactly. But if it lacks a loop, OK. If there are no heater rectifiers, OK. The thing to do is have it ready on the bench, and when it fails, start checking voltages THEN. Note if the heaters are on in all tubes. If the symptom doesn't fade in and fade out, I guess the heaters would be OK. If the problem comes and goes as fast as flipping a switch, it ain't the heaters. Verify high voltage is present on all plates and screens in the output tubes. AN intermittent choke would kill screen voltage, and all preamp tube voltages, so missing or present screen voltage at pins 4 of power tubes tells that tale. Use an insulated probe - fancy words for a "stick" - a chopstick works great - push on each little component on th thing to see if any respond to touch. Looking for a loose connection. If plate voltages are present through the preamp, check the cathodes - pins 3 and 8 of the 12AX7s. Expect a volt or two each except for the cathode followers. Go to Schematic Heaven and in teh Marshall listings under "90s reissues" open the 1959SLP (2nd). Is that pretty close? The second page of it shows the 1959SLP-01 which adds the FX loop, but if you just tie W3 and W4 together, it is the same as no loop. Note that pin 2 grid is tied directly to pin 6 plate. That would explain the high voltage on the grid you found. The right side of that tube is a cathode follower, so the high voltage is OK on that grid. If all the DC levels seem OK, then apply a fairly strong signal to the input, and trace it stage to stage with a scope. No scope? Set your meter to AC volts and trace with that.

09-16-2006, 10:42 AM	#17
Satamax Senior Member Join Date: Jun 2006 Location: Southern french alps, right by the Italian border Posts: 752	My usual reply! http://www.geofex.com/ampdbug/ampdebug.htm Once you've checked all that, then your amp has a more complex problem worth going to a tech. Bye. Max.

09-16-2006, 04:28 PM	#18
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks guys for your help. I have all of the info from amp debug first, and I have been working through it before I started posting. Heres the deal now. All of the heaters are constant, no dipping out there. I have retested the amp with the tubes in place and found some strange results - 12ax7 V1 - 0v at pins 3 & 8, It should be 1 - 1.5v 12ax7 V2 - 0v at pin 3, it should be 1.1v & 156v at pin 8 it should be 175v 12ax7 V3 - 24v at pins 2 & 7, it should be 0v & 36v at pins 3 & 8 it should be 24v Voltages are around 50v lower than specced on pins 3 & 4 of the EL34's pin 5 has a -39v reading on all EL34's I have tried testing when it cuts out, but applying my test leads to the pins seems to bring it back to life!! especially on pin 3 of 12ax7 V1! Any suggestions? Again, thanks for your input. Pete.

09-16-2006, 04:44 PM	#20
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Cheers Satamax, Ampdbug seems to make me think it is something to do with the coupling caps? The problem only started this bad when I put new valves in, I have removed the new ones and put the old ones back in while I test so as not to blow the new ones up. Cheers, Pete.

09-16-2006, 04:51 PM	#21
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,263	Well we ought to know what the plate voltages are as well. With zero on the cathodes, that means the tube is not conducting. If there is no B+ on the plate, then of course it won't conduct. If B+ is present and the tube is not working, then the B+ will be very high, in fact it will climb up to the supply voltage instead of the typical plate voltage. No current means no voltage drop across the load resistor. If there is HV present, then the tube is not conducting because it is bad, or because the socket is not making contact with the tube pins, or the heaters are not lit. SO next time it screws up, check the plates first. If touching pin 3 of V1 restores sound consistently, perhaps the physical motion of touching the meter probe to it moves it enough to restore contact. Pull V1 and look down the socket holes closely and see if any of the female socket pins have spread. Another idea would be to flick V1 with your fingernail, like flicking a bug off your sleeve. If that wakes it up, that is further evidence of a loose socket or bad tube. Or at least a loose connection somewhere nearby. Flipping the standby off and on sends a little mechanical joly through the chassis. SO this theory is still consistent.

09-16-2006, 04:55 PM	#22
Ray Ivers Lifetime Member Join Date: Jun 2006 Location: Connecticut Posts: 442	Pete, I think your problem may be at the point labeled 'BK1' (below R1) on the SLP schematic, which grounds V1a and V1b cathodes. Your amp may have a different arrangement, though. Ray

09-16-2006, 05:10 PM	#23
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks again gents! The plate voltages are V1 pin1 = 139v pin 6 = 145v V2 pin1 = 156v pin 6 = 276v V3 pin1 = 221v pin 6 = 211v I have tried trpping all of the valves while I have sound and it makes no difference (V1 you can hear the taps). It seems like when I connect my meter I am creating a circuit which brings the amp back for a bit. I need to check the valve seats a bit more I think now, but the heaters are on all of the time. Cheers, Pete.

09-16-2006, 07:10 PM	#25
tim Senior Member Join Date: Sep 2006 Location: Boscawen, NH Posts: 206	Have you verified that every wire going to every pin on the tube sockets are actually soldered to the pin? I have seen many times when the wire was simply never soldered (wire wrapped around the pin but no solder) or was soldered and looks like it's soldered but the solder never wetted to the socket pin....I had a Fender Custom shop amp here a few weeks back and there were two "no solders" on one of the preamp tube sockets...right from the factory folks. Wire was wrapped around the pin and was making contact most of the time but rap the amp with your fist and it would have spasms. Just got done refurbishing a Fender Twin silverface last week and two! cathode bypass caps were installed backwards from the factory...I had to do a doubletake on that one.

09-17-2006, 02:58 AM	#26
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks Tim, I am going to check for high resistance from the the wires connecting the valve sockets to the PCB, I recon I may just give all of the valve connections a bit of new solder to make sure and treat the valve pins with progold as Ray mentioned. If it still plays up, I will then look at taking it into a tech to help me, but I really want to fix this myself!! Thanks for your help!! Pete.

09-17-2006, 03:57 AM	#27
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Hi Max, I use a fairly good Isotech meter, which does have a mv setting. I saw a slight fluctuation on the meter when I touched the cathode pins, and I will re-test in the morning with a fresh head (and mv setting!). I will post the results here, but you have all given me food for thought, it may take me a while to test and try everything I have been told. Keep posting, I will try all of the advice that is listed!! This is my learning curve! Thanks again guys for your support, and even if I dont solve this problem myself (with your help of course) I will let you know what the local tech finds to be the problem (last resort!!) Cheers, Pete.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Similar Threads

09-17-2006, 12:35 PM	#28
Satamax Senior Member Join Date: Jun 2006 Location: Southern french alps, right by the Italian border Posts: 752	Don't forget to retension the sockets! Nother thing which could cause zero conduction on the preamp is if you had a very negative voltage on their grids! Bie. Max.

09-17-2006, 03:57 PM	#29
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Hi all, Continuity is good from the valve sockets to the PCB components I have re-teststed the cathode voltages on V1 (pins 3 & 8) and they are 0.89v, which I think seems acceptable. V2 (pin 3) is at 1v which seems good. The thing that is really bugging me at the moment is the 24vdc on the 2 grids (pins 2 & 7) of V3 (the PI), my voltage chart shows that these should voltage free. The voltage goes when I test without the valve in place. I have swapped the valve positions, so it is not a bad valve. Any advice? could this be a leaky cap (C9 or C8)? Thanks, Pete.

09-17-2006, 04:17 PM	#30
Ray Ivers Lifetime Member Join Date: Jun 2006 Location: Connecticut Posts: 442	Pete, Your +24V reading on the PI grids is most probably correct; it should actually be a bit higher, but this is a super-high-impedance circuit node and it's likely your meter is loading the reading down a bit. If your voltage chart is like mine (the "genuine Marshall" item), you've noticed that certain tube pin positions have a horizontal line drawn through them. This does not necessarily indicate a 'voltage free' or 0V condition, but rather a 'no-read' (i.e., don't bother taking a reading here) recommendation. The 50W MK2 chart actually specifies 0V for pin 8 of the output tubes, although the others don't. Ray

09-17-2006, 04:44 PM	#31
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks Ray, that helps. This is what has just happened - I could not get a sound out of the amp, not even intermittent, so I put the new power tubes in. Still no sound, until I plug into input 1 high, and turned the vol up to about 5, back comes the sound. I plugged into input 2 high and turned the vol to about 2 and backed off the high gain vol. All is good, I am giving little thuds on the srings to see if the amp is going to cut out but it never. Then a very loud bang bang popping noise coming out of the cab and when I looked at the valves the 4th power valve (V7) had huge arcs and sparks inside it. I have switched off the mains to the amp now. Any ideas on what has just happened? Will my brand new output tube be fried now? Cheers, Pete.

09-17-2006, 05:13 PM	#32
Ray Ivers Lifetime Member Join Date: Jun 2006 Location: Connecticut Posts: 442	Pete, As Max mentioned, it may be time to take your amp to the shop. In the hands of a seasoned tech with years of troubleshooting experience, a DMM can be a consummate revealer of problems both big and small; I've seen this with my own eyes, more than once. OTOH, it can be very misleading; for example, the 0V reading at the V1 cathodes - which seems to come and go (does that sound like a problem to you? - it sure does to me!); is that caused by an open cathode resistor - which I mentioned earlier, but was evidently never checked as "the meter reading was OK" that time - or a shorted cap? An oscilloscope would tell you in a heartbeat, but a meter... well, I surely don't have to tell you, obviously you know the shortcomings of DC-only troubleshooting as well as anyone right about now. Is V7 toast? I don't know... but the real question IMO is, "did you damage your output transformer - or something else - during the 'big bang'?" If you're intent on continuing using just DC meter readings, I would pull the power tubes - no sense in repeating a high-power failure event. FWIW, I still believe the problem is somewhere in the cathode circuits of the first 2 or 3 gain stages, and the output tube arcing could have been caused by a huge preamp-generated transient (or a number of other things). Ray

09-17-2006, 05:57 PM	#33
Peter Nerves Junior Member Join Date: Sep 2006 Posts: 16	Thanks Ray, I think the time has come to get this into a repair shop as you have said, I would have loved to have solved this, but I dont have the experience or equipment of a seasoned tech. I will get it in tomorrow, and ask the tech to give me the full lowdown on what the problem is and post their findings here. The mains fuse blew when V7 started arching, I replaced the fuse put all of the old tubes back in, and it has gone back to the way it was when I started, just cutting in and out when it feels like it (I can make it come back by turning the vol to above 5, and it stays in when I turn down to 1, it stays in while I play softly, but whack a chord and it cuts out, repeat the above to get sound back). I wish I had hold of some Progold, as the problem was not anywhere near as bad as this before I pulled the original tubes, so it screams out to me that disturbing the tube seats has made the problem worse. Tapping the tubes does not cause the sound to cut in or out, and the heaters are lit all of the time. Thanks for you help Ray and everybody else who has given me advice. Its is going into a shop before I cause some major damage Cheers, Pete.