Marshall 1959SLP Problems - Please Help

[Enzo]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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