Marshall Major Blowing Power Fuse

[cjenrick]

go ahead and stick the tubes in there, you are ready to go, yes secondary resistance is usually .25 ohms for the 4 ohm wind and increases slightly with each tap so you need a 4 wire meter to measure the DCR.

vinyl glass sleaving is real handy for covering splices, you can buy it on evilbay, you could also used glass cloth tape from Scotch 3M , Permacel, etc
.scrape the enamel off with a razor blade, tin the wires, stick them side by side and hit them with the iron and they stick nicely. a few turns of black tape and them some sleaving and you are good to go.

that might be a Radio Spares xfmr, the Partridge was the one we were hoping to rewind, they sometimes use a tape wound core which is very expensive compared to EI lams but has less leakage.

and as luck would have it, we had a Sovtek Mig 50 come in today with a blown OPT so we get to find out what makes this thing tick,

cj

[catalin gramada]

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[catalin gramada]

Also, I'm considering replacing the can caps in this major but the 200uF cancaps are oversized. Modern cans are smaller. Does anyone know of way to modify the large clamps to accommodate the smaller cans? BTW, I know these cans are 47 years old. ESR meter was giving me a reading of .67 which should be good? Is it worth replacing these caps?

Put some paper arround. And be aware in this type of series connection you have the first can cap shell at half potential. It is compulsory to have extra care for isolation between fixing clamp and capacitor shell. Don.t do it and have a permanent risk to shock you or put into a short. Isolate the top of the capacitor as well if metalic shell is exposed
If you clamps are extra large and have room enough you can use polyprop. motor RUN capacitors against electrolythics instead
Nice amp

[cluster]

I notice that the big 200uF cans have 3 terminals with two tied together, so I'm guessing they are actually 100+100uF? Is that right? The schematic says 2 cans of 200uF are in series but doesn't say that they are internally stacked. I would expect the can to read 100+100 instead of 200uF or else why have the tree terminals.

Also, could some explain the power filtering section in this amp. The 2 375uF cans seem to feed the bridge rec? The 2 x 200uF to the plates via the OT? But it also feeds the plates of V2 preamp and PI.

[catalin gramada]

Just supose: big ones could be 200+200 meant 400uF in parallel, pretty close to 375 schematic shows. Also I saw an error in power supply drawing. 56k/2w should be tied over each cap to ensure balance. There are corectly tied in you amp but schematic didn.t shows. You dont.t need balance resistors over the big ones as time HT center tap was tied in between

[cluster]

with 118VAC via the variac I can see 631 VDC on the plates (no tubes). Hooked up those diodes from pin 3 to ground for extra precaution. Next step is bias the amp with the tubes in. Take measurements. Listen for hum and/or strange noises. Bang the OT and PT a couple of times to see if I get any noise. Feel for any excessive heat on the OT. Let it sit for 15 minutes or so then pug in my lester for it's final test.

[dstrat]

Hi, those caps usually are marked outside the can but may need to turn them to see it. might be a good idea to check that.
also know that there are errors on most of the marshall schematics.

dave

[Leo_Gnardo]

Don't forget, it would be a good idea to alter that bias supply before you put tubes back in. The adjustment pot is between the rectifier & filter network. If it fails open, as pots do, the bias voltage will fall & you'll be popping fuses or worse. The adjustment pot on mine was intermittent. You may choose to trust yours but if it does become intermittent you'll wish you had changed it. I simply bypassed the factory pot with a 10K fixed resistor, and put a 10K trim pot plus fixed resistor in series with the 10K to ground. Also I changed the value of the 820 ohm resistor so I had a useful voltage range. The bias voltage is well up there, somewhere around -70V.

[catalin gramada]

Don't forget, it would be a good idea to alter that bias supply before you put tubes back in. The adjustment pot is between the rectifier & filter network. If it fails open, as pots do, the bias voltage will fall & you'll be popping fuses or worse. The adjustment pot on mine was intermittent. You may choose to trust yours but if it does become intermittent you'll wish you had changed it. I simply bypassed the factory pot with a 10K fixed resistor, and put a 10K trim pot plus fixed resistor in series with the 10K to ground. Also I changed the value of the 820 ohm resistor so I had a useful voltage range. The bias voltage is well up there, somewhere around -70V.

That.s a good advice. Have to add: First, without any voltages turn it very,very gently from a end to another. Do it twice, that will broken the oxide film due to wearing. Than put it into the most negative position (eg. if works between -50 to -80v put it into -80v position) That will ensure power tubes will draw minimal current at first. Than you can start normaly, let it warm for a while and adust the bias to proper value. Recheck over half an hour or so for final adjustment

[Familyortiz]

This is something I do when troubleshooting anything.

1) Look for the obvious. It sounds like you did.
2) If you can power the amp, you can measure the output voltages for all of the supply outputs. All of them. If not, measure the resistance to ground for all supply outputs. The shorted guy will standout.
3) When the shorted supply is found, at that point you can try to isolate which connection to that shorted supply is the culprit. There are only a number of connections to each supply so it's not a huge task.

Good luck!

[cluster]

Good news at last. Put the tubes in and variac'd the amp up to 118VAC. Bias the KT88's to 35ma/tube and got the following measurements:

V4 plate: 590VDC screen: 585VDC
V5 plate: 587VDC screen: 586VDC
V6 plate: 591VDC screen: 587VDC
V7 plate: 590VDC screen: 585VDC

V1a Plate: 177.7VDC
V1b Plate: 233.0VDC
V2a Plate: 205.0VDC
V2b Plate: 305.5VDC
V3a Plate: 292.0VDC (PI)
V3b Plate: 281.0VDC (PI)

Bias feed = -66.7 VDC and -69 VDC at each grid pin

I would have thought the screens to be lower that what I measured but those were the readings.

Leo, could you explain a little more how I should mod my bias pot. I do want to do this before putting the amp back together. I also have a PPIMV for Metro that I've been thinking of putting in but part off me wants to leave it original. Still haven't decided.

Thanks everyone for the great help. Could have done it with out you guys :-)

Joseph

[cluster]

Here is a close crop of the bias circuit. Where should I move the pot?

Also, if there are any other mods I could do to make this amp more stable and less likely to blow up, I would appreciate it. Thanks.

[Leo_Gnardo]

Here is a close crop of the bias circuit. Where should I move the pot?

What I did was snip the connections to Marshall's pot, bridge over it with a fixed resistor. Don't remember value exactly it may have been 2K4 or 3K3. Left the pot physically in place, it's a bear to remove. Next resistor (820R) swapped out for 10K or 15K. Finally the existing 10K snipped out & replaced with a 10K or 20K trimpot in series with a fixed resistor, value chosen so that bias voltage could be swung say -60 to -80V. The fixed R may have been 56K - 68K, somewhere around there. These higher value resistors allow better filtering of the bias supply, and if there's a failure of the bias pot, bias voltage rises to a point the output tubes cut off. Marshall's way, if that pot opens up you lose the bias supply and output tubes go red plate.

I use a 6 amp slow blow mains fuse in this amp. Marshall's 10 amp slow blow imho isn't protective enough.

Not a bad idea to replace any original electrolytic caps there may still be in your Major. Someone already had replaced the caps that pass signal to the output tube control grids. If you suspect leakage here or think it's a good idea to replace them as preventive maintenance, now's the time.

Unlike that da'esh chappie mentioned by G1 ( http://music-electronics-forum.com/t...36/#post457839 ), you managed to dodge the bullet with this one, finding & repairing the fault in the OT. Good work indeed!

[cjenrick]

screen voltage is usually pretty close to plate voltage in a four tube amp as they usually use a choke or UL transformer which has lower DCR than a 1 K voltage dropping resistor like you might see in a Princeton Reverb, chokes are usually around 100 ohms DCR,

so figure a few ma across 100 ohms plus maybe a 470 ohms screen resistor , .002 * 570 = about 1 volt, 4 ma = 2 volts,

when you start cranking that screen voltage will come down due to the AC current,

what kind of KT88's are those? 585 plate volts in kind of scary, maybe drop the screen voltage with some additional resistance or switch to 7027/6550 , Music Man runs 750 on the EL34 plates but the screens sit at about 350, or use the Jerome Brown trick (PV, Kustom) and insert cathode resistors in the pwr tubes, you get a mix of self bias and cathode bias for a bit more compression and you offset the plate voltage by whatever cathode voltage you have, this also helps prevent thermal runaway (red plating) and can also balance the tubes a bit, power comes down a tad depending on the cathode resistor value, but that can be a good thing on a brute like this,

good to hear you are up and running!

[catalin gramada]

Be aware with this types of ppimv. Usually the pots replace grid leaks resistors of power tubes.(and usually this was designed for EL34 tube type going to 2x250k-2x500k value). You don.t want to exceed nominal value of 68k which you KT88 Marshall did (it is allready high), so be sure you PPIMV didn.t and check first.
If you don.t have to remove grid leaks and you ppimv is decoupled from power stage (eg. Orange ppimv type) it is not a concern