Marshall Major Blowing Power Fuse

[Justin Thomas]

They crunch without using the "Distortion." Eventually. And they're not <THAT> loud. >
I'm sure this Major can do the same. Actually, gimme a Pig. No Master.

Justin

[catalin gramada]

thanks for this tip but could you be more specific as to what point in the power supply to wire up the screens? Also, would I need an extra can cap to to preform this task? Would this take some pressure off the OT??

You don.t put 'pressure' on you OT if you don.t try to 'squeeze it. Anyone wants a little bit overdrive from power stage meant salt and pepper. Unfortunately that goes to premature wearing. Doesn't.e xist a recipe for this. I think you problem is you amp not distort enough for you taste. It will not do it. It.s not those kind of amp. It.s a beautiful jewel for other task. Please Don. T mod it. You will regret. Put something in front.

[cluster]

Could someone here explain the two black secondaries off the PT going to the 200uF can? I don't see this anywhere on the schematic. As far as I can tell the 2 red secondaries off the PT go to the standby switch -> rectifier -> HT fuse -> 200uF -> 200uF. Here I see these 2 mysteries black wires connected to the positive lug of the 2nd 200uF to secondary of PT. The other strange thing about this PT is that the heater wires seem to be connected on the primary side. I thought heater wires are always off the secondary. Maybe the heater secondary was fed to the back of the PT to make it closer to the last power tube at the end? Still this mysterious black wires.

[dstrat]

hard to say without measuring but i would think that is the HT secondary center tap, those two wires are connecting the two half's of the secondary.
that is what it looks like to me, its a voltage doubler circuit.


oh yeah nice amp btw.

dave

[catalin gramada]

[ATTACH=CONFIG]43879[/ATTACH]
Could someone here explain the two black secondaries off the PT going to the 200uF can? I don't see this anywhere on the schematic. As far as I can tell the 2 red secondaries off the PT go to the standby switch -> rectifier -> HT fuse -> 200uF -> 200uF. Here I see these 2 mysteries black wires connected to the positive lug of the 2nd 200uF to secondary of PT. The other strange thing about this PT is that the heater wires seem to be connected on the primary side. I thought heater wires are always off the secondary. Maybe the heater secondary was fed to the back of the PT to make it closer to the last power tube at the end? Still this mysterious black wires.

Like David said...Just suppose....I think those tranny have not a secondary center tap.but two equal windings in series. One is the end of first.the other is beginning of second. If are tied together can be considered as a center tap for the whole secondary. No way for doubler btw. Those CT is there just to keep balance over the first caps.otherway you can missed it but have to ensure the balance with resistors like second caps did.But is just my suppose.Need to check to be sure

late edit- please, take very serious my advice: be very,very sure the top caps aluminium shell from series bateries have not any chances to touch the fixing clamp and/or the chassis ! You can put some extra paper arround. Sometime the cap isolation is so thin as can be very easy punched by a burr!

[g1]

As far as the heater wires, you can't tell what is primary or secondary just from the outward appearance. Some transformers may have all wires coming out of the same area, both primary and secondary. Normally you would look at the wiring diagram to figure out what colour wires are primary or secondary. The physical location does not really mean anything.

[pdf64]

thanks for this tip but could you be more specific as to what point in the power supply to wire up the screens? Also, would I need an extra can cap to to preform this task? Would this take some pressure off the OT??

A UL OT (assuming that's what it is) takes a lot of stress off the power tube screen grids, which are often the electrodes that (in regular amps) overdissipate and cause power tube shorts.
Your increased value of the screen grid current limiting resistors will give them an even easier life.
Yes there will be a bit more current in the OT primary with UL but I think it is inconsequential and that retention of UL will give best reliability / tube life, all else being equal.

Yes, dropping UL and properly changing to regular pentode/tetrode mode would require an additional HT node with 2 more stacked filter caps.

[catalin gramada]

Yes, dropping UL and properly changing to regular pentode/tetrode mode would require an additional HT node with 2 more stacked filter caps.

...and a choke filter too

[catalin gramada]

Still blowing fuses ?

[cluster]

Still blowing fuses ?

Thankfully no. The amp is up and running as it should. However, I still have it on the bench just to keep an eye on it. Want to make sure it's good and stable. I'm also considering replacing the filters caps but I don't know yet. Next, I want to trace the circuit and compare it to the schematic. I've already found a couple of mistakes in the schematic (incorrect component values). Once I trace it all I'll try and post a revised schematic with the corrected errors.

I've read that the original Dagnalls were crap (Partridges were very expensive). That the insulating material that was used was really thin and cheap. Because of this, arcing would occur between windings and the OT would smoke. Later OTs were better made and didn't blow.

[catalin gramada]

I know there are 3 models of Majors which comes after original "pig". 1966,1967 and 1978 maybe something fit with you amp





From you pics I can imagine the PS looks more like this:



Horrible drawing have any sense for you ?

late -No need to redraw the schematic. I checked for you. Except 100k which not looks factory instaled NFB resistor instead 47k and extra screens resistors everything in you amp shows original 1967 schematic values probably made in "68-69.What seems to be suspect for you ?
Changing electrolythics is a good movement. Take your time and buy the best you can afford.
...and well...yes, there are few little modern design improvements you can do it against 20th century standard industry but didn.t worth it, are not completely justified... just if you amp shows signs of instability or is excessive noisy

[cluster]

I know there are 3 models of Majors which comes after original "pig". 1966,1967 and 1978 maybe something fit with you amp

[ATTACH=CONFIG]43897[/ATTACH]
[ATTACH=CONFIG]43898[/ATTACH]
[ATTACH=CONFIG]43899[/ATTACH]

From you pics I can imagine the PS looks more like this:

[ATTACH=CONFIG]43906[/ATTACH]

Horrible drawing have any sense for you ?

late -No need to redraw the schematic. I checked for you. Except 100k which not looks factory instaled NFB resistor instead 47k and extra screens resistors everything in you amp shows original 1967 schematic values probably made in "68-69.What seems to be suspect for you ?
Changing electrolythics is a good movement. Take your time and buy the best you can afford.
...and well...yes, there are few little modern design improvements you can do it against 20th century standard industry but didn.t worth it, are not completely justified... just if you amp shows signs of instability or is excessive noisy

Thanks so much for that input. My mode is the 1967 Major. The errors I found so far are (left to right on the schematic): see the 25uf bypass cap on V2a, just before the tone stack? On mine its a 50uF; see where it says 2 resistors at 270 just before the PI. Those should read 270K; plate resistors at the PI that say 47k/2watts, in mine are actually 56k/2watts. My screens have an added 750R in series with the existing 250R for a total of 1K (probably a good thing). Also there seems to be 2 caps attached to the CT of the HT secondary just after the standby switch which I don't see on mine.

So far that is what i've found.

Also, I've read that in some modes the bias secondaries has a centre tap. Not in mine.

The red plastic cover on my indicator light has broken off, so I'll need to get a new one for that.

Question: would it be worth just replacing the 2 x 375uf cans (i can get these) and the 50+50uF can (preamp) but leaving the massive 200uF cans? No-one sells those massive cans and I have no idea as to how to safely install a set of 100+100uF cans there instead. Any ideas??

[catalin gramada]

what diameter have those fixing clamp? Take a look at F&T . There are large cans at 40-45mm dia. not necessary double or 400uF as they are in amp. Search for 105*C series 680 or 820uF, 450-500V Are pretty large. It.s not a big deal to put a little bigger value. Usually electrolythics have extralarge tolerances. Two 680 or 820uF series meant 300-400uF average. Good quality caps, really German made stuff

http://www.ftcap.de/en/products/elec...der-terminals/
http://www.ftcap.de/en/products/elec...rew-terminals/

680uF/450V have 40mm = 1.575"
820uF/450V 45mm = 1.77"

[cluster]

So I pulled out some old caps from another Major I used to own. Surprise, the big caps are actually 200uF + 150uF. I able to read the values on the ones in my amp but now that I have these, I can clearly read the values.

Would these work?? -> Found these JJs at the tube store 390uF @ 450VDC www.thetubestore.com - JJ Can Capacitor 390uF / 500V

[catalin gramada]

JJ sound very nice in my replacements and projects. I cannot tell you nothing about their reliability, but this can be a concern when are exposed to excesive heat and it is not you case.(btw are rated at 85*C). But are so cheap and sound so good it is hard to beat, so I can consider a good option. Change all elcap. in you amp , included bias and bypass cathodes. Be aware I found standard 35mm dia. can is a little bit large, I think 36-37 mm. It fit in standard 35 mm clamp but need to be mounted it in clamp first and then plant to chassis. Clean very well from oxide the ground lugs and chassis ,You don.t need to have residual contact resistance here.