Marshall Major Blowing Power Fuse

[cluster]

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!

Tubes JJ KT88's. Here is the data sheet for them: http://www.thetubestore.com/lib/thet...re/JJ-KT88.pdf

Question; How does dropping the screen voltage effect tone and feel of the amp??

[cluster]

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

Thanks for that info. I'll skip the MV at this stage. Try to keep the amp original.

I've heard some people say that Marshall Majors don't take overdrives well (especially Fuzz) and some blow up on a count of square wave signals hitting the OT (something to do with it's ultra linear character). I have used a transparent overdrive (a TIM) on this amp and it really makes this amp come alive at volume 4-6 or so. Any thoughts, pros/cons on using OD pedals with a Major.

[cluster]

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!

Thanks for this. I'll try to draw out a little schematic and check back with you. Amps that red-plate are scary.

[catalin gramada]

Is hard to predict as time are tied in ultralinear connection and its effect is related by OT but I saw from you pics you screen resistors are allready oversized from original. Seems someone tied a series 750 ohm with original 250 going to 1K. This mean more voltage drop over when dimmed. The screens draw very litle current at idle and in consequence at idle the voltage will be as same as the plate,more or less. When dimmed they draw more, so more voltage drop over screen resistors. Bigger screen resistors meant less voltage to the screens on "dynamic" mode.

[Leo_Gnardo]

maybe drop the screen voltage with some additional resistance

That's what I did with the Major I was working on, 1K 10W instead of those 250 ohm hockey puck resistors.

Power figure dropped to 168W at clip instead of 186, still plenty enough to rock the house.

I've heard some people say that Marshall Majors don't take overdrives well (especially Fuzz) and some blow up on a count of square wave signals hitting the OT (something to do with it's ultra linear character). I have used a transparent overdrive (a TIM) on this amp and it really makes this amp come alive at volume 4-6 or so. Any thoughts, pros/cons on using OD pedals with a Major.

As long as you're not overstressing the power amp, I don't see why not use OD pedals to get your fuzzy tone. A Major, being driven well into square wave zone by dialing it up & wailing on it, that's what wrecks them. I consider them "friendly giants", they make good bass amps, or clean guitar to earsplitting volume. Treat 'em gently and you'll get good results. If you like the tone of a real overdriven Marshall, get or build a late 60's - early 70's 50W with a B+ in the 360-400V range. Real power will top out around 30W at clip with EL34 or 35W with KT88 or 6550, you'll get plenty of power amp clip tone & save a lot of headaches. Luv those amps!

[catalin gramada]

...And it is very easy to recoverable mode if you feel is to loud. Just connect it into triode mode. It will sound slightly different but maybe is in you spot.
You amp looks in pretty original condition. Don.t mode it. Play it like a Major damn... Love it.

[cluster]

Took a photo of the bias pot underneath the bias board. Looks like a heavy duty sucker of a pot. As an alternative to redesigning the bias circuit, would it be possible to put a fuse in the existing one so if there is a fail, the fuse blow instead of red plating the tubes??

[catalin gramada]

[ATTACH=CONFIG]43875[/ATTACH]

Took a photo of the bias pot underneath the bias board. Looks like a heavy duty sucker of a pot. As an alternative to redesigning the bias circuit, would it be possible to put a fuse in the existing one so if there is a fail, the fuse blow instead of red plating the tubes??

No, no fusing into bias supply. This circuit should be as robust as possible. Take in account bias load pot/resistors are directly in series with grid leaks and is better to keep this impedance path as low as possible. You don.t want to break this connection in any circumstances. The pot/trim should be as hefty in respect with power dissipated by itself

[g1]

As an alternative to redesigning the bias circuit, would it be possible to put a fuse in the existing one so if there is a fail, the fuse blow instead of red plating the tubes??

No, like Catelin said, no fuses in bias circuit. If anything you would fuse the power tube cathodes to avoid excess current, such as used in JCM900 and other examples.

585 plate volts in kind of scary, maybe drop the screen voltage with some additional resistance or switch to 7027/6550

Just a warning that unless you are talking about NOS 7027's, modern production 7027's are built to 6L6 specs (if they are even anything other than re-pinned 6L6's). Possible exception being TAD which may be a re-pinned modern 7581 with 35W rating. Check the actual manufacturer spec. sheet.

[Enzo]

Fuse in the bias? Fuse blows you lose bias completely, making whatever else was wrong even worse.

[cluster]

Fuse in the bias? Fuse blows you lose bias completely, making whatever else was wrong even worse.

Could you recommend a less intrusive way to safe guard the bias circuit without having to completely redesign it or disabling the existing pot?

[Enzo]

Well, safe guard it from what? There is no current stress on a bias circuit. SO mostly I concern myself with the failure of the pot wiper. If that loses contact, our circuit reverts to some condition. We want that condition to be the maximum bias voltage.

Most bias supplies rectify something to form a base negative supply. The a voltage divider adjusts the portion of it you want. I never make a bias control that takes the bias off the wiper. I always wire the pot as a variable resistor. If that pot is used as the top resistance in a divider, the loss of the wiper means the resistance maxes, which throws the bias voltage to the least - the opposite of the desired goal. SO if I make the pot the lower element, then an open wiper reverts the supply to the maximum negative voltage.

So your circuit in post #72 for example I would make the 10k resistor the variable and the 3k just a resistor. I would not use a 10k pot, I would chose a pot value plus a resistor, so maybe a 5k pot with 5k resistor to make 10k. That way if the wiper opens, we have 10k resistance for the max voltage. And the resistor means if someone turns the bias control all the way down, it won;t go to zero, it will only drop to whatever 5k represents.

My numbers are arbitrary, you might find a 20k resistor with a 3k pot works well. The exact values would be determined by the base supply voltage and make up a divider that gives you the adjustment range of bias voltages, say 40-60v. Whatever.

That is how I would protect it.

[cjenrick]

doh on me, i thought those were 66 tubes instead of 88's, so you are alright with that B+ voltages and screens too.,

dropping screen volts can be bad for tone if you take it too far,

UL transformers might be a little too clean for some people, if so, simply safe off the taps and put a screen dropping resistor to the pwr supply, wire up some 470's or 1 K five watt resistors and rock on,

ever play the Fender Super Twin? (UL transformer) Great for steel guitar but mega clean with guitar, and they are real back breakers,

[cluster]

doh on me, i thought those were 66 tubes instead of 88's, so you are alright with that B+ voltages and screens too.,
UL transformers might be a little too clean for some people, if so, simply safe off the taps and put a screen dropping resistor to the pwr supply, wire up some 470's or 1 K five watt resistors and rock on,

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??

[pdf64]

...ever play the Fender Super Twin? (UL transformer) Great for steel guitar but mega clean with guitar...

Note that the OTs in those late 70s/early 80s Fenders had their screen grid taps at ~12.5% (so operate basically near regular pentode/tetrode mode), way off the UL sweet spot ~40%.
I think it's better to refer to them as distributed load.
I think that they seem super clean as they have a high stiff HT, and very good mitigation for bias shift under overdrive.