Marshall Major mains fuse value

[Pedro Vecino]

I look for recommendations for the value of the Marshall Major mains fuse. In the original scheme appears 10A for 120V. In my case they are 230V and I think that 5A is excessive value. With 4A it allows to connect without problem but the owner told me that at a very high volume (with Les Paul and volume at 7) melted the fuse. It is a thunderous volume, just to test it, not of normal use.
Could it be associated in some way with lack of amperage in the fuse?
Another thing that I have seen is that it came equipped with a 12AT7 instead of a 12AU7 in the phase splitter. General condition is fine. 650 volts and discrete bias (26mA per tube through the plates) with a Winged C KT88 quartet.
Thanks!

http://drtube.com/schematics/marshall/1967u.gif

[Chuck H]

Was it a slow blow fuse? I can't say about the Major, but fuses in this application typically are.

[Pedro Vecino]

Yes. It's a slow blow fuse (T4A).
Thanks.

[pdf64]

If the owner seems credible then it's probably best to take their word that a T4A blows at high volume.
A 4A fuse may not provide much more protection than a 5A fuse anyway, and there's a HT fuse to back things up.

[dstrat]

Seems to me the designer set that value for a reason.
Heaters:
4 - KT-88 heaters = 1.6A X 4 = 6.4A
3 12a* .9 A = 7.3 A

datasheet says .230ma max signal.
so add another .92 A
a quick estimate I'd say 8.25A if you run it full out..

[Dave H]

That 7.3A is at 6.3V it will only pull 7.3 *6.3/120 = 0.38A from the 120V mains (ignoring transformer losses).

[dstrat]

oops YES very good point! thanks

[J M Fahey]

I look for recommendations for the value of the Marshall Major mains fuse. In the original scheme appears 10A for 120V. In my case they are 230V and I think that 5A is excessive value.

Why?
10A@120V is exact same power as 5A@230V.
You are changing primary side according to local mains, but secondary is always the same, no matter what (even in Japanīs 100V land) , power consumption is always the same.

With 4A it allows to connect without problem but the owner told me that at a very high volume (with Les Paul and volume at 7) melted the fuse.

4A is nunder rated, donīt look further into it.

It is a thunderous volume, just to test it, not of normal use.

Itīs a Marshall!!!!
Itīs *designed* to be used at thunderous volume.

Could it be associated in some way with lack of amperage in the fuse?

Of course, itīs missing 1A capacity.
It "works", sort of, but nuisance blows often.
And the nuisance word is well applied, it will let you tune and warm up but blow in the middle of a scorching solo.

Another thing that I have seen is that it came equipped with a 12AT7 instead of a 12AU7 in the phase splitter.

Place whatīs asked for, designer used 12AU7 for a reason, specifically 12AU7 can handle more current.
Typically 10/12mA vs 12AT7 5mA and 12AX7's meager 1 or 2 mA.
Notice smaller amps (Fender or Marshall) usually have 220k power tube grid resistors, 6550 equipped ones prefer 150k , these Majors use 68k.
And I guess when fully overdriven those grids will love to get a few mA into them.
Ampeg SVT used 12BH7 to provide even more current, and not even 12AU7 work well there.

General condition is fine. 650 volts and discrete bias (26mA per tube through the plates) with a Winged C KT88 quartet.

Cool

Just as |a reference: since slow blow fuses are not popular here in Argentina, I have to specify "some" standard fuse value that works, because even if I can fit a slow blow one which I can get at a couple shops, users will invariably fit a "regular" one if needed, because thatīs what *they* will find at the friendly neighbourhood shop.
Given that, my 300W SS amplifiers (which eat about half of what a Tube one would) "should" work with 2A fuses ... these guaranteed blow at turn-on
3A fuses work fine and sometimes I fit 4A ones (mind you, in an SS amp) , the few times there was an amp short or something, they dutifully blew protecting the PT so they are not dangerously overrated.

Whatīs my point?
If 3A fuses are fine for a 300W 220V SS amp, then 5A is most reasonable on a similar 230V one, but tubed.

Long story to confirm by daily practice what Math already answered.

[Pedro Vecino]

Thank you very much to all. Especially to Juan Manuel. His assertions are always impetuous and clear all doubts
I knew the voltage and intensity relationship. Surely I expressed myself badly (the logical thing with my English level).
The calculation I made to think that 4A fuse with 230V was enough in the Major (and 5A probably unnecessary) was related to the consumption of a 100w Marshall with EL34 (marked with 375W in rear panel). Something like that (an aproximation):

Marshall 100 watts:

Heaters EL34īs 4x 1,5A = 6A x 6,3V = 37,8W
Heaters 12AX7īs 3x 0,3A = 0,9A x 6,3V = 5,67W
HT 140mA x 4 = 0,560A x 500V = 280W
Total = 325 wats. + 16% = 375W (16% is what I add to get 375W)
With 2A fuse: 230V x 2A = 460W (375W x 1,22)

Marshall Major:

Heaters KT88īs 4x 1,6A = 6,4A x 6,3V = 40,32W
Heaters 12AX7īs 3x 0,3A = 0,9A x 6,3V = 5,67W
HT 240mA x 4 = 0,960A x 650V = 624W
Total = 670 watts + 16% = 777W
With 4A fuse: 230V x 4A = 920W (777W x 1,18)
With 5A fuse: 230V x 5A = 1150W (777W x 1,48)

I'll look to see if I have slow 5A fuses. This amplifier imposes a lot of respect on me with those voltages and intensities. That's why I've consulted it. Thanks again!

[Chuck H]

Remember that the purpose of THAT particular fuse is to prevent MOST of the catastrophic failures that might cause fire. It's NOT there to protect the amp. Some manufacturers (and it appears this amp made an early attempt) have other internal fuses for protecting circuitry. It's entirely probably that Marshall used a 10A fuse because it was a convenient value and met a required safety criteria. Notice that on the schematic it clearly shows the 10A fuse AND the selectable primary voltages. I might just stick a 10A 120V fuse in there.

[Pedro Vecino]

I do not know why I remembered this phrase from "Forged in fire" in the strength tests: "Itīs not what iron does to bamboo, but what bamboo can do to iron"