Traynor YBA-1 MKII fusing

[LarBal]

Just had one of these come back to me for the second time.

Initially I had modded the preamp to a 3 stage +CF high gainish thing, and installed new EL34s. It had had a cap job before I saw it. 575VDC plates. Amp sounded great and lasted a bit.

Owner reported that the "circuit breaker" kept popping, and he could reset it, and it'd be fine for a while. So it came back, and I replaced the breaker with a normal 3A fuse and fuse holder, as I had read the breakers can be unreliable after decades.

Now it just came back to be today, owner reported it was working fine at rehearsal, and when he turned up reeeaaaalllly loud to match another guitarist, the fuse blew.

So first thing I do is put in a 4A fuse, it it's working again. Same EL34s since I put em in...

At this point, I don't want to see this amp again, and I'm interseted in ensuring that I don't.

Now, the scheme that's taped on the top states 3A fuse, but I'm now realizing there are other schematics for this amp that show "circuit breaker", and I need help determining what fuse I should put in this amp.
I don't think it's the correct scheme attached to the amp, I think someone put it there at some point in it's life.
Anyways, I've attached some relevant pics, including the little sticker inside the amp that even adds to my confusion.

Should I ditch these JJ 34s and get some KT77s? I already gave each screen 470r 5W, but should I up that? What size fuse should I put in here?
I can't find anything else wrong with this amp that would cause fuses to blow, other than possibly being under spec'd by me.

I believe this to be the correct one. 7A for a pair of EL34??? : https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

If you look at your posted schematic, the 3A fuse is for a 240V mains input. Your profile doesn't say where you're located, which you may want to update so we know where you are. If you are running on 110-120V mains, your fuse should be roughly double, so 7A would be closer to correct. I don't see any reason to ditch the EL34's..

[LarBal]

Oh my goodness, I sure missed that one. Thank you. Location updated.

[g1]

If it's a MkII, that is not the correct schematic. Also with it being taped in implies someone added it later.
It should use the circuit breaker listed on the sticker, or a 5A slow-blow fuse.
Bias is quite cold and adjusted for voltage across the screen resistor as stated in note #2 (note #7 of correct schem attached). If it is run hotter, you may see nuisance blowing of the fuse.

schematic on pg.11 of attached.

[mozz]

Traynors always had the schematic taped to the top inside. So i would say it's the proper one to use if it matches. Far as i remember, mine had a 7A breaker.

[g1]

Traynors always had the schematic taped to the top inside. So i would say it's the proper one to use if it matches. Far as i remember, mine had a 7A breaker.

Yes, the sticker for the breaker is in there. But it should have a schematic with the breaker, not a fuse. And that schematic is for a strictly 240V unit, which this is not.
I should have been more specific, the schematics were always glued to the top, never taped. It was added to production at some point I'm not sure about. Earlier units like the older YBA-1 did not have them.

[Helmholtz]

I assume the circuit breaker has a faster reponse than a slo-blo fuse.
So it will be rated for a higher current to take account of the turn-on surge.

[mozz]

Ok, mine is a YBA-1A MARK II. His is marked the same on the schematic but 240v?. Mine also has, near the power cord, 115v 2A. Took the top off and there's a piece of galvanized metal, no schematic. I think it was there, i probably put it in a manila envelope to never be found again. Mine was modded into a plexi, bias adj and test points, master vol, there are 2 fuses (not stock) and a 5A breaker (which i had a NOS mil surplus) but i think it was 7A from the factory. I can't recall if my breaker was a problem but i do know i changed it because i had that new one here.

I would be using a slo blo fuse maybe 3A or 4A at the most. Does it have the fan?

[g1]

The breaker is 4.55A Hold 7.5A break. They show it as 7A breaker on schematic. The earlier MkII prior to the breaker used 5A slow blow fuse.