Fuse Sizing for Amp With Crazy Heater Current

[The New Steve H]

I'm building an amp with 3 12A_7 tubes, 4 6BM8s, and a 5Y3. The heater current is insane. I come up with 0.9 amps for the preamp, 3.12 amps for the power tubes, and 2 amps for the 5Y3. That adds up to 6.02 amps, total, before I even look beyond the heaters.

The 5Y3 tops out at something like .125 A, so I assume the maximum draw is around 6.15 A, apart from the instant after the amp is turned on.

What size fuse do I need? I have some 7.25 A slo-blos, but that seems pretty close to the maximum current. On the other hand, it's only close in a relative sense, since amps with smaller fuses have fuse ratings that are close to the maximum draw in absolute terms. I think.

[The New Steve H]

Now I'm seeing different values for the preamp currents. I saw sources saying 12AX7s took 300 mA, but now I'm seeing 150 mA for both 12AX7s and 12AY7s. This amp will have one 12AY7 in the chain.

If what I'm seeing is right, the total current is about 6.0 A.

[The New Steve H]

Oh, for God's sake. Now I'm seeing .150 A for each SIDE of a 12AX tube. Arggh.

[Cliff Schecht]

That ain't shit. I'm about to dig into a build that's 8.4A on a 6.3V tap alone (4xKT88, 6 preamp tubes). Here's how I will be fusing this amp:

8.4A*6.3V=52.92W
550V*0.5A = 275W (overestimating the KT88 anode current draw a bit for some margin)

52.92W + 275W = 327.92W

327.92W/120V = 2.73A

So for this amp the mains fuse will be a 3A slow blow fuse. With your setup it sounds like you can use a 2A slow blow but it's never a bad idea to run the numbers..

[ToneMerc]

Cliff's got it, but I typically use fast blow on the mains and slo blow in the HT CT winding.

Simply

Total VA/Line Voltage= Fuse Rating

TM

[The New Steve H]

So I just try to figure out where the watts are and how many, convert it back to 125VAC, and use the resulting current, multiplied by some safety factor of which I am not yet aware?

[The New Steve H]

Is this even close to right? The plate voltage is 275.

PT: 275 V x .125 A (max 5Y3 current) = 34.375 W

6.3 V heaters: 6.3 V x 4.02 A = 25.33 W

Rectifier heaters: 5 V x 2 A = 10 W

Total: ~70 W

125/70 = 1.78 A

Figure 2.5 A fuse?

The actual voltage here is 125.

[Cliff Schecht]

2A would be better. Remember you already calculated in some margin so you shouldn't need much more.

Tonemerc, I use slow blow on the mains but try to use a fuse that is rated "properly", meaning not too far over my calculated requirements. If Steve was to use a fast blow on his mains at 2A it would probably blow when he flipped the power at least once in his life. If he used a slow blow, it can handle the instantaneous current draw from the heaters but will still blow if you manage to actually short something to ground, even momentarily. These aren't timed release fuses, they are simply designed to handle transient current spikes without popping instantly.

[katopan]

Even just the PT inrush current will pop a mains fast blow fuse unless it's ridiculously oversized.

Steve H, the assumption so far is you're talking about sizing the mains fuse, not a separate fuse on the heater supply?

[The New Steve H]

I wasn't planning to have two fuses. I am very brave.

[diagrammatiks]

2-3 should be fine.

your heater current isn't all that big at all.

[dass101]

You got the final calculation wrong: it's 70W/125V=0,56A

Also, you didn't take into account the current draw of the power tubes.

[Structo]

Cliff's got it, but I typically use fast blow on the mains and slo blow in the HT CT winding.

Simply

Total VA/Line Voltage= Fuse Rating

TM

I always thought we were supposed to run the Slo Blo on the Mains and the Fast Blo on the HT?

Slo Blo on the Mains because of in rush current momentarily exceeding the rating..

[The New Steve H]

I can't believe I divided volts by amps instead of the other way around.

But I believe I accounted for the power tube draw when I put the 5Y3 in there. It powers the output transformers, no?

[dass101]

If you measured the current between the rectifier tube and the reservoir cap, it is correct.