bucking transformer rating

[Randall]

I am going to build a bucking transformer in a box. I have done it before, but for a big Marshall, so I used an 8 amp Triad transformer to be safe. Thinking more towards a smaller amp(s), a pair of 6L6s or less, do I really need an 8 amp transformer and if not what would be a safe and smaller one? I am using RG Keen's example, and he shows a 3 amp 12vac rated transformer. The amp that will be using this has a 3 amp fuse, should I go to 4 amps to be safe?

And should the bucking box fuse be the same rating as the tranny, or would dropping down an amp be a good idea?

[The Dude]

All of your post sounds good to me. A 4A transformer should be fine for your example. Fusing it at 3 also seems fine. I might consider a slow blow to allow a little turn on surge current.

[Randall]

As it turns out, the little brother Triad 4A transformer is only $3 less and not in stock, so I'll just go with the 8A and be done. Also, I hadn't noticed, but Triad actually recommends fusing 1A slo=blo above rating on the secondary side for full performance. I don't want two fuses, so can I just fuse at 7A on the primary?

[Tom Phillips]

...can I just fuse at 7A on the primary?

By calling the unit an "A transformer" I'm thinking you mean the current rating of the secondary. Assuming you are using a 120V Primary to 12V secondary bucking transformer a 7 A fuse on the primary would indicate that the transformer is allowed to draw 840 Watts from the line input. That would only happen if the transformer failed or the load on the secondary was drawing ~70 Amps! So...NO... a 7A fuse on the primary is not appropriate.

Time to google the basics on this.

[Randall]

"Time to google the basics on this"

C'mon Tom. I have googled the basics on this, and that's why I ask you here. RG Keen's diagram only has a Fuse on it, with no value or explanation. That's what I am trying to figure out. If you know the answer, couldn't you just enlighten me?

[R.G.]

In the absence of real data, we have to think, and that always hurts.

The 6V6 "little amp" apparently needs up to a 3A fuse. Presumably, it work OK with that value as long as the primary voltage is right. We don't really know what current overload for what amount of time will make a "3A" fuse blow, we only know that it is guaranteed NOT to blow with 3A through it.

That's with the original voltage through it. A bucking transformer will drop the voltage a little. This ought to make the little amp use a smidgen less current, but maybe not proportionately less. You're thinking about bucking the line voltage down by 5% to 10%, so the 3A fuse that was OK at the higher line voltage will still be OK with the bucked-down voltage, maybe a bit more generous. So there's some fat left in a 3A fuse because the amp will pull a little less current.

Countering that, we have the current used in the bucking transformer to make the voltage drop a a bit. If you're using a 12V bucker, either CT for 6V or full for 12V, you're dropping about 5% or 10% respectively off the AC voltage. To do that, the primary of the bucker transformer has to make power equal to the current in the secondary times the voltage of the secondary. If you're dropping 12V off the line, and there's less than 3A in the line being bucked, the secondary of the bucker sees that same 3A.

Transformer action means the bucker primary has to carry (Vs/Vp)*Isec = (12/120)*3 = 300ma. That power comes through the incoming AC line and is added to any current the amp sucks through the bucker secondary and its own current needs.

So the little amp current isn't well know other than being "somewhat below 3A" and "drops maybe 10% because of the 10% drop in voltage it sees". On the other hand, the bucker transformer has to eat some current out if the incoming line to make the bucking voltage. This is (purely coincidentally in this case) about the same amount the amp might drop. Maybe a bit more, maybe a bit less. But the lowered current of the amp with lower line voltage is in the direction that offsets the primary current of the bucker.

My best guess here is that 3A is still fine for the 'bucked' little amp.

Notice that if you are using a bucker to >>increase<< the voltage to the little amp, the math works the other way and both the little amp needs more current at the higher voltage it then gets and the bucker needs more current to make that additional power for the load getting a higher input line.

The way you do this is to estimate the current for the load eating the modified line, figure the secondary current of the bucker, then the primary current of the bucker, and do the math to add the bucker to the real load. Then you guesstimate the fuse.

Notice that the primary fuse in an amp is NOT to protect the amp. If it does that, it's largely an accident. It's there to prevent fires and electrocution assuming the amp has already melted down and is shorting and blowing smoke and flames already. So the larger philosophical question is whether a 3A versus 4A fuse offers any real difference in fire/electrocution safety.

[Dave H]

The secondary of the bucking transformer is in series with the amp transformer primary. The amp is fused at 3A so there's up to 3A through the bucking transformer 12V secondary which is only 0.3A through the bucking transformer 120V primary. The fuse is common to both so it needs to be 3A + 0.3A = 3.3A. Take your pick, use 3 or 4 amp as R. G. said.

8A is overkill for the bucking transformer secondary rating as it only needs to supply the amp transformers primary current which is less than the amp's 3A primary fuse rating.

[Mike Sulzer]

I wonder if the power consumed by the bucker is less than I think you are saying, R.G. It seems to me that the current flow in the "secondary" of the bucker is opposite the usual sense for a secondary, and therefore power is flowing from the "secondary" to the "primary". Then the "primary" is reducing the current flowing into the circuit, and then the only losses added by the bucker are its resistive and core losses.

What I am saying might not be of any practical significance in any case. Nobody makes a 2.9 amp fuse!

[Dave H]

I wonder if the power consumed by the bucker is less than I think you are saying, R.G. It seems to me that the current flow in the "secondary" of the bucker is opposite the usual sense for a secondary, and therefore power is flowing from the "secondary" to the "primary". Then the "primary" is reducing the current flowing into the circuit, and then the only losses added by the bucker are its resistive and core losses.

What I am saying might not be of any practical significance in any case. Nobody makes a 2.9 amp fuse!

I think that should be a 2.7A fuse.

If the main transformer used 3A at 110V or 330VA and it's now being powered from 122V mains with a 12V bucker then the system needs 330VA from 122V which is 330/122 = 2.7A. The bucker primary is providing 0.3A of the current used by the main transformer primary.

A 3A fuse it is then. I got it backwards. It would 3.3A if the 12V transformer was used as a booster.

[Mike Sulzer]

I think that should be 2.7A.

If the main transformer used 3A at 110V or 330VA and it's now being powered from 122V mains with a 12V bucker then it needs 330VA from 122V which is 330/122 = 2.7A. The bucker primary is providing 0.3A of the current used by the main transformer primary.

A 3A fuse it is then. I got it backwards. It would 3.3A if the 12V transformer was used as a booster.

Right you are. I just thought the 2.9 amp fuse sounded even more unobtainable.

[mikepukmel]

Any chance you could post a link to a source for the transformer you're going to buy?
thanks
Mike

[J M Fahey]

[rant mode] DRAW THE F*CKING CIRCUIT!!!!!! [/rant mode]
A LOT of the confusion in this thread would be self cleared just looking at the NOT DRAWN circuit and tracing the current paths.
Just sayin´.


Hint: you are all talking about "a" Fuse or "the" Fuse as if it were only one or just one (or at most two) wiring possibilities.
Think again.

[g1]

Agreed. This is from R.G.'s article on his geofex page : Vintage Voltage Adapter
(for as standalone unit mentioned in first post)