Troubling idle current draw from unloaded 60's Super Reverb

[pdf64]

Hammond have a primary magnetising current limit of 400mA for that PT https://www.hammfg.com/files/parts/pdf/290D2X.pdf

[catalin gramada]

I don't think the iddling, no loaded current have some in common with magnetising
Iimit current. Think more thermal characteristics do a difference. I don't have the knowledge but always wonder why smaller trannies are made with more turns per volt ? And my beliefs is they just doing another compromise to deal with thermal abuse which the bigger ones may support easier. ...just my thoughts.

[pdf64]

If primary magnetising current isn’t the primary current with the secondaries unloaded, what is it?

[catalin gramada]

You're talking about a curent magnetising limit meant premagnetisation current which is tens time more as nominal regime curent. That I understand.
Supposed is a 400mA primmary rated. We don't know what kind of limit it is. What happen if we draw more? It overheating ? It saturate ? I thought is a time function in respect was rated for a infinite working time. For short periods of time it may use over rated currents without any problems , and think not saturate at all. That lead me as conclusion is a thermal deal.

[Helmholtz]

I think pdf64 is right here.

While 400mA looks rather high, it would explain the no-load current draw and that the PT still doesn't start to smoke.
Magnetzing current is largely reactive (inductive) and reactive power doesn't heat the transformer.
It's value is mainly determined by the primary inductance.

I've never seen a PT datasheet that specifies a max. saturation current.

[Helmholtz]

With all the tubes pulled, as well as the lamp, this essentially opens the filaments as the center tap is grounded vs the 100Ω resistors.

I didn't understand the whole sentence as only tubes (and lamps) have filaments.

[SoulFetish]

I didn't understand the whole sentence as only tubes (and lamps) have filaments.

Okay, what I mean by that is. With no tubes or lamp installed, there (should be) no load on the filament. The winding should be an open circuit.

[Helmholtz]

Okay, what I mean by that is. With no tubes or lamp installed, there (should be) no load on the filament. The winding should be an open circuit.

Yes, after re-reading today I figured what you meant.
Still a transformer can't have a filament.

[pdf64]

You're talking about a curent magnetising limit meant premagnetisation current which is tens time more as nominal regime curent. That I understand.
Supposed is a 400mA primmary rated. We don't know what kind of limit it is.

It's a max, above that the unloaded transformer will fail its acceptance test

What happen if we draw more? It overheating ? It saturate ?

If we draw more implies load current? It's a 260VA transformer, it's fine to draw load current, and in that case the primary current will probably increase above its magnetisation level.

I thought is a time function in respect was rated for a infinite working time.

No time limit is mentioned in the test condition for Iex.

For short periods of time it may use over rated currents without any problems , and think not saturate at all. That lead me as conclusion is a thermal deal.

I don't think you're discussing magnetisation current above, seems more like you're thinking of the VA rating?

...Magnetzing current is largely reactive (inductive) and reactive power doesn't heat the transformer...

Unloaded PTs do tend to warm up somewhat though, so it seems like there must a 'real' component to the primary current?

[Helmholtz]

Magnetizing current Iex does not depend on the load.
It would only increase at a higher mains voltage or a lower mains frequency (like 50Hz instead of 60Hz).

For the manufacturer a higher than rated Iex would indicate e.g. a poorly assembled core having increased effective airgap.
So it is an acceptance criterion for QC.

[g1]

Unloaded PTs do tend to warm up somewhat though, so it seems like there must a 'real' component to the primary current?

I had a boogie that the customer wanted a new PT installed. Dual Rect. I think. He said it had been humming badly though it seemed fine to me. It was all good except it got pretty warm even unloaded, and the unloaded primary current seemed high. I went along with it as the customer insisted it be replaced. I was hoping the new one would idle cooler unloaded, and less current, and the owners suspicion would be proven correct.
The brand new replacement was exactly the same as far as unloaded current/heat.
However, I never heard back from the owner, and for a very costly part, I thought I would have if he didn't hear a difference in the hum level.

[Helmholtz]

Unloaded PTs do tend to warm up somewhat though, so it seems like there must a 'real' component to the primary current?

Yes, a transformer has core losses due to hysteresis and Eddy currents.
The core losses cause a phase shift of the magnetizing current resulting in an active/real current component.
Core losses can be modelled by a loaded secondary which adds a real current to the reactive magnetizing current.

Percentages vary with design.

A PT may have 5% core loss, meaning that the core would be heated by 13W with a 260W transformer.

[catalin gramada]

I had a boogie that the customer wanted a new PT installed. Dual Rect. I think. He said it had been humming badly though it seemed fine to me. It was all good except it got pretty warm even unloaded, and the unloaded primary current seemed high. I went along with it as the customer insisted it be replaced. I was hoping the new one would idle cooler unloaded, and less current, and the owners suspicion would be proven correct.
The brand new replacement was exactly the same as far as unloaded current/heat.
However, I never heard back from the owner, and for a very costly part, I thought I would have if he didn't hear a difference in the hum level.

Had a Peavey Bass with a PT who get in 20 min idling as hot as it can. Remembered asked Enzo about possible failure he told me to not worry about, and he was right, the tranny had very good "regularisation"- 5% about. It looks like a 300VA but was rated for 550VA.

[SoulFetish]

Just wanted to follow up. Turns out everything worked out okay. The PT was probably on the high end of acceptable no load current. After reinstalling all the tubes and setting the bias, the loaded transformer was operating as expected.
This was another super which sounded fantastic, by the way. With the right compliment of 4 x 10" speakers, there is no Fender which sounds better than a Super Reverb, IMO.