Single ended triode strapped pentode amplifiers.

[Lavane]

It means the transfomer was probably designed with 50Hz in mind.

That seems a bit odd being designed a 115V primary. I get it for the different transformers with 110V primaries. They arent really meant for use in the US anyway. But you see a lot of them being used in Chinese amps. Close enough is good enough I guess. The 6.3V filiment windings in these 110V transformers measure around 7.2V when plugged into 120V mains.

 

[whoaru99]

It's not odd at all, and has nothing directly to do with the nominal voltage. They say 50/60 Hz. 50Hz transformer needs more "meat" than 60Hz transformer for any given VA. That's why they are saying it's 20% more capable at 60Hz.

 

[Mr Boochie]

I use fast blow fuses in almost everything. At the moment the only equipment I have with slow blow fuses is my VTA ST-70. I haven't had a fast blow fuse pop in a long time. For one thing I never fast cycle power switches.

If you decide on that big toroidal PT, I would go with good wire-wound or metal-oxide resistors on the power supply rail. 5-watt Mills, Ohmite, or Dale wire-wound ones would be nice. At least in the first filter stages. I put those in the bigger Fender amps, and they last forever.

 

[gadget73]

Fast vs slow-blow fuses is largely about the inrush current. Its fairly high with a transformer, and even higher if its solid state rectified since its also charging the caps. Toroids tend to be worse than an EI core too. That lasts a very short time, but its enough to cause nuisance failures with a fast blow unless its sized larger than would be ideal for over-current protection. Thats where the slow-blow has an advantage. Or you can have a thermistor in there to soften the inrush and allow use of a reasonably sized fast blow without the nuisance failures.

 

[Mr Boochie]

Fast vs slow-blow fuses is largely about the inrush current. Its fairly high with a transformer, and even higher if its solid state rectified since its also charging the caps. Toroids tend to be worse than an EI core too. That lasts a very short time, but its enough to cause nuisance failures with a fast blow unless its sized larger than would be ideal for over-current protection. Thats where the slow-blow has an advantage. Or you can have a thermistor in there to soften the inrush and allow use of a reasonably sized fast blow without the nuisance failures.

I would not use a Slow-Blow fuse with a 454VA-rated power transformer for a push-pull, two-6L6GC amp or single-ended 6L6GC (I'm not sure what amp he intends on using this PT). Its tubes, resistors, or power supply caps may fail before the fuse does.

Yes, if it were a "normal-sized" 250VA transformer.

That's what I would do. It's better to keep replacing fuses — than to keep replacing tubes or soldering in new caps or resistors.

 

[Lavane]

Fast vs slow-blow fuses is largely about the inrush current. Its fairly high with a transformer, and even higher if its solid state rectified since its also charging the caps. Toroids tend to be worse than an EI core too. That lasts a very short time, but its enough to cause nuisance failures with a fast blow unless its sized larger than would be ideal for over-current protection. Thats where the slow-blow has an advantage. Or you can have a thermistor in there to soften the inrush and allow use of a reasonably sized fast blow without the nuisance failures.

That was kind of my thinking originally. Thats why I posted about the use of a SS WZ-34 coppertop instead of a GZ-34 tube rectifier. The WZ-34 has a thermistor inside. Using a SS bridge rectifier should get me closer to the B+ I need though. I have thermistors, I can add one or two for a softer start. I have a good assortment of glass fuses in various amperage ratings and all are fast blow types. I just haven't had issues with them blowing at start up unless there is a fault in the piece of equipment.

 

[Mr Boochie]

That was kind of my thinking originally. Thats why I posted about the use of a SS WZ-34 coppertop instead of a GZ-34 tube rectifier. The WZ-34 has a thermistor inside. Using a SS bridge rectifier should get me closer to the B+ I need though. I have thermistors, I can add one or two for a softer start. I have a good assortment of glass fuses in various amperage ratings and all are fast blow types. I just haven't had issues with them blowing at start up unless there is a fault in the piece of equipment.

Lavane, it's not the rectifier (tube or solid-state) — it's that big power transformer you plan on using. You have to design the power supply downstream of that PT to handle the inrush current that that thing has in reserve. I'm not sure the thermistor in the WZ-34 will handle sudden high-current demands using that PT.

Think of it kinda like using a car battery as a 12-volt power supply for an appliance that only needs 1 amp RMS to work. If, for some reason, the unit has a momentary short or malfunction and draws 10 amps, the car battery will supply 10 amps instantly, prolly beating most fuses — and most likely causing damage. A 2-amp RMS, 12VDC supply will not. A fast-blow 2-amp fuse would be the only thing to save that unit.

You are prolly better off with a 250VA PT. Well, that's my thoughts on this.

 

[whoaru99]

I would not use a Slow-Blow fuse with a 454VA-rated power transformer for a push-pull, two-6L6GC amp or single-ended 6L6GC (I'm not sure what amp he intends on using this PT). Its tubes, resistors, or power supply caps may fail before the fuse does.

Yes, if it were a "normal-sized" 250VA transformer.

That's what I would do. It's better to keep replacing fuses — than to keep replacing tubes or soldering in new caps or resistors.

Personally I do not think the trafo presents any real risk.

Slow blow fuses/breakers are standard fare for handling inrush/startup loads without needing to use unnecessarily high values of fast blow to cope.

The big peak inrush currents are, by some measurements I did in the past, of very short duration typically less than one cycle. As well, peak inrush is sorta luck of draw depending when the unit is switched on.

If switch on happens to coincide with the peak of the mains cycle that's the highest. Every other place on the sine is something less.

 

[Lavane]

Lavane, it's not the rectifier (tube or solid-state) — it's that big power transformer you plan on using. You have to design the power supply downstream of that PT to handle the inrush current that that thing has in reserve. I'm not sure the thermistor in the WZ-34 will handle sudden high-current demands using that PT.

Think of it kinda like using a car battery as a 12-volt power supply for an appliance that only needs 1 amp RMS to work. If, for some reason, the unit has a momentary short or malfunction and draws 10 amps, the car battery will supply 10 amps instantly, prolly beating most fuses — and most likely causing damage. A 2-amp RMS, 12VDC supply will not. A fast-blow 2-amp fuse would be the only thing to save that unit.

You are prolly better off with a 250VA PT. Well, that's my thoughts on this.

It's what it is and what I have on hand. A fast blow fuse is definitely what I'll use like I usually do. I get that if there's a fault there will be a lot of available current from that transformer. The thermistor or thermistors would just be for a bit of a softer start.

 

[Lavane]

Personally I do not think the trafo presents any real risk.

Slow blow fuses/breakers are standard fare for handling inrush/startup loads without needing to using unnecessarily high values of fast blow to cope.

I haven't had to use unnecessarily high value fast blow fuses yet to cope with inrush at start up. The few fast blow fuses I have lost were due to a couple new manufacture rectifier tubes failing along with a dual triode driver tube that had one side short out.
When I build something I use a variac with the first start up and monitor voltages and current draw as I bring the amp up for the first time.

 

[Mr Boochie]

Personally I do not think the trafo presents any real risk.

Slow blow fuses/breakers are standard fare for handling inrush/startup loads without needing to use unnecessarily high values of fast blow to cope.

The big peak inrush currents are, by some measurements I did in the past, of very short duration typically less than one cycle. As well, peak inrush is sorta luck of draw depending when the unit is switched on.

If switch on happens to coincide with the peak of the mains cycle that's the highest. Every other place on the sine is something less.

Well, just my opinion. I will leave this thread to the others.

 

[whoaru99]

I haven't had to use unnecessarily high value fast blow fuses yet to cope with inrush at start up.

The point of saying "unnecessarily" was relative to slow blow.

 

[Lavane]

The point of saying "unnecessarily" was relative to slow blow.

Ah, ok.