Transformer Rating

[GregS]

I'm currently working on a high power amp design which will have about 160W output. I'm planning to use the Hammond 278CX power transformer and the Hammond 1650T output transformer. These are the same transformers that the Traynor Custom Special bass amp used which put out 160W with 4 EL34's.

I want to use either 6550's or KT88's. The specs of the power transformer are:

400V-0-400V, 465 mA secondary
6.3V, centre tap 6A heater winding

My concern is the current rating of the heater secondary only being 6A. The heaters for 4 KT88's will draw 7.2A. The heaters for either 4 6550's or EL34's will draw 6.4A. Pre-amp tubes will add even more to this. It would seem obvious that the heater winding isn't up to the task. However it isn't so obvious. It seems hard to believe a PT capable of 160W output can't supply the heaters of 4 big bottle power tubes. Also how did it work in the Traynor amp without the heater winding being over taxed? Is it that this heater winding rating is extremely conservative?

It's not a big deal to get a filament transformer to share some of this load. However, I'd rather not do this if it isn't necessary.

Any input would be much appreciated.

Thanks,
Greg

[Chuck H]

I'd bet that the Hammond is over rated on that wind by enough to handle it. But don't take my word for it. Call Hammond and ask. They were very helpful with tech info when I called them a couple of years ago.

[R.G.]

The specs of the power transformer are:

400V-0-400V, 465 mA secondary
6.3V, centre tap 6A heater winding

My concern is the current rating of the heater secondary only being 6A. The heaters for 4 KT88's will draw 7.2A. The heaters for either 4 6550's or EL34's will draw 6.4A. Pre-amp tubes will add even more to this. It would seem obvious that the heater winding isn't up to the task. However it isn't so obvious.

It is affirmatively NOT obvious. It's a problem in heat transfer, and those are nearly always complicated.

It seems hard to believe a PT capable of 160W output can't supply the heaters of 4 big bottle power tubes. Also how did it work in the Traynor amp without the heater winding being over taxed? Is it that this heater winding rating is extremely conservative?

The power rating of a transformer is something like the current rating of a fuse. That's not where it will blow, it's where it's guaranteed not to blow.

The limit on power out of a transformer is a sloppy, foggy thing to define with any exactness. It's really a thermal limit. The inside insulation of the transformer is what fails when transformers die. These are rated for maximum internal hot spot temperatures in various classes, often 105C, 130C, 155C, 180C and perhaps more. The materials in the various classes are good for up to that temp. Maybe more, but it's unspecified. If the hottest spot inside the middle of the transformer gets too hot, the insulation gives up, the coils short, arcs form, and the smoking death spiral begins. What's hard to figure out is
- what is the external temperature? Every degree outside raises the insides a degree.
- what is the heat rise per watt from each position inside the transformer? Each winding occupies a different volume inside it, and has a different path to the external cooling. A 10% overload on one winding may not hurt at all, and may cook another winding further inside where heat has a harder time getting out.
- what are the loads on the other windings (generally as a function of time); if one winding is overloaded, it can be cooled by a less-than-fully loaded neighbor
- How long does the oveload go on? Transfomers may take hours to come to final temperature. Short term (e.g. an hour) may not ever let the thing get fully warmed up, so on short gigs it never gets fully hot, but dies when it's run for several hours as the house amp at a festival.

And, just like men walking in delayed-action mine fields, transformer designers are conservative.

It's not a big deal to get a filament transformer to share some of this load. However, I'd rather not do this if it isn't necessary.

Think of it as a "feature" or a chance to run the preamp tubes on DC, or some such rationalization to make it feel better.

[loudthud]

I have a Traynor Custom Special that has been converted to 6550s. The heater winding is a little low on voltage. The way the stock amp is wired, the heater winding goes to the pilot lights, then a twisted pair goes quite a distance to the power tubes. There was a significant voltage drop in that twisted pair, you could feel the wires getting warm. I replaced the stock wiring with some #16 aircraft wire and now IIRC the voltage reading at the first 6550 is 6.2VAC. The original CS amps used 7027s that didn't require as much heater current.

[GregS]

Thanks for the informative replies.

I think I will get a filament transformer to be on the safe side.

Thanks again,
Greg

[Wombaticus]

Thanks for the informative replies.

I think I will get a filament transformer to be on the safe side.

Thanks again,
Greg

Just as an aside, really, the current draw for a KT-88 filament is 1.6A both on the old MOV datasheet, as well as at least on modern production one I looked at -- not the 1.8A implied by your reckoning of 7.2A total for 4. But whats 0.8A between friends?