Couple other transformer questions

[nsubulysses]

1. Why is Hammond 278CX 800VCt and the 6.3V winding is only 6A???

What are you supposed to do with it? it is too high voltage for 6L6 so you have to use 6CA7/EL34/KT88/6550, which are all 1.5A heater tubes. So you are already at your 6A heater winding limit without even adding any pre tubes yet.

ALSO, strangely, people talk about using 6 power tubes with this PT. Kevin Oconner for example. I appreciate KOCs books and have learned from them but I also take it with a "I don't know if I should 100% trust this guy" approach. For instance here he recommends Hammond 278CX with 6x6550. Once you add the pre tubes you'll be over 10A heater draw. what am I missing?:


Also, if this is supposedly same transformer used for Traynor Custom Special that amp is 4x6CA7 and 3x12AX7 so 6.9A heater current. They exceeded the heater current on all these amps for many years and they still work fine?

I know this sounds strange but someone may remember. I remember reading on this forum some time a few years ago someone posting saying they called Hammond and they said 278CX is rated at 6A but can actually handle 10A heater draw fine. Anyone remember this hearsay?

2. hammond 278CX is 800VCT at 535mA and Hammond 278X is presumably the two power tube version at 800VCT at 230mA. Would you expect higher B+ in the two power tube version because the 800VCT winding is drawn down less by 2 power tubes vs. 4, or is this accounted for in the 230mA vs 535mA power handling of the HT winding and B+ would be same?

[nickb]

There is a note on the dwg for the heater supply that states "augmented by 167N6" to bring the current up to 9.9A. It's not stated how the augmentation is done. Shared load?

My experience with heater windings is NEVER, EVER, exceed the rating (unless you like smoke / flames).

I don't feel the 535mA winding it up to the task for 200W amplifier. I'd be looking at over 600mA. More it you want 100% duty.

[nsubulysses]

Obeying KOC recommendation and "going for it," I have run Hammond 278CX on 4x6ca7, 4 or 5x12AX7 amps, so 7.2 and 7.5A for heaters.

Strangely, I bought transformers from Heyboer and asked that they be built to exact same spec as Hammond 278CX but asked for 8A heater winding instead of 6A since I was worried about it. The Heyboers get more hot than the Hammond when you run same signal into two exact same amps but only difference is one has Hammond PT and one has Heyboer PT. I let them both crank for about an hour so they got good and hot and test by hand and also by thermal probe on the top of the PT. I let them play the radio and measured both on a scope so the output signals of both amps was the same.

I have a personal amp and a few amps that friends have that have defied 6A heater current for about 2 years. Wonder if they will all fail in 10 years? or if they made it this far is it probably ok?

If the lore is to be believed Traynor custom special ran 6A heater winding at 6.9A for about 4 decades without known failures.

[Leo_Gnardo]

1. Why is Hammond 278CX 800VCt and the 6.3V winding is only 6A??? - - - - - I know this sounds strange but someone may remember. I remember reading on this forum some time a few years ago someone posting saying they called Hammond and they said 278CX is rated at 6A but can actually handle 10A heater draw fine. Anyone remember this hearsay?

Yes, the heater winding spec is worrisome on this PT. I built a 4xKT88 amp with one about 20 years ago. Its owner ran it with no problems but at one point I did measure filament voltage and it was sagging to 6.0V or less. So I added a "helper" filament transformer. Great, more weight and barely enough room to wedge it in. OTOH I sure did not look forward to possibly having to replace my customer's PT. Then I heard about Hammond's "silent approval" to run over 6 amps. OK I guess if you don't mind filament voltage sagging noticeably. I bet they'd sell a few more of these behemoth tran's if they would simply fatten the filament winding just a fraction.

BTW I had the amp in for a checkup a couple weeks ago. Runs a solid 130 watts at 100 Hz, and bass sounds very solid through it. Wallops the legs like a slightly small SVT. Then I got to hear its owner's band outdoors. He was using a single EV 15, didn't go through the PA (that was just speakers-on-sticks OK for vocals). That rig held its own at 100 feet distance, with double drummers in competition. It's nice when things work right!

[nsubulysses]

There is a note on the dwg for the heater supply that states "augmented by 167N6" to bring the current up to 9.9A. It's not stated how the augmentation is done. Shared load?

Ah I missed that. Here it is, 4A heater supply transformer. Maybe KOC assumes or calls Hammond 278CX 6.9A heater winding because of Traynor Custom Special which he seems to like a lot and reference a lot

https://www.mouser.com/ProductDetail...%2F4jevQ%3D%3D

[loudthud]

The original Traynor Custom Special used 7027 tubes which are only 0.9A each on the heaters. I used one of those amps with 6550s and the heater Voltage was below 6.3V. I had to replace the long wires from the pilot lamps to the first power tube with 16 gauge aircraft wire to get the heater Voltage up to 6.3VAC. Transformer ran a little warm, good thing it had a fan.

[nsubulysses]

I called Hammond and they were very down to talk for a bit and drop some knowledge on their transformers!

I asked if it would be worrisome to run 6A heater winding on 278CX at 7.2 or 7.5A and they said it was not worriesome at all. 10A would be pushing it and probably shouldn't be done they said.

They said they build very high quality transformers that are overdesigned and do not have a good profit margin, but it doesn't matter because transformers have never been one of the big money makers for Hammond. The reason this came up is because I asked them how is it possible I could get a Heyboer transformer designed to exact same specs as Hammond 278CX but ask for 8A heater winding instead of 6A, and the Hammond runs cooler. They said, cheaper laminations and cheaper materials in general for heyboer so the transformer has to be probably physically bigger and it runs hotter, but still works fine, but is slightly lesser quality technically. Yep, he is correct the Heyboer is a bit bigger. They assumed Heyboer, as solely a transformer manufacturer, probably makes a less overdesigned transformer with cheaper materials because they have to live off transformer sales. Whereas for Hammond it is just a small operation within a bigger operation, and they don't live off selling transformers, so the materials and safety margins are probably a bit better.

They said if you test DC resistances on the windings it is likely that Heyboers will be higher since they have more windings and a bigger transformer with poorer regulation to achieve the same thing, so I guess that also means a higher no load voltage.

They also said they will have a new website in about a week haha. so hopefully it will be easier to navigate.

[nsubulysses]

Also, like LT they said if you want a really high quality transformer it would be overspeced so it will not get very hot, but it will be twice as big and cost probably twice as much. They said the only thing that really kills a transformer is heat, so thousands of cycles of hot and cold would be limited with a way overspeced PT that does not get very hot.

[Leo_Gnardo]

The original Traynor Custom Special used 7027 tubes which are only 0.9A each on the heaters. I used one of those amps with 6550s and the heater Voltage was below 6.3V. I had to replace the long wires from the pilot lamps to the first power tube with 16 gauge aircraft wire to get the heater Voltage up to 6.3VAC. Transformer ran a little warm, good thing it had a fan.

For anyone who's building a big amp like that from scratch, or replacing a PT, run the filament leads direct to the power tubes, then you can run a pair of skinny wires to the pilot from there. Just because the major leaguers do it the half ass way doesn't mean we have to follow suit.