PT Rating

[Blackburn]

I'm curious what most of you guys here that build EL84 quad power sections (AC30/Rocket/Liverpool) have your PTs spec'd at on the B+ winding. I see that 250ma is typical and what some are rated at, such as Edcor. Mercury Magnetics are at 300ma. I'm having another PT made by Heyboer to replace one that isn't working out and have spec'd it at 250ma, but I'm sure there's enough time to call back and knock it up to 300ma. Liverpool and AC30 circuits are biased real hot and are too close for comfort for me, so that's why I ask. What do you guys think?

David

[Firestorm]

260mA max in Class A, so it's probably fine either way. But why not call if you can bump it and be nice to the PT.

[matt h]

(deleted)

[martin manning]

I'm going to say that at typical EL84 plate voltage (300) and load impedance (4k), you don't really need more than about 175 mA.

[Blackburn]

Thanks for the insight, guys. I'm assuming a plate load around 320v, or so. The OT is an original Dynaco A-470. I was hoping to leave it at 250ma, because going up to 300ma would mean larger wire and possibly a larger footprint, so I was told anyway. I really need to keep the footprint the same. The amp's built and I don't fancy drilling new holes in a chassis with everything in it...

[pdf64]

The early AC30 PT B+ spec seems to be 280-0-280 at 160mA, see http://www.webphix.com/schematic%20heav ... 301960.pdf
It seems rather low but despite being stuck in that oven of a cab, plenty are still working 5 decades later.
Pete

[tubeswell]

The Vox AC30 1960 schematic indicates that there is 12.5V dropped across the shared 50R RK under a 30W output condition. Thats 250mA for the quad of EL84s alone (not counting the ~10mA for all the pre-amp tubes). I'd say that the old PT ratings were conservative.

[Blackburn]

The Vox AC30 1960 schematic indicates that there is 12.5V dropped across the shared 50R RK under a 30W output condition. Thats 250mA for the quad of EL84s alone (not counting the ~10mA for all the pre-amp tubes). I'd say that the old PT ratings were conservative.

This is part of the reason I asked about the rating of PTs in amps like AC30s. My numbers show pretty close to spec, but most PTs rated at 250ma seem to do fine. I'm not planning on voltages as high as old ACs, more like Rockets. I hope Heyboer gets this next one right.

[martin manning]

The original question referred to Rocket, Liverpool, and AC30. These are two apples and an orange, IMO.

Rocket cathode voltage is at 8.2 per the chart with a 50R, so that would be 164 mA for the power tubes at idle. Liverpool is around 10V but with 2x 130R, so about 154.

AC 30 is a different animal, with higher plate voltage and current. The 1960 schematic shows 10V at idle on the cathodes through a 50R, so 200 mA. The schematic has "160 m/a" written next to the PT secondary, which makes no sense unless it applies to each half of the winding, for a total of 320. Not sure how they get away with 50 mA idle current through each tube at somewhere around 360V on the plates. That would have them idling at somewhere around 125% of max dissipation. For one data point, MC PT for AC30 is rated at 270mA.

[Blackburn]

Thanks, Martin. Kinda got sidetracked on Vox for a sec and forgot about the TWs. The spec of 160ma on the AC30 schem had me scratching my head. What you said makes a lot more sense. I prefer my plates low, around 300v. I plan on doing a Pool soon and am expecting 335v, or so, and that's the max I'm planning on ever running 84s. The new one from Heyboer may very well be the one I use for the Pool and will be rated at 250ma. Good to know about the MC tranny. I have an Edcor that will suit the lower side very well with a GZ34.

[Firestorm]

Haven't actual AC30s been known to catch fire? Says something about the power (and the design). O'Connor goes on about this: 245mA being necessary to get 30W into a 4K load, bypass cap on the output allowing additional current for the driven side, etc. Without reopening the Class A/not Class A debate, none of these designs will stay in Class A when we push them hard. Apparently we like the sound. But it suggests that transformer specs should not be marginal. I'd rather have a PT that could deliver more than I'd ever use, rather than being "close" under extreme demand.

[Blackburn]

They certainly have!

I agree about overspec'ing things when able, David. This particular Heyboer PT will be 250ma because I need the same footprint as the one they screwed up. I don't dime these things, so it isn't a concern for me, since this is a personal amp. In the hands of someone else however, that's another story.