Using two PTs for HT supply

[andrew_k]

Background
I'm about to finish off a twin KT77/KT88/6550 amp (AX84 October Stage w/ SS bridge recto), the perfect time to consider a major redesign to the power supply! hehe

My current PT has plenty of heater current but only a 310V@250mA HT supply (no CT), offering only ~390v of rectified DC once under load (310*1.4*0.9) which isn't adequate to take advantage of the big tubes or OT (5K/100W Hammond). The PT was very cheap, I can't afford to buy a more suitable PT, but I do have a salvaged PT with a 270-0-270 HT.

To the point...
Rough calcs-
540v HT supply = ~680 DC under load (~756V max)
310 HT supply = ~390v DC under load. (~434V max)
I want to use one PT to provide the HT for the plates and the other for screens + PI + preamp. Are there any design considerations I should be aware of when using two PTs in one amp?

Can they share a single fuse? If I was going to use a 1A fuse on each, would combining them on a 2A fuse be safe?

The 310V HT supply has 450V filter caps already and I'll use 2 450V caps in series for the 540V HT supply, but what else should I be aware of? I think an HT fuse is possibly worthwhile for the plate supply - 500mA slow blow?

I'm honestly studying theory daily and trying to answer my own questions but you can't learn everything overnight. Thanks for being understanding of a newb with more enthusiasm than education

[Steve Conner]

What is the power rating of your 270-0-270? You can usually reckon about 100VA per kilogram of weight. So, if it weighs less than about 2kg, I'd expect it to burn out under the strain of cranked 6550s. Of course if you know the current rating of the HT winding that's even better: for a 100W amp it should be around 400-500mA.

You probably want just one fuse for both transformers. If the plate supply fuse blew but the screen supply kept going, it would burn out the tubes' screen grids.

Having said that, a nice feature would be a switch to select running the plate supply off either 400V or 700V. This would act as a 1/3 power switch for situations where you want to crank it but the full 100W is just too much. Or viewed the other way, a 100W turbo boost for the 30W amp you've just built :-)

With a little more thought, maybe you could add a couple of diodes in the rails so that if the plate supply blows up, the power tubes keep running off the 400V rail instead of losing plate voltage altogether and burning out their screens.

[andrew_k]

Thanks for your reply Steve.
I knew nothing about the 270-0-270-equipped PT but I had a hunch it was made by Trimax so I went searching and eventually tracked down the details in their 1964 brochure -- even though this PT weighs 2.2kg and has a 3A 6.3v winding the HT is only 60mA. It's a PT from a valve tape machine, never intended to run a power amp.
Drats.

I'll finish this build with the other PT for now. In a month or so I should be receiving a gigantic old tube 'scope in unknown condition. It may end up donating it's PT to this amp depending on various factors.

Thanks for your points about switching and protection, I will take them on board.