UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 120W PP EL34 amplifier re-born.

Diabolical Artificer · 27th Sep 2018, 9:42 am

"have you decoupled the input of the regulator with an 0.1uf capacitor and the output with a 1uf capacitor, as close as possible to the pins?" Yes Al.

You make some good point's there Andy, will have a look at the LD1086, it's easy to get stuck in your ways I guess and use LM317's as default.

Managed to fault the circuit repeatedly, by jumping a bigger load onto the PSU, it is probably as Chris says, the reg is going into foldback current limiting mode. I scoped the OP and got this odd "signal", hash whatever you call it.

So any sudden current demand is what is causing the "fault". Odd thing is, the tfmr winding is beefy enough and the 317 should cope with what's asked of it, so either the 317 is faulty or the circuit is wrong.

The trouble I have with the LM317 and other 3 pin regs I've used is figuring out what IP voltage to use. Too much IP voltage the reg is working too hard and can fail, too little and OP voltage drops. It's a fine balance. I'll admit to struggling to deciphering some parameter's on the datasheet.

Back to bench...Andy.

27th Sep 2018, 9:42 am	#16
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: 120W PP EL34 amplifier re-born. "have you decoupled the input of the regulator with an 0.1uf capacitor and the output with a 1uf capacitor, as close as possible to the pins?" Yes Al. You make some good point's there Andy, will have a look at the LD1086, it's easy to get stuck in your ways I guess and use LM317's as default. Managed to fault the circuit repeatedly, by jumping a bigger load onto the PSU, it is probably as Chris says, the reg is going into foldback current limiting mode. I scoped the OP and got this odd "signal", hash whatever you call it. So any sudden current demand is what is causing the "fault". Odd thing is, the tfmr winding is beefy enough and the 317 should cope with what's asked of it, so either the 317 is faulty or the circuit is wrong. The trouble I have with the LM317 and other 3 pin regs I've used is figuring out what IP voltage to use. Too much IP voltage the reg is working too hard and can fail, too little and OP voltage drops. It's a fine balance. I'll admit to struggling to deciphering some parameter's on the datasheet. Back to bench...Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far.