audio-talk

Higher forward voltage so a bit more headroom for the cascode.

Got it.

Having decided to go for a two box solution with umbilical, are there any layout issues I need to be aware of? Can the two toroidals interact?

Should be fine. Keep the heater reg and shunt reg in the same box as the phono.

Will do.

All coming together nicely now, just awaiting the arrival of a couple of cases from China.

Is there a specific value for the VDR across the mains input?

Will this be a suitable VDR?

https://www.rapidonline.com/epcos-b7221 ... or-51-2926

Yep, that will be fine.

Thanks Nick!

Nearly there. Checking the current draw through the various stages, the whole phono board is pulling about 5ma, and the 60ohm resistor is passing 31ma, so 26ma is being shunted through the gas tubes which seems about right. HT is a little high at 260v, but I guess that's down to the tolerances of the gas tubes.

I'm puzzled about the earthing arrangements. Does the 100ohm earth lift resistor go to the HT 0v rail? If so, does that mean that the 40v lift on the heater supply is also biassing the 9v windings of the transformers, and if so, does it matter?

Yes. The heater is raised wrt the valves cathode and the ground is linked to earth via the 100r. The 9v tx doesn't care.

Excellent.

'''Tis done. And, amazingly, 'twerks!









Not the neatest job in the world, but I took heart from Richard's Doc Mods recommendation for amplifiers to make sure wiring looks as bird's nest like as possible, and apart from a brief hiatus over which leg of a transistor is which (haven't worked with three-legged beasts before), it all went together pretty smoothly.

Sound-wise, I'm really pleased. Imaging, focus, pace and timing are all good, with a lovely sense of flow which was missing before, but but but....

It hums. Not loud, but enough to intrude on quiet passages. It's a 100Hz hum, totally unaffected by position or proximity to other equipment, and still present with the inputs shorted. The hum disappears totally as soon as the HT is switched off, the music gently fading away over s surprisingly long time, so I'm pretty sure it's HT ripple, although I haven't 'scoped it yet.

I don't really know how to check the function of the two HT regulating transistors. I assume they're conducting otherwise there'd be no HT, but could one or other be s/c?

I don't really know how to check the function of the two HT regulating transistors. I assume they're conducting otherwise there'd be no HT, but could one or other be s/c?

Check the voltage between their emitter and collector, and check the voltage across the 50R sense resistor.

shane wrote: ↑Mon Jul 03, 2017 7:02 pm The hum disappears totally as soon as the HT is switched off, the music gently fading away over s surprisingly long time, so I'm pretty sure it's HT ripple, although I haven't 'scoped it yet.

Could it be heater related? The heaters are raised about 40V or so aren't they to keep the cathode follower heater/cathode insulation happy innits?

DIET. Congrats on the build by the way.