Another 4 (nearly 5) months pass but I've had a concerted effort the last couple of evenings since Owston and we have ignition!
The voltages look to check out okay from the quick look I had before I made tea. Something was smelling rather hot though so close scrutiny is required. I've only hooked it up to cheap speakers to test but it makes the right sort of noises which is encouraging .
At least one of the transformers buzzes a bit though, might be the filament transformers, and I can feel vibration in the first choke of the LCLC power supply. I haven't snubbed it yet but would it be worth trying a 100nF cap in front of it?
I don't know what it sounds like yet but I have to say I could fall in love with GM70s just for how they look. Just a shame my energy consumption meter was reading 370W!
Currently reading 0 notes per minute as they're not hooked up to anything at the moment. I couldn't resist them when I saw them, I thought they might go well with the GM70s .
SteveTheShadow wrote:Good stuff Simon.
Ought to be boxed up and ready by the next Owston then?
Now that's very funny Steve . When was the last time I put something in a box? I have had severe words with myself since the last Owston and I'm full of good intention but somehow I can never bring myself to say yep, it's finished. I will redouble my efforts.
Yep it's 12GN7 very close to Nicholas Chua's circuit. Hearing Andrew's amp last Owston was good enough for me. I've only got 480V across the B.c. and I'd like a bit more really, but I'm working with the iron I have.
The raw DC supply is choke input as per Rod's design. Hammond 185something trafo into 0.0025R cap choke cap. The amp hasn't been on long enough yet to know hot the heatsinks fare. I haven't even set the voltage with HT applied yet. Something was getting rather warm though, might just be the vitreous enamel resistor breaking in. I hope.
Last edited by simon on Thu Nov 14, 2013 5:57 pm, edited 1 time in total.
Aye, Steve makes it sound like that was a good thing .
I've snubbed the first choke with a couple of 220nF caps but it hasn't made any difference to the vibration. Not too keen on that, just doesn't feel good to me.
I can report thought that after about an hour the heatsinks are only a little warm to the touch. I think I've got a better idea where the hot smell might be coming from. The GM70s give off a surprising amount of heat, but more concerning are the dropper resistors to the driver decoupling cap. I've mis-rated them - there's 8W through them and they're only 14W rated. Good job it's on a breadboard. Really they should have been paralleled 30k 14W. I could use a couple of 0A2s but goes against what I'd like to keep as essentially a simple amp.
So how does it sound? I've only heard it on the cheap bookshelf speakers upstairs so far which are fairly mid-fi but it does seem to be rather beguiling with a pleasant ease. So far so good.
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. When was the last time I put something in a box? I have had severe words with myself since the last Owston and I'm full of good intention but somehow I can never bring myself to say yep, it's finished. I will redouble my efforts.
