audio-talk

Ray P wrote: ↑Tue Jan 01, 2019 5:19 pm I see the amp has acquired a small label, screwed down to the front edge of the breadboard?

Yes, it now has a broadcast receiving licence plate.

There's another couple of tricky aspects about fuses I forgot to mention. One is that the blow current is usually double the rated (printed) current. So your 500mA will need 1A to break them. Or thereabouts.

EXCEPT, when high ambient heat derates the fuse so if a fuse keeps blowing while it's supposed to be well within limits, it' because it's filament has been heated and weakened. Guess how I know.

Used to have this problem in enclosed chassis which are heated by transformers etc. The simple answer is double the rating. But then you wonder what the blow current actually is.

Thanks for that info Mike. I reckon any short or arcing will try to draw a lot more than one amp, so I would think the half amp rating is probably right for this application. So going by your info, at least I shouldn't get nuisance blowing.

Well I'm now bereft of amp.
It has now left the living room and is awaiting its time to be put into a box.
I think I've taken it as far as I can, so next time it appears it'll be the boxing up bit.
I've now got the computer back online, so I'll be in a better position to do the schematic before the weekend.

Meanwhile, there's Ray's build to watch.

Yep, fuses have a i squared t measure. A T fuse is rated to take 100% of its rated current for ever (though thats only the IEC rating, the UL rating is 75%), above that, its time and current that matters. THis graph shows that a T fuse will take 3 times its rated current for 1 second, and 2 times for 100 seconds. But there is also the repeated use characteristic, so the time will add up as the fuse weakens.



A T fuse will withstand 10 times its rated current as a inrush.

More info here

https://www.schurter.com/content/downlo ... ection.pdf

Cressy Snr wrote: ↑Wed Jan 02, 2019 3:47 pm Meanwhile, there's Ray's build to watch.

No pressure then!

Cressy Snr wrote: ↑Wed Jan 02, 2019 3:47 pm I've now got the computer back online, so I'll be in a better position to do the schematic before the weekend.

Steve, to help with my peace of mind, can you label/number the pins for the 13E1 when you update your schematic. Cheers

Ray P wrote: Steve, to help with my peace of mind, can you label/number the pins for the 13E1 when you update your schematic.

Cheers.

Audio act schematic with pin numbers for 13E1:

Probably not the time for a question like this but I'll ask anyway. Prolly covered previously tho I don't remember. Why is the driver biased with less current than the input?

izzy wizzy wrote: ↑Fri Jan 04, 2019 3:20 pm Probably not the time for a question like this but I'll ask anyway. Prolly covered previously tho I don't remember. Why is the driver biased with less current than the input?

I like the sound with it like that.

Good enough reason ; but actually they're almost the same, aren't they ?
Do you also need more grid-cathode headroom on the driver ? - is the drive level quite a lot, before feedback ?

IslandPink wrote: ↑Fri Jan 04, 2019 5:09 pm Good enough reason ; but actually they're almost the same, aren't they ?
Do you also need more grid-cathode headroom on the driver ? - is the drive level quite a lot, before feedback ?

Yep the drive level has to be high before feedback. The drive level I have is the best compromise I could squeeze out of it, whilst keeping distortion down. There was a lot of wrangling and nudging things to get the amp to sound as good as it does.
Wolfgang and me discussed this a lot if one cares to look back at the thread.

Cressy Snr wrote: ↑Fri Jan 04, 2019 4:42 pm I like the sound with it like that.

Thanks.
Excellent. The 5687 is a great sounding valve.
Look forward to hearing it at Owston if you and it are going.

Cheers,
Stephen

Yep the drive level has to be high before feedback.

Short question: Why

Secondary Question: Its normally gain that is needed, rather than the ability to swing voltages it wont be swinging. Feedback will reduce the voltage swing, so does it have to be there in the first place.

Longer Question: I would love to see what that amp works like by looking at the voltage level into the output valve with a load on the amp. I can only assume the grid and cathode move a long way apart, but it would be nice to see it

Nick wrote: ↑Fri Jan 04, 2019 8:13 pm
Secondary Question: Its normally gain that is needed, rather than the ability to swing voltages it wont be swinging. Feedback will reduce the voltage swing, so does it have to be there in the first place.

Maybe I was getting confused with open-loop gain.

Nick wrote: ↑Fri Jan 04, 2019 8:13 pm

Yep the drive level has to be high before feedback.

Short question: Why

Secondary Question: Its normally gain that is needed, rather than the ability to swing voltages it wont be swinging. Feedback will reduce the voltage swing, so does it have to be there in the first place.

I'm only going on what Bruce Rozenblit says, as I understand it, in his books with the OTLs in as projects.

I don't seem to see all these nuanced terms that are obvious to others. I'm very, very literal and figure that you need more swing in the first place so that there is sufficient in reserve for the feedback to bring it back down again so the amp amplifies to the degree you want it to. For me voltage swing and gain are interchangeable (but apparently not) To the way I think, big swing resulting from tiny swing = high gain. Only slightly bigger swing from tiny swing = low gain. That's how I see it.