A2 experiments.

thomas · Post by **thomas** » Sat Jan 12, 2013 11:13 am

Oooh that looks lurvely! Large hunk of graphite, top cap and its thoriated!....

pre65 · Post by **pre65** » Sat Jan 12, 2013 11:18 am

thomas wrote:Oooh that looks lurvely! Large hunk of graphite, top cap and its thoriated!....

Yep, and when I looked on e-bay they were neither cheap nor plentiful.

Dave the bass · Post by **Dave the bass** » Sat Jan 12, 2013 11:24 am

They do come up cheap as singles occasionally but the trove of them I have has been amassed over a good few years TBH Phil. Yeah, they look pricier now. Meh! I don't think they're lusted after that much.

DTB

andrew Ivimey · Post by **andrew Ivimey** » Sat Jan 12, 2013 11:45 am

A cursory glance at the spec sheet foprces me to say, 'Why not - onwards indeed' partic. if you have manage to get them cheap.

just thinking a tad about grid bias ... have you seen the pages from Sushido, how and what he does in the bias dept of various valves? Something to be gleaned from that p'rapps.

Dave the bass · Post by **Dave the bass** » Sat Jan 12, 2013 1:24 pm

No I haven't MrI but I will, ta for the pointer.

Onwoodz.

DTB

IslandPink · Post by **IslandPink** » Sat Jan 12, 2013 4:39 pm

Dave the bass wrote: http://scottbecker.net/tube/sheets/049/8/830B.pdf
DTB

Hefty !
What kind of volts can you feed it, Dave ?

Dave the bass · Post by **Dave the bass** » Sun Jan 13, 2013 12:07 pm

Sorry for the delay Mark, well, before I breakdown The Didcot I thought I'd do a bit of note making and very basic measurement.

Output across my 8R dummy load...

and P-P swing across the grid to cathode of the TZ40....

I think I've calculated the power output to be approx 9W into an 8R load driven to just before clipping. I also think there's a lot of 2nd harmonic distortion there because either half of the sine isn't the same.

I can swing 54V pk-pk into the grid of the TZ40 before the driver stage clips. I also think thats where some of the 2nd harmonic distortion is coming from, not too sure, I'm still learning here.

DTB

Mike H · Post by **Mike H** » Sun Jan 13, 2013 1:57 pm

Yep is compressing on one half cycle. I also notice the driver is doing it too, maybe all of it, going by the polarity I presume the compressed peaks correspond to large positive anode Volts, large negative grid Volts and lowest anode current.

Of the driver that is.

Dave the bass · Post by **Dave the bass** » Wed Jan 16, 2013 9:05 pm

andrew Ivimey wrote: just thinking a tad about grid bias ... have you seen the pages from Sushido, how and what he does in the bias dept of various valves? Something to be gleaned from that p'rapps.

I've been swotting up on different ways designers introduce and generate +ve bias MrI, ta for pointing me to Shishido.

http://lagarto-ex-infoseek.jp/shishido/801a.png

He uses a 317 here to regulate the +ve bias voltage, but another thing thats interesting is the primary impedance of the o/p TX, thats very low for an 801 isn't it?

DTB

Post by **Nick** » Wed Jan 16, 2013 9:41 pm

but another thing thats interesting is the primary impedance of the o/p TX, thats very low for an 801 isn't it?

You weill find when you start drawing A2 load lines, that lower primary impedance gets you into a better place as long as you can cleanly swing the +ve signal.

Dave the bass · Post by **Dave the bass** » Wed Jan 16, 2013 10:01 pm

Nick wrote: You weill find when you start drawing A2 load lines, that lower primary impedance gets you into a better place as long as you can cleanly swing the +ve signal.

Wow wee, so much to learn here, I didn't know that Nick. Ta's.

When you say "as long as you can cleanly swing the +ve signal" thats when the XC of the O/put valve starts placing big demands on the driver/driving circuitry yes?

DTB

andrew Ivimey · Post by **andrew Ivimey** » Wed Jan 16, 2013 11:19 pm

It don't mean a thing if it ain't got the swing.

I take this to mean, build it and have a listen, keep your nose peeled incase something smells like its frying ... but if it sounds good!

I went to an amazing lecture yesterday where digital processing was shown to give more clarity to the hearing impaired but that was fine for speech. With such processing harmonics got crunched so listening to music through these processors which are built into the best and latest hearing aids makes musical appreciation horrible as what was even harmonics (typically 2nd) gets trashed into odd harmonics (3rd and fifth) - total yuk city! It was a very articulate and funny presentation. The cheese and wine afterward (we're talking Cambridge University here oooo-er) made it all seem very worthwhile, Larks.

Then Anna took me for a splash up meal at Brown's - this is the way to go.

Post by **Nick** » Wed Jan 16, 2013 11:27 pm

Dave the bass wrote:
Nick wrote: You weill find when you start drawing A2 load lines, that lower primary impedance gets you into a better place as long as you can cleanly swing the +ve signal.
Wow wee, so much to learn here, I didn't know that Nick. Ta's.

When you say "as long as you can cleanly swing the +ve signal" thats when the XC of the O/put valve starts placing big demands on the driver/driving circuitry yes?

DTB

Yes, but its not XC, remember in A2 the grid becomes an anode.

Dave the bass · Post by **Dave the bass** » Thu Jan 17, 2013 8:02 am

Aaah, OK, from a thread years ago about the phrase 'difficult to drive' I remembered capacitive reactance reared its head and thought that was at play here too.

So in A2 with the grid become anode like and current flowing is it the impedance of the grid requiring a driving power (both voltage and current drive) that makes driving in A2 a bigger issue here then Nick?

DTB

Post by **Nick** » Thu Jan 17, 2013 8:32 am

Dave the bass wrote:Aaah, OK, from a thread years ago about the phrase 'difficult to drive' I remembered capacitive reactance reared its head and thought that was at play here too.

So in A2 with the grid become anode like and current flowing is it the impedance of the grid requiring a driving power (both voltage and current drive) that makes driving in A2 a bigger issue here then Nick?

DTB

Yes, thats THE big issue for A2. Also, when the grid goes -ve (if it does) it goes back to being a grid again, so the driver stage has to cope with both conditions. Hence the need for low impedance,

One way is to drive the cathode. Its still low impedance, but its a more consistent low impedance.

audio-talk

A2 experiments.

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