On Fri, 9 May 2008 00:06:50 +0100, "Ian Iveson"
<IanIveson.home@[EMAIL PROTECTED]
> wrote:
>flipper wrote:
>
>>>>Is that one mirror or two?
>>>
>>>One per channel. One per tube tubes but two in total.
>>>
>>>> I have wondered here in the past
>>>>whether it's best to use one of the valves as a
>>>>reference,
>>>
>>>It has an interesting effect, one bad and one good, sort
>>>of. If the
>>>tube on Rk is leaky then the current mirror drags the
>>>other with it.
>>>On the other hand, if the leaky one is on the 'controlled'
>>>side the
>>>current mirror adjusts so it works.
>>>
>>>Ran into that on the first one I did, using 6EM7s.
>>>Unbeknownst to me
>>>one of them was leaky but, by happenstance, it was on the
>>>'controlled'
>>>side and things appeared fine. But in the process of
>>>making changes,
>>>the tubes got swapped and idle current went through the
>>>roof on both.
>>>
>>>>or to use a separate SS reference and two mirrors.
>>>
>>>Well, that's separate current sources, the mirror just
>>>being how you
>>>get two current sources, and bias pulls apart more quickly
>>>that way
>>>because the CCS is fixed and 'insistent'. With this
>>>approach the Rk
>>>side is as soft as the Rk, meaning it will 'allow' a
>>>current increase,
>>>and as bias ****fts up from the increased current the
>>>mirror expects
>>>more current too. So the mirror would lower bias if not
>>>for that tube
>>>also conducting more, like the Rk side, and bias doesn't
>>>change as
>>>much (on either side) as it would with an obstinate CCS
>>>insisting that
>>>idle is 'it' and no more.
>>>
>>>
>>>>> Preamp uses a common Rk PFB setup between the gain
>>>>> triode
>>>>> and
>>>>> concertina phase splitter for a gain of about 140. The
>>>>> amp
>>>>> is then
>>>>> closed with 18dB of.global NFB.
>>>>>
>>>>> I used a 1/4 inch jack for the speaker connection to
>>>>> load
>>>>> switch
>>>>> internal 10W sand block dummies, so the amp is never
>>>>> unloaded, and,
>>>>> more to the point, the dummies provide a load when
>>>>> using
>>>>> headphones.
>>>>
>>>>How does that work out SPL-wise? If you switch from
>>>>speakers
>>>>to 'phones is the apparent volume similar?
>>>
>>>Depends on the headphones but they're not straight on the
>>>dummy load.
>>>I use a series R, like normal, and you pick it for 'best
>>>fit'.
>>>
>>>
>>>>> Might not be a monster amp but it's cute, compact, and
>>>>> fun
>>>>> as all get
>>>>> out. The whole ball of wax, sans the wallwart of
>>>>> course,
>>>>> fits in a
>>>>> 5"x7" chassis.
>>>>>
>>>>> For speakers I worked up a 300cu ****ted enclosure
>>>>> wrapped
>>>>> around the
>>>>> MCM 1853 4 inch driver. They seem to have a response
>>>>> peak
>>>>> around 2kHz
>>>>> but rather then smooth that out with passives in the
>>>>> cab I
>>>>> just use
>>>>> the PC's equalizer and they sound amazingly good for 14
>>>>> buck cheapies.
>>>>>
>>>>>
>>>>> Darndest thing I ever saw, not that I've seen
>>>>> 'everything', but one of
>>>>> the 6AW8s I got in a batch has a heater short. Not to
>>>>> cathode, or any
>>>>> other pin, the heater itself. She pulls heavy current
>>>>> but
>>>>> won't light,
>>>>> as if there's solid wire between the pins. Can't
>>>>> complain
>>>>> too much, I
>>>>> guess, what with that batch being 50 cents each. Just
>>>>> wish
>>>>> he'd have
>>>>> had more of them.
>>>>>
>>>>> I'll post pics in alt.binaries.schematics.electronic if
>>>>> folks would
>>>>> like to see it.
>>>>
>>>>Circuit would be nice too. Don't you have any web space?
>>>>Easier for us.
>>>
>>>Well, I do but it's all screwed up right now. I'll look at
>>>it tonight
>>>and see if something can be made to work.
>>
>> Ok, I found the problem. I don't have the MAC address for
>> that
>> server's NIC and for some inexplicable reason the DHCP
>> server decided
>> to 'move' it so the firewall rules didn't route to it any
>> more.
>>
>> Anyway, that should be back up, assuming the server
>> doesn't crash, and
>> I moved the schematic and some pics over to it.
>>
>> Web page is http://flipperhome.dyndns.org
The 6AW8 amp is
>> the last one
>> in the projects list.
>>
>> I did notice one thing, though. I'm not sure that
>> particular zoble is
>> what I finally ended up with and same with the headphone
>> resistor.
>> Those were my "start with something" values. But that's
>> minor tweaking
>> stuff.
>
>Thanks. One or two details might be worth discussing; I'm
>always interested to know how ppl make decisions.
>
>Why did you decide not to direct-couple the voltage amp to
>the concertina,
I've done it both ways, and a third (bootstrap) but that's a different
story.
DC coupled the plate sets the concertina operating point, and tube
variation causes that to ****ft, but it's independent when AC coupled.
> and to share the valve halves by channel
>rather than by function?
Well, the tubes are 'by channel', all right, but I have no idea what
alternate "by function" arrangement you could mean. What would be the
point of, say, using V1's triode in the other channel?
Having said that, unless there is some compelling reason to do
otherwise, I just naturally arrange by channel. Wiring is usually more
direct and it reduces the odds of crosstalk. Probably of no
consequence in a power amp but it's a habit.
>What's going on with the connection of the concertina's
>cathode load to the cathode of the voltage amp, rather than
>to ground?
As I mentioned in the brief description, that's positive feedback and
is how it gets AV 140 from an amplification factor 70 triode.
> How well is the splitter balanced, given the
>different total loads?
The 'imbalance' is less than typical component tolerances of the load
resistors themselves.
It's also opposite to the direction you probably think because the
100k/1,2k junction is not DC, it is a small signal in anti phase. So,
increasing the *lower* load to around 102k gives (in theory anyway)
'perfect' balance but, of course, the output tubes aren't 'perfectly
identical' anyway so, in practice, the miniscule PS 'imbalance' is
negligible.
>I can't see how the output valves' bias is adjusted.
One does not normally 'adjust' cathode bias. In fact, that's why I
used cathode bias, "no user adjustments required."
>Why do Zobels go to anode supply, rather than to screens?
I don't know why you'd take them to the screens. Strikes me that might
be some kind of feedback, if there's a screen stopper, but not a
'zobel'. Well, come to think of it, if screen is tied to B+ (well,
after the filter) then some folks may have done it that way as a
convenient tie point. Hmm, that might solve a layout problem I've got
on another amp.
> I
>have read quite a lot of waffle around this subject but
>nothing very convincing. What, exactly, are those Zobels
>for?
They compensate for OPT leakage by giving the anodes a decent load at
HF.
>Using a resistor in series with headphones isn't ideal, to
>my mind.
Well, it's pretty common technique with power amps and I didn't dream
that one up, just copied it.
It also has the advantage of at least partially compensating for
different headphone impedances since drop across the series R is more
with low impedance headphones and less as the HP impedance increases.
Not quite so effective with only 3 watts Po to begin with but it
'helps'.
> My opinion is that a head amp should be have an
>output impedance of no more than 5 ohms, or one tenth of the
>'phones' impedance.
How did you arrive at that?
>Nice pictures.
Thank you.
> Speakers are beautifully done.
Does my heart good to hear you say that because I'm not 'super'
pleased with them. They're fair enough considering they were built
with junk white wood but, then, they were 'concept' speakers and I
didn't feel like forking over money for something exotic only to maybe
discover the idea didn't work.
To be honest, they look worse in real life because I had the lumber
yard cut them, thinking their laser guided saw could do better than my
fumble fingers, but the cuts are poster boys for how not to do it.
The backs look worse than the fronts because I lined them up best I
could on the 'working end' at the expense or the rear
> Amp looks
>functional. Valves look a bit gloomy.
What? You want a picture with them lit up?
Gee, and I went to all that trouble selecting Sylvanias so the tube
labels would color match the OPTs. LOL. Just kidding. I had no idea
what they were and the only question I asked when I saw 50 cents was
"how many have you got?" I'd have had something to think about if he'd
said 2,000 but 9 was a no brainer.
> Transformers look
>cheerful but very small.
Well, they're 5 watt for a 3 watt amp.
> How's your bass?
Through 4 inch drivers?
Surprisingly good, actually, and it's improved a bit with playing as,
I presume, they loosen up.
>Ian
>
>
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