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, and to share the valve halves by channel
rather than by function?
What's going on with the connection of the concertina's
cathode load to the cathode of the voltage amp, rather than
to ground? How well is the splitter balanced, given the
different total loads?
I can't see how the output valves' bias is adjusted.
Why do Zobels go to anode supply, rather than to screens? I
have read quite a lot of waffle around this subject but
nothing very convincing. What, exactly, are those Zobels
for?
Using a resistor in series with headphones isn't ideal, to
my mind. 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.
Nice pictures. Speakers are beautifully done. Amp looks
functional. Valves look a bit gloomy. Transformers look
cheerful but very small. How's your bass?
Ian
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