On Wed, 7 May 2008 19:13:08 +0100, "Ian Iveson"
<IanIveson.home@[EMAIL PROTECTED]
> wrote:
>Flipper said:
>
>> That's the latest project I finished, a little 'PC
>> Speaker' Tube
>> amplifier using, per channel, a pair of 6AW8s PP into 20k
>> OPTs. Does
>> between 3 and 4 Watts Po per channel.
>>
>> Similar to my mini guitar amp it's powered off a 12VAC
>> (40VA in this
>> case) wallwart with a reverse 20VA tranny inside
>> generating about 240V
>> (loaded) for B+. Screens are at 200V.
>>
>> PS filtering is 33uF --> OPT common --> 4.7k 33uF --->
>> screens --->
>> 10k 33uF ---> Preamp. Hum is only detectable if you stick
>> your ear on
>> the cones but I'm thinking about increasing the primary
>> filter to 47uF
>> or 100 uF.
>>
>> Idle is 10mA and, as has become my common practice with
>> cathode bias,
>> I use a bypassed current mirror to keep a tight DC balance
>> through the
>> OPT.
>
>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 seperate 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.
>Thanks, Ian
>>
>
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