On Wed, 07 May 2008 23:14:21 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>On Wed, 07 May 2008 15:12:27 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>
>>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.
>
>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.
>
Btw, since I got the server back up and added the 6AW8 stuff I went
back in today, cleaned up the rest a bit, and added pics for the
earlier 'low voltage' 6GM8 hybrid PC Speaker Amp I built a while back
and the 12AX7-6BQ5/6GK6 PP amp.
>>>Thanks, Ian
>>>>
>>>
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