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Old 5th August 2004, 07:12 PM   #1
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Default DIY scrap tubebuffer with ECC88

Hi guys n' gals!
westerday i put together a tubebuffer more or less as an experimental platform, and i could use some tips&tricks to make it sound better, i think it sounds pretty OK already, but i'm sure theres plenty of modifications that will make things even better,

here's the schematic:
http://home.swipnet.se/dice/tubemania/buffer_sch_L.jpg

and a picture of the guts:
http://home.swipnet.se/dice/tubemania/IMG_3422_L.jpg
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Old 5th August 2004, 07:41 PM   #2
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First, you need a ground reference on the tubes' grids. Take a 470K resistor from the grid to the common. Now since you've got the cathode resistors returned to common, it would probably be better to return the grid resistors to some reference voltage, somewhere in the neighborhood of 40-50% of the B+, with the cathode resistors sized to get you your desired plate current.

Second, voltage multipliers are not reputed to have great regulation. And you've got a circuit which, inherently having mediocre PS rejection, needs a stable, quiet DC rail. Splitting and regulating the filament rails isn't important at all in comparison with getting the B+ right- proper raw supply, regulation, filtering.
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Old 5th August 2004, 09:03 PM   #3
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ooooops! I forgot to mention something important- if you bias up the grid to a positive voltage as I suggest, you need to capacitively couple in so that the grid can float properly and you don't inject DC into the source.
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Old 5th August 2004, 10:57 PM   #4
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Hi,

I'm at a loss as to why this circuit would be called a buffer??

Clearly this design amplifies, quite a bit too, signal is taken off the plates so Zout will be on the high side as well.

Cheers,
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Old 5th August 2004, 11:04 PM   #5
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You're right, Frank, I looked at it much too quickly.

Please ignore my comments about biasing. But my comments about the power supply are hereby doubled.
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Old 5th August 2004, 11:16 PM   #6
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Hi,

Quote:
But my comments about the power supply are hereby doubled.
I'm more than happy to second you on that one....

Fortunately it doesn't take too much to get it started; just a powerxformer with 2*120VAC secondaries (or thereabout), a pair of FWBs, some filtering caps and resistors to make an ordinary Pi filter would already adorn the circuit with a dedicated PS per channel.

Chances are I can have something cooking along the intitial lines within a couple of hours....Way too hot to sleep anyway.

Cheers,
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Old 6th August 2004, 12:06 AM   #7
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Why not just build a simple cathode follower as a buffer? There are many examples on the web...even low voltage ones when it comes to a 6DJ8.
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Old 6th August 2004, 12:25 AM   #8
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Hi,

Quote:
There are many examples on the web...even low voltage ones when it comes to a 6DJ8.
Because this is DIY, not CIY ( copy it yourself)...

Either way, if it's a buffer that's needed then, sure there are plenty to be found even here on the forum.
If some gain is still needed, I think I posted a schematic of mine using a plate follower direct coupled to a cathode follower only a week or so ago...

Ah...Found it on my own HD:

Cheers,

P.S. Mr. Triatic, if the pic of the innards of the preamp is showing shiny plates for the Chinese ECC88/6DJ8s then they are really Chinese 6N1Ps NOT true ECC88s. This implies that the Zout of that stage is even higher than for a European ECC88 or Russian 6922.
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Old 6th August 2004, 12:37 AM   #9
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Quote:
Originally posted by fdegrove
Hi,



Because this is DIY, not CIY ( copy it yourself)...

Yup, that's what I is alright. Copy this, copy that, steal this, steal that.

Hopefully, it all comes out semi-interesting in the end.
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Old 6th August 2004, 08:57 AM   #10
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hi guys, thanks for ur help!
-yes, this is DIY, not CIY... hence all my mistakes.. 8-)

@SY-> i know that the PSU isnīt good, but i couldnt finda any suitable HT-transformer, so i thought i should try to double it six times, which gives me about 150Volt and i have a drop about 25-30 V in the PSU-filter, (since the ECC88 are spec. to 130V ?) and use the filter to reduce hum, although i canīt hear any hum in my speakers so obviously it works 8-)
The split and reg. of the filaments are due to my transformer choice, i didn't have another..8-/

@fdegrove-> yes, i guess it amplifies about 6dB or so.. i would gladly lower the amplification a bit.
yes, these tubes are Golden Dragon, i also have a pair of Billington gold, but there isn't very much that differs between them regarding sound..
--
now i changed the capacitors at the output to better ones, that helped, i also put an inductor i series with the resistor in the PSU-filter

-do you think i should pick the output signal on the cathode side instead? (like ordinary buffers..?)

i experienced some ringing/slicht echo in voices earlier, i wonder what caused these oddities?

now my amp have been "on" for about 20 hours, with no degradation of sound, without any fire or blown-up capacitors,
-thats good!

my main goal with this amp/buffer is to learn me more about construction, i do have some experience, but not as much as needed.. so even more help is truly appreciated.. 8-)

PS @fdegrove, your muting seems a little bit rough for the tube?
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