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Old 19th August 2002, 12:01 PM   #1
marjan is offline marjan  Slovenia
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Default 6922

I need low noise (as low as possible) 6922/ECC88/6DJ8 double triode for use in MC step up modul. Any recomendation based on expirience with this tube. Whitch one should be the best for that purpouse?
rgds Marijan
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Old 19th August 2002, 07:38 PM   #2
dice45 is offline dice45  Germany
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marjan,

to answer your question: the best and lowest-noise stuff available as new tubes seems to be the Sovtek 6922. Allen Wright uses them throughout in his preamps and reports only the best.
Forgetabout vintage NOS stuff, famous E88CC from Telefunken, Philips, Amperex and the even more fancy CCa from Siemens. They all sound slightly better than the Sovtek but are vastly inferior as far as long term noise behaviour is concerned.

Sorry in advance for my naysaying

You are bold to try that, MC step-up, do you know that?
ECC88 and family will soon turn from a new tube into a noisy tube.
Tube wizards having forgotten more that i ever will know failed in doing unless they paralleled ECC88 section, many of them.
I abandoned this plan and decided to use SS devices or better, input transformers.
Jensen JT-346-AX to be specific.

Point is: tubes have both thermal noise and shot noise and the shot noise is going to drive you crazy. S/N ratio of single tube sections is too bad for MC signal level: the signal partially drowns in the noise floor. If you parallel sections, this gets a bit better but you sacrifice detail resolution, so much that you better switch to MM cartridges. Reason for the MC cartridge is detail resolution and tone colour distinctability and saturation and exactly that you sacrifice by paralleling sections.

I have some planar ceramic triodes in my collection, G_m about 12mA/V, about 115. If i have much time surplus, i will try if the do the MC job. But i seriously doubt it.
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Old 20th August 2002, 12:01 AM   #3
lohk is offline lohk  Europe
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Marjan, Bernhard,

I do not want to highjack that thread - but it seems the right place right now:
Do you - or anybody else - have a schematic (or a link) where I can use a single E88CC in a two channel preamp.
I just made the passive part of a pulteq eq and I want a simple amp stage with tubes for it.

greetings
Klaus
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Old 20th August 2002, 05:59 AM   #4
marjan is offline marjan  Slovenia
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Bernhard
Thank you for your sugestion regarding choice of the tube and certanly for sugestion regarding MC step up solution. I tried Lundahl 9026 and Ortofon TO-20 transformers, old PS audio solid state MC preamp (sounded the best), sort of Hiraga`s mc stepup with unsuficient powersuply..., and I was not satisfate. Combination of those devices with my Paravichini`s diy EAR835 riaa and my Ortofon mc25 on Kuzma turntable, simse to fall in reproduction. I was hapier with my old Grado entry level prestige on the same setup.
I`m shure that you are familiar with the design but anyway, this is what I`m trying to build (see Hampton schematic). It does use paralleled sections, design is extremly simple and what I was able to found about this preamp was encourageing. I`m aware of the problems I`ll hawe with the noise but I`ll take a risk and try.
Thank`s again!
rgds Marijan
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Old 20th August 2002, 06:04 AM   #5
marjan is offline marjan  Slovenia
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And the PS
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Old 20th August 2002, 06:29 AM   #6
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Boy did I see this schematic before?!

Marjan, If I'm not mistaken it uses a 200V B+?

I had this Hampton MC amp bookmarked but lost it last month. I think the web URL somehow did not serve anymore.

I have been using a similarly designed MC pre for the past 2+ years. It has a B+ of 24V. paralelled TFK 6DJ8's. The result has been satisfactory. With my MC Gold 0.4mV output, the loading resistors are 110 R. Noise has never been an issue. Neither was tube life.

Some say the 24V B+ is too low and not optimal for second order harmonic distortion and I should use 55V - I haven't had a chance to try yet. Hampton's schematic is worthy for a try and if you have a variac perhaps you would adjust to a 55V B+ and let us know your results.

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Old 20th August 2002, 08:57 AM   #7
marjan is offline marjan  Slovenia
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This is original hampton suply. Basicly the same, changed in the heater dc section. B+ is the same 84V as on modification. I`ll try it on friend`s variac anyway. TFK diamond or normal?
BTW I have few toroids at home.What do you think about using separated transformers for plate and heater Planing to use 100V/150VA and 12V/60VA
rgds Marijan
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Old 20th August 2002, 04:31 PM   #8
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The new heater supply uses a LM317/LT317? That should be better than the 7806 alone. Don't forget to heatsink it.

I would use Scottkies diodes in both B+ and heater supply.

For "experiments" with a variac you have to use 2 transformers. Good luck and let us know your results.

Steve
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Old 21st August 2002, 12:45 AM   #9
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Lightbulb MC Stepup using 6dj8

Sorry I spotted your problem this late.
I use a circuit that may just do the trick.
Basically I don't like to use transformers at this very critical frontend since any flaws will be amplified througout the audio chain.
Here is what you can do :

- 2 x 6dj8 in parallel per channel (dual mono) at 24 vdc regulated with a 7824 ic.
- regulate the 12 vdc heater supply with a 7812 ( 2 x 6 vdc)

- use a fet such as BF 244A as a current source for the anode current regulation ( or similar fet, idss must be the same ).

-cathodes are grounded.

-signal is taken from the anodes.

I used MIT metalized polysterenes since I could not find any colourations.(Metalized teflon is best on paper here but was not commercially available at the time)
Keep in mind to keep the signal path as short as feasable.

This sounds very well,noise is no problem unless perhaps if you'd
use ridiculously low output MC's.
Low level information is excellent provided the rest of the system is up to it.Transients are lightning fast,I use OTL amps so nothing there in the way of ringing or overshoot.
Frequency response is deliberately limited from 16 Hz to 1MHz in order not to burn the woofer coils.

The cartridge is not loaded and no input cap is used for ultimate transparancy.I used a Jan Allaerts MC hand built cartridge but others may require loading.Anyway that's simple enough to solve.

Best rgds,

Frank De Grove.
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Old 21st August 2002, 07:09 AM   #10
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Cool!

The schematic I have was given by a friend and I had it for years. Anyway I think its not really complicated to build - routing of power supply hookups and grounding would be important in deciding the quietness of the amp.

Mine uses a 2SK30A ( 'GR" is said to sound best but I can only find 'BK') as current regulator. Other areas are identical except the ouput coupling has a total cap value of 10u and a 1M ohm output loading resistor.

The 24V was provided by a 25V EI transformer/scottkies bridge and a single LT317 adjusted to the voltage.

How would a cartridge work 'without a loading resistor'?

TFK 6dj8 (<>)has been my choice. I have tried a pair of sovtek 6922 and 6H23p and don't like them personally.
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