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Old 20th October 2009, 10:34 PM   #1
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Default 27 & 6SN7 Preamp - Crossover

This idea has been bouncing around in the back of my head for some time now.

Linkwitz alignment 12db @ 100hz

All the math has my head swimming!

Please check/suggest, etc.

Only issue now is how to elevate the 27's heaters since the followers cathode is at 120V ??

BTW, Input impedance to the amps is 100K and the 8hz HP point is intentional.
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Old 21st October 2009, 03:46 AM   #2
kevinkr is offline kevinkr  United States
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You'll need separate heater supplies and you should not elevate the filaments too much beyond what is on the cathode - also make sure that the time constant on the filament bias voltage is similar to the supply and heater time constant so that the voltage difference between them is not large. I have had 27 cathode to filament insulation failure at a differential of about 12 - 15V IIRC. One of the recommendations of the time was to use a 2.5Vct filament winding and connect the ct directly to the cathode. This might work with some resistance to isolate the shunt capacitance between the filament transformer and the cathode circuit.

You might want to consider replacing the second 27 with something else, and note that the 56 is not better in this regard. You might try a 6C5 or 6J5 type as a follower.

I use a variety of early dht and idht in my designs and they have some less than endearing shortcomings, but can sound great.
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Old 21st October 2009, 01:34 PM   #3
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Kevin,

The "older" numbered tubes do have some "shortcomings". This is my first real "design" effort using tubes like this and heater/cathode voltage tolerances this small were not on my mind.

I have decided that rather than run the risk of a catastropic failure of my (4) NOS 27's (mesh plate) I will re-design using 6SN7's.

An all Octal Preamp/Crossover sounds like a nice idea to me.

BTW, your avatar is great! See the attached 829B PP "Triode" Williamson I am building in dribs and drabs.
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File Type: jpg 829B PP Triode Output & Power Supply.jpg (84.3 KB, 829 views)
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Old 22nd October 2009, 12:36 AM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by coldcathode View Post
Kevin,

The "older" numbered tubes do have some "shortcomings". This is my first real "design" effort using tubes like this and heater/cathode voltage tolerances this small were not on my mind.

I have decided that rather than run the risk of a catastropic failure of my (4) NOS 27's (mesh plate) I will re-design using 6SN7's.

An all Octal Preamp/Crossover sounds like a nice idea to me.

BTW, your avatar is great! See the attached 829B PP "Triode" Williamson I am building in dribs and drabs.
I've always thought your avatar was pretty cool too..

The 27 mesh plates are very nice sounding, perhaps you could keep the first stage a 27 and make the rest 6SN7.. The 829 looks like a cool project.

One of the other issues with these early tubes is that they are incredibly microphonic by modern standards. I haven't had much problem with that, but have had significant issues with electrostatic pick up so I now run 26/27 and their kin in WE style shield cans - kills that problem stone dead, but unfortunately completely hides the tube. It's obvious to me now why WE shielded similar tubes in their theater amps.
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Old 22nd October 2009, 12:45 PM   #5
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Quote:
Originally Posted by kevinkr View Post
I've always thought your avatar was pretty cool too..
One of the other issues with these early tubes is that they are incredibly microphonic by modern standards. I haven't had much problem with that, but have had significant issues with electrostatic pick up so I now run 26/27 and their kin in WE style shield cans - kills that problem stone dead, but unfortunately completely hides the tube. It's obvious to me now why WE shielded similar tubes in their theater amps.
Kevin,

Your comment about "microphonics" leads me to something I wanted to try. I think it would be "cool" to place some sort of "glass bell" over the tubes. I am thinking that I could use some sort of glass "hurricane lamp" chimney. If there were large holes in the top plate of the chassis surrounding the socket the glass might "shield" the tube from vibration but still allow cooling and visibility??
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Old 22nd October 2009, 02:39 PM   #6
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Default UPDATE Starting to "Breadboard"

Kevin,

I took your advice and re-designed to include 27 direct coupled to a 6SN7 follower, 150V worked out nice. (141V on the 27's Plates)

Attached is one channel schematic and PS schematic.

Anyone please CHECK and make any suggestions. I am going to start making one of my "Wood and PC board hybrid chassis". I think I will do separate PS since I still have about 20MA's or so to "spare" for an RIAA preamp. (6SL7???)

Anyone have suggestions as to a SIMPLE RIAA preamp? Using an Octal tube?
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File Type: jpg Preamp-Xover PS.JPG (50.7 KB, 258 views)
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Old 22nd October 2009, 02:47 PM   #7
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Coldcathode,

The idea on post #1 is also an idea on my mind for long time. Did you finish and have it running yet??? I really would like to hear the result from you.
I have a pair of 18" Goldwood sitting for the open baffle project and would love to have a active crossover to fit into.

Albert
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Old 22nd October 2009, 04:07 PM   #8
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albertli,

I am starting this now. It might be a week or so until I can have it running to some extent as I need to source the resistors & caps.
I spent a lot of time on the math, etc so I should have little trouble getting it to run, SOUND will be the fun part.

I would be interested in knowing about the goldwood Open Baffle project?

Speakers are my next foray after completing the preamp and xover.
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Old 22nd October 2009, 04:09 PM   #9
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albertli,

The ULTIMATE end to all of this is an ALL TUBE, 3 way system,

The Power Company is going to love me!
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Old 24th October 2009, 04:04 PM   #10
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Compare to SS class A. I don't think you would pay more than that.

Albert
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