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Old 16th March 2005, 07:17 PM   #1
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Default line-stage preamp tubes? which ones to use...

I'm thinking about building a preamp and have several tubes on hand which I would like to use. I have some 12AU7, 12AX7, 12BH7A, 6SN7, and 5687. I don't need any gain so a buffer stage or a low gain design is what I'm after. The amp I am thinking about using is the 6BM8 in Super Triode mode. I kind of like this amp but it is very SS sounding and needs some 'warming up". My preference is for a very clean detailed sound with lots of imaging and depth.

I have a few circuits in mind for each tube but wanted to narrow down the tube first. If anyone is familiar with each tube's characteristics as a preamp, please offer you experiences.

PS. My point of reference is the Bottlehead maxed out 12AU7 Foreplay. Its nice but needs a bit more warmth with the ST 6BM8 amp.

TIA...
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Old 16th March 2005, 07:27 PM   #2
JimW is offline JimW  United States
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Default 12bh7a

I am presently using 12bh7a's biased to run at 8ma in a srpp line stage. They give an overall gain of 10 without cathodes being bypassed. While I understand that many do not care for srpp stages, I have been very pleased with the results.
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Old 16th March 2005, 07:31 PM   #3
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Parafeed with a medium mu tube like the 5687. With the transformers step down, you'll have just a modicum of gain.

Her'e how to do it on the cheap .
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Old 17th March 2005, 12:32 AM   #4
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Hi,

6BM8 in STC mode sounds to SS-like, huh?....

Oh well...Never mind.
Of all the tubes you list the 5687 is the darker sounding of the lot.
In SRPP with a well filtered PS and unbypassed cathode it won't have too much gain.
SRPP = mu/2 with cathode bypass, a little less without.

Should the Zout be too high for the load you're driving you can still bypass the cathode resistor but while it will lower Zout it will inevitably take the gain up a good notch.

Cheers,
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Old 17th March 2005, 01:55 AM   #5
SY is offline SY  United States
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That's funny, a book I have here claims the 5687 is harsh and bright. If you ever wonder why I believe in controlled listening tests, here's yet more evidence.

Anyway, for lowest distortion (which may not be what you want), the 6SN7 is champ for that application. At 8V p-p output, grounded cathode, the open loop THD is down in the 0.00x% range.
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Old 17th March 2005, 02:28 AM   #6
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EF86 configed as triode is currently one of my favorite building blocks for low noise and simplicity and uniform sound.......run them at least 3mA.. ...although it seems there are alot of people who like to snub EF86's thumbs down.
The bandwidth of a pentode friged as a triode way better than sluggish ECC83's.

richj
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Old 17th March 2005, 06:52 AM   #7
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Quote:
Originally posted by richwalters
EF86 configed as triode is currently one of my favorite building blocks for low noise and simplicity and uniform sound.......run them at least 3mA.. ...although it seems there are alot of people who like to snub EF86's thumbs down.
The bandwidth of a pentode friged as a triode way better than sluggish ECC83's.
In a recent series of SE EL84 amps we ended up with a triode connected EF86. Too much gain but excellent sonics (only running it at about 1.8 mA, we'll have to try cranking it up a bit)

dave
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Old 17th March 2005, 10:15 AM   #8
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RD,

Another tack you can consider is a buffered buffer using 1 twin triode/channel. You want to use a LINEAR type; so, the 6SN7 and 5687 are the candidates. Wire a section up as a cathode follower with an intentionally small load resistor (about Rp). Use a 6 V. Lithium battery in the grid circuit for bias. The under loaded cathode follower generates some 2nd harmonic distortion that will warm up the overall sound. Couple the tone generator to a conventionally loaded cathode follower. The 2nd CF isolates the tone generator from the load, which insures reliable voicing. If things are not warm enough for you, reduce the tone generator's load resistor value. OTOH, if things are "over the top", increase the tone generator's load resistor value.

BTW, there is a lot to be said in favor of HIGH gm/LOW Rp types, like the 5687, for cathode follower service.
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Old 17th March 2005, 01:07 PM   #9
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Quote:
Originally posted by SY

Anyway, for lowest distortion (which may not be what you want), the 6SN7 is champ for that application. At 8V p-p output, grounded cathode, the open loop THD is down in the 0.00x% range.

In Building Valve Amplifiers, Morgan Jones says that a better tube is the 12SN7, not for its electrical characteristics (6sn7 and 12sn7 are the same tube...) but because the lower heaters current it is useful in order to avoid humm problems, large loop areas and and electromagnetic connections.
It is cheaper too...

What is your opinion?

Mark
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Old 17th March 2005, 01:16 PM   #10
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I've never tried a 12SN7. But if there's any hum issue with the 6SN7 in line-amp or power amp service, I have yet to be able to hear or measure it. Admittedly, I've never tried to use a 6SN7 as the input tube to a phono amp...

When I bought them, the 5692s I use were cheap. Ahhh, those were the days.
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