• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

line-stage preamp tubes? which ones to use...

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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...
 
diyAudio Senior Member
Joined 2002
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, ;)
 
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.
 
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
 
frugal-phile™
Joined 2001
Paid Member
richwalters said:
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
 
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.
 
SY said:

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... :cool:

What is your opinion?

Mark
 
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.
 
mark_titano said:



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... :cool:

What is your opinion?



Hi there........arguement not valid if DC heater supply is used...as for electromagnetic connections....that's a new one on me.....I'd always thought the mains tranny was the culprit....perhaps I've misunderstood the 50 Hz wavelength.....

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

My guess to why it sounded SS is maybe because I used a discrete transistor? Didnt have a 6AK5 on hand at the time, which is what the original circuit had, so went with it the transistor instead.

http://www.ne.jp/asahi/evo/amp/6bm8/zu2.gif

Maybe I should try to find a tube to use as a driver instead. Eli had a good thought, though. But, whats with the 6V battery?
 
But, whats with the 6V battery?

RD,

The battery is for grid bias. You cap. couple at the I/P, use a 1 MOhm grid resistor, connect the "bottom" of the grid resistor to the - side of the battery, and connect the + side of the battery to the triode's cathode. Connect the small load resistor between the cathode and ground. By using battery bias, you avoid calculations and get good bass. 6 V. of bias cleanly handles a 2 VRMS I/P signal, as produced by a CDP.

A final point, connect the O/P of the source selector switch to the "top" of the volume control and ground the "bottom" of the volume control. The wiper of the volume control connects to the CF I/P cap. Use any volume pot. you like that's 100 KOhms or less.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.