Long tail 6sl7 for Dynaco stereo 70?

[Original Burnedfingers]

http://www.aikenamps.com/LongTailPairDesign.htm

I was wondering if this would have enough guts to be used as the front end of a Dynaco Stereo 70.

[Miles Prower]

I don't like that design. He's using equal plate resistors with a passive tail load. You can get away with that when using transistors, but 6SL7s have onehelluvalot less gain than any transistor. The output voltage won't be of equal magnitude unless you force balance by unbalancing the plate resistors. Even better is an active tail load to force constant current, and present the cathodes with a lot more effective impedance than the 47K tail resistor he does use.

As for drive capability, you need to pay attention to the Ci + Cmiller + Cstray of the driven stage if you are to aviod slewing at the top end. A 6SL7 may, or may not, be up to that task. If directly driving the grids of the finals, I'd say not too likely. If driving a cathode follower grid driver, or a subsequent small signal stage, then a 6SL7 could do that.

[korneluk]

Originally posted by burnedfingers
http://www.aikenamps.com/LongTailPairDesign.htm

I was wondering if this would have enough guts to be used as the front end of a Dynaco Stereo 70.

Yes it can. Alex Peychev uses this approach in his modified ST-70s which sell for a pretty penny.

-- josé k.

[Original Burnedfingers]

Yes it can. Alex Peychev uses this approach in his modified ST-70s which sell for a pretty penny.

Is there a site? schematic?

[Eli Duttman]

Originally posted by burnedfingers
http://www.aikenamps.com/LongTailPairDesign.htm

I was wondering if this would have enough guts to be used as the front end of a Dynaco Stereo 70.

Dude,

While the 6SL7 is a fine tube, it leaves (IMO) MUCH to be desired as a LTP. Slew limiting considerations indicate a high gm type as a LTP. The 'SL7 is high R_P/low gm.

A 12AT7 as the LTP will yield as much gain as a 'SL7 would, but it has the requisite low R_P/high gm.

The "classic" Mullard circuit with a 6GK5 as the voltage amplifier and a 12AT7 as the LTP is worth looking into. You get plenty of open loop gain and the high gm of both small signal types protects against slew limiting.

[JoshK]

Originally posted by Miles Prower
I don't like that design. He's using equal plate resistors with a passive tail load. You can get away with that when using transistors, but 6SL7s have onehelluvalot less gain than any transistor. The output voltage won't be of equal magnitude unless you force balance by unbalancing the plate resistors. Even better is an active tail load to force constant current, and present the cathodes with a lot more effective impedance than the 47K tail resistor he does use.

As for drive capability, you need to pay attention to the Ci + Cmiller + Cstray of the driven stage if you are to aviod slewing at the top end. A 6SL7 may, or may not, be up to that task. If directly driving the grids of the finals, I'd say not too likely. If driving a cathode follower grid driver, or a subsequent small signal stage, then a 6SL7 could do that.

What's the Ci stand for in your post? The Cap between the 6SL7 LTP and the next stage?

[Boris_The_Blade]

I guess I can say this since I have all the 6CL6s I ever need. Using a 6CL6 LTP in pentode with a SS BJT CCS, I managed about 150 Vpp (single ended) out with <0.5%THD (my measurement limit) at 1KHz. Power supply was +340 and screen was 150V. Plate resisters were 12K and CCS was at 28mA. Single ended gain was 35V/V. It drove PPP EL34.


What it went in.http://www.audiokarma.org/forums/sho...d.php?t=140121

[tubewade]

Will it work? Yes. Will you be happy with it? Hmmm. I tend to agree with Miles that the anode resistors cannot be the same value if you want balanced output and that you really need a buffer between this stage and the output valves.

Also agree with Eli D. that the 12AT7 is better suited to the task. The Mullard circuit he recommends is proven and up to the task.

[ray_moth]

A 12AT7 as the LTP will yield as much gain as a 'SL7 would, but it has the requisite low RP/high gm.

Eli is right. However, you won't have very much gain with just a 12AT7 or, indeed, any double triode LTP as a front end. You need more stages to get enough gain for NFB. (Depends on how many holes you want to drill!)

For example, you could use a 6SL7 LTP splitter, followed by a 6SN7 differential driver (an all balanced design) which I like; or you could use a single-ended triode voltage gain stage, like a 6SL7 or triode-strapped EF86, followed by the 12AT7 LTP splitter à la Mullard.

[sorenj07]

Originally posted by Miles Prower
I don't like that design. He's using equal plate resistors with a passive tail load. You can get away with that when using transistors, but 6SL7s have onehelluvalot less gain than any transistor. The output voltage won't be of equal magnitude unless you force balance by unbalancing the plate resistors.

Originally posted by Randall Aiken
Note that in most cases, the out-of-phase output plate resistor will have to be made around 10% smaller than the in-phase output to achieve perfect balance between the two outputs.