6sn7 LTP values

[andrew]

The math is over my head on determining the values for this 6sN7 in a LTP. I usually just find a existing schematic a copy it but I cannot find a guitar amp schematic with an LTP 6sN7. As far as I can get is the 6sN7 has a plate resistance of 7700. 7.7 X 2 = 15.4K for the plate resistors. What should the other component values be? Thanks for any help.

[martin manning]

What supply voltage are you going to use?

[soma_hero]

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

[andrew]

hi, it has a 330-0-330 PT with a choke and Express dropping resistors. I need to order wire and some parts so I can't get a voltage reading now. At one time I had the 6sN7 wired like the 6sL7 on Aiken's site and it sounded just fine but I would like the correct component values. Thanks.

[martin manning]

Assuming a 300-350V supply, my put would be:
Plate loads 18k (inverting side) and 20k (non-inverting side)
Bias resistor 0k92 (920 ohm)
Tail resistance (total from bottom of bias resistor to ground) 6k2

[tubeswell]

... and probably 470k for the grid load resistors

[martin manning]

... and probably 470k for the grid load resistors

Pete, which resistors are you referring to, and why that value? I guess you mean grid leak? Could be 470k-1M...

[andrew]

should I keep the same Express presence control values? Thanks again .

[tubeswell]

... and probably 470k for the grid load resistors

Pete, which resistors are you referring to, and why that value? I guess you mean grid leak? Could be 470k-1M...

Yes grid leak is what I meant. In terms of the 470k vs 1M, I'm just taking a stab - thinking that because of the higher tube current (and therefore grid current?) I'd be tempted to use 470k for (possibly) reduced hiss. I have found 470k to have less hiss than 1M in LTPs

[martin manning]

Tubes, yes maybe so... I was just reviewing Blencowe and I see that he suggests reducing LTP grid leak resistance to reduce noise.

Andrew, I'm still not quite sure what you are up to (maybe it's a secret ;^), but if you are putting this in front of an Express power stage, then the circuit below should work. It's the values from above with the presence added in. If you can't find a 2k pot, you can use a 5k and put a 3k3 across the outer lugs. That will get pretty close to the same response out of the presence.

[andrew]

Thanks Martin, this has been a great help.

[tubeswell]

Tubes, yes maybe so... I was just reviewing Blencowe and I see that he suggests reducing LTP grid leak resistance to reduce noise.

Yes that's where I first saw it, and so I tried it, and he was right.

[andrew]

I'll try the 470s also. Thanks tubeswell

[martin manning]

Let us know how it works out!

[andrew]

I used the 6sn7 LTP values and it made a nice solid sounding amp. I did not use a presence or cut control. I have a 12ax7 in parallel for V1 and a 6sL7 in V2 and a 6sn7 LTP. The rest is is pretty much a Rocket 66. The 6sn7 pulled a lot of volts so instead of dropping power resistors I have each of the power resistors coming off of the screen supply. I tried a 6sL7 in V1 but I like the 12ax7 better (100k anode resistor/820 cathode resistor). I'll probably try the 6sL7 later and play with the values some more. The preamp tubes are on DC and it is buzz free. Thanks for the help guys.