Help with 6SN7 w/ CCS

[tubesmuggler]

I have about 185V B+ for a 6SN7 input stage in my headphone amp. I started with the CCS set to 9mA, LED bias -2V which gave a Vp ~130V. Does this seem decent?

This is my first ever scratch design and the sound is pretty good, but there is some harshness and is very un-tube like, and very obvious. This is not a taste issue. I expected to have issues with it being too warm sounding but it's the opposite. I want to rule out this OP as a problem before I look at the output stage.

TIA.

[analog_sa]

Originally posted by tubesmuggler
I expected to have issues with it being too warm sounding but it's the opposite.

6SN7 is like that. Some people really dislike it.

[DougL]

-2 volts on the cathode will only allow about a 64 Vpp output before grid current on the 6SN7. That's only about 20 V (minus CSS stall) from hard clipping of the stage.

To Check my math, -2V bias is a max input of 4 Vpp. Since CSS loaded, Av = Mu = 16, so 4 * 16 = 64 Vpp
130 Vp+ -(64/2)V = 162 Vp Max, 98 Vp min.

The rest of your circuit would be helpful.
If you need more than 10 Vrms for your finals, you probably need to re-design your input stage. If possible, higher B+ might allow more bias voltage.

Sorry for the drive-by.

Doug

[tubesmuggler]

Hi guys,

analog_sa, I've had a couple of 6SN7 linestages in the past and none were anything but warm sounding, very different than what i'm getting now.

----------------
DougL, thanks for your time.

I have a headphone amp where each channel is a SE triode-strapped 6G6-G output tube, with one 6SN7 triode for input and drive duty. I don't have a schematic to post because I dont' have software.


Also, when is a Rg necessary? I've looked at various 6SN7 pre schematics online and some have the grid to the 100K volume pot and some have a 470K to 1M resistor for example. Is this just for safety if a pot fails?

[DougL]

If you need to draw a schematic, you can use these symbols.

Joel Tunnah Vacuum tube audio

Very unusual, and Very cool Output tube.

Rg is necessary if you use a classic potentiometer because they could fail open. Not as necessary with a stepped attenuator, but still a good thing.

Doug

[SY]

I have about 185V B+ for a 6SN7 input stage in my headphone amp.

Way too low. 6SN7 needs volts. With that limited B+, you'd be better with something like an ECC88. The harshness you hear could well be grid current distortion.

[tubesmuggler]

I'm happy to try a different twin triode but I'd prefer an octal if possible. Know of any that are suitable?

I saw a schematic by dick olsher that shows a ~B+ of 205V and 4.5K Rp, and 157Vp. Rk is 300R for -3V. After seeing this I thought I might be OK.

I also noticed the heaters are at 6.75V so I'll fix that up too.

[SY]

160V at the plate is a reasonable value. I wouldn't go lower. At that voltage, you'll want more like 3-3.5V at the cathode.

4.5k plate load? That's an effects box, not an amplifier!

[Jeb-D.]

I'd first try ditching the CCS if your going for a warm sound.

[tubesmuggler]

Here's the initial schematic.

I've change the LED to a 360R which brings about -3.25V and Vp to 144V or so. I couldn't hear any noticeable difference.

Again I don't think this is my opinion of the sound quality, it's not a taste thing - rather it sounds like there is something not quite right.