Long Tail Pair Question

[Cressy Snr]

Hi everyone,

In my 6AS7 push-pull amp, I have two differential stages before the output valves, one voltage amp and one driver using cathode bias via a 2k resistor for the voltage amp and an 820R for the driver. All very nice, I suppose but I can't help thinking there should be a tail resistor somewhere in each section of around 470K.

I don't want to complicate matters with a CCS (not yet anyway)
The question is, where do I put the tail resistor ie between where and where? I can't eventually put in a CCS if I don't even know where to connect the thing.

The amp sounds rather good as it is so I reckon it will be even better if the LTPs are done properly.

Hope someone can answer this.
Thanks
Steve

[Paul Barker]

You have to loose some voltage headroom. Decide how much you don't mind losing. Say 50v then calculate the quiescent current of the pair of valves, devide 50 by the current in amps and you have your tail resistor value.

The tail resistor goes between the cathode resistor and ground, the grid resistors instead of going to ground join the node between the cathode bias resistor and the tail resistor.

Have you forgotten to re-evaluate the second stage anode and cathode resistor value to accomodate 6sn7?

[Paul Barker]

By the way I would only alter one stage in the first instance to check for quality before deciding to continue.

You might go different values for each stage, the first stage could afford to loose more headroom so maybe calulate for 100v across the tail resistor mking it greater value, the second stage maybe just sacrifice 50v.

[Paul Barker]

I believe Slagle had good results in times past using chokes instead of tail resistors. Less voltage drop, similar result of forcing balance.

[Cressy Snr]

Have you forgotten to re-evaluate the second stage anode and cathode resistor value to accomodate 6sn7?

Thanks for that Paul.

I recalculated the anode resistor value for a 6SN7 but forgot to change the titles on the valves so the values are actually for a 6SN7. I showed the 6SL7 crossed out so that Nick's reply in the "Everyone's Projects" post would still make make sense.

I was going to make the second stage a choke-tail pair but I only had the one suitable choke, so that's on hold until I get another one from Philip@Bluebell.

Cheers
Steve.

[Cressy Snr]

I modded the amp to put in the tail resistors on the first stage only. This tightened up the bass and cleaned up the top end somewhat. A very useful improvement.
Next I ragged a choke out of my 12E14 single ended amp to go with the other spare one I had. These were used to mod the second stage for choke tail pair operation.
This also worked very nicely as the stage did not suffer any significant voltage drop. I did not want to lose drive to the finals, so the choke tail was the ideal solution here. Bass tightened up a bit more and the clarity further improved at the top end.

I would think that these improvements were due to the improved balance in the first two stages, enabling better cancellation of second harmonic distortion products.

As I understand it a diff amp will cancel even but sum odd harmonics. This is why I was keen to use valves from the SN7 family as these produce predominantly second harmonic distortion and extremely low levels of odd harmonics. Acording to Morgan Jones I will probably now have around 0.007% third harmonic distortion of the signal leaving the driver stage, which is negligible. A nice clean signal therefore to drive the output stage.

Here is the circuit as I have it now. A nice blend of resistors and iron I think.
Steve

[Paul Barker]

Now you're cooking on gas.