New SE With Triode Mode KT120/150

[Cressy Snr]

Deleted.
Note to self.
Stop being a prat.

[Nick]

One small point, it may be the op points are just fine, but if you were to permit yourself one more cap you could pick what on paper looks like better ones.

[Cressy Snr]

One small point, it may be the op points are just fine, but if you were to permit yourself one more cap you could pick what on paper looks like better ones.

May have a look in a couple of weeks. At the moment, I’m enjoying one of those ‘once in a blue moon’ kinds of sound: the sort that you get scared sh*tless of losing and not being able to get back.

[Mike H]

Glad somebody else has had the same thing happen. So I inadvertently created a white noise generator for a few seconds every time I switched the amp on and off.

Yep.
I remembered what I did, changed the Zeners to lower Voltage ones.

[Cressy Snr]

OK some thoughts on this driver stage, which could be wrong:
Because the load resistors are equal, it is basically a cathodyne phase splitter with one output in use. On the Sun Audio 2A3 amp, the cathode resistor is bypassed, so we don't have a phase splitter with the top output in use, instead we have an ordinary, direct-coupled pair of valves.

IMG_0592.JPG (31.98 KiB) Viewed 2748 times

So in my case, I think this 'half of a cathodyne' setup is operating under 50% internal feedback, which makes it very linear.

12AU7.jpeg (101.48 KiB) Viewed 2748 times

Looking at the characteristics above, we have an AC loadline of 36K due to the loads being in parallel with the 220K grid leak resistor of the power stage so we have a rough total swing of 110V. Except the gain of the stage is halved compared to the chart because of the 50% internal feedback caused by the high value, equal to the anode, unbypassed cathode resistor. Taking that into account, the driver is swinging 55V pk-pk. The 807, as I have it configured, wants 40V pk-pk, so the driver is just about adequate.

Bypassing its cathode resistor would give more drive but more distortion, so really, the reason I get such a good sound and distortion performance is that I've been slightly lucky with the gain. It’d drive an EL84 in ultralinear mode beautifully.

These are my thoughts, so feel free to pick holes. How it all works seems perfectly logical to me, but my idea of logic is usually poles apart from everybody else’s.

[Nick]

That does beg the question, would it sound any better or not with the output from the cathode instead of the anode.

[Cressy Snr]

That does beg the question, would it sound any better or not with the output from the cathode instead of the anode.

It wouldn’t be too difficult to find out.

[ed]

That does beg the question, would it sound any better or not with the output from the cathode instead of the anode.

just for a bit of fun it would be good to have a few guesses from the current observers before Steve gives the empirical result.....

My guess is the cathode...but it is a bit of fun and Steve's ears are not what they were etc etc

[Cressy Snr]

OK,
Tried moving the output to the cathode.
It's not a night and day difference, but nevertheless it is an improvement. There's a smidge more fine detail and transients are a bit faster. Noise floor also seems to have dropped a bit. It looks like this now.

12au7caathodeout.jpg (108.1 KiB) Viewed 2632 times

[Nick]

Pedantic hat on. The voltages on the cathode and anode of the second valve are inconsistent. If there is 81v across the cathode resistor then there has to be 81v across the anode resistor which means either B+ is not 300v or the anode voltage is not 210v

[Cressy Snr]

Pedantic hat on. The voltages on the cathode and anode of the second valve are inconsistent. If there is 81v across the cathode resistor then there has to be 81v across the anode resistor which means either B+ is not 300v or the anode voltage is not 210v

Guilty as charged, but TBH I’m fed up with correcting things. You make changes, the voltages change and I seem to have got out of step. These seem to be mixed in with the 6SN7 readings. Sorry but I just cant seem to get these things right. In does tend to take the shine off things.
I’ll take the advice you gave a while ago and just refrain from posting voltages and component values. It’ll be better all round I think.

[Nick]

Fair enough, just letting you know.

[Cressy Snr]

Fair enough, just letting you know.

Cheers Nick.
This is probably the best way going forward:

12au7cathodeout.jpg (103.95 KiB) Viewed 2601 times

Just the HT voltages. Can't really cock that up.

[Cressy Snr]

Anyway, half a cathodyne splitter is an interesting way to make a good sounding single-ended driver if nothing else. Could only have come from a fool brain like mine

[IslandPink]

Nice project . It would be interesting to see what Spice shows as the distortion spectrum on this, now.