Partial Feedback EL84PP - Not quite as expected (yet)

[Svein_B]

I have rewired an EL84 PP amp to use "partial feedback", a'la Yves' ECL84PP and Gingertube's Baby Huey.

So far I have only managed to produce a little over 1.5W of clean power (<1%THD) without GFB, and a little over 3W with GFB.

The circuit diagram:


I know that it will be difficult to find an optimal operating point with a starved 12AX7 as driver tube. The effect of the anode-anode feedback will change the load on the 12AX7 to a considerably lower impedance than the static load-line. I have however not yet figured out exactly what the load impedance will be, nor the effective Rp of the driver tube.

A rough estimate of the effective load-line is attached. The oscilloscope also shows that the top half of a sine curve start to look flattened with increased volume.

Amplification in driver tube is measured to 19 (withot GFB). The funny thing is that shorting the 15K shunt resistor does not seem to affect the overall gain.

Any ideas of what to try next?
I mean, it should be possible to make it work, since Yves and Gingertube have done it. Yves have also published his measured power and THD.

SveinB

[gingertube]

SveinB,
I ran into low power output on one of the Baby Hueys. In that case it turned out to be a wiring fault causing so much feedback that the front end differential amp saturated before generating enough drive to the output tubes. It is VERY suspicious that shorting the 15K shunt resistor did not cause any change in gain. That is also suggestive of a saturating front end. Check that the diff amp anodes have not less than 140V on them. If the voltages are too low you will need to reduce the tail current. Check that the resistor from the EL84 anodes to each end of the 15K are 47K (2 watt). Check that the resistors from the 15K to the anodes of the 12AX7 are 220K AND *** IMPORTANT **** that the PULL side EL84 Anode is feeding (via the resistor network) the PULL side diff amp anode AND that the coupling cap from that side diffamp anode feeds that same side EL84 grid. That is, check that you have not inadvertently wired positive local feedback.

The fact that the power went up when gNFB was applied suggests that maybe you managed to wire POSITIVE local shunt (partial) feedback. If so swap the feeds to the output tube grids.


Can't think of anything else to check.

Cheers,
Ian

[Svein_B]

I admit that I sometimes do the occasional wiring error, but I do not think that is the problem here.

It is quite obvious that the rising distortion at higher power is caused by driver stage unlinearity/overload. Traditional output clipping start at around 8W as it should.

I think I will have a closer look at the effect of the 15K shunting resistor. The design idea is that the middle of this one will have zero AC signal (virtual ground). This may hold true as long as the output tubes operate in class A, but I am unsure of what happens when transitioning to class B. I will replace the single 15K with two 6.8K resistors and measure what happens in the middle. Maybe also reduce the values a little to reduce the signal feedback.

Any one know what the effective Rp of the 12AX7 will be as a diff-amp in this circuit?
This value seem to be important to estimate the feedback and signal levels.

SveinB.

[Salas]

I don't know the effective Rp, but I have measured gain 19 in my 2X12AT7 6mA total, CCSed phase splitter. Without an anode to anode resistor. How come the 12AX7 & resistor does not give you something different for gain?

[Svein_B]

A few measurements show that the circuit works - sort of, but the 12AX7 seem to be under a bit of strain.

My earlier statement that shortening the 15K shunt resistor did not produce any noticable effect in gain is not entirely correct according to my scope. It does give a difference of 4dB, and lowers distortion with the resistor un-shorted. at least at low to moderate volumes. The shunt resistor is now composed of 2 x 5.6K = 11.2K.

Attached is a scope picture of the anodes of the 12AX7, and the midpoint of the shunt resistor. It is a couple of dB below max power with distortion >5%.
It show unlinearity at the 12AX7 anodes. The lowest trace is the midpoint of the shunt resistor, not quite a true virtual zero.

The circuit seems to operate very close to the limits of the 12AX7 capability, and needs very careful tweaking. I wonder if a more conventilal wiring of the driver with anode resistor to B+ and separate feedback ressitors might be easier to work with. Also a different driver tube (12AT7 ?) might be happier under the strain of the anode-anode feedback.

I am still looking for a formula for the effective Rp of the diff pair.

SveinB

[SY]

r_p (eff) = 2r_p + R_L

[Wavebourn]

This "partial feedback" means a local feedback (any feedback is "partial", so I don't understand why this particular parallel feedback by voltage is called such a way) from anodes of tubes to their grids. As a feedback by voltage it decreases output resistance, but as a parallel feedback (applied counter - phase to an input signal, in parallel) it decreases input resistance of the stage loading the previous stage harder, on a lower resistance. I've always tried to load triodes by more resistance to obtain less distortions from them. In this particular case decrease of distortions in the final stage increases distortions of a previous one. I still don't understand why somebody would like to do that...

[Shoog]

I still don't understand why somebody would like to do that...

Because experience shows that it works very well, especially if the "partial feedback" resistor can be designed to have a high value and so maintain a reasonable load on the triode. It is a hard balancing act, as I am discovering with my latest design.

Listen to one of these amps and you will understand why people recommend them.

Shoog

[Wavebourn]

Shoog, there are different ways to skin the same cat, more elegant, without torture of driving triodes.

[Irakli]

Hi Guys,

Is there any particular reason for so much suffering trying to use AX7 as a driver? May be it is easier to change a tube and use something like 5687 etc.

Regards, Irakli