Cathode feedback

[mark_titano]

I'd like to use balanced cathode feedback in a push pull output stage like Audio Reaserch did in the d40:

http://www.drtube.com/schematics/ar/d40.gif

My question is: I've got an Hammond 1608, 8ohm speakers and el84 power stage. Can I use the secondary to this purpose? There are two windings in the secondary, 0-4ohm and 0-4-8 ohm. Look here:

http://www.hammondmfg.com/1608.htm

What's your thought guys?
Thankyou for your support

Mark

[plovati]

Hi Marco,
I've read now Your email (my server is down).

To use this topology the winding of the transformer must be balanced and this strongly depends on the manufactoring.

Electrically, simply tie the 4 ohm (Gn/Yell) wire to ground and connect the 8 ohm speaker to 0 and 8 ohm wires is the only possibility I see.
The other 4 ohm winding can be paralleled experimentally to one of the previus or left floating or connected to some compensation reactance, to find out the best balance between the two halves of the amplifier.

Ciao

[EC8010]

Yes, you can do it, but you need to connect the transformer in a slightly non-optimum way because of the way that Hammond implements 8 Ohms.

Connect the two windings in series as if for 16 Ohms as per Hammond's diagram. so that BLK/YEL goes to GRN. This is your centre tap. Connect BLK to one cathode and GRN/YEL to the other. If you have a shared cathode bias resistor, it goes between centre tap and ground. If you have individual cathode bias resistors, the centre tap goes to ground and the resistors are between the cathodes and transformer. Connect your 8 Ohm loudspeaker between the centre tap and YEL.

Check for instability with a 10kHz square wave on an oscilloscope.

[mark_titano]

Thankyou guys.

Ec8010, that's the way I've thought to connect the secondary. The question is, which are the problems or performance losses that I'd probably meet in that way? And the differences in power handling or frequency responce? I'm thinking to connect the speaker to the 16ohm taps, I'm only afraid because the reflected impedance should be too low for the 84s.
Or, due to the fact that 8ohm is only a nominal impedance, to connect the speaker to the 4ohm tamp... I think that only a test with my equipment could answer to these questions.

Piergiorgio...what's the optimum load for the 6005?

Mark

[EC8010]

If you connect an 8 Ohm load across the 16 Ohm winding you will halve the load the output valves see. The cathode feedback will make them a little more tolerant of that load but you will still lose power and gain distortion. Unless I know exactly what the impedance of a loudspeaker is (having measured and plotted a graph of Z against f) I normally consider all loudspeakers to be 4 Ohm. The loss in power is insignificant. Using the transformer as I have suggested is likely to increase leakage inductance (affects HF response) and increase DC resistance (slight loss of power and reduction of loudspeaker damping). It doesn't cost to experiment, so try it and see what you think.

[SY]

One other point is that the drive voltage requirement for the output stage will roughly double.

[PRR]

> the drive voltage requirement for the output stage will roughly double.

In this case: Not even. The secondary voltage is about 15V peak, the 6L6 are biased about -30V. That's not much increase in necessary drive voltage, nor is it much feedback.

To get a big increase in drive need (and lots of local feedback) you need hi-Z cathode winding or tube(s) with much higher power sensitivity than a couple 6L6. Massively parallel works great with 4/16Ω winding.

[plovati]

6005W is like 6V6, apart slighty less ratings on voltage and power. Any load suitable for 6V6 will fit also 6005.

The reccommended Raa is in the range 6600-10000 ohm, most application user near 8000ohm anode-anode.

Optimum for power or for distortion? I think that with some cathode feedback You can use profitably a lower load than the usual tabulated one, getting more power at the same distortion.

Pay attention not to exceed the max rating, especially on the screen current.

[mark_titano]

Thankyou guys.

Sy maybe the drive voltage requirement will be double but, if I don't use the local feedback, then I will need some global feedback...and the driver stage should be in any way with higher gain. So I'm planning to use a driver stage that could provide enough gain to allow the use of some kind of feedback. I'll split the signal with an input transformer ( I'll use the Lundhal 1544A), a differential driver stage dc coupled to the power stage. I think that an ecc88 should be enough to drive the 84s with a little amount of cathode feedback.
I'll do the calculation using these formulas:
Amplifiers-and-Superlatives.pdf

But...how to use them? I'll use both ultralinear connection and cathode feedback. So, to find the gain, have I to use first the formula for the ultraliner then the one for cathode feedback? Have you other formulas? I need to learn to use microcap...

Mark

[SY]

Sy maybe the drive voltage requirement will be double

No, PRR is right, the drive voltage requirement will go up but in this case, it won't double (with AR's bigger amps, it does double). But at the same time, the cathode feedback will be very small.