KEL84 Mod

[acorn]

Hi All

lately I have been trying to work out why I can't obtain the 80 volts on the cathode of the concertina phase splitter of the KEL84 amp
I have been getting silly readings like 45 volts - 55 volts when it should be around 80 volts.

I have read that others on previous posts have had similar problems with the ECF80s, I can only put it down to the Mutual conductance of these valves varies widely, so as to counter this I changed the cathode resistors R14/15 from 27K to 33K and woe an behold the cathode voltage came up to 65 volts on some of the valves but on the better type I got the 80 volt reading as stated on the original circuit and as a result it sounds much better. I thought that with most phase splitter you had to have equal voltages on each half ie 200volts on the anode and 100 volts on the cathode in order to obtain a good balance so as to drive the valves to give the least distortion.

It would be interesting to hear from the panel regarding comments on my findings or any suggestions as to why they may think otherwise.

Regards Acorn

[Ianm2]

just as an aside, I have heard the concertina becomes unbalanced at high frequencies, but can't really find any references to this, anyone shed light on this, I think its ameliorated by a carefully placed cap.

[acorn]

Hi Ianm

Yes thats correct it goes out of balance at high frequecies as the valve wares out over time to counteract this place a 68 to 100 Pf across the 470 grid leaks and that will help this as been done in the past but I assume do to cost it as been left out in recent designes but Morgan Jones as incoparated in his EL84 amp.

Acorn

[NickG]

AFAIK, you have to have equal resistors in a concentena otherwise you will not get equal output.

I thought MJ showed the output impedance of the splitter was the same for both sides, so I am trying to work out how it can be unbalanced. I guess it would have to be a result of the loading, without load, it can't fail to be ballanced, as the same current is passing through both resistors.

Hang on, though. The lower resistor is seeing Ccg and the upper Cga so that could cause different loading. Maybe a large (by comparison) cap across the grid leak resistor swamps the effect of these two capacitances being different forcing it into ballance.

[Ianm2]

I have also seen the odd concertina with different value R's, presumably to counteract this, some of the 60s amps were aware of imbalance, fisher or scott I think

[acorn]

Hi Nick

I assume the idea of equal resistors is to split the voltage equaly but this does not seem to work in practice on the KEL84, all though Morgan Jones EL84 circuit did show nealy an equal split between the anode voltage and cathode voltage this is what I am trying to achieve with this mod it as made quite an improvment to the sound so am I on the right track, I have also seen different resistor values on some other Push - Pull circuits I have seen, can any one else shed any light on this, as it really does work.

Acorn

[NickG]

its not so much a matter as splitting the voltage. The idea, is the same signal current must pass through the upper and lower resistor. As the same value is used, the same signal voltage must be developed across them. If they are not the same value, the signal will not be the same, so there will be an error in the phase splitting. You should check that the ( HT - the top voltage ) is the same as the lower voltage, thats how its equally split, not just the HT divided by three. Ok, thats a good idle place for them to be, but not essential for the correct operation of the phase splitter.

By altering the resistors, you are gatting a nice voltage that seems like a good idea, but its not, its actually breaking the correct operation of the phase splitter.

If you prefer the sound, good, but I think if you measured the result you would see an increase in distortion due to unbalanced drive to the output stage.

If you have seen other circuits with different values, then I would suspect its not a concertina splitter, do you have a link to such a circuit?

Hey, what do I know, I prefer SET's anyway :-)