why is such a high idle current needed in class AB1 modulator stages ?

[w4bfs]

Jim WD5JKO put this statement in another thread ....  

 Many folks use the Hi-Fi output tubes in a Ranger like the 6550 or EL-34. You will find that an idle current that approaches the plate dissipation of the tubes is necessary to get low IMD. So for EL-34's, the Pd is 25 watts. That would be about 40-50ma per tube idle current when the B+ is around 500v. With the Gonset arrangement and sweep tubes G2 driven (G1 grounded), the idle current can be about half what I mentioned. There are many Valiants in the NE heard on 10M that use this arrangement. They sound sweet too.   (end of quote)

I have been puzzling over this for a few days .... it should not be necessary to do this .... this practice is undesirable for several reasons especially shortening the life of expensive components .... I restored many guitar amplifiers (mostly Fender) over the years and I found suitable AB1 operation for most 6L6 family tubes at 30 to 35 mA cathode current .... basically the bias is adjusted until the crossover 'knee' disppears and a few mA added for aging .... happy happy players

so why is it necessary in a modulator stage to move closer to class A push pull ?   the guitar amp has a speaker(s) for a load .... the actual impedance model for a speaker is complex and hardly resistive BUT it is reasonably constant over the + to - swing of the output ac waveform ...otoh the single ended class C modulated stage approaches its calculated modulated impedance only on the + audio peak and is higher elsewhere .... so it appears that the higher conduction angles of the class A pp modulator MAY provide a load as well (my speculation)

a possible solution is to experiment with negative cycle loading for the modulator .... it just might lower the observed distortion .... the simplest is just a diode and a resistor across the secondary of the modulation transformer .... this could also help with end-of-xmission transients

any ideas ?

ADDED: 10:30 am   I have talked with W2CQH and now understand that the class C modulated impedance is actually resistive and holds its value over most of the waveform (he has verified this) with the only discontinuity at the negative peak .... we hold a mostly daily sked and will talk about it some more ....mo' to come

[Steve - K4HX]

The amount of idling current also depends on the tube. IIRC, 828s (as one example) could be run in AB1 with very low idling current.

[WD5JKO]

I have been puzzling over this for a few days .... it should not be necessary to do this .... this practice is undesirable for several reasons especially shortening the life of expensive components .... I restored many guitar amplifiers (mostly Fender) over the years and I found suitable AB1 operation for most 6L6 family tubes at 30 to 35 mA cathode current .... basically the bias is adjusted until the crossover 'knee' disppears and a few mA added for aging .... happy happy players

so why is it necessary in a modulator stage to move closer to class A push pull ?   the guitar amp has a speaker(s) for a load .... the actual impedance model for a speaker is complex and hardly resistive BUT it is reasonably constant over the + to - swing of the output ac waveform ...otoh the single ended class C modulated stage approaches its calculated modulated impedance only on the + audio peak and is higher elsewhere .... so it appears that the higher conduction angles of the class A pp modulator MAY provide a load as well (my speculation)

    My experience is that all tubes approach a zero plate current bias with non-linearity. At the other end of the dynamic range we have compression leading to saturation. I call this the 'S' curve. Some tubes can be biased close to cutoff and still be very linear. The 805 triode is one of them, as these tubes make good fidelity in class B service. Another tube that sounded terrible at a low idle current was the 8417 Beam Power tube. With this tube, the sound kept clearing up as the bias raised the DC operating point. The 8417 is unique in that its transconductance is off the charts; somewhere around 2X the other big bottles in the same power category.

  The 'S' curve can be straightened to some extent with negative feedback

   I have since learned that sweep tubes with the drive to G2 can be exceeding linear, and in class B with zero bias, and a low DC idling point. The 807/6L6's can be backed off some too...more so then some of the big Hi-Fi tubes that came later.

   What often happens is the greater bias for low idle current creates the need for more peak grid driving voltage. Lets say as an example, you have a bias of -20V on each tube in a P-P pair running class Ab1. The driver must therefore provide 20v peak to each grid (referenced to ground). If we were to throttle back the power tubes to a lower DC operating point, perhaps moving the bias to -25V, then the driver must provide 5v peak additional drive, or a 25% increase. In many amplifiers the driver distortion will go way up unless the headroom is there.

  I am attaching two files:

File 'e' is an EL-34 running 8 watts AM as a class Ab1 RF linear amplifier. The trapezoid sides are not straight indicating non linearity, and an example of the 'S' curve. As I raised the idle current, the linearity improved. The driver had the headroom. The EL-34 was just not very linear unless I increased the DC operating point. This tube ran 700V B+, and about 35MA plate current. I later used a Teslovok (from Penta) E-34L which had the big 6L6GC tri-metal plate. This tube was more stable at a higher Pd resting state.  

File '130_Modulated_Trapezoid' is from a pair of old Fisher branded 7591's set for 12.5 watts carrier on AM. These tubes are extremely linear, and the trapezoid sides are very straight. With this tube, lowering the idle current is OK as it stays linear. Unfortunately the new production 7591's are down on power, and have the same linearity issue as the EL-34. On the scope plot, the high modulation percentage brought the + peaks off the scope grid, and the negative peaks made those wavelets typical of a low level balanced modulator setup.

  The two tube types referenced were in a highly modified Central Electronics 20a.


Jim
Wd5JKO
  

[Opcom]

For tubes that do not do well near cutoff, and when the overall negative feedback does not correct it well, a trick that works, and i have used on several power amplifiers, is to use a low voltage center tapped winding on the output transformer to buck against the push pull cathodes. Because this winding can deliver power, not just voltage, it has a very good effect on improving linearity of beam tubes. A good setup is one where the peak voltage applied against the cathode is about half to three quarters of the value of the peak grid drive voltage. It is a degenerative setup. The requirement is more drive, and the driver's plate voltage may have to be increased to obtain it, as well as adding gain to a preceding stage. The overall effect of this is to cause the amplifier output voltage to be much better regulated, and it uses some of the amplifier's power to make up for the output transformer's shortcomings, up to a point. If the circuit is a modulator, a 25VCT or other low voltage winding (like using the 0-4-16 Ohm taps for the purpose) may not be available on that transformer.

This scheme has worked very well on inexpensive, high current tubes like the 6CD6, operating at 450V plate, 125-150V screen, and about 20mA idling current, quite low. When loads become odd, including speaker loads, A triangle test waveform will show the improvement in linearity over a standard setup. The use of the cathode feedback just around the output stage helps eliminate crossover distortion by the simple fact that during the usual crossover period, only a little negative feedback is applied to the cathodes, effectively increasing the gain of the output stage, but during the rest of the half-cycle, the full effect is there reducing the gain appropriately according to the stage's output.

Sweep tubes are nice because the screen voltage is low allowing larger plate swing yet they can really conduct hard. High current is important whenever the load is funky, or when it may change due to a selection of different RF section operating conditions (different Z). Everyone is concerned about having enough voltage swing, but when the requirement goes the other way, the extra current is a big help.

In a situation with a modulation transformer, I have not tried using a separate output transformer for cathode feedback of this kind but it ought work if the modulation transformer is of good quality, and if not, the feedback transformer (which can be the standard hi-fi audio output transformer) can be placed across the modulation transformer secondary so that that unit is inside the loop. Because of the low amount of power going through the feedback transformer, there should not be any big problems with phase and added distortion assuming DC is kept out of these windings, but it's all experimental at that point.

As far as negative cycle loading, I have a big problem with it when going 120%+ positive, because all I have is a keep-alive with a single diode and that's about 1000 Ohms impedance. It really overloads the negative cycle when pushed hard and instead of 250-300mA peak plate current in the modulator, those negative peaks can hit 600-800mA. For this reason I do not intentionally overmodulate, and just let the keep alive do only that which it is intended to do. I could use more diodes and resistors, but I'm partial to a vacuum diode the 8020, so there is but one. And there are no switching transients from other diodes.

[WD5JKO]

Beefus:

From Timtron himself:

"Modulator resting current should be set for 65 - 70 mA for 6L6 tube types, 80 - 90 mA for 6CA7 and 90 - 100 mA for the 6550 and 8417."

http://www.amwindow.org/tech/htm/rangtron.htm

Pat,

   I have tried the cathode feedback myself with a single ended tube output circuit. The results certainly were helpful. In fact most if not all single ended output setups could benefit from this. Think of the usual 6V6 to 8 ohm output, common to many BA receivers. Just return the cathode to ground at the speaker terminal (keep the cathode bias + parallel bypass cap in the current path)...as long as the phasing is correct, gain goes down slightly, and linearity goes up a lot more.

Was just reading this from Patrick Turner:

http://www.turneraudio.com.au/OTL-amps-pros-cons.html

Here he used cathode feedback with a 6C33 big triode single ended output stage. Scroll down to the last circuit presented.

Jim
Wd5JKO