Load impedane for KT88/6550 in UL with Tertiary Windings (600v plate, 300v screen)

[GordonW]

OK. I'm getting ready to experiment here.

I have a pair of Bogen amp chassis... voltage doubler supply, with about 600v plate voltage. Each has an OPT, with about a 4.2K primary. They also have available tertiary windings, which would give about a 25%, ultralinear, connection for the screens... which can be connected to a lower B+ voltage (300v, easily had from the center of the voltage doubler power supply).

From looking around at the UL and pentode curves... it looks like this might be a good alignment. The UL ratio is less than "normal" (25% vs. 43%- closer to pentode mode than normal), but the primary impedance is higher than normal UL (4.2K here, as opposed to about 3.3K recommended for the 6550, in most tube manuals for UL).

It looks like this might be a good load-line for this tube... 4.2K, 600V plate, 300V feeding 25% UL from tertiary tap to screens...

Anyone see any big problems with my off-the-cuff derivations here? My only concern is possibly the lower screen voltage affecting the output impedance of the tube... but, it looks like it should be much less change than straight pentode mode would be, compared to "normal" UL mode (plate and screen at the same voltage)?

BTW: I'm probably going to use a set of Valve Arts 6550s in this amp, at least as a start. I got 'em, and they're supposed to be pretty darn good...

As for circuits for these- I'm still waffling between the Citation II and Heathkit W6 type architectures (Hegeman vs. Williamson, in essence)... though, with the reduced screen voltage (higher gain?) on the output tubes, a Mullard circuit might work?

Regards,
Gordon.

 

[jaymanaa]

I think you will need a good tough 88, and could always go to a KT 90 if need be. You know more about this than I ever will, so i'll be watching and learning.:yes: Douglas is probably the go to guy on different UL tap spots. If I'm not mistaken he has done some experimenting on that very topic.

 

[GordonW]

KT90s are my go-to, if the 6550s can't take it. I don't trust too many KT88s these days.

I'm thinking of dropping the idle current down to less than 50ma per tube, as a start. That would get plate dissipation down to below 30w per tube. I've seen amps like Rogue Audio and the like, get good sound from 6550/KT88/KT90 class tubes with as low as about 40ma per tube at idle...

EDIT: BTW- Jay, did you get a PM from me a little while back? About some tweeters? Lemme know...

Regards,
Gordon.

 

[jaymanaa]

KT90s are my go-to, if the 6550s can't take it. I don't trust too many KT88s these days.

I'm thinking of dropping the idle current down to less than 50ma per tube, as a start. That would get plate dissipation down to below 30w per tube. I've seen amps like Rogue Audio and the like, get good sound from 6550/KT88/KT90 class tubes with as low as about 40ma per tube at idle...

EDIT: BTW- Jay, did you get a PM from me a little while back? About some tweeters? Lemme know...

Regards,
Gordon.

Yeh, I've been trying to decide what to do about my W90 tweeter situation. Do you know of a current production unit with the same or better specs? Sorry for the OT.

 

[dcgillespie]

Gordon -- Typically, with 600 volts plate, 350 volts screen and fixed bias operation, 6550s in strict pentode operation with a 4.2K plate load will deliver 100 watts at the plates, and represent optimized operation. Using a similar scenario while adding any kind of UL influence on the screens will only act to raise the required plate load for optimum performance, and of course, reduce power somewhat.

Does the 25% tertiary winding represent 25% of the winding, or the impedance? That can make a huge difference in what is being represented.

One suggestion to consider if you are trying to go conventional UL; lower your plate voltage to about 550 volts. If the tap represents 25% of the winding, that will help align the tubes better to the load you are using, and allow you to (possibly) develop the drop across a regulator which would really help to maximize performance in a reduced load scenario.

Just remember that any transformer operating in UL requires extremely tight coupling between the plate and screen windings, or very weird and usually bad problems crop up.

Likely the best configuration to consider is to use the tertiary winding in the cathode circuit of the output tubes. Doing this has a number of advantages. First, if the screens are supplied from a regulated source, traditional UL is effectively in place. Second, it allows degeneration directly against the input signal for even greater distortion reduction effectiveness. Finally, it also raises the effective load that the tubes operate into, which in this case you need. This would minimize/eliminate the need for plate regulation, which is always a bigger task that simple screen regulation. This scenario will however require a stout driver circuit, which typical driver stages can usually supply easily enough, but remember that 50 K DC resistance grid #1 requirement.

Just some thoughts to ponder. I've been down this road with similar tubes and voltages, using an Acro TO-345, which is a TO-340 that also includes a balanced 500 ohm line winding that I used in similar fashion. Keep us posted on how your efforts progress!

Dave

 

[battradio]

You could also use the tertiary windings in the cathode circuit if they are completely seperate winding if it would simplify it any.

 

[GordonW]

I know the tertiary windings can handle the current of the screens... but I'm not at all sure they would be OK for the ENTIRE cathode current (plate plus screen).

I certainly don't mind running them in pure pentode mode. I could always add some plate-to-plate feedback to lower the influence of varying speaker impedance. Pentode mode would start out with higher gain, so I could "lose" some by adding feedback, back to the drivers. Looks like that might be the best go-to.

BTW: The tertiary windings are 25% impedance. But, I'm not absolutely sure how well they're coupled to the primary. There are two single windings, that I was considering connecting in a center-tapped fashion, to make one screen winding. One of them was originally a feedback winding (global NFB), which was the one I'd think might not be able to handle cathode current. The other one of them, was originally an "alternative" secondary for constant-voltage speakers... I'd expect that to NOT be that closely coupled. Hence, my thinking that pentode mode, with some enhancements with respect to local feedback, might be the way to go...

Regards,
Gordon.

 

[GordonW]

Here's the schematic of the original Bogen design:

That'll show how the original output transformer windings were set up.

I did determine that the feedback winding and the 70v secondary were the exact same ratio, by voltage-testing the transformer... coincidence, I guess... :scratch2:

Regards,
Gordon.

 

[jaymanaa]

I certainly don't mind running them in pure pentode mode. I could always add some plate-to-plate feedback to lower the influence of varying speaker impedance. Pentode mode would start out with higher gain, so I could "lose" some by adding feedback, back to the drivers. Looks like that might be the best go-to.

Now you're talking, I love pure pentode, and if you get some exrta umph for added feedback, that's a heck of a bonus. I'd do pure pentode with all the voltage I could muster from some Hexfreds, then do a sand regulator for the screens, ala Morgan Jones.

 

[jaymanaa]

That 4.2k pri z should rock man!

 

[battradio]

If the winding where used for the 70 volt out they should be able to handel the entire power out of the entire amp at a higher current than the cathodes , due to the step down ratio of the transformer .

 

[dcgillespie]

Gordon -- All you can do at this point is experiment with those windings to see how they can best be used to good advantage. I question that they have the necessary coupling to perform properly as UL taps, but stranger things have happened. I would try them in the cathode position first. If they will work there without the output stage becoming a multivibrator circuit, then possibilities exist.

However, something must be off though in the percentage of impedance these windings represent. The design of the output stage has it operating under the guidelines shown in the RCA Receiving Tube Manual for 100 watts output (plate) into a 4.2K load. If the primary winding does in fact represent 4.2K ohms, that means that roughly 650 vac is developed across the full primary winding when developing 100 watts at the plate. Assuming a perfect transformer (no loss -- which low wattage winding ratio tests emulate), that means that a 70.7 volt secondary would represent 50 ohms for 100 watt operation. Since each half of the primary winding represents 25% of the total plate-to-plate impedance, that means that each half of the primary is 1050 ohms. Since the the 70.7 volt winding represents 50 ohms and 1/2 of the "FB" winding, that then, compared to one half of the primary winding impedance means that the FB windings represent about 4.76% of the primary impedance.

On the other hand, both FB windings together (since they are the same ratio) would represent an output voltage of 141.4 vac when developing 100 watts. Against the ~ 650 vac required at the primary to develop that power (again, assuming no loss), that would place the FB windings at about 21.75% of the primary winding, which as you suggest, is low for UL operation.

In any event, I am concluding from all of this that the FB windings you are looking to use represent about 22% of the primary winding rather than 22% of the primary impedance -- all of which is very close to the 25% (though not by impedance) figure you indicated. Is this the case -- or am I not understanding your comments correctly?

Dave