E.E. Design Help Sought, on Ef Biasing

dowto1000

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Hello All,

I am not sure what sort of VDC value should be used, to apply to the center tap of a separate Ef XFR, to bias (above ground ) a 6AQ5's filament, when that 6AQ5 is operating as the Finals....... in a two -stage DC SEP amplifier.

I seek conservative long-term reliability. I would love to get suggestions, from EE types, who can assist me, a DIYer, with no E.E. formal training.

Any and all well-meant help will be very much appreciated. I am " hazy" on estimating " DC and PEAK " voltages, and could truly use help.

Many many thanks !!

Dowto1000



Heater to Cathode SPECS .....  6AQ5.JPG
 
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What are the cathode voltages on your other tubes? If this is all a common heater string you'll need to find a happy medium that works for all the tubes. If the only thing on this heater string are the output tubes, honestly the easiest thing to do is simply tie the center tap of the heater winding to the cathode of the output tubes. V H-K would be zero at that point. Failing that, a voltage divider off B+ to land it where you need it would be the sensible choice.
 
What are the cathode voltages on your other tubes? If this is all a common heater string you'll need to find a happy medium that works for all the tubes. If the only thing on this heater string are the output tubes, honestly the easiest thing to do is simply tie the center tap of the heater winding to the cathode of the output tubes. V H-K would be zero at that point. Failing that, a voltage divider off B+ to land it where you need it would be the sensible choice.


Thanks for responding !! Yes, I was planning of using a simple voltage divider off the main B+, and a 10 uF film cap to ground.

I have 1.75 VDC on the cathode of half a 6SL7 driver tube. But that 6SL7 has its own separate Ef supply sir.

I plan to use a separate Ef XFR specifically for the 6AQ5s, to try to reliably optimize the Finals' operation..

Thanks, helpful, lets see what others may suggest. Conservative reliability is the long term goal here, avoiding tube problems on audio peaks, etc etc..

I don't KNOW how applying 177 VDC will apply to audio peak voltages, and to the 200 VDC maximum . Would applying something like about 120 VDC be better, more conservative long-term, to stay away from the 200 VDC spec?? Ideas??

Where in Southern N.J. ?? I was born and raised in Trenton.

Dowto1000
 
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Basically exit 1 on the turnpike, then about 15 minutes further south in Salem county.


The peak voltage is basically just the steady state DC bias plus any audio signal. If you have the heaters at +177v you're safe. That would actually allow the cathode to swing from -23v to +377v (relative to ground) and still be within what is acceptable for the tube. The actual voltage relative to ground isn't important, its the voltage difference between the heater and the cathode that matters.
 
Let's say you were to do a voltage divider from the B+ rail. The problem there is you may temporarily exceed max heater-cathode voltage if the B+ rail comes up faster or slower than the cathodes of the output tubes (for example you used silicon rectification and your B+ supply came up within a few seconds, but it takes the output tubes ~30 seconds to fully warm up). The best approach is to bias up the filament from its own cathode. That way the difference in voltage between heater and cathode will always be zero (in other words it would be just like having the cathodes tied directly to ground with a 6.3V supply driving the filaments), and the filament and cathode will always be at the same voltage potential regardless of the state of warmup of the amp.

If you have a center tapped filament transformer, simply connect the cathode directly to the center tap. If you don't have a center tapped transformer you can connect it to either end of the filament, or you can simulate a center tap by connecting two resistors of the same resistance value to the secondary ends, and connect the other end of those resistors together, and tie the cathode to the junction. Any size resistors from say 100Ω to 1KΩ will work just as well.
 
Let's say you were to do a voltage divider from the B+ rail. The problem there is you may temporarily exceed max heater-cathode voltage if the B+ rail comes up faster or slower than the cathodes of the output tubes (for example you used silicon rectification and your B+ supply came up within a few seconds, but it takes the output tubes ~30 seconds to fully warm up). The best approach is to bias up the filament from its own cathode. That way the difference in voltage between heater and cathode will always be zero (in other words it would be just like having the cathodes tied directly to ground with a 6.3V supply driving the filaments), and the filament and cathode will always be at the same voltage potential regardless of the state of warmup of the amp.

If you have a center tapped filament transformer, simply connect the cathode directly to the center tap. If you don't have a center tapped transformer you can connect it to either end of the filament, or you can simulate a center tap by connecting two resistors of the same resistance value to the secondary ends, and connect the other end of those resistors together, and tie the cathode to the junction. Any size resistors from say 100Ω to 1KΩ will work just as well.

Thanks Kward,

Its a all-tube DC SEP amp. So, I always employ tube rectifiers, for their sonics, and, also for start up considerations, typically - a directly-heated 5U4GB.

In a Direct Coupled tube amp, I start-up everything, all-together, at once, NO delays of any kind are used.

So far, from what I get out of this thread, it looks like I won't have to build a voltage divider and use a 10 UF film cap.

Lets see, however, if some EEs offer a different insight.

At this moment, I am all for connecting the center tap of the 6AQ5 Ef XFR to the top of the 5K 50 Watt Rk resistor.

I am very appreciative of you first two posters, sharing your initial thoughts with me. Thanks so much !!

I think this will become an important amp for me to build, for a variety of very good reasons. Mainly, I have " had the dance " already, past four years, with sonic shortcomings in DHT 45s and 46s on the living room's VOTTs, and this inexpensive pentode tube design may possibly.... easily....... whomp em both. A 6AQ5 is a superb sounding tube !!

That surely would be much fun to accomplish !!

Dowto1000
 
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