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Old 25th June 2012, 01:31 PM   #1
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Default 12vac heater - dc elevated question -

I've got an old projector amplifier that uses 12vac CT tap for 6V6 filaments (push pull) in a funky way - one 6v6 gets the "start" side + CT (1/2 of the 12vac), and the other 6v6 gets the "end" side + CT (the other 1/2 of the 12vac).

Now, that all worked fine, with one of those going on to feed 6.3vac to the PI and preamp tubes that follow (12au7, 12ax7, and 12ay7).

However, in my brilliance I wired the following 12a?7 tubes for 12vac - pins 4 & 5. I did not initially tie in pin 9 to anything, but the hum tells me that I have to.

So, thinking that pin 9 is a center-tap to the filament, shouldn't I be able to wire that to a small DC source (output tube cathode) to "elevate" the heaters?

In other words, why bother with the typical 100 ohm pair of resistors to make an artificial center tap, when I have pin 9 available.

Also, I'm inclined to think that I need only tap pin 9 on one 12a?7 tube. Or would it be better to tie all the pin 9s together and then to DC.

I tried ground referencing them to their individual star points, but it wasn't much help. I then briefly jumped one to DC and it was a vast improvement.

BTW - I've avoided grounding the CT of the filament winding primarily because of how this amp uses it for the 6V6 heaters. But maybe I'm not thinking clear there.

Thanks for any help!
Jeff
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Old 25th June 2012, 03:38 PM   #2
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Yes, although it's rarely done, pin 9 can be a center tap. I believe this was the original intention, although historically, always ignored.
I have used pin 9 as a center point in DC heater setups, supplying 12 VDC to pin 4 and 5. My conclusion was to tie pin 9 with a capacitor, just from the standpoint of lower AC noise. Although, I like your idea.
I am not sure what the result of that idea will be, I have never tried it that way.
There is a surprising lack of attention to AC entering the audio path at the first preamp stage(s). And this AC ingress is amplified thousands of times by the following stages.
In lower gain amps, this will be less of a problem, in high gain amps, it becomes a real headache.
Normally, the output tubes and phase inverter will function on AC heaters. It is the preamp stages, especially the first one or two, that deserve the most attention with a DC heater setup.
The best DC heater setup is probably in the old McIntosh tube hi fi preamp. The warmup of the heaters is slowed, but the reduced AC noise is more than worthwhile.
There are a variety of sources, that cause AC hum in the preamp, besides the heaters, and each one of these is worthy of it's own discussion.
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Old 25th June 2012, 03:53 PM   #3
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Well, long ago this had 12vdc for the preamp tubes. unfortunately now I've so firmly secured the twisted pairs to the chassis, and beneath the rest of the point-to-point "rat nest" (though a tidy one) that I dread changing any wires down there. I suppose I could bridge rectify the 12vac and with some careful wire snipping have a series wound filaments, though.

but, the DC entering pin 9 was such a big improvement that I'm more likely to go that route for sake of ease. I suppose when I'm back at the bench I'll tie it to both preamp tubes. I may try the 100 ohm resistors to 4&5 to compare. I know I only have to tap in at one place in that method.

Thanks for the response!
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Old 25th June 2012, 05:32 PM   #4
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Results will depend on the circuit.

Does the use of opposite phase windings for the 6V6 heaters mean that the hum cancels in the PP operation?

The ECC8x valves with optional series/parallel heaters connect pin 9 to the junction of two heaters, one for each of the two valves in the bottle. It is not a centre tap in relation to either of the two.

A centre tap is used on a heater so that its average potential does not vary with respect to signal ground. Your "centre tap" does not achieve this, AFAICS. However, if the two valves are used in parallel, the ensuing hum might cancel.

Raising the average potential of voltage-amp valves can make a difference of its own. You may be confusing two issues.
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Old 25th June 2012, 08:33 PM   #5
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Does the use of opposite phase windings for the 6V6 heaters mean that the hum cancels in the PP operation?
Possibly the intention of the original design.

The ECC8x valves with optional series/parallel heaters connect pin 9 to the junction of two heaters, one for each of the two valves in the bottle. It is not a centre tap in relation to either of the two.
Thanks for this perspective

A centre tap is used on a heater so that its average potential does not vary with respect to signal ground. Your "centre tap" does not achieve this, AFAICS. However, if the two valves are used in parallel, the ensuing hum might cancel.

Raising the average potential of voltage-amp valves can make a difference of its own. You may be confusing two issues.


yes, adding DC at pin 9 does not regulate the potential with respect to ground.
however it is another means to reduce the hum that is present.

do you see a problem if I connect center tap to ground on the transformer - my concern being that it uses the CT as one leg of the 6v6 filaments?
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Old 26th June 2012, 02:41 AM   #6
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Quote:
Originally Posted by brilliantblue View Post
do you see a problem if I connect center tap to ground on the transformer - my concern being that it uses the CT as one leg of the 6v6 filaments?
I would leave the 6V6s wired as is with the centre tap elevated to say 40V DC and have the 12A*7s wired for 12V. This way the low level tubes get their ground reference about the centre tap. The 6V6s, handling higher voltage signals, don't care that their reference is not centred about their heaters.
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Old 26th June 2012, 02:53 AM   #7
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Default thanks for the responses

I tried tying pin 9 of both 12a?7 (skipping the PI) together and alternately to ground, CT of the heater taps, and to DC. I also tried tying them to the CT and then to DC. Ground was noisy, but the DC elevated worked great to pin 9. No noticable change with CT attached, so I left it unattached (one less wire).

Thanks for the input!
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