12.6 vac filament wiring 12AX7s

[mresistor]

I have a 12.6 vac transformer that is not center tapped. Can I use this to power the filaments of 12AX7s in series? How is it implemented?

[Willabe]

Yes.   

12A _ 7's can be run at 12.6acv, just use pins 4 and 5, 1 leg of the heater wind to pin 4, the other to pin 5, leave pin 9 open. (Pin 9 is the CT of the tubes heater.)

For the heater CT use 2x100 ohm 1/2w metal film R's, just like with a 6.3acv heater wind with no CT. Wire the 100 ohm R's, 1 each from 1 leg of the 12.6acv wind to ground.


              Brad     

[mresistor]

Thank you Brad. To use a little of Merlins terms...    it appears to be enormously simple.   

[sluckey]

I seem to recall 12AX7 filaments being wired like this , but I can't remember where. Pretty sure it was some old digital circuits. Anyhow, pin 9 was used as a ground reference like a center tapped winding or artificial CT.

[mresistor]

That's interesting Sluckey. Another idea to try.  Willabe - is that method you suggest for SERIES wiring of more than one small bottle?

[sluckey]

Willabe - is that method you suggest for SERIES wiring of more than one small bottle?

All tubes would be wired parallel. IE, if you have three 12AX7s, pin 4 of all three tubes would connect together and to one transformer lead. Pin 5 of all three tubes would connect together and also to the other transformer lead.

[mresistor]

Sluckey, thank you sir.  I understand the parallel wiring aspects. What I am looking for is any info on wiring the 12AX7's in series with 12v, as in my original question.  I assume that one side of the transformer secondary winding will have to be grounded and the other pass through all the filaments. In Merlins discussion he says;

(quote) "Heaters which are normally designed to be run in parallel can be run in series provided you ensure their current demands are met correctly. For example, an EL84 and ECC83 could be run in series from a 12V supply."

(quote)" Because the heaters are in series it doesn't matter in which order the valves are wired. However, if one end of the (AC) heater chain is to be grounded then the most sensitive preamp valves should be closest to the grounded end of the chain where the voltage swing (i.e. hum field) is smallest. "

ref:Heater/Filament Supplies

So, is it possible? What is the upside and downside of wiring series heaters with 12vac?

[kagliostro]

So you mean 12v tubes placed in series (some 12ax7 with internal filament in series, as an example)

and not only the internal filament of one 12a*7 tube placed in series as to be connected to a 12.6v source

Some old amp used to connect that way the tubes, like some Geloso PA Amp

but usually this was done because they use that higher voltage also for some other purpose

the bias circuit on some Geloso PA





---

This if I've correctly understand you

If you want to know if is good practice to connect one branch of the 12.6v to ground, it isn't (the same for 6.3v)

avoid the use of the chassis as part of your heater circuit, don't connect it to ground if there isn't a CT or if you want less Humm

use a Fake CT build with 2 x 100R (or 2 x 220R or 1 x 100-500R pot with wiper connected to ground)

K

[sluckey]

I have a 12.6 vac transformer that is not center tapped. Can I use this to power the filaments of 12AX7s in series? How is it implemented?

Yes. By operating the 12AX7s in 6.3V filament mode. You can only run two AX7s in series like this. One transformer lead will connect to pins 4/5 of tube one. Pin 9 of tube one will connect to pins 4/5 of tube two. Pin 9 of tube two will connect to the other transformer lead. You can add more pairs of series connected 12AX7 across the transformer leads.

So, is it possible? What is the upside and downside of wiring series heaters with 12vac?

Yes, it's possible. I don't see any upside. Down side is it's complicated. Just because you can do a thing doesn't mean you should.

[kagliostro]

May be I'm misunderstanding what you mean

What is the upside and downside of wiring series heaters with 12vac?

if you mean to put in series two 12ax7 wired for 12.6v, the upside is you can have a higher voltage winding usable for other purpose (bias as an example)

if you mean to put in series the internal filament of a 12ax7 to operate it at 12.6v instead 6.3v, the upside is you have less current requirement and less current mean ideally less humm predisposition

K

[mresistor]

Thank you Kagliostro and Sluckey - that was what I was looking for ..  That Merlin guy is pretty knowledgeable but doesn't give practical examples sometimes.

[sluckey]

Please notice that the examples K posted are for series filaments using a DC source. That's pretty common, but not what you were asking for.

And there is nothing practical about using 12AX7s in 6 volt mode and stringing two in series across a 12.6VAC source. In fact, it's very impracticable and the wiring will be unduly complicated. Just wire it simple and straightforward so each tube uses 12.6VAC to heat it up. Very simple and clean.

Now if you had two 6BQ5s and only had a 12.6V heater source, by all means, wire two in series across the 12.6v source.

[PRR]

> wiring the 12AX7's in series with 12v

quote "Heaters which are normally designed to be run in parallel can be run in series.... if one end of the (AC) heater chain is to be grounded then......

You are confusing two situations.

You have a 12V transformer.

You have tubes which can eat 12V (or 6V, but that's moot here).

You will wire all the 12V tubes and the 12V transformer all in *parallel*.

We do not really care that *inside* the tube we really have two 6V heaters in series.

Merlin (and I agree that he knows so much he sometimes gets ahead of less experienced readers) is talking about "series-string heaters", a concept we mostly do NOT see in classic amplifiers. (However it is common on some newer jobs.)

Heaters have to be robust, so they must be short and fat, which means low voltage and high current. However we have this handy 110V wall outlet! And we have a lot of tubes. Can't we wire the heaters in series to make-up a high-voltage load? Say nine 12.6V heaters in series.... that's about 110V-117V. Yes, that will work, if they are all the same current. More likely we have some bigger tubes, the rectifier and power tube. These can be built with high-voltage same-current heaters. A radio may use a 35V rect, a 50V power tube, and 12V 12V 12V for 1st-audio, IF, and converter. 121V!

In this case the most hum-sensitive tube (1st-audio) should be on the groundy end of the string, the rectifier on the top, and probably the power bottle just below the rect heater.

However this SHOULD not interest us, because a guitar-amp without a power transformer is *deadly*. (Not so much for radio or TV, where the user *never* has to hold-on to the circuit, the way you hold the ground of the guitar-jack.)

You could use a 36V transformer to power three 12AX7 in series, but why? (I can contrive some found-parts situations, but that's getting desperate.)

A somewhat different case: amp on PCB and DC heater power. They go together. PCB construction does not favor the twisted-lead wiring that makes AC heat tolerable. DC supplies are now fairly cheap. However not dirt-cheap. And also DC supplies have losses which are about a Volt for each diode. If you aim for 6V, but need 8V to cover diode losses, there is 33% waste. Adds cost and heat. If you wire three 12AX7 at 12V each for 36V, you need 38V, only 5.5% loss, 1/6th the heat and lower cost. Also series-wiring is easier to sneak through the PCB puzzle. However, being presumably very-clean DC, Merlin's note about *AC* series-string radios and TVs does not apply.

[jjasilli]

I additon t Merlin & PRR, checkout this thread:  http://ampgarage.com/forum/viewtopic.php?t=20060&view=previous&sid=e434179cc4c4dce0db8038dfa9d36850


As to sluckey's idea of grounding pin 9, Clever.  But this assumes that the resistances are equal on the 4-9 heater and the 5-9 heater, or there will be a hum imbalance.  Also the the guru's say, essentially, to ground the 1st preamp tube.  But that links 60Hz AC to the wrong side of the chassis; maybe better to run that ground wire back to the other side of the chassis.  OTOH, no fault, no foul.

[mresistor]

I was just interested to know if series wiring could be accomplished and how to go about doing it. Merlin wasn't real clear on how to do it with two 12_7s. I see wiring fils in series with two 12_7s it's like a voltage divider.
I don't know exactly the amp I plan to build with it yet. Sluckey I noticed the DC filaments in those schemos also noticed they are basically unregulated.
I conclude that its way more practical and easy to use an artificial ct. and wire fils parallel with 12v.

I might use this in a SE build with a KT88.

I assume one would have to ground all the pin 9's and I'd use the main grounding point. I wouldn't do a Leo with it.

[sluckey]

I might use this in a SE build with a KT88.

How will you heat the KT88?

[jjasilli]

re grounding ALL the 9 pins in series filaments:  No.  Just one ground reference point. 


In the series circuit, the guru's are suggesting to ground the 9 pin of the 1st 12a_7 preamp tube.  (But the signal input side of the chassis may be a poor place do that - so I suggested running a ground wire back to the other end.)

[mresistor]

I might use this in a SE build with a KT88.

How will you heat the KT88?

Probably with a PT that has filament winding.

JJ I get it now.   

[PRR]

> suggesting to ground the 9 pin

IMHO, that's a "cheap trick".

Nothing very wrong with it. The two-halves likely balance as good as 10% resistors, and hum-balance should not be critical.

But it is non-obvious, which will confuse someone some day. There's enough confusion in the world. Saving two 12-cent resistors is not IMHO worth it.

> 60Hz AC to the wrong side of the chassis

If the heater winding on the PT is *isolated*, there "can't" be any current in that ground return.

The real heater-winding will have ~~~300pFd of stray capacitance to primary, but this is insignificant.

If the heater winding has appreciable leakage from somewhere, then you could have trouble.

I'm not sure what all this discussion is in aid of. Merely to avoid spending $10 for a 6V filament transformer? When OP now proposes a 6V power tube?

[mresistor]

The KT88 requires 1.6a filament current, that in combo with two small bottles would be 2.2a. A PT with a 2 amp winding would probably work, but I personally wouldn't want to run a filament winding at over 75-80% of it's rating. So, in that case I would use this 12v trans. That makes sense doesn't it?

[sluckey]

Probably with a PT that has filament winding.

   

I think this thread has gone to the Borg!

[mresistor]

PRR    thank you sir for your input. I was just curious as to how to use a 12v trans to wire 12_7 filaments in series. That aid has been given and received and now I understand. So all is not lost. I do have a 12v trans and would prefer to use it. I have a PT whose 6.3vac filament winding is only rated at 2a so this 12v trans may come in handy. Then again, it may sit on the shelf for a long time.

[sluckey]

In the series circuit, the guru's are suggesting to ground the 9 pin of the 1st 12a_7 preamp tube.

I'm the one that mentioned grounding pin 9. But that was in a pure parallel circuit. I don't think it was suggested to ground pin 9 in the series circuit. If I said that, I sure didn't mean it.

[jjasilli]

The KT88 requires 1.6a filament current, that in combo with two small bottles would be 2.2a. A PT with a 2 amp winding would probably work, but I personally wouldn't want to run a filament winding at over 75-80% of it's rating. So, in that case I would use this 12v trans. That makes sense doesn't it?

No.  The first post is about running 12ax7 filaments in series @ 12V (and we're talking VAC).  When combining in series you need to add the voltages for ea tube: 2X 12ax7 tubes wired for 6V strung is series = 12V.  Add a 6V power tube; now you need 18V (not 12).  OTOH, it would be futile to run the 6V KT88, in parallel, off a 12V heater supply.  May as well put 2X KT88's in series 


BUT, the filament resistances (impedances!!) ALSO have to match when strung in series.  No problem for the 2X 12ax7 tubes.  A resistor will have to be added to balance-out the different filament resistance of the power tube.  See the Merlin site.

[jjasilli]

I have a 12.6 vac transformer that is not center tapped. Can I use this to power the filaments of 12AX7s in series? How is it implemented?

I think confusion has arisen from the way this question was asked.  Should it be taken literally!?!


A 12a_7 tube's filaments can be run, at your option, either at 6V or at 12V, depending on how you decide to wire pins 4, 5, and 9.  With a 12V transformer, you can wire 2 such tubes for 6 volts and run them in series for a 12V supply.  Or, you can simply wire them in parallel for 12V operation.  But, given a 12V supply, as sluckey asks above, how will you supply the 6V power tube filament?


EDIT:  a solution is to run 1X 12ax7 in parallel with a [12ax7 in series with the power tube].  This will require a balancing resistor.  I am not actually recommending such a circuit, but it is doable.  See the Merlin site.

[mresistor]

JJ - this 12vac trans would be an auxiliary filament supply to augment the PT that has a filament winding in it not rated with enough amps to drive all the tubes.

[jjasilli]

OK, then the thread I posted above from the other forum tells you how to make a virtual CT using a pair of 100 Ohm 1 W resistors.  Then wire your tubes in parallel for 12V operation (Pins 4 & 5)


But, statistically speaking, it seems that no one is doing this.  Why?  Probably because with 12VAC, you can rectify it to a regulated DC supply.  12VAC X 1.414 = 17VDC.  That's enough to regulate down back to 12VDC and have a real quiet supply.  Contrast with 6VAC X 1.414 = 8.5VDC; hardly enough to regulate.  AS regulation squishes the voltage back down, it also squishes away hum & noise.

[mresistor]

JJ - that's a good idea, I'll have to make sure to get a chassis that has more real estate. For my tired old eyes and for the added components. Appreciate the info.

[jjasilli]

@ drgonzonm:  Your points are well-taken but are no longer an issue.  mresistor has further explained that his 12V tranny is an auxiliary needed only to supply the filaments of 2X 12ax7 tubes.  The power tube and any other tubes are already "covered".

[Leevi]

Old topic but thought to inform that the following wiring is now tested and it works.
The grounding of pin 9 is needed in order to avoid hum.


/Leevi

[VMS]

Did you ground pin 9 on every tube or just on one tube?

[tubeswell]

Some more examples

[Leevi]

Did you ground pin 9 on every tube or just on one tube?

It is a single ended amp with one 12AX7 preamp tube. Power tube has own 6.3v supply
grounded with 100 Ohm resistors.

/Leevi

[VMS]

ok, thanks.


I was just wondering if grounding the pin 9 works as a CT for the heater supply, maybe its enough to just ground one pin 9 if there are more than one tube.