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Author Topic: Peavey Classic 30 Output Tube Filament Wiring  (Read 7594 times)

Hawk

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Peavey Classic 30 Output Tube Filament Wiring
« on: April 21, 2015, 09:09:50 AM »
From what I understand the EL84 output tube filaments in a classic 30 are wired in series so if one tube malfunctions then that stops the voltage going to the other tube filaments so the other tubes will also stop working. If that is true is there a way to wire the filaments in parallel? Or is that something we wouldn't want? Was the output section designed that way for a good reason?  Thanks.

Matec

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #1 on: April 21, 2015, 10:54:49 AM »
Yes your assumption is correct, if a valve fails, all other will turn off too.

The 12ax7 also have their filaments in series, if we remove one 12ax7, no pre valve will work.

To connect all the EL84 in parallel you need a source of 6.3V AC. (another transformer).

The reason, well, there is the Peavey that can answer ...

Cheers

Hawk

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #2 on: April 21, 2015, 12:16:13 PM »
Thanks Matec.So when an EL84 fails is it possible that it's filament will still be intact and feed voltage to the other series connected output tubes?  :-\

Matec

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #3 on: April 21, 2015, 01:22:41 PM »
Yes, it is possible.

Despite the valve need the heaters to work, are independent circuits. So, in the EL84, the heaters may work, but the valve can still be damaged.

But one thing is certain; not keep damaged valves in an amplifier. :grr

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Roly

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #4 on: April 21, 2015, 02:41:20 PM »
Quote from: Matec
if a valve fails, all other will turn off too.

Hi Matec, welcome; but that needs a little qualification.

In old b/w ac/dc TV's the entire heater chain was in series so all those valves could be heated directly by mains voltage, and if one failed they would all go out (like xmas tree lights).

In the typical guitar amp situation however the 12.6V is applied to each twin triode and to the two output valves in series (they are not "independent").  If one of these heaters fails then its companion goes out, but not all the heaters - they are series pairs in a parallel string.  In a 12A?7 this means that one heater failing will make both in that tube go out, and that if one output valve heater fails they will both go out.  All other intact series strings will remain alight.


Quote from: Hawk
So when an EL84 <filament> fails is it possible that it's filament will still be intact and feed voltage to the other series connected output tubes?  :-\

No.  Valve heaters are like light globes, they are either intact and passing current, or they are open circuit with no current flowing, and terminally stuffed.  You will get the entire heater supply across the open heater, and nothing across the good one (because no current is flowing, the failed valve has effectively switched the good one off).

One basic reason for using a 12.6V heater circuit rather than the more traditional 6.3V circuit is that the transformer required is marginally less expensive (current capacity pushes up transformer cost slightly faster than voltage).  To the owner builder this is of no matter, but when belting out hundreds or thousands of a design the bean counters get to have the final say over engineers.

At first glance there may not seem a lot to choose between running a heater circuit at 6.3 or 12.6 volts, but from a design PoV there is quite a big difference.  It has been traditional valve amp practice to run the heaters on 6.3V and to ground the center point of the heater winding, meaning that it's actually two 3.15V circuits in anti-phase.



Inside the almost universal twin-triode used in the preamp stages this means that there is a virtual earth point in the middle of each section cathode, and therefore the AC voltage applied to the heater, being anti-phase, cancels out capacitive coupling of heater mains hum to the cathode.



Apart from using a center-tapped heater winding the same effect can be obtained using a phantom mid point created by grounding the mid point of two 100 ohm resistors in series across the heater supply.  In fact in high quality valve amps this is often a 100 ohm pot with a grounded wiper across the heater supply, known as a "hum dinger", which is adjusted for minimum hum output, best AC balance to ground.



12.6V heaters give up this possible benefit because the phantom mid-point, the virtual earth, now occurs between the two heaters, not in the middle of their cathodes, so hum balancing is impossible.

When building, the commonly available transformers with a 6.3v 2Amp winding are often larger and somewhat more expensive than 12.6V 1Amp trannies, however the lack of high voltage trannies these days mean that many owner-builders use a pair of transformers back-to-back, and one of the more common and therefore fairly cheap options are transformers designed for QI downlights, mains voltage to 12.6V at an amp or several, and given that this arrangement normally has spare 12.6V current capacity it makes sense to use this intermediate voltage for the heaters.



I want to introduce you to a dead German bloke, Kirchhoff (1824 – 1887, who came along not long after Ohm, and built on his Law).


Gustav Kirchhoff

Kirchhoff formulated two Laws that carry his name; Kirchhoff's Current Law (KCL) and Kirchhoff's Voltage Law (KVL).  Now while these look a bit complex they are really very basic in concept, almost "obvious" - electricity does not accumulate anywhere in a (resistive) circuit.

Quote from: KCL
The principle of conservation of electric charge implies that:

    At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node

or equivalently

    The algebraic sum of currents in a network of conductors meeting at a point is zero.



If that were not the case we would be getting excess electrons in some places and a shortage in others, but that doesn't happen because what flows in also flows out.

From this we also get that in a simple circuit loop, such as output valve heaters in series, the current is the same everywhere in the loop.


Similarly;

Quote from: KVL
The directed sum of the electrical potential differences (voltage) around any closed network is zero, or:

    More simply, the sum of the emfs in any closed loop is equivalent to the sum of the potential drops in that loop, or:

        The algebraic sum of the products of the resistances of the conductors and the currents in them in a closed loop is equal to the total emf available in that loop.

http://en.wikipedia.org/wiki/File:Kirchhoff_voltage_law.svg

The sum of the voltage drops around your circuit is equal to the supply voltage.  Again, if it wasn't then electrons would be in excess or shortage somewhere.

The normal student reaction at first is "gee, that's heavy", followed a few minutes later by "what?  That's so obvious why does it need a Law?"  Maybe, but an intimate working understanding of both KVL and KCL are essential to effective faultfinding.

If you say theory and practice don't agree you haven't applied enough theory.

Matec

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #5 on: April 21, 2015, 03:28:36 PM »
Hello Roly.

Thank you very much for your concern for the readers of this forum.

Really his explanation was huge and I'm even having trouble understanding all text content, due to English not being my native language.

But do not worry about my qualification.

The Peavey Classic 30, whose schema I have relied to give my answers, this is it.

And in them we can see that all EL84 have their filaments in series with AC power (probably a 25v). And the 12ax7 have their filaments powered by -36V DC.

It is unusual, and the principle is strange, but it is a problem of Peavey and not mine.

The statements I made, were specifically related to Peavey Classic 30

I always take care of my answers, and I think everyone should do the same.

Cheers

Roly

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #6 on: April 21, 2015, 04:17:10 PM »
Now we have the actual circuit, I stand corrected.  This particular amp uses a singular 36VDC heater circuit.

My remarks above relate only to the almost universal use of 6.3 or 12.6VAC heater circuits in valve guitar amps.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #7 on: April 21, 2015, 04:44:27 PM »
Yes, two separate series strings in that amp, one for power tubes, and the other for preamp tubes.

Yes, if the heater on one tube opens then the rest of the tubes go dark.  However, open heaters is really not a very common tube failure mode.  Can one tube fail but the heater remain intact?  Of course it can.  A tube can fail in many different ways.  If the screen shorts to the plate, that won't open the heater.  When you say a tube malfunctions, it doesn't specify the heater opening.

Don't worry about it.  If all your power tube heaters are dark it is FAR more likely that there is a failed solder joint or failed jumper wire between boards, than for a tube heater itself to be open.

The reasons Peavey did this is pretty straightforward.  Many amps over the years, Peavey included, have had trouble with the high current heater supply burning contacts in inter-board connections.  By running the tubes in series, they can run 24v at one quarter the current, making the amp much more reliable.  Could you rewire the heaters in parallel?  Sure, all you need is to add a 6v transformer to the amp, then isolate the heater wiring traces from the board and run wires.  But I see no value in doing so.

Hawk

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #8 on: April 22, 2015, 09:11:52 AM »
Quote
My remarks above relate only to the almost universal use of 6.3 or 12.6VAC heater circuits in valve guitar amps
Roly I've always had it in my head that tube heaters were only capable of handling a small voltage but not a large one--I checked a 12AX7 data sheet and it showed max rating Vh-k 180volts.

Sorry  I didn't post the schematic immediately and that created some confusion.However, out of that came same great info and learning. Thanks Enzo, Matec and Roly (nice long detailed answer :tu::tu:

Matec

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #9 on: April 22, 2015, 11:20:06 AM »

Hawk

I think you are confusing information from the datasheet.

The tension of the heaters of a 12ax7 is 6.3V in parallel, or 12,6v in series.

The VH-K refers to the voltage of the cathode in relation to the voltage of the heaters. Since the cathode and the heaters are very close, this tension should not be above 180V under a risk of voltage leak or electrical sparks between these parts of the valve. (I hope this has been easy to understand).


Cheers

Roly

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #10 on: April 22, 2015, 12:11:02 PM »
Well I keep saying that our assumptions are what bring us unstuck - I made an assumption and promptly came unstuck.


Quote from: Hawk
had it in my head that tube heaters were only capable of handling a small voltage but not a large one--I checked a 12AX7 data sheet and it showed max rating Vh-k 180volts.

We need to be clear here about the difference between the heater voltage and the heater-to-cathode withstand voltage, Vh-k(max).

I've encountered valve heaters that range from 1.2V right up to 115V and lots of different voltages in between depending on the intended duty (but never a 240V one  8|), but by far the most common are 6.3VAC.

This is just the heater voltage, the bit that lights it up, and is pretty similar to a light globe, only duller.  As a Rule of Thumb for the valves we encounter with thoriated tungsten cathodes this should be within +/-5% of the manufacturers specification - at the socket.



The heater is within a long thin cylinder that is the cathode and is insulated from the cathode with what looks like a ceramic material.  Like all insulation this has an upper voltage limit and that is what the Vh-k(max) rating is, the maximum heater-to-cathode voltage allowed.  Typically 100-200V this also can vary over a wide range depending on the intended service of the valve.

With the types of valves we commonly encounter in guitar amp circuits the heater-cathode voltage rating is hardly ever an issue, but the split-load phase inverter is a common situation where it might come a bit close.



This stage idles with about two-thirds of the supply on the anode and about one-third on the cathode.  Assuming a 300V supply that will be Va=200V and Vk=100V.

On positive signal swings the valve will turn on and at full drive the anode and cathode voltages won't be much different, somewhere around half the supply or 150V, and that's not far off the maximum 180Vh-k rating for 12AX7's and the like (keeping in mind the actual heater power is normally at ground potential).  The implication is that for that valve the supply shouldn't be higher than 400V 360V.

The cathode follower, say used to drive a tonestack or as a Line Out or Fx Send buffer is another case where we need to take care because it may be possible for the cathode to go almost to the supply voltage on positive signal peaks.

Note that these are design considerations, say if you are building your own amp, but heater-cathode failure is quite rare in general amp servicing.


(ed to correct voltage)
« Last Edit: April 23, 2015, 05:40:57 AM by Roly »
If you say theory and practice don't agree you haven't applied enough theory.

Matec

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #11 on: April 22, 2015, 01:20:34 PM »

Roly

I say, without reservation, that this post is perfect.

Thank you very much!

Matec

Roly

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Re: Peavey Classic 30 Output Tube Filament Wiring
« Reply #12 on: April 22, 2015, 01:36:20 PM »
If you say theory and practice don't agree you haven't applied enough theory.