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Old 6th July 2012, 02:11 PM   #21
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Is it just me? - but I'm pretty horrified at the thought of a guitar amp with series heaters, I've never seen one - is it an exclusively Peavy thing?.
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Old 6th July 2012, 05:10 PM   #22
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by Nigel Goodwin View Post
Is it just me? - but I'm pretty horrified at the thought of a guitar amp with series heaters, I've never seen one - is it an exclusively Peavy thing?.

Why? No reason why it shouldn't work well that I can see - what is it that concerns you?

Of course I think that current control for series connection is a good idea, and that ramped start-up for all valves is quite possibly valuable, if it can be achieved economically, and more importantly, reliably.....
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Old 6th July 2012, 05:45 PM   #23
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Something to verify,

I think the original 12ax7's were designed to have their heaters connected in series.

But the 12ax7a versions, allowed to have the heaters connected in parallel.

Incandescent light bulbs, usually burn out when first switched on.
The same thing for the heater section of a tube.



BTW: When power is applied to a tube, the heater can draw more than double what the spec's state for current draw.

So for this reason, current regulation for the heater element prolongs the tube life
much better than voltage regulation.
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Old 6th July 2012, 06:35 PM   #24
Simon B is offline Simon B  United Kingdom
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The US valve/tube numbering system approximately indicates the heater voltage in the first number - thus a 12AX7 has a 12.6v heater, whilst a 6L6 is 6.3v. The "12" series valves were apparently designed for use in car radios, with HT from a "vibrator" type supply.

The lightbulb analogy is worth considering, not because heater failure is common; as Enzo has pointed out, it isn't, but because of the underlying cause. Mechanical stress due to high thermal gradients in materials with none-zero temperature coefficients of expansion.

The glass of a valve envelope is very well matched for tempco with the material of the pins. But when there's a difference in temperature between the glass and the pins, there's still a problem.

This same principle will apply throughout the whole valve.

I can't begin to quantify this, but the slower a valve heats up and cools down, the smaller this effect will be.
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Old 6th July 2012, 09:42 PM   #25
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Simon,

I suspect you are right - Valves (tubes) don't like thermal [or physical] shock.
Even Wine or Beer doesn't like it.

There have been tests on tubes - and it seems that using current regulation for the heater - instead of voltage regulation - prolongs a tube's life.



Hopefully going a slightly off topic is OK.

Have you seen any curves that describe how the transconductance of a tube changes with use/time ?

Thanks,
.
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Old 7th July 2012, 12:12 AM   #26
Enzo is offline Enzo  United States
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I mostly participate in pro audio forums, so there is something I need to be cautious of here. Here we have a mainly hifi membership. The needs of a guitar amp are different from those of a hifi sytem. (or a radar or a 1950s computer for that matter). SO someone bringing a hifi sensibility to the discussion may have different assumptions than someone like me.

In a guitar amp, we will be wearing out tubes a lot faster than anyone else would, and we expect to replace power tubes yearly or more often. Just as tires for your car (that is tyres for some of you I suppose) will present diffferent concerns if they are on mom's family car versus junior's drag racer. SO concerns over arcane things like cathode poisoning, or heater lifetime and such are really not an issue to us. The tubes will have ended their useful life long before those things show up. The hifi guy on the other hand will expect a longer life from his tubes and will be operating them in a much more polite manner. And in the case of that ancient computer with its roomful of thousands of tubes, well anything providing even a small incremental improvement in tube life matters.


Nigel, this particular whole amp series string is fairly recent even for Peavey, as far as I know. I can't think of anyone else doing it offhand, but there may be others. Preamp tubes, mainly 12AX7s, have been run in series by any number of amp makers for some time. I am a bit more aware of Peavey designs as I do more repair volume on their products than most others. They sell just tons of amps. They and Fender. Other brands don;t sell nearly as well. But a quick look shows 12AX7s in series across 24vDC in the Peavey Rockmaster preamp, and that was designed in 1991. So it is nothing new.
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Old 7th July 2012, 12:52 AM   #27
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by Enzo View Post
........someone bringing a hifi sensibility to the discussion may have different assumptions than someone like me.
........
In a guitar amp, we will be wearing out tubes a lot faster than anyone else would, and we expect to replace power tubes yearly or more often
.......
SO concerns over arcane things like cathode poisoning, or heater lifetime and such are really not an issue to us. The tubes will have ended their useful life long before those things show up.
......
that ancient computer with its roomful of thousands of tubes, well anything providing even a small incremental improvement in tube life matters.

Good points Enzo, though in fact the increase in life achieved by Tommy Flowers on the Colossus was up to about three times the expected figure, which did stick in my memory.

It's precisely because guitar amps get through tubes so fast that this interests me. I fully accept that actual heater failure is rare - I never did finish the christmas-tree-lights-out-of-dead-valves project, a house move consigned them to the garbage. My dead-bin is growing again though, so
maybe this year...they were series connected too, come to think of it.

The physical properties of at least some of the materials used in valve construction, like hardness, ductility etc change a fair bit over the big range of temperature change that valves experience. So it seems possible that over
the first 100C of temperature rise a valve might have a different sensitivity to the speed of that change, compared to over the last 100C. And of course some bits get hotter than others.

I'm trying to phrase this in non-partisan terms as far as possible, as I'm interested enough in this to be willing to lose the bet to learn a little.....

In the meantime it seems that the only reliable information applicable to guitar amps is.................

Never turn them off!

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Old 7th July 2012, 02:35 AM   #28
Enzo is offline Enzo  United States
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well, I claim no particular expertise in the matter other than experience. In the guitar amp world there are always debates about whether to have a standby switch and where to put it and how it is to be implemented, and in those someone always brings up cathode poisoning or stripping, and other tube life things. There may be something to it, but in my experience, no partiicular approach to amp circuits seems to make more of a tube-eater than any other style. Like guitar strings that will continue to function long after they have lost their tone, power tubes wear out and lose their tone long before they get weak emissions or suffer other failures. And unlike hifis, guitar amps are thrown into trucks and hauled down the highway time after time, not to mention the combo style amps have the speaker in the same box as the amp and her tubes, vibrating away.
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Old 7th July 2012, 08:34 AM   #29
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Quote:
Originally Posted by Simon B View Post

Why? No reason why it shouldn't work well that I can see - what is it that concerns you?



For an obvious start, the inability to unplug valves and leave it working

There just seems no advantage to it?, and the valve series used aren't even intended to be used in that way - hence the problem over their different heater currents.

There are specific valve series intended to be used in this way, the P series (300mA) and the U series (100mA), the E series (as commonly used in guitar amps) are designed for 6.3V parallel heaters. The double triodes (ECC8x) have the major advantages of centre-tapped heaters, so you can wire them in various ways (it might be sad, but I can still remember the pin connections on the double-triodes).

Quote:

Of course I think that current control for series connection is a good idea, and that ramped start-up for all valves is quite possibly valuable, if it can be achieved economically, and more importantly, reliably.....
You could just as well soft-start parallel heaters as well. but is it really worth it? - the technology is all but obselete, about 100 years old, and hasn't needed soft-starting in all that time.

Admittedly valve heaters do VERY occasionally go O/C, and soft-starting 'may' help to reduce that - but I feel the soft-start circuit is likely to be far less reliable than the heaters it feeds.
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Old 7th July 2012, 09:57 AM   #30
Enzo is offline Enzo  United States
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I made a case for it earlier. A common problem in guitar amps is the high current connections for 6.3VAC burning up on the inter-board wiring and even the transformer wires plugging onto the board. By running them in series like that, they need only a 0.9A connection, and the resulting higher voltage winding can be lighter.

Commercial amps are not designed for running with tubes missing. Many do run that way, but they are not designed with that in mind. When they want to disable a tube, they turn it off via the active elements, not the heaters.

Those particular tubes may not have been designed for series strings, but it works fine and causes no problems, at least in the examples we see.
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