What is the purpose of paralleled triodes?

[12AX7]

I noticed a lot of amps that are considered exceptional have this. I was looking at a JMP marshall combo schematic just now that has v1 paralleled with different cathode values for each side but the plates both meet at the grid of V2. Others i see the cathodes are tied as one or the plates are or both. Is there a particular reason tonally they use any variations of these paralleled triodes?

[SILVERGUN]

Search...this has been covered extensively.
 
Excerpt taken from this ampage discussion:
http://archive.ampage.org/threads/1/gagd/011345/Parallel_preamp_tubes-1.html
 
"....it's rare to find a 12AX7 that has both sections bearing identical characteristics. So when the two sections are paralleled, there are two slightly different output signals being blended at the common plate load and coupling capacitor. This can appear as a sort of "thickening" of the sound, which can help make a circuit sound "fatter". "
 
 
The 2 different cathode values (in your JMP example) will ensure that even perfectly matched 12AX7 triodes will produce unbalanced outputs 
 
https://books.google.com/books?id=9kMDBAAAQBAJ&pg=PA107&lpg=PA107&dq=merlin+parallel+triode&source=bl&ots=Oy6i0oqfdJ&sig=48objcpalfI7X8-9hH3hMt-MVGs&hl=en&sa=X&ved=0ahUKEwi429eapJfRAhVl_4MKHbbVCXQQ6AEIVDAG#v=onepage&q=merlin%20parallel%20triode&f=false
 
 
 

[12AX7]

Thanks. So possible slight reduction in noise but moreso, a thicker tone. Not sure i want thicker, but if i wanted to try this any reason i can't use a different tube rather than the same tube? I ask because i don't wanna rewire the amp just to try this, but i have a extra unused AX7 in my amp and i was thinking if simply clip leading the cathode and plate pins to V1A just to see what the result is then if anything interesting result maybe try messing with plate and cathode values by doubling them which apparently would be like thier current values. 

[SILVERGUN]

You'd also have to connect grid to grid.
 
If the unused triode was on the same socket, I'd say go for it...
But, with the unused triode on a different socket it scares me to have plate voltage laying across the amp on a test lead (not intended for HV).
This method will also surely induce some serious noise, especially in your high gain preamp.
 
You 'can' apply this technique to any preamp tube, it just makes the most sense at the input stage.

[12AX7]

Yeah, that was just a typo. (missed mentioning tying grids) And yes, its a different tube but i can just do a solder wire for that one. I've clip leaded HV before but with very good solid and HD clip leads. Ones i have now are too flimsy tho. I'll give it a go one of these days when i have the amp open again. Just curious.

[HotBluePlates]

... I was looking at a JMP marshall combo schematic just now that has v1 paralleled with different cathode values for each side but the plates both meet at the grid of V2. ...

I always recommend linking to the schematic you're asking about.

The JMP circuit (IIRC; which is why a link would help) has two channels, one Normal (really a dark channel) and one Bright (really an icepick channel).  As far as the circuit is concerned, they are two independent channels.  That the outputs are eventually tied together is only a byproduct of the overall architecture, and they are intended to act as independent triodes.

That said, folks often play through both at the same time (by using an external jumper cord between jacks) because it helps balance the different sounds available.  Or the jumpering is provided internally in modified amps or spin-offs.

I wouldn't consider the triodes "paralleled" unless each shares a single plate load (or sometimes a single cathode resistor).

I noticed a lot of amps that are considered exceptional have this. ... Others i see the cathodes are tied as one or the plates are or both. Is there a particular reason tonally they use any variations of these paralleled triodes?

... Excerpt taken from this ampage discussion:
http://archive.ampage.org/threads/1/gagd/011345/Parallel_preamp_tubes-1.html
 
"....it's rare to find a 12AX7 that has both sections bearing identical characteristics. So when the two sections are paralleled, there are two slightly different output signals being blended at the common plate load and coupling capacitor. This can appear as a sort of "thickening" of the sound, which can help make a circuit sound "fatter". " ...

So I didn't read the Ampage reference (shame on me).  Here's my take in a nutshell:

In my opinion, the notion of "balancing different tones because the sections aren't identical" is logical but incorrect.  Most every dual-triode has some amount of difference between the triodes, but it doesn't have a significant impact in most applications.  Selected tubes with balanced triodes were available in the old days (at additional cost) when the circuit had a valid need for such selection.  Usually, that "valid need" amounted to instrumentation work, or where the absolute least distortion was required.

Sometimes, Fender amps (and the first Marshall amps) shared a cathode resistor (of half the normal value) because it saved the cost of another resistor.  Simple as that: economics of large-scale production.  $0.10 saved over 10,000 amps made a difference in Fender's annual profit.

But where a dual-triode is used with shared plate and cathode resistors, you get something different from using the triodes individually.  You've effectively created a "composite tube" which has twice the transconductance (Gm), half the plate resistance, and (as a result) a bit more gain when using same/similar plate/cathode resistor values.  Tubenit & others have found that parallel triodes seem to yield about 30% more gain (with a 12AX7) than using a single triode.  The sonics are changed, and the composite tube sounds a bit fatter than the same triode used singly.  And lastly, the extra Gm helps slightly in lowering the noise floor.

While you get about 30% more gain than using the triode singly, it is very much less gain than using the same two triodes cascaded.  That's because in cascade, the individual stage gain multiply. So something like a gain of getting a gain of 60 with a single 12AX7, a gain of 78 with two 12AX7's triodes paralleled, but a gain of 3600 with the same 12AX7 triodes cascaded.

Yes, paralleled triodes could share a common plate load but use different cathode components (R or C) for different tones/textures which are then combined.

Mostly, paralleled preamp triodes (or other tubes) are a recent invention in the past couple of decades, probably springing from hobbyist experimentation.  Designers in the old days probably would have seen this as wasteful* except where paralleling tubes to deliver more power (as in the output section) was common practice.

[HotBluePlates]

* There were special-application circuits in the old days which look like paralleled triodes, intended for wide-band amplification with fairly modest gain (20dB or 40dB, which equates to x10 and x100).  The triodes which appeared to be paralleled were separated by impedances between each triode's plate and each triode's grid to deliver signal "down the chain" rather than from grid to plate as you'd expect.  The Hewlett-Packard 460A is an example of this type of wideband amplifier circuit.

[dennyg]

My first build was an 18watt Marshall clone which has parallel triode with two input jacks - one sends signal only to first grid, other to both.  The parallel input definitely has a thicker, more punchy tone which overdrives the PI (and possibly the EL84s?) with a great crunch when dimed.  This amp shares cathode resistor and cap with both triodes.  Merlin also does a nice explanation of the benefits of parallel triodes in his book. 

[tubeswell]

Paralleling triodes of the same type gives you lower plate resistance/higher current, higher transconductance, lower output impedance and higher gain compared to a single triode of the same type.

Whether or not you might consider parallel triodes to be advantageous depends on the situation.


e.g. if you are building a typical Supro Thunderbolt (which typically utilises 3 triodes) and you want to use up the extra triode, you can wire both triodes of V1 as parallel inputs with the input jacks wired to activate as a single triode or parallel triodes, depending on which jack you use, or as parallel inputs for different effects trains going into your amp. In parallel mode, they will drive a fancier tone stack better that a single triode will. (See schematic)

However, if you want to use two triodes to drive a heavily-loaded tone stack, you can get even lower output impedance and nicer asymmetry with a DC coupled pair utilising a cathode follower. If you want two triodes to get even more gain, you can cascade them. If you want really high transconductance and more gain, you can totem pole them into a cascode stage.

[SILVERGUN]

..e.g. if you are building a typical Supro Thunderbolt (which typically utilises 3 triodes) and you want to use up the extra triode, you can wire both triodes of V1 as parallel inputs with the input jacks wired to activate as a single triode or parallel triodes, depending on which jack you use, or as parallel inputs for different effects trains going into your amp. In parallel mode, they will drive a fancier tone stack better that a single triode will. (See schematic)

Now there's a cool use of 2 input jacks.
I'd be inclined to separate the cathodes and experiment with an exaggerated 'low' and 'high' input....or 'normal' and 'boost'.
I'll hang onto that one...thanks tubeswell

[PRR]

> single triode or parallel triodes, depending on which jack

In the plan shown, the "unused" triode is a heavy load on the single triode. It may be a fine sound. It isn't a "fair" A/B comparison between parallel and the usual single. Obviously done this way because the "right" way would need a switch in the plate circuit, which can't be done with the common switch jacks.

[Ed_Chambley]

Paralleling triodes of the same type gives you lower plate resistance/higher current, higher transconductance, lower output impedance and higher gain compared to a single triode of the same type.

Whether or not you might consider parallel triodes to be advantageous depends on the situation.


e.g. if you are building a typical Supro Thunderbolt (which typically utilises 3 triodes) and you want to use up the extra triode, you can wire both triodes of V1 as parallel inputs with the input jacks wired to activate as a single triode or parallel triodes, depending on which jack you use, or as parallel inputs for different effects trains going into your amp. In parallel mode, they will drive a fancier tone stack better that a single triode will. (See schematic)

However, if you want to use two triodes to drive a heavily-loaded tone stack, you can get even lower output impedance and nicer asymmetry with a DC coupled pair utilising a cathode follower. If you want two triodes to get even more gain, you can cascade them. If you want really high transconductance and more gain, you can totem pole them into a cascode stage.

Yea buddy, you sure do come up with some cleaver designs.  I believe you may have paralleled triodes on a 5G9 or at least I think you did.  After seeing it, I did the same switchable as I prefer the tremolo with only one, but without it is nice.