V1A or V1B for input

[pjd3]

Hello.

I've been seeing varied opinions, or facts on differences between a 12ax7's two gain stages.

Is anyone aware of a difference between them as far as what might be expected in noise, etc.... performance?

My amp is going to be always in a clean low/medium volume so, no high gain noise excascurbation here.

Thank you,
best
Phil D.

[martin manning]

This has been discussed before. As I recall, one or two manufacturers data sheets say one section is lower noise, but others do not differentiate. I wouldn't worry about it.

[drewspriggs]

I've always just used whatever makes the layout neater

[pdf64]

The pins 6,7,8 triode is more favourable in regard of hum https://frank.pocnet.net/sheets/010/e/ECC83.pdf

[martin manning]

I don't think that is any guarantee that that is the case for all manufacturers.

[pjd3]

Thank you.

Its looking in my case that using the triode that makes for the best wiring/routing layout will be fine enough.

But I surely will keep the "pin 6, 7 and 8" claim in my head.

Thanks everyone,

Best

Phil D.

[solderhead]

You can get better signal to noise from a 12AX7 by running both triodes in parallel.

Of course, if you're really worried about noise then a 12AX7 is the wrong choice of tube for the first gain stage.

Some people who need to keep an AX7 on the input try to improve the noise problem with DC heater supplies.

[pdf64]

You can get better signal to noise from a 12AX7 by running both triodes in parallel.
...

But why bother devoting an entire noval socket to a 12AX7 parallel triode stage when a pentode could achieve much better results?

[Helmholtz]

The pins 6,7,8 triode is more favourable in regard of hum https://frank.pocnet.net/sheets/010/e/ECC83.pdf

That's interesting, because it means that heater-to-cathode leakage is not balanced between the 2 filament sections (at least with old Philips ECC83s).
Also explains why a humdinger in a non-centered position often gives a better result than a grounded CT.

[solderhead]

You can get better signal to noise from a 12AX7 by running both triodes in parallel.
...

But why bother devoting an entire noval socket to a 12AX7 parallel triode stage when a pentode could achieve much better results?

um.... because it's not possible to find an EF86 that isn't microphonic?

[Helmholtz]

But why bother devoting an entire noval socket to a 12AX7 parallel triode stage when a pentode could achieve much better results?

Acc. to Merlin pentodes are somewhat noisier than triodes because of the additional plate-screen current partition noise.
Also tend to be more microphonic as mentioned above.

[solderhead]

I have Merlin's books and they're definitely worth reading.

My experiments with pentodes in the first position has yielded the results that he's warned about. And I think it's worth noting that Vox has abandoned the traditional use of EF86 in the first position in favor of the 12AX7 in their modern designs because of the problems with them that we've been talking about.

I have to confess that I haven't tried the cascode topology (yet). For me the shortest route to success has been to follow the lead of the Matchless designs that run the first pair of triodes in parallel. In my experience that's been working pretty well -- and is especially useful in those cases in modding amps already have an unused triode. I have to confess that I haven't tried adapting to guitar amps the hybrid pentode/triode tubes that are popular in HiFi designs. There may be some fertile ground for experimentation there but those tubes tend to be expensive because the guitar amp industry has so heavily favored the 12AX7.

[jabguit]

You can get better signal to noise from a 12AX7 by running both triodes in parallel.
...

But why bother devoting an entire noval socket to a 12AX7 parallel triode stage when a pentode could achieve much better results?

um.... because it's not possible to find an EF86 that isn't microphonic?

plenty of other pentodes to choose from...

cheers,

[pdf64]

I recall that the 5879 has a better reputation for microphonics than EF86 https://tube-data.com/sheets/127/5/5879.pdf

Also that the input grid stoppers are noisier that the input stage valve, with the tiny Miller capacitance of pentodes perhaps persuading the designer to treat the input stage grid arrangement a bit differently, if the same bandwidth etc characteristics are to be achieved.

[solderhead]

I've always wondered why Vox and Matchless chose to abandon pentode inputs when there are so many pentodes to choose from.

So Pete, do you have any first hand experience with the 5879 in this application that you could share, or are we just having a blue sky discussion at this point? I don't see any of the recalculations that you referenced as being a roadblock, but I do see current production/future availability as being a significant roadblock.

From my perspective there aren't really very many pentode options available unless you're willing to take on the risk of designing around obsolete/out of production amplifiers. Even while looking for "modern" parts it's very common to run into parts that are still commercially available but are listed by the manufacturer/distributor as "not suitable for new designs." Those parts are considered obsolete by the manufacturer and are already out of production or slated to be phased out of production; commercial designers avoid them. We're kind of stuck in that situation for those NOS tubes that aren't in current production. To me, it's not worth bothering to design around any extinct parts. It may work for others, but that's an individual choice that I have made. At least the 12AX7 is in reasonably widespread production, which has to be part of the reason that every amp company seems to prefer it.

I've always wondered if there's an official reason that Vox abandoned the pentode inputs. I seem to remember lots of complaints about microphonic pentodes at the time that Vox switched over.