5751 Preamp tube

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cGil
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Post by cGil »

My GDS 18watt's trem channel (w/Richie mod) prefers Chinese or Sovtek 12ax7WA variety tubes. My trem speed will do a slow crawl; however, I've noticed that if it's already dialed down that slow when I hit the foot switch, it may not get started oscillating again. I usually have to bump the trem speed up a little from the zero point on the knob before it'll get rolling.

I've got Sylvania GB-5751's and Phillips JAN 5751's, and neither tube has enough plate resistance to get my trem rolling, even with the speed knob on "10". With Sovtek 12ax7WXT+ and LPS tubes , the trem just won't slow down enough for my needs. JJ 12ax7's just push the trem into wild oscillation overload on my amp, so I use JJ's only on the other channel and the PI.

Gil...
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zaphod_phil
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Post by zaphod_phil »

The Marshall 18W tremolo seems to also be very fussy about the gain of the tube. It has trouble oscillating with some brands of 12AX7 as well.
StarGeezers wrote: BTW, It is in an authentic Marshall head cab, if that means anything here ... :roll: :lol:
Yes, but is your authentic Marshall head cab RARE? If it's RARE it can go for real big money on eBay. :lol:
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StarGeezers
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Post by StarGeezers »

I couldn't possibly think of selling it... What would I put my 18 watt in ... besides, not rare, just a funky old 100Watt head cab , too big for the current amp really...but impressive size and MOJO is why I kept it for so many years waiting for a suitable amp... and it's about the only thing on the amp thats even remotely related to a Marshall... given the AB Mins mutations of the classic 18 watt... with pentode front end , ZP PI ,and 6V6s ... Otherwise I wouldn't be allowed here at all... :o :lol:

Our Tremolo is a 3SB on the floor , always works , besides who uses one anymore except us old guys playing Oldies...??? :?
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Post by dotfret »

If your trem is oscillating, the quick fix is to use a 12AZ7, then you can forget it, if it works. Otherwise, take it all apart and start again!

There's a lot of other valves that sound nice in P1, apart from the 12AX7. The 12BZ7 is one, but a pair of EC91 can sound really scary, as long as the filament transformer can handle the extra current (like for the 12BZ7 anyway).

The 5751 is a good sub for the 7025 in a lot of Fender amps, to moderate the attack - and in those conditions, it drives into overload easier than a 12AX7. In an 18W design, where the thing will "bite yr leg" without effort, a 5751 just tames the volume. But there are other alternatives for the Fenders - they can be very tractable with other members of the "family".

The real question is - how far can you deviate from the original 18W design and still call it an 18W?
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kleuck
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Re: 5751 Preamp tube

Post by kleuck »

WaZaK wrote:I don't want an opinion based on authenticity, etc, etc, just the facts:
What is the overall effect at all levels, of using a 5751 tube (as a sub for a 12AX7) in the preamp?
Is it it more clean headroom? Earlier break-up at lower volumes?
Or would it be just useless in an 18watt (single channel) with drive (PPIMV) control?
Yes, I know the term "useless" could be subjective, but if the amp just won't work ..........
(I don't have one to try - or I would!)
A 5751 is not at all a 12AX7 with a lower µ (except the moderne Sovtek 5751 wich is a tamed 12AX7LPS), but a different tube, with a different structure, different curves and therefore working points.
Obviously they DO sound quite differently from the 12AX7 in many amps and postions.
So the point imho is not the gain (actually a 12AX7 EI silver plate has more or less the same "low" gain in my amp), but the sound.
I found that it is (at least the GE Black Plate from the 50's) by far the best tube in the V1 position (and i tested TFK, Mullard, ****, EI, Svetlana, JJ, RCA, GE, bunches of 12AY7 etc)
Just give it a try, you will know by yourself.
Last edited by kleuck on Sat 06/27/09 3:35 am, edited 1 time in total.
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WaZaK
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Post by WaZaK »

Ne vous inquiétez pas votre anglais. Vous devriez entendre mon français! De toute façon, je suis un étranger: un Brit dans les États-Unis .......

I am aware from the specs that a 5751 is not a 12AX7 - but due to it's own specification, it is a drop in alternative. I also now know that the NOS JAN Phillips 5771 has a similar gain to some 12AX7's - so they can't even be generally described as "low gain" 12AX7's either - just different! And yes, I have tried one now.

Thanks for your reply.
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dotfret
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Post by dotfret »

It is a matter of personal taste, if you have some genuinely old valves that can still perform. I have Philips Miniwatt smooth greyplates, made by Ei in the 50s, that have to share first place with some Tesla "folded plates" (like the RFT style, but made in CZ - look like a 12AT7 internally).

But do not make the mistake that the 5751 is a different entity to the 12AX7.

There are 12AX7 which resemble the typical 5751 design, physically. Sylvania turned out a lot in the early 60s, and they show up often on eBay - usually branded Baldwin, or some other organ manufacturer. I have some Raytheon 12AX7A grey plates made in Korea in the 80s which are also 5751 style.

There are a lot of oddities out there - I remember throwing out a side-getter 12AX7 from an AC30 about 30 years ago, and thinking that I had never seen anything like it (I did find 3, NOS, a while ago: Gabi has those, they were part of the MD donation effort).

France is a place where the viewpoint is restricted, especially when the local producers made wonderful valves. One of my prize possessions is a 5 star CIFTÉ 5751, silver plates - and if you can buy any of the military silverplate 12AX7 from the 50s produced by MBLE or CIFTE, you have feathers in your hat!
But the view point is confined - you can only see the current market, and buy within it. You miss the old Radiotechnique (and FIVRE) valves, because you were not there to collect them when they were cheap. And because France has always been a "partisan" place, your countrymen ignored the obvious advantages of Mullard, S&H, and inappropriately branded Philips valves.

Be careful not to ignore the old record player on the fleamarket, the old amp that might have something you want inside it.

Comme je ne suis q'un vieux soulard anglais, je sais quelquechoses ...
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kleuck
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Post by kleuck »

Héhé, j'ai une ECC83 Mazda Silver plate.

I know that some old 12AX7s were actually 5751s, and some 5751s were re-labelled 12AX7s (like the modern Sovtek, nothing changes).
I have a bunch of USA-made tubes (aaahh the Old 50's Tung-Sol withe the long plates !), and one of the firsts to pretend that a lot of american tubes are often better in guitar amps than the sought after Mullard, Brimar and TFK (etc).

Hi-Fi stuff is another animal i suppose.
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dotfret
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Post by dotfret »

Well ... some US made valves are undoubtably good. You may wonder why people often pay double the money for RCA, compared to other US manufacturers, and also why RCA did not outsource valves very often (and when they did outsource valves, the supplier was usually Mullard).

And because the US market was/is so large, the prices of US valves remained low, while the prices for old stock went up in Europe, as old valves became scarce.

But I am really here to post this link to show these old Bharat Indian-made 5751 which clearly derive from the 12AX7 design -
http://www.tubemonger.com/BEL_5751_1970 ... E_p/16.htm

but as the blurb says, they are not exactly the same.
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Post by zaphod_phil »

kleuck wrote:I know that some old 12AX7s were actually 5751s, and some 5751s were re-labelled 12AX7s...
In that case how come 5751s seem to have less gain than 12AX7s...? :?
dotfret wrote:But I am really here to post this link to show these old Bharat Indian-made 5751 which clearly derive from the 12AX7 design.
Yes, the "ladderplate" structure is clearly visible in that picture.
dotfret wrote:You miss the old Radiotechnique (and FIVRE) valves...
I happen to have a very nice FIVRE EL34 I use in my single-ended P1 eXtreme BTW.
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kleuck
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Post by kleuck »

zaphod_phil wrote:
kleuck wrote:I know that some old 12AX7s were actually 5751s, and some 5751s were re-labelled 12AX7s...
In that case how come 5751s seem to have less gain than 12AX7s...? :?
EI Silver Plates or Matsu (why can't i write the whole Brand ?) exhibits quite the same gain as a 5751 for example.
But i don't know exactly, perhaps were they only "selected" 12AX7.
Anyway according to Hi-Fi specialists, 5751 are meant to draw more current than 12AX7.
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Post by dotfret »

I still can't take good pics of valves - I've put these in my scanner!

The Korean Raytheon 12AX7A that look like typical 5751 internally -

Image

Y'know, "selected" is a strange term where valves are concerned.

With most types, the gain is a nominal figure, and production types can often have a higher gain than specified. "Selected" valves are usually selected for low noise (high Signal to Noise ratio) - and for some obscure reason, the lower gain valves in a batch are usually the ones with the best S/N ratio.

Most designs do not run valves at "full tilt", they under-run the valves so that they last longer. Couple that with the fact that you have to "double the overall power" to get a 3dB difference in output, and it can become very tricky to distinguish the difference in loudness between a 5751 and a 12AX7 in a given application. I've got a couple of old Laney PA amps that have a single 12AX7 for each of 6 channels, which makes comparisons easier - but it gets hard to compare when you have to switch everything off, then change the valves over.
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Post by marshman »

I've absolutely no experience to base my thoughts on, but I suspected that when the manufacturers needed specific tubes for military contracts, they just set a couple gals down at a tester of some sort and set 'em to work testing 'gain'...if it scored exceptionally high, it was labelled 7025...if it went low, they labelled 'em 5751s. Can't prove anything, but it seemed as likely as anything.

There are so many variables involved with tube tasting of preamp tubes that it's difficult to really compare things, but it always seemed that using a lower gain pre-amp tube just shifted the point at which distortion sets in around the volume knob a bit...so grind sets in at "5" with a 12AX7, "6" with a 5751, maybe "7" with a 12AY7. What I did find is that using a 5751 or 12AY7 gives me a little better control over that 'edge of distortion' area that I like to play in, in exchange for maximum distortion and power.

But I'm a bassplayer, so hey, I may just be incoherent.
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Post by dotfret »

Nearly right - truth is that, in std valve manufacture, the high gain valves were usually noisier than the low gain valves. So the manufacturers used those gals to select the low gain valves that were inside the spec, but would have a better S/N ratio.
Then they would be subjected to further testing to ensure that they would last out the longer "industrial lifetime" (Standard lifetime test for a 12AX7 was 1000h, industrial versions were expected to last ten times longer) - and at this point, they knew if they had 12AX7 or 7025. Mostly, they didn't "label down" between types. They would "label down" if they had made top spec valves that did not cut the mustard, so you can occasionally find some 12AX7 which have very strange plates (eg. down-labelled Mullard M8137). But you are very unlikely to find low gain 12AX7 down labelled to 5751 - the manufacturers were not usually THAT far off in estimating what they were making.
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