5751 Preamp tube

[WaZaK]

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!)

[Chubsman]

The gain factor of a tube is how much it amplifies the input signal. A 12AX7 type has a gain factor of 100 and a 5751 a gain factor of 70.

So a 5751 in a socket that expects a 12AX7 will have 30% less gain. This will make the amp quieter, cleaner and will break up later.

[Raja]

You should order a couple, they're good tubes to have in your arse!!

[zaphod_phil]

IMHO I don't see much of a use for them in 18W amps, but each kind of amp is a different story.

[WaZaK]

I read good reports on the Phillips tube and just wondered ......... Yes, I am aware of the gain factor. But I don't have much interest in spec sheets: just wondered what they did to the ears!

I'm not trying to achieve a particular sound - just trying to build up "an arsenal" of goodies that can be tried in different circumstances.

To me, half the fascination for amp building is the opportunities to try out different things. But I don't expect everything I try to be an improvement!

[Chubsman]

Well, I am not telling you that the gain factor is 70% to be mathmatical... But you should be able to figure out what it will sound like to the ears....from that information....

[jaysg]

a) If used in the 18W trem channel, the trem probably won't oscillate
b) lower overal hiss from any amp if replacing a 12ax7 in the first stage
c) GE 5751s are a bit darker than most 12ax7s -- which I like in a BF Fender

[WaZaK]

But you should be able to figure out what it will sound like to the ears

Maybe I could ........ but that was not my question: I got the percentage mumbo-jumbo from the Tube Depot site (and specs don't necessarily mean much) and I was asking for someones "audio experience". But thanks anyway.

As I said earlier jaysg:

would it be just useless in an 18watt (single channel)

Trem channel oscillation (or lack of it) is not a problem!
Are you speaking from experience with the hiss level? Is it a significant reduction? (Not that I have much hiss to start with!)

[zaphod_phil]

With less gain, you will generally get less his. No rocket science there.

Now, if you want to get hold of interesting valves to try out, I strongly recommend shopping around for some 12BZ7s. They're like a high current version of a 12AX7, or actually a pair of 12AX7s in parallel. They have the same voltage gain, but twice the current capacity and half the anode impedance (aka "plate resistance") of a 12AX7. This means they get loaded down less by the impedance of the following stage, so for example, are great for gain stages in front of a tone stack. And they will also give slightly higher gain than a 12AX7 when plugged into a typical gain stage circuits. As with 12AT7s and 12AU7s, they bias differently from 12AX7s, and therefore you may need to use a larger cathode resistor value. And they sound freekin' amazing!

[grndluxe]

a 5751 in V1 makes a lite IIb run a bit quieter. like has been said, it has less gain than a 12AX7 therefore doesn't drive the circuit as hard.
i had a really nice GE 5751 in the lite IIb that i built. that combined with some power tubes i "thought" would be really good (Generals i got on ebay) made the amp actually TOO clean compared to all the other sound clips i have heard. maybe a bit more clean headroom, but too clean for what the 18 should sound like.
that what you wanted to know?

[WaZaK]

but too clean for what the 18 should sound like.

Yes! That's the sort of comment that is helpful! Sadly it is what I really DIDN'T want to hear - but sort of expected!

Another new tube to find ... 12BZ7.

you may need to use a larger cathode resistor

What sort of ball-park figure, ZP?

[zaphod_phil]

Around 1.5k to 1.8k. Same with 12AT7s and 12AU7s. A good idea is to measure the anode voltage at idle. For maximum clean swing from the stage you should aim for around 2/3 of rail voltage at the anode. For more asymmetry and distortion, bias towards 1/2 of rail voltage, or anywhere in between the 2/3 and 1/2 mark.

A word of warning about 12AT7s in preamps. At one time I did a lot of tests with them, along with a few other guys on AX84. I/we found that they produce an unmusical muddiness in the tone when overdriven, which is particularly noticable with humbuckers. They do sound fine when run clean. Turns out they generate a surprisingly large amount of odd-order harmonics when overdriven, like 7th, 9th and 11th, with the 9ths being particularly strong. This occurs with many different brands of 12AT7s, although appears to be somewhat less of a problem with JJs. Not sure why.

[StarGeezers]

FWIW.... been using a 5751 in the PI of my Ab Min 18 watt... Works Great with ZPs specs... Had waaaaaaay too much PI gain with the 12AX7... Now ,it's just INSANE!!!

[zaphod_phil]

But you've got a higher gain pentode preamp, right? So probably the lower gain of the 5751 is compensating for the higher gain of the preamp.

So, hey we've found something you can use a 5751 for in an 18W.

[grndluxe]

glad i could help. i hate that it is what it is. if i end up doing a lite IIb again, i'll probably go with plain old JJ 12AX7s and EL84s (or 6V6s).

[WaZaK]

Well, on balance it would appear that it will do no harm to get a couple of NOS 5751's while they still exist ($20 @ Tube Depot) - just to keep in the old "goodie" box!
Thanks for the replies - as usual.

[zaphod_phil]

$20 sounds like a lot of money to me, but certainly if you've got the cash then it's probably a worthwhile investment. But I might suggest scouting around on eBay as well. I've bought quite a lot of good NOS tubes at very reasonable prices that way.

[StarGeezers]

QUOTE: "But you've got a higher gain pentode preamp, right?"
ZP, EXACTLY!!! and much THANKS to you, a whole Lot of Gain!!!!! Had to tame it a bit so I could get some Cleans when I first cracked the Volume on ...hahahahahaha Sounds really nice with the Pentode front end... Really Sings!!! Thanks Again!!!
BTW, It is in an authentic Marshall head cab, if that means anything here ...

[Phil_S]

a) If used in the 18W trem channel, the trem probably won't oscillate

Since I don't have a trem 18W, I can't test it, but I've done quite a bit of reading on the subject and worked with this in a VibroChamp homebrew.

To trigger the tremolo oscillation, the tube needs to have a high plate resistance. 5751 has Ra=58,000, 12AZ7 has Ra=62,500. These are so close together that I can't imagine why the 5751 wouldn't trigger the tremolo. Even the 12AY7, Ra=22,800 might be high enough to trigger oscillation in a tweaked circuit. 12AT7 and 12AU7 have Ra way too low.

So, I ask, has anyone actually tested a trem 18W to see if the 5751 will trigger oscillation? Mostly, I'm just curious.

Overall, though, I agree with others. Save your 5751's for a F type amp where you can get clean tone in spades. Don't waste it in an 18W, which is designed for grind. I think it's Dartanion who says, "the distortion is the tone." I stuck one in V2 of my 'wreck express to tame that circuit a bit, but I keep thinking it's wasted there. Every time I look at it, I get the itch to remove it.

[StarGeezers]

Phil, it's just a tube , might well use the good stuff now ...If it sounds like you want it to, then you're not wasting anything...

"The only enemy of Good is Better! "