5751 Preamp tube
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- WaZaK
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5751 Preamp tube
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!)
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!)
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- Chubsman
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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!
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!
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I'm the "wrong" side of the pond: 4,500 miles from real ale. Help!
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- WaZaK
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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.But you should be able to figure out what it will sound like to the ears
As I said earlier jaysg:
Trem channel oscillation (or lack of it) is not a problem!would it be just useless in an 18watt (single channel)
Are you speaking from experience with the hiss level? Is it a significant reduction? (Not that I have much hiss to start with!)
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- zaphod_phil
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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!
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!
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Nature abhors a clean tube amp
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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?
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?
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- WaZaK
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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!but too clean for what the 18 should sound like.
Another new tube to find ... 12BZ7.
What sort of ball-park figure, ZP?you may need to use a larger cathode resistor
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- zaphod_phil
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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.
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.
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Nature abhors a clean tube amp
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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 ...
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 ...
Last edited by StarGeezers on Wed 06/10/09 8:42 am, edited 1 time in total.
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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.jaysg wrote:a) If used in the 18W trem channel, the trem probably won't oscillate
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.
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