5751 Preamp Tubes...

Nostalgic Distortion

Nostalgic Distortion

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Anyone use these???

Recently I acquired a new Deluxe Reverb Reissue! While I'm quite happy with the amplifier overall I'd like to try and tame it's inherent upper mid spike just a touch if possible? Originally I had planned on doing half a "Trey Mod" to achieve this, for anyone who doesn't know what a Trey Mod is in regards to the Deluxe Reverb, it's just a 12AT7 in place of the 12AX7 in V1 & a Mid bump/cut knob in place of the second vibrato input jack.

Since I'm not entirely sure if installing a Mid bump/cut knob is even possible with a reissue's printed circuit board & if it is I can't afford to have it done now anyway, I figured I'd see how far I got just by swapping out V1 with a 12AT7. While reading up on the subject matter & looking at valves the 5751 was brought to my attention...

Apparently it's essentially just a 12AX7 with 30% lower gain so it has the same basic effect as the 12AT7 in V1 but doesn't lower the amplifiers volume as much & is less noisy? Just kinda wondering if anyone else out there has tried this tube in place of a 12AX7, & if so what their thoughts were???
 
Re: 5751 Preamp Tubes...

I use 5751 tubes frequently to 'tame the gain' on amps having more than I care for. Haven't tried one in a DRRI, but I'm always pleased with the results in other amps. Besides, they're not expensive and you can just put the 12AX7 back if it doesn't appeal. Try one.
 
Re: 5751 Preamp Tubes...

ive used nos ones in amps with good results, cant hurt to try
 
Re: 5751 Preamp Tubes...

I've always felt that switching from a 12ax7 to a 5751 resulted in a weak, bland tone.

But everyone's different.

Can't hurt to try, and it's good to hear these things for yourself.
 
Re: 5751 Preamp Tubes...

I would sooner go with the 5751. 12at7's and 12ax7's are not very plug and play compatible. The 12at7 is a much higher current valve than the ax7 and will need different biasing to get the most out of it, and it doesn't actually have much less gain in circuit than an ax, a gain of 50 versus a gain of 60. People often think the 12t7 is cleaner due to lower gain, but it's mostly because the at7 can swing a much larger input signal without distorting. The confusion stems from the fact that an ax has a higher potential amplification factor on datasheets, but it's much higher internal resistance throws a lot of that away. You can substitute them for each other with out worrying about ruining anything but you won't really get an even comparison unless the circuit is adjusted to better accommodate the at7.

A 5751 for most purposes is just a de-rated 12ax7, so it biases up with the same values - its pretty much plug and play.
 
Re: 5751 Preamp Tubes...

Personally I would try the 12AT7 first, but experimentation leads to expanded knowledge ;)
 
5751 Preamp Tubes...

I recently stumbled across PM Tubes....I took a chance and ordered a couple and I LOVE them.
tubeboutique.com
 
5751 Preamp Tubes...

jeremy said:
so who makes pm tubes?
Click to expand...

http://thetoneking.com/wp/the-king-of-tubes-peter-watson-and-pm-tubes-make-their-way-to-us-shores/

From what I understand they are a Chinese tube somehow PM selects a higher quality. The inner construction is different from a standard 12ax7b. It looks more like a short grey plate RCA or something. They don't have that metal band inside like the Chinese tubes do.
I have them in my Quickrod and was able to A/B them with the standard Chinese Sino 12ax7b. I found them to be smoother and have a rounder top end.
They made a definite difference to my ears anyway. I know the tube debate will go on and on. And to be honest, I've rolled a LOT of tubes in my Splawn from the new ones to NOS. Right now the PM are my favorite followed closely by the Chinese 12ax7b....
My .02 anyway.
 
Re: 5751 Preamp Tubes...

Joey Voltage said:
I would sooner go with the 5751. 12at7's and 12ax7's are not very plug and play compatible. The 12at7 is a much higher current valve than the ax7 and will need different biasing to get the most out of it, and it doesn't actually have much less gain in circuit than an ax, a gain of 50 versus a gain of 60. People often think the 12t7 is cleaner due to lower gain, but it's mostly because the at7 can swing a much larger input signal without distorting. The confusion stems from the fact that an ax has a higher potential amplification factor on datasheets, but it's much higher internal resistance throws a lot of that away. You can substitute them for each other with out worrying about ruining anything but you won't really get an even comparison unless the circuit is adjusted to better accommodate the at7.

A 5751 for most purposes is just a de-rated 12ax7, so it biases up with the same values - its pretty much plug and play.
Click to expand...

I've never heard of biasing a 12AX7 V1 pre-tube before. Hear something new every day.
 
Re: 5751 Preamp Tubes...

preamp tubes arent fixed biased so they dont need to be adjusted, its plug and play
 
Re: 5751 Preamp Tubes...

jeremy said:
preamp tubes arent fixed biased so they dont need to be adjusted, its plug and play
Click to expand...

No they are not all plug and play, Not in the sense they will both share the same q point with the same values for rk and ck. while it wil work in the sense that it will make sound, it has nothing to do with the method of biasing, but with the valve itself, (whether a fixed - v is applied to the grid, or resistively self biasing is used is irrelevant) most preamp tubes are actually cathode biased, with both the plate resistor and cathode resistor setting the quiescent operating point. They don't need to be adjusted so long as they are roughly the same type, like a 5751 and a 12ax7, however when switching to one of a different type, especially one with a much higher transconductance like a 12at7, the same values used for a 12ax7 do not reflect the same bias point or where the load sits in the grid curves. While you won't hurt anything, it's not a fair comparrison.

Take a look at the ep/ip charts for both the 12ax7, and a 12at7 and you. Will see the huge disparity between them.

Edit: the attached plate characteristics

http://www.jj-electronic.com/images/stories/product/preamplifying_tubes/pdf/ecc83s.pdf
http://www.jj-electronic.com/images/stories/product/preamplifying_tubes/pdf/ecc81.pdf
 
Last edited:
Re: 5751 Preamp Tubes...

Get a N.O.S. 5751. Word is that the current production 5751's aren't anything more than 12AX7 "rejects" (low GM). The GE's are usually cheapest and not my favorite but I'd take one over a current production 5751 any day.
 
Re: 5751 Preamp Tubes...

Darg1911 said:
Get a N.O.S. 5751. Word is that the current production 5751's aren't anything more than 12AX7 "rejects" (low GM). The GE's are usually cheapest and not my favorite but I'd take one over a current production 5751 any day.
Click to expand...

I've seen this stated before. All I can say is I've tried both the current production Tung Sol 5751 and the JJ 5751. The JJ was more to my liking as it made a more noticeable difference from the 12AX7 and accomplished exactly what I set out to do.
 
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