Using a 12au7 in place of a 12ax7 in a phono pre-amp??

[Frank Sol]

I hope this is not breaking any AK rules... But I'm bring this topic from the Turntable Forum to here.

I, bigdirty & some other AKers would like to get more feedback on this tube swap. How 'dangerous' could it be using a 12au7 in place of a 12ax7 in a phono pre-amp, the Jolida JD9 specifically?

Thanks!


Below are two posts from TT Forum

Did you guys know that Underwood HiFi suggests rolling 12au7/ecc82 tubes along with 12ax7/ecc83 ones to find the 'right ones' for your personal taste!?

Personally I've tried RFT ecc82... Not bad and they tone down the highs when using a MM cart from the "High Output" connection. I didnt run them too long because I was told by online gurus that the 12au7 could or would overload the plates and cause problems or even unit failure.

I thought about using 12au7 to tame some of the gain of the 12ax7, but have the same reservations about overloading the plates. Im sure it would be as simple as changing some of the values of the resistors around the tubes. Thats a bit beyond me though.

maybe some AK tube gurus could chime in on this subject?

 

[dshoaf]

This will probably leave you with more questions but here goes:

Short Answer: It depends on the circuit they are being used in.

Longer answer: Bigdirty alludes to the fact that circuit changes _may_ be needed. You need to understand that the 12AX7/AU7 tubes are sufficiently different in their operating characteristics that they are still in production 60 years since they came out. There's clearly a reason that a medium and high mu tube is available to them for that long.

The key, here, is that each circuit designer will make a number of design trade offs in order to use one tube over another. The circuit topology involved also affects the final decisions.

So, to determine if there's any danger for you using one tube over the other in your Jolida, that's a matter of the circuit topology. You'll have to post a schematic to really determine what the problems might be. Without looking, I'd suspect oscillation, noise floor rises, and poor use of feedback (and the resulting increase in distortion) would be likely candidates.

So, I have to ask, why is using a 12AU7 over a 12AX7 so inviting?

Cheers,

David

 

[mhardy6647]

No danger, but way less gain from a 12AU7 than a 12AX7.

 

[BuzzK]

I wouldn't do it. Design points aren't close at all. You might get away with a 5751 sub, but even that is geared for more current and has lower gain.

 

[swjanitor]

FWIW, I've gone the other way in some old Motorola console amps I've converted for guitar... swapping out 12AU7s for 12AT7s and 12AX7s. No problems so far.

 

[Kegger]

As others allude, it depends on the circuit, and where there at,
but more then likely in phono pre it won't workout very well there.

 

[BuzzK]

FWIW, I've gone the other way in some old Motorola console amps I've converted for guitar... swapping out 12AU7s for 12AT7s and 12AX7s. No problems so far.

Distortion is often acceptable -- even desirable with many guitar amps. The same doesn't hold true for HiFi gear.

 

[Frank Sol]

So, I have to ask, why is using a 12AU7 over a 12AX7 so inviting?

David, below is your answer. Many JD9 owners prefer to use the "High Output" even with MM carts...

We were looking for a way to reduce the gain.

No danger, but way less gain from a 12AU7 than a 12AX7.

As mentioned... I've used the 12au7 for maybe 2hrs without any noticeable complications. So I guess if they meet a need go ahead with the 12au7s

 

[Frank Sol]

Distortion is often acceptable -- even desirable with many guitar amps. The same doesn't hold true for HiFi gear.

I will run a pair of 12au7 tonight and listen for distortion playing a few highly familiar LPs

I'll begin with RFT ecc82. Nice tubes

 

[gadget73]

You can plug it in and nothing will blow up. How well it works is totally dependant on circuit as others have mentioned, but the smoke will not come out by doing this.

 

[diamondsouled]

It's possible to sub a 12AU7 for a 12AX7 in a line state, better to stay closer in gain though, but in a phono pre where RIAA equalization is designed into the circuit it's a non starter. There are a lot of different kinds of 12AX7s to sub though.

Here's a handy gain factor chart:

http://thetubestore.com/gainfactor.html

Cheers

Lar

 

[swjanitor]

Distortion is often acceptable -- even desirable with many guitar amps. The same doesn't hold true for HiFi gear.

Absolutely correct. I swapped 12AU7s out for 12AX7s intentionally to increase gain and combined with other mods (increasing feedback resistor value) allow distortion at lower volume levels.

I mentioned it only from the standpoint that swapping these tubes in what was originally a Hi-Fi design hasn't caused it to blow up.

 

[hammr7]

I know that 12AX7s are higher gain, but that gain does allow for additional distortion. 12AU7s have higher current along with their lower gain, so tend to be more linear over the ranges they can handle. 12AU7s tend not to distort anywhere near 12AX7s, even when pushed to their limits.

At the risk of stereotyping, guitar players, especially old tweed and blues types, hate 12AU7s. They can't push the tube to get that crunchy distortion they desire.

For old tube gear manufacturers, the use of 12AX7s instead of 12AU7s was part of a cost trade off. The higher gain of the 12AX7 allows the use of a less powerful amplifier, meaning less powerful power tubes, to get the same volume.

Organ companies were the largest market for vacuum tubes in the late 50's and early 60's. Companies like Lowrey and Conn often paired 12AX7s in tone generation and pre-amplification with 10-15 watt tube amps. Companies like Allen went with 12AU7s, believing the tubes produced better sound quality. Allen felt they could do this because they had 20, 75 and even 90 Watt tube amplifiers. They used so many 12AU7s tested for low microphonics that they had their own tube designation, PN-11.

 

[gadget73]

There is a lot more to consider than simply more gain = more distortion. Actually that isn't even always true, but we'll ignore that for a moment. The operating points of the tubes are different enough that if you plug a 12AU7 into a circuit designed to take a 12AX7, it may distort despite the lower gain.

 

[Frank Sol]

I will run a pair of 12au7 tonight and listen for distortion playing a few highly familiar LPs

I'll begin with RFT ecc82. Nice tubes

*Update*

Have put about 14 playing hours (24 total hrs of power-up time) on my JD9 w/12au7 tubes.

All appears well and the better news... They sound great even when running a MM cart from the 'High Output' rear outs.

Cautionary note: A friend suggested that perhaps Underwood HiFi did a little tweaking when they preformed their mods on this unit to allow for 12au7 use!?

 

[paulorr1]

12AT7 would be a better substitute.

 

[seta1]

Hi,

The 12AU7 biases differently and if installed in a normal 12AX7 circuit, will pull considerably more plate current. I have seen guitar players do this swap in Fenders only to have the amp fail because of blown 100K plate feed resistors. Some amps are tough enough to handle the swap. Others are not. Bottom line is that to properly use the 12AU7, you will need to change the plate and cathode resistors so that the tube is working correctly. The 12AU7 is not a direct replacement for a 12AX7 or even a 12AT7.

 

[bigdirty]

learning out loud

Im interested in figuring this out using the JD9 as the reference. Ive attached 2 datasheets for the tubes in question.

noticing some discrepancies.

plate resistance (nominal) -
12ax7 - 54.1k ohms
12au7 - 7.5k ohms

plate current -
12ax7 - not more than 9mA
12au7 - not more than 22mA

besides these, there isnt much difference between the two, is there?

 

[bigdirty]

seta1 thanks for your post above.

 

[Brett a]

*Update*

Have put about 14 playing hours (24 total hrs of power-up time) on my JD9 w/12au7 tubes.

All appears well and the better news... They sound great even when running a MM cart from the 'High Output' rear outs.

I wonder if you could give an update, Frank. Have you moved on from the 12au7s, or are you still running them?

I'm asking b/c I just won a pair of RCA 12au7 clear tops from one of Grumpy's AK auctions and am planning on trying them in my JD9.