Help. Are there any preamp valve experts out there?

[Shark_Jump]

Can anyone give some preamp valve advice?

I have a Conrad Johnson PV2 Pre amp. Very happy with it but the line stage is giving more amplification than I need. With CD input the power amp clips at less than 1/3 turn of the preamp volume.

It has a standard tube set up of 12AT7>5751>12AT7.

My question is, are there any tubes that I can swap with the standard ones that will lower the gain?

 

[Uncle Erik]

I also have a PV2, but don't know what else you can try. The 5751 is a lower gain version of the 12AX7, but I don't know if there are other substitutes.

If anyone has some suggestions, I'd be interested in hearing them, too.

 

[Slash47]

Those tubes (12AU/T/X7 etc) all have loads of gain, I don't think you'll get it low enough for your liking.

Sound should be pretty sweet though

 

[Shark_Jump]

Quote:

Originally Posted by Slash47 /img/forum/go_quote.gif sweet

and bouncy (has anyone used that before?) and LOUD

 

[Shark_Jump]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif If anyone has some suggestions, I'd be interested in hearing them, too.

So do you have the same issue? I think I could quite easly blow my amp/speakers if the volume was accidently turned more than half way.

I did try using inline 12db attenuators on my inputs from the CD player but it had an effect on SQ. On reflection attenuating the signal before amplifying it is pretty dumb (reduced signal/noise ratio).

 

[Skylab]

Did you try the 12AU7? It has a lower amplification factor than the 12AX7. The 12AU7 is a medium-mu tube, with an amplification factor of 17, versus the 12AX7 being a high-mu with an amplification factor of 100. They draw the same amount of heater current, so it is almost certainly safe to try, although it's always best to call the amp maker and ask them.

That does NOT mean it will sound good. But is should provide less gain (depending on how the circuit of the C-J is operating). It may sound great - but it might not. Depends on how the 12AU7 works in that circuit.

FWIW I prefer the 12BH7 in circuits that use a 12AU7 - it's also medium-mu, but it does draw twice the heater current of a 12AU7, so in this case you really must check with the amp maker (or do the transformer load math is you know what the trafo is rated for) to make sure the trafo can handle the current draw load.

 

[Uncle Erik]

Quote:

Originally Posted by Shark_Jump /img/forum/go_quote.gif So do you have the same issue? I think I could quite easly blow my amp/speakers if the volume was accidently turned more than half way.

No. It works fine for me. I wonder if you might have had your potentiometer replaced at one time - replacing it with a different value might do that.

 

[Shark_Jump]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif Did you try the 12AU7? It has a lower amplification factor than the 12AX7. The 12AU7 is a medium-mu tube, with an amplification factor of 17, versus the 12AX7 being a high-mu with an amplification factor of 100.

Thanks Skylab, the 12AU7 is certainly appears to be a possibility. However just to clarify my valve line stage set up is actually 12AT7, 5751, 12AT7, there is no 12AX7.

Is the 12AU7 interchangable with the 12AT7?

 

[Uncle Erik]

The 5751 is a version of the 12AX7 with tighter tolerances and lower gain.

 

[Skylab]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif The 5751 is a version of the 12AX7 with tighter tolerances and lower gain.

Right.

The 12AT7 is also a high-mu tube. But I'm not sure subbing a 12AU7 for a 12AT7 is a good idea. You should check with the manufacturer first.

 

[Shark_Jump]

I just checked a couple of websites and apparently the 12AY7 with a gain of 44 is quite a common swap for both the 5751 and the 12AT7 (gain 60). I will probably try switching the 12AT7 in V1 with this.
Fuzzy's Dead Mouse - 12AX7 Family Tube Substitution

The GE spec for the 12AY7 states it was specifically designed for the low level stages of high gain amps and is especially low noise and it looks very compatible looking at the relative spec sheets.

Any recommendations on a 12 AY7? I normally go for Mullard tubes, but it doesn't look like they did a 12AY7.

BTW. I noticed the lower gain AU7 you recommended requires a slightly different plate resistor which also makes it less suitable as I don't want to start modding the standard circuit .
Lower Gain Tube(s) in V1 [Archive] - GuitarsCanada.com - The Canadian Guitar Forum

 

[Skylab]

All of these tubes have slightly different operating points in addition to the difference in gain. Gain is the product of the transconductance and the plate resistance. This is why if you don't know the exact details of the circuit, and have an understanding of tube electrical properties (which I am only just beginning to really understand myself), you need to exercise some care.

All of the tubes under discussion are close enough electrically that nothing dangerous is likely to happen when subbing them, since they all draw the same current from the trafo, have the same pin configuration, etc. But again that doesn't mean they are a good choice in that circuit, or that they will sound good in it.

You can either call C-J and see what they say (which again is where I would at least start), or you can try the closest ones and see how they sound.

To answer your last question, though, I have never used any 12AY7's. There appear to be lots of the usual US Brands - Sylvania, RCA, GE - but it seems to be a more expensive tube generally than the 12AT7 or 12AU7 (but less than 12AX7).

 

[Shark_Jump]

Thanks Skylab, much appreciated and understood.

Curiosity will probably kill this cat, I am on ebay looking for a 12AY7 as I type this. Apparently the 6072 and 6072A is a 'ruggedized' version of it.
Index of /tubedata

A while ago I found a web site that explained very well in basic electronics how to interpret the GE spec sheets, if I find it again I will post it. If I remember correctly a straight swap with a valve with a lower spec gain (mu) can actually increase amplification if the required input resistance is different (by working the valve harder). Although I don't think it applies in this instance.

 

[Skylab]

These look nice, and are actually pretty fairly priced it would seem:

1960 GE JG-12AY7 / 6072A

Not cheap, but tubedepot is a completely legit/reliable seller, versus the wild wild west of EBay tube buying.

 

[Shark_Jump]

Great call. Just ordered it. Surprised at the cost of international postage nowadays - $30 to Australia - owch!

Will update when I swap it in, in case it is of use to anyone.

I notice they are ex-Learjet, an added bonus is I can use it in my private jet if it doesn't suit my amp.