Help. Are there any preamp valve experts out there?

[Skylab]

Those tubes looked good enough that I felt I needed a pair for the collection

 

[Frank I]

Quote:

Originally Posted by Shark_Jump /img/forum/go_quote.gif 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.

I owned CJ Amps. I would use only what CJ tells you to or call them or Kevin Deal at Upscale Audio he will give you the right information.

 

[Shark_Jump]

OK , I have done a bit of homework and had another check of the specs on the unlikely professor. http://www.unlikelyprofessor.com/tub...reamp%20Tubes/. Now I am beginner at this so if you know what you are talking about please correct me if Im wrong anywhere.

The CJ PV2 preamp line stage valves are 12AT7>5751>12AT7

Assuming the plate supply voltage of the PV2 is approx 300V (This is a guess, I would appreciate if someone could tell me the actual). Looking at the graphs for Plate Current vs Plate Voltage, I come up with the following approximations.
12AT7 - Assume grounded plate current is 25mA, plate load resistor Rc is then 300/.025=12K, Gain is 38 (+-1V either side of -1V on the load line)
5751 - Assume plate current is 4mA, Rc is 75K, Gain is 52.

Looking at the 6072 spec. ('Ruggedized' 12AY7) it looks safe to swap in for either valve with an associated gain drop of approx 20%. However due to its lower plate current it is more suited to switching with the 5751 than the 12AT7 at lower grid voltages.

Inspecting the 12AU7 spec. recommended by Skylab , it does seem compatable with the 12AT7 due to the higher plate current and lower plate resistor value they share. It also would give a much greater drop in gain of approx 15 instead of 38.

So it looks like the 6072 is a safe swap in my PV2 for either the 12AT7 or the 5751, but it won't make much difference in gain. Now how about this $600 12AU7 instead? http://www.tubedepot.com/bsct-ecc82-mullard50s.html

 

[Skylab]

You don't want to spend $600 on those, I assure you. Mullard CV4003's (which are 12AU7's) shouldn't cost more than $100 a pair, and that is a lot for 12AU7's, although they have been going up in price. The very nice Tung-Sol black glass 12AU7's are now about $100/pair too.

Please note these are examples, and I cannot vouch for these sellers:

2 Mullard ECC82 12AU7 CV4003 boxplate close matched TOP - eBay (item 370311715568 end time Jan-25-10 07:14:02 PST)

TUNG-SOL JTL 12AU7 ECC82 Black Glass D Getter USA Lot7 - eBay (item 130283607924 end time Jan-19-10 17:49:29 PST)

 

[Shark_Jump]

OK, the GE Lear Aerospace 6072 tube arrived and I swapped in. The major reason for the 6072 tube purchase was to try reduce the gain.

My standard tubes in the CJ are 2x 12AT7 (Mullard CV4074) and 1x 5751 (Sylvania Triple Mica Grey Plate). The sound is both detailed and musical with a quite background, this CJ PV2 preamp really brings my whole system alive, I like it a lot.

Swapping for the 12AT7 gave only a marginal reduction in gain, swapping the 5751 gave a little more attenuation, but only by a minimal amount. Overall the effort to reduce the gain by swapping in this tube was a fail.

Now sound quality, my musical 'memory' is not good, so by the time I swapped the tube and waited 30 seconds for the CJ's warm start circuit to turn it back on, I would have forgotten any subtle differences.

I would say that the 6072 didn't sound good in the 12AT7 spot, there was possibly a bit of distortion, so it didn't stay in there for long. Switching for the 5751 was a different story, there was no distortion, a subtle boost in the lower frequencies and a delicate feel, perhaps to the detriment of some of the detail. If you are looking to calm an amp down and you have a 6072 or a 12AY7 lying around, swapping out a 5751 may be worth a try.

Now where was that site with the Mullard 12AU7's .......

 

[Skylab]

I got mine here: http://www.langrex.co.uk/images/valv...%20Mullard.jpg

 

[Shark_Jump]

Thanks I like those prices, I might even upgrade my standard tubes. Can't see a 5751 on there but will pop them an email.

If you push any more links like that I can see myself developing a serious tube rolling habit!

 

[Skylab]

Quote:

Originally Posted by Shark_Jump /img/forum/go_quote.gif If you push any more links like that I can see myself developing a serious tube rolling habit!

I resemble that remark

 

[vajrasattva]

it is ok to put a 12au7 into the place of the 12at7, you get lower gain, and current through the au7 is dependent on your current plate resistor on the 12at7. since the 12au7 can handle more current than the 12at7, it wouldnt hurt it to run it at 12at7's Rp.

 

[Pricklely Peete]

How old is the CJ preamp ? Might be worth getting it checked out by CJ to see if everything is in spec. If it's already been certified then forget about the suggestion...

What V are you seeing at the preamp outs anyway ? What amp are you using with it ?

Peete.

 

[Shark_Jump]

Prickly Pete. Just to clarify, there is nothing actually wrong with the preamp, it actually sounds fantastic, I am only trying to reduce the gain a bit by rolling tubes, and as a side effect, seeing what effect this has on the SQ. It probably does need a service though. l understand Bill Thalmann is also a specialist at this, postage from Australia is a bit of a pain for me though.

Power Amp is Myryad Z160. Spec says input sensitivity at max rating of 50W is 0.7V, I don't have an Avo or Voltmeter.

From doing a bit of research I understand in 1981 when the CJ was made, the output levels of record and tape players were a bit lower than the output of CD players/DAC's today. I'm not sure why this is the case but it does make a strong case for using a passive preamp in a modern system without a turntable.