[defec]

First off, my apologies. I posted the first paragraph below over at Audiokarma but after figured here would be a better place for it sense there is a lot more traffic. I'll cut and paste/post any answer I get over there if it's okay.

Hi, I have an old Fet Valve EC preamp and was wondering what should I consider when selecting the right replacement 12ax7's for the best sound and most linear output from both the line and phono stages. I've noticed over the years that certain tubes will seem to push up the low end and/or roll off the highs a bit. Is it distortion that I'm hearing? My complete uneducated guess is that it could be due to some 12ax7's being lower gain then others and the circuit likes a certain gain range but of course I really have no idea.

Also, I finally changed over the 12AT7's in a Fet Valve 300EX from that of Chinese origin to Electro Harmonix brand, adjusted the internal pots via instructions that I found in this forum to 235ma and it was a large step up in sound quality.  It was a great treat. I was expecting either a minor one or no difference.

Any input is greatly appreciated. Thanks.

[Lefty052347]

I would recommend JJ EDD803's from Tube Depot.  2 pair matched with 1 pair also culled for low microphonics.  That would total about $72 for the four tubes.  For comparison, Gold Lion ECC83/B759 would cost $196.

Regards,
Dean

[defec]

Thanks for the response. I have some JJ short plates somewhere that I should dig up. The Gold Lions may be wasted in this application but I'm not sure. The Fet Valve preamp seems to interact with tube change a lot differently then a traditional circuit so I figured that I'd ask to see what was the best fit. My guess is what's best for other things could be not the best fit in this case.  In the distant past I tried some pretty revered NOS stuff and I remember a lot of 'em sounding a lot worse than Chinese 12ax7's.

[trackball02]

I have a Fet Valve CF preamp, after my stock JJs failed within 6 months, i replaced all 4 tubes with Gold Lions. They sound great and very reliable. I would use them again.

[defec]

It may be different with the EC being older and I'm guessing that the 12ax7 is not used as a cathode follower in it. I'm hoping Frank will weigh in and set me straight.

[mark funk]

For the last few years i have been running a nice combo.  Been running Telefunken ECC81s in my Fet Valve Ultra + 550 and a pair of Telefunken ECC 801s in my Trancendence R +  DAC and a nice pair  of 1966 Valvo 6201s in my Trancendence 5 preamp and all is well in my cave.

[avahifi]

If this is a really old FV preamp built in a Dyna PAT-5 chassis, then it uses four 12AX7 tubes, one in each line channel and one in each phono channel.  Newer versions use 12AT7 tubes in the line sections.

The tubes don't have to do any work as they are in our patented transimpedance circuit where all the load drive is done by the companion power mosfets.

We use JJ 12AX7/ECC83s tubes here with good results.  A tested set of four is $59 plus $10 shipping.

Frank Van Alstine

[defec]

Hi Frank. Thanks for the reply. It's not in a PAS chassis but your own 17"ish one with smaller tone control knobs etc. It does have a sticker that states to use four 12ax7's though.  I'll give some JJ's a try again.

[avahifi]

Defec. your AVA preamp uses two 12AT7 tube in the line section, two 12AX7 tubes in the phono section.

Don't use 12AX7 tubes in the line section.

A set of two matched pairs is available from us for $59 plus $10 shipping.

Frank Van Alstine

[defec]

Frank, thank you for the response. Hmm, I swear that the preamp has a sticker saying to use four 12ax7's but you'd know better than I  on what to install where. The wrong tube installed may explain why I perceive the highs to be a bit rolled down or bass more dominate. What's funny is that I've had this preamp forever and it came to me used with a pair of Sovtek 12ax7wxt+'s in the line and Chinese 12ax7's in the phono stage

 I'll post some pictures in a few days to just to be 100% sure. I'd do it now but I messed up my lower back working on a friends heavy Ampeg SVT Bass amp head:(. 

[avahifi]

Many long years ago we switched from 12AX7 to 12AT7 tubes in the line sections of our preamps to improve current drive capability.

I would need to see some close up photos of your line board to see plate resistor values to make sure.

However, if your back panel label says use four 12AX7 tubes then believe that.  The change from 12AX7 to 12AT7 tubes requires some resistor value changes.

Frank

[defec]

Frank, I know that this is an old thread but I'd love to try using 12AT7's in my preamp section if you'd be okay with sharing the valve of what resistors to change with me. I've built many a tube amp kit over the years and even adjusted my Fet Valve EX amp when I changed 12at7's after reading howto on a old thread here so I'm comfortable with doing the work. If something went wrong I'd just revert the circuit back to how it was originally.

Also, while I have you, how much capacitance is installed at the preamp's phono input?

Thank you.

[avahifi]

I will have to dig through my record archives to find the answer for you.  Note that you will have to pull the mother board to change the needed part values to use 12AT7 tubes in the line section of that older preamp.  The phono section must stay with 12AX7 tubes as 12AT7 tubes do not have enough open loop gain for that application.

Bug me with an email in the middle of June and I will research your question.

frank@avahifi.com

[defec]

Thank you. I really appreciate it.