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Conrad Johnson CAV 50

Just acquired this beauty from my father in law. I've never owned a tube amp, so not sure if I should sell it, or keep it and pay him. Anyway, a question for you tube heads. One of the EL34 tubes in slot V7 ( a tube which can have it's bias adjusted, which I assume means the power transmitted) has it's red light constantly on. I turned the bias all the way down and it still flickers. However, I noticed that my left speaker sounded blown/distorted. When I turned the bias up, the distortion decreased. The instructions say to turn the bias down/counter clockwise, until the red light goes out. But, if I do this, it sounds bad. So, my question is this. Is it simply a tube I need to replace to fix that problem, and if I change one tube, should I change them all. Next question is I searched for the EL34 tubes and of course there are a large variety of them. Any thoughts. To me the amp sounds little bright with the Celstion SL6's I am using, so any warmer tubes would be nice. Also, if I were to sell this setup, any idea on what I might ask on pricing? Thanks.
Roly
rolyasm
jond's avatar
jond
3,400 posts
 
Roly the CAV-50 is an integrated so it's an amp and preamp combined in one chassis though it doesn't have an active preamp stage so it's really a passive preamp. So what I'm sure Jazz is referring to are the small signal tubes you have 1 12au7 and 2 12ax7's both types are very easy to find and plentiful. You also have 2 6SN7's that are probably input tubes of some sort for the amp stage. Rolling any of the tubes can potentially change the sound just like changing the power tubes will, so have fun and welcome to the world of tubes!
jazzonthehudson's avatar
jazzonthehudson
277 posts
 
I have had most CJ tube gear, including the PV10 and MV50 and I am pretty sure the CAV50 is based on the PV10/MV55 circuitry (I talked to the tech dept before buying the CAV50 as I liked the MV50 and had it for nearly a decade). In fact, the power stage of the CAV50 (12AX7 line stage for the gain, 6SN7 dual-triode driver and EL34 in push/pull for the power) is IMO (almost) identical to the MV50. 

The line stage of the PV10 (w/o phono stage) was based on a single 12AU7 (one half for each channel) which can be easily upgraded to a nice ECC82, IOW, that tube has a lot of influence on the overall sound. I recommend the Siemens E88CC Germany if you like great vocals, other reasonably priced are Philips E88CC Holland, Philips/Mullard E88CC UK and Amperex 6922 USA (orange label). Then you could upgrade the  12AX7 / ECC83 / 7025. 

As for output tubes,  JJ E34L has indeed great value/money. 

At the end of the day, the circuitry is based on the PV10 which is a good but not very resolving model like the PV12 at that time so one should not go crazy with tube rolling.

Jond, you are probably confusing it with the CA200, aka little brother of the venerable Premier 350,  solid state control amp that has indeed a passive preamp. 




jond's avatar
jond
3,400 posts
 
Jazz no confusion I read this on c-j's website regarding the CAV50:

"Because each element of an audio circuit introduces its own sonic colorations, a simple circuit should be expected to yield superior musical performance. At conrad-johnson, we have maintained a commitment to designing simple, straight forward circuits for audio components. The CAV50 elevates this design philosophy from the circuit level to the system level by eliminating the preamplifier altogether and incorporating the control functions on the amplifier chassis, hence the term "control amplifier". This approach completely eliminates the inherent colorations of the active circuitry embodied in a preamplifier stage, and of the cabling between preamp and amp. The result is a significant improvement in system performance and convenience, combined with a reduction in cost."
jond's avatar
jond
3,400 posts
 
However re-reading it, it's rather confusingly written as there seems to indeed be an active preamp stage incorporated into the CAV50.
rolyasm OP
14 posts
 
A rep from a tube store wrote me and here is what he had to say about a few tubes I was looking at.....
"The EL34 is a pentode. A pentode has a suppressor grid in addition to the control grid and screen grid. The power pentode was developed by Philips in the 1930s. RCA developed the beam tetrode which has a set of beam plates instead of the suppressor grid. At the time Philips held the patents on the power pentode anyone who wanted to build power pentodes had to pay Philips to obtain licensing to build them. RCA developed the beam tetrode so they could build tubes that functioned like a power pentode and not pay the licensing fee. The 6L6 was one of the first beam tetrode tubes. Compared to the EL34 pentode, the 6CA7 and KT77 beam tetrodes will have a little more bass response and sound warmer."
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