[iclark]

On reading a previous post, Frank was extolling the virtues of the EH 6CA7"s.

"Right now I am happiest with the Electro-Harmonix big bottle 6CA7 tubes in our tube amplifiers.  This unit appears to have JJ EL34 output tubes, which work well too, but to my ear are a tad brighter sounding." (Topic, U70 on Ebay)

I took this to mean that I could just yank out the EL34's and replace them with 6CA7's.
So I ordered the EH6CA7's from the Tube Store. I pulled the EL34's, turned the bias controls all the way down and installed the new tubes. Turned the unit on in breathless anticipation. Everything lights up, lots of hum and the bias shoots up to 1.8 V DC, this is with the bias turned all the way down. My target is 1.56 V. Shut the amp down realizing that something is not right, Time to rethink. I put back the EL34's. Power on, rebias and my U70 is back to its full glory. No Damage done.
Obviously I have to do some mods to use the 6CA7's.
Any advice Frank?

[boycephoto]

Good morning Ian,

I sounds like you have a defective tube(s).  The U70 does not need any modifications to use 6ca7 tubes.  I would try them one more time, if you have the same results I would contact The Tube Sore.  I have used Ei 6ca7 in my U70 and it worked fine (didn't sound anywhere near as good as NOS Mullard EL34's or Winged C EL34's).  I am happy with with I have but have wondered how the re-issue Genalex KT77's would sound. 

Good Luck,
Dave

[avahifi]

I too suspect you got a bad tube in that new batch.

Look at the bottom side of each tube to make sure the locating lug is not missing or broken off on one or more of them.  That would allow them to be inserted in a socket improperly.  I have seen some replacement 8-pin sockets that had a center hole too big out of spec. That would allow a correct tube to be installed wrong too.

Regards,

Frank Van Alstine

[rlee8394]

I have been running the EH 6CA7 tubes in both of my U70's without any issues at all. Rock solid bias, great sound! Try replacing just one channel with the EH 6CA7's. Did any of the four start to glow red? That would indicate a bad tube.

Ron

[iclark]

Hi Guys,
Thanks for all the advice. All is well.
I tried them again, this time with a little shot of Detox-it in each tube socket. The bias started at 1.8 V as before even with the pot turned all the way down.
This time I thought what the heck I'll give them a listen. After 5 min. the bias dropped lower to the point where I could make an adjustment. The hum has gone.
Nice tubes, but I'll have to do more listening.
Interesting thing about this quad, one is constructed differenly from the other three. I always check my tubes after installation, before powering on. When I saw this I thought I had missed the notch, as Frank said. But it was installed correctly.
Photo here.
http://www.audiocircle.com/index.php?action=gallery;area=browse;album=1941

Also what threw me was the 6CA7's have a pin at position 6, where as the EL84's don't.

Tubes as so much fun, always an adventure.
It's nice to know that when I run into a problem, real or imaginary, I can rely on this forem to help me out.

Ian

[gjs_cds]

One wants to believe that the Tube Store does indeed test when they sell matched quads.  But it is also obvious that these tubes came from different batches.  So this begs the question, of they're from different batches, will they age similarly?

[iclark]

The Tube Store says that using their matching numbers you can request the same number should a tube fail and put it in your amplifier with out having to change your bias. I have found this to be the case with this current quad of tubes. When trouble shooting my current problems I did a lot of switching tubes from side to side and the bias never wavered. All my 6CA7's have a lable on them marked 57. Regarding lots I have the following
07 01
07 01
07 10
06 11
as marked on the tubes, I assume this is the lot no. or date code.
The tube in question was a 07 01 so I assume some Russian QC guy was asleep that day. However different in appearance or not I cannot detect any difference in performance.
I don't think it is practical for the Tube Store to give you all the same lot. Testing each tube is the most reliable way to ensure they are all similar in performance. Tubes being tubes are going to fail for various reasons and generally only one will fail regardless of use or age.

Ian

[boycephoto]

Ian,

Did the Tube Store take care of your problem?  If so are you happy? Any issues with their customer service?

Dave

[boycephoto]

OOPs, I looked back and I see you have taken care of the problem   

Dave