NAD. Just picked up a Roadster!

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mastora

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I haven't got a chance to crank it yet but I'd like to ask a few qustions about the tubes I found in it. In the V positions all are mesa tubes except for V3 and V5 which are JJ's. Why would JJ's be in those positions. There are Groove Tubes GT 6L6 R in all 4 positions and Mesa 5U4GB in the other 2 positions. What do the groove tubes do that the stock ones don't? This is my first Roadster that is replacing my Triple recto. I'm not that familiar with Roadsters. If I put a Tung Sol in V1 like I did in my Triple will it improve the cleans?
 
V3 is the cathode follower, which is basically an amp stage into a buffer with over 200 volts on the cathode. I'm guessing the JJ was placed there after another tube failed. Many tubes are not rated to handle that much of a voltage difference between the cathode and filaments. In fact, the JJs are not rated for it (180 V) and will fail prematurely. From what I understand, the Chinese 12ax7s are a little more robust for this.

V5 is the cathode follower for an FX loop. That will also need to be changed to prevent failure. The free fix would be to swap two other tubes for those. I think the stock Mesa tubes are good to go in those CF circuits.

The Tung-Sol swap is still a good choice. :D
 
Thanks for your information and input. I've replaced the JJ's with Mesa's and put a Tung Sol in V1.
 
JJ ECC83's (12AX7) are fine in the Cathode follower position.

The current Mesa branded 12AX7 is a JJ.

Avoid any New Sensor brand tubes in the CF position.

New Sensor brands include:
Sovtek (aka Mesa "Russian-1")
Electro-Harmonix (aka Mesa "Russian-2")
Tung-Sol Reissue
Mullard Re-issue

Dom
 
Thanks Dom. I'd read in a few places that JJs fail, but I know you have a lot of experience.
 
Thanks for the info. You said that current Mesa are JJ's. What are older ones? I think the Mesa 12ax7 I have are about 10 years old. Comparing the 2 Mesa branded 12ax7's, they look different.
 
The old Mesa 12AX7A (if they have dimpled square getter) are Chinese tubes (made from late 1980 to early 1990) but that would be older than 10 years. I pulled the stock tube from my Mark IV(purchased in 2000) in 2012 (never changed them until then) same as current Mesa 12AX7A. The Chinese square getter tubes should hold up okay in the CF circuits. The new version of the Chinese tubes look similar with the dual mica spacer above the plates but have a halo getter (they are not as good as the older versions). I have a few in good condition that I use in my RA100 and one in the Mark V (V2) position. I feel they have a better sound quality than the current Mesa 12AX7A. However, I actually prefer the current Mesa 12AX7A in the Roadster. You will not notice much of a change in tone using the Chinese tube in V3 (since it also uses half of the tube for a gain stage). I have experimented with V1 and V2. I settled on using a Mesa SPAX7 in V1 and stock in V2. The SPAX7 is similar to the standard tube, not sure if there is a slight variation in construction but almost same tube as the Mesa 12AX7A.

Old Mesa tube from 1987-1991 Original from Mark III combo, also had many replacement tubes.
P4160046_zps1a7719b1.jpg
 
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