[coldfogey]

Has anyone ever tried a 12AT7 variant   5965 or 7062.  The claim is they are identical except for a slightly lower gain.  Is there more to it than merely boosting the volume control?  I am using 600R power amp.

[underdawg]

you have to be careful with alternates even if just lower gain. Ill look at my books after work to give my analysis but others may repond first.

[avahifi]

Installing a low gain tube in a Fet Valve amplifier won't change the overall gain of the amplifier, unless the tube gain is very very low.  All it will do is increase distortion.  The voltage amplifier section of the Fet Valve amplifiers use some loop feedback to insure uniform performance with a variety of normal output 12AT7 tubes.  Lowering the tube gain substantially just reduces the amount of feedback and increases THD.  With a really low gain tube bass control and dynamics will mud out.

We urge that you avoid low gain tubes in AVA products.  If you must try tube rolling in a Fet Valve amp, try a set of old new stock Telefunken 12AT7 tubes.  Of course they won't make much difference as they have the same gain as the JJ 12AT7 tubes we supply.  We have tested them in the Fet Valve amps and can tell you that they do not require bias adjustments.  Use of low gain tubes probably will as they will cause the bias current to droop too.  This is not an adjustment to be attempted by a casual amateur!

Frank Van Alstine

[coldfogey]

Thank you.  I appriciate the detailed answer.

[underdawg]

I did extensive testing on several different 12at7 tubes for my amp.The ones I settled on were rca command black plates 12AT7.They are reasonable priced still and my pair has zero microphonics. I was impressed with the tubes I kept them after the amp was sold.My second favorite was the Sylvania gold brand 6201 tubes with gold pins.

[underdawg]

one of the better books I have found for tubes is :
tube substitution handbook authors william smith and barry buchanan by prompt publications.
other good books :
rca recieving tube manual rc 30
vaccum tube valley for listening tests - opinioons so keep that in mind

[tonyptony]

I did extensive testing on several different 12at7 tubes for my amp.The ones I settled on were rca command black plates 12AT7.They are reasonable priced still and my pair has zero microphonics. I was impressed with the tubes I kept them after the amp was sold.My second favorite was the Sylvania gold brand 6201 tubes with gold pins.

underdawg, what did you like about the RCAs over the GB?

[ltr317]

I did extensive testing on several different 12at7 tubes for my amp.The ones I settled on were rca command black plates 12AT7.They are reasonable priced still and my pair has zero microphonics. I was impressed with the tubes I kept them after the amp was sold.My second favorite was the Sylvania gold brand 6201 tubes with gold pins.

The RCA black plates are my favorite 12AT7s also.