[avahifi]

You are not "stuck" with one brand of tube, but I am very leary of giving detailed info to amateurs regarding adjusting bias on the Fet Valve amplifiers because I don't want any dead customers, they don't do good repeat business.

I can note that all the reports are that tube rolling does not affect the sound of the Fet Valve amps as much as the preamps.  It is really good no matter what tubes you use.  We use the JJs because they are really low noise and low microphonices, very reliable, high gain, and sound really good to us compared with everything we have tried here.

Frank Van Alstine

[tonyptony]

We do see the need to do this when changing tube brands here.  It is not a end user easy do it yourself project.  You need an ammeter and the ability to access the small trimpots on each audio board without zapping yourself or the amplifier (300V or so DC floating around there, really nasty if you slip).

Frank Van Alstine

Frank, let me ask a slightly different question: so if a FetValve is supplied with a certain tube, let's say the JJs, how much variation is allowed if changing tubes of the same type - before rebiasing is needed?

Case in point: as you know I am waiting on delivery of a used FetValve 550. This amp is being supplied with the original JJ tubes that were provided. Becuase I was worried about tube damage during shipping I bought a new set of JJs from you. Do I automatically assume that it's plug-n-play? Is there any way for the end user to know that rebiasing is necessary?

[avahifi_lj]

Hi:

Let me take a stab at the question:  You should be okay putting the new JJs in the amp.  Most of the JJs I have used set to the same bias point (there is an odd exception, and that's why we test each amp).  As for the stock tubes that are coming in your used amp, they likely will be okay.  Note that we ship amps all the time with the tubes installed.  The tubes are pretty small and light weight compared to the EL-34 output tubes (used in the U70 amp).

Where I see the biggest problem in amp adjustment is using old tubes in the amp.  A lot of the "new old stock" (NOS) tubes are many years old, and simply act a bit different.  Some require higher bias settings and some lower settings than our stock JJ tubes.  The interesting tubes are the ones where the tube heater glows like a light bulb when first turned on, only to settle down a few seconds later.  Maybe it was some type of "quick start" tube used back in the 1960s.

Frank is correct in that we have found VERY little difference in the amp sound when we have tried different tubes.  In my home system I tried NOS tubes, that when used in an Ultra SL preamp had a significant warm/mushy sound, but made very little difference in my Ultra 550EXR.  FYI:  Another reason we like the JJs is they are very consistent compared to other tubes we have tried.

A couple of interesting points:  When we select tubes we always choose based on the sound from the preamp -- we never select a tube based on how it sounds in the power amp.  We have tried almost every 12AT7/12AX7 tube currently in production (JJ/Tesla, Ei, EH, Svetlana, Sovtek, Chinese tubes, etc.) and several NOS tubes.  While it's fun to try the NOS tubes, we have to select current production tubes due to availability and cost.  We go through so many tubes that even if we do find a NOS tube we like, the chances of getting quantities we need are slim.  Also, the cost of NOS tubes are many times higher than what new tubes cost.

I have spent a few evenings (and beers) rolling tubes in my preamps.  It can be great fun to experiment with different tubes; however, if you want to tailor the sound of your Ultra system try changing the tubes in the preamp, and don't worry too much about the power amp.

Larry

[ricmon]

Hi:

The interesting tubes are the ones where the tube heater glows like a light bulb when first turned on, only to settle down a few seconds later. Maybe it was some type of "quick start" tube used back in the 1960s.  ...

What exactly do you mean by "interesting" as the Siemens in the 550 do exactly that. "glows like a light bulb when first turned on"

[avahifi_lj]

Hi:

I'm not too keen on seeing tube's heating element light up then settle down to normal.  If the tube is designed that way then it's likely no concern.  If the tube is not designed that way it seems to me that the hot flash is not good for the heater.  Is the heater not able to handle the turn on surge?  Is the tube's heater being stressed to a point that one should be worried about longevity of the tube?  We have seen this behavior in some of the NOS tubes we tested.  Over the years I have not seen many tube heaters fail, however.

Thanks

Larry

[skrivis]

Hi:

I'm not too keen on seeing tube's heating element light up then settle down to normal.  If the tube is designed that way then it's likely no concern.  If the tube is not designed that way it seems to me that the hot flash is not good for the heater.  Is the heater not able to handle the turn on surge?  Is the tube's heater being stressed to a point that one should be worried about longevity of the tube?  We have seen this behavior in some of the NOS tubes we tested.  Over the years I have not seen many tube heaters fail, however.

Thanks

Larry

AFAIK, the resistance of nichrome is different when it's hot than when it's cold. That's why this is possible.

As for those particular tubes, it's hard to tell. Heater shapes differ, and a straight wire will behave much differently from a coiled wire.

The composition of the thorium coating can differ too. It's quite possible that the particular design allows or even requires a white hot burst at startup. (Or maybe they just didn't know what they were doing. hehe)

I've seen old tubes in a table radio flash like this, and they did it every day for 5 or so years that I know of. I thought it was really neat when I was a kid.

[tonyptony]

This last part of this thread has been a great source of info. Thanks to Frank and Larry from AVA and the rest of the gang here on this circle.

[ricmon]

This last part of this thread has been a great source of info. Thanks to Frank and Larry from AVA and the rest of the gang here on this circle.

I agree thanks guys.

[skrivis]

If anyone is interested in learning more about tube circuits, I can recommend this:

http://www.audioxpress.com/bksprods/books/bkaa54.htm

I have an old surplus copy, but AudioXpress/Old Colony does a pretty good job on reprints.

[WEEZ]

While I know nothing of tube design or amplifier design, I can confirm what skrivis said regarding the resistance of ni-chrome heater wire. Cold resistance is less than hot resistance. It varies with the exact alloy- ranging from 7 to 10 %, or so.

FWIW

WEEZ