2. amps-preamps
  3. 1241747902

Can someone explain

I lost a tube in my Cary SLP98L last night. Actually, one channel has been fading away for some time and I just figured out that it was one of the SLP98's line stage tubes. I replaced the line stage pair with spares and all is well again.

I am wondering about the second pair of tubes, though. The so-called "buffer cathode followers." All four tubes are the same age (close to 8 years of use) and I'm thinking I should go ahead and replace them as well. I know from experience that the line stage tubes make a big difference in the sound of the preamp, as they are actually carrying the signal from my source, but can someone tell me what the "BCF" tubes do, and if they also have an important role in the sound? I have one pair of NOS VT-231 tubes left and don't want to use them in the buffer position if they don't make a major difference.

Hmmm, thinkin' that I should dig out the Chinese tubes (maybe Ruby?) that came with the SLP98 and see if they make a difference I can hear in the buffer position....

Still, if anyone can explain the function of the BCFs to me I'd be grateful. A link to an explaination of tubes for the electronically challenged would be nice also, if anyone knows one.

Thank you!
abysmillard
unclejeff
1,313 posts
 
Sometimes I will replace all the tubes with 'matched' tubes when one blows. then, I keep the still functional (mis)matched tubes as replacements for when I might need them. Yes, this can make sense.

Still, I do think of those times from the 1960's, etc. when the brand and vintage really did not matter at all. As in, when running 6550 tubes, replace with a 6550...unless all you have is the KT88.
mechans
3,048 posts
 
You may want to keep tube of similar parameters in the buffer stage which is to my mind equal to an electron removal system. Since your VT-231s are almost certainly GTs then I would use GT buffer tubes. RCA made civilian GTs by the billions and you can get true NOS tubes if you want for fairly low amounts. However a true NOS tube is not needed and a used "good" pair will suffice. If you can't get them for $15 to $20 each email me. I may (can) be able to help as in I own a ton and don't use them.
almarg's avatar
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almarg
9,670 posts
 
In a cathode follower stage, which is analogous to an emitter follower in a solid state circuit, a resistor is placed in series with the cathode, the input is applied to the grid, the plate is connected to the usual high dc voltage (but directly, rather than through a resistor), and the output is taken from the cathode, rather than from the plate. Various other resistors are used for biasing.

A result of this is that the voltage gain is very close to 1 (actually, slightly less than 1). The fact that the cathode (output) voltage very closely follows the input (grid) voltage accounts for the name "cathode follower."

A second result is that the output impedance of the stage will be much lower than for a typical amplifier stage which provides its output from the plate. That enables the stage to, among other things, drive cables and loads which have significant capacitance, or relatively low impedance. The term "buffer" refers to the fact that the stage acts as a buffer between the amplifier stage, with its high output impedance, and whatever the buffer stage is driving.

In this case, the "cathode follower buffer" tubes are undoubtedly placed between the line amplifier stages and the outputs of the preamp, and so they are directly in the signal path. Intuitively I would expect the sonic effects of age-related degradation of their parameters to be somewhat less significant than for the line amplifier stages, but I don't have a feel for the degree to which that may be true.

Regards,
-- Al
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