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I’m newish to tube amps, have heard what I can and read a lot. Based on all that, I’ve chosen a particular amp with parallel output tubes.This is the Transcendent Sounds T16 OTL. I’ve satisfied myself by searching that these OTLs have overcome higher maintenance and short tube life problems of earlier OTLs.
But when replacement time comes around, they need 8 * 6C19 PIs a side - the only supplier I know (The Tube Store) does not sell them matched. I imagine you could need to buy two or three times its compliment of 16, to get matched sets of 8.On a smaller scale this applies to other designs with parallel output tubes.
So how critical is matching of tubes for stability, and what tolerance is there?
Follow Ups:
There is no need to match the output tubes in your amp
Alan
Alan
Thanks for your reply. Based on my (small) knowledge of tube amps, I can't think how.
Could you explain how/ might that shorten the component life or require more maintenance?
"Bruce Rozenblit 12/30/03 at 09:13 AM--------------------------------------------------------------------------------
The amps are designed to work better with tubes that are not matched.I will sell 6C19pi tubes to customers who bought amps for $9.
As far as tube life in concerned, I've only sold a few tubes. The T16 has been out for several years. They last thousands of hours."
http://www.websitetoolbox.com/tool/post/tubehifi/vpost?id=73726&highlight
I've read that.
I know Bruce is an expert, but he hasn't explained how. Being a sceptic and logical, I thought someone would know?
The Futterman output circuit (used I think by Transcendent) is not balanced. Futterman invented or was the first to implement a "totem pole" output topology, where the plates of the tubes forming the "bottom" or ground side of the circuit are essentially connected to the cathodes of the tubes on the "top" or positive supply voltage side. It may be that any two tubes of the same type and matched within random standards will share the current adequately and well enough that selecting tubes is not necessary. Many years ago, Julius Futterman used to sell replacement 6LF6s that he had merely tested for function to customers for $5 each, no matching necessary according to Julius. (Interestingly, in later years after Julius passed away, I had to go to NYAL in the person of Harvey Rosenberg for a single replacement tube. He told me that he alone had Julius' "data" for my amplifiers and that if I gave him the serial numbers he would select a matched 6LF6 for me at the cost of $50. Go figure.)
is that each BANK of output tubes (positive or negative) is balanced against the other, using the balance pot which, in the case of the T8 OTL, adjusts the positive bias (the negative is fixed). Balancing individual plus/minus tube pairs is obviously wasted in this arrangement.Don't know how the bias is balanced in the T16, but it's probably something similar.
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