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In Reply to: Re: schematics for 6C33 single ended OTL? posted by Lew on July 7, 2006 at 06:44:46:
The output impedance of a cathode follower is approximately Rp/1+u. A 6C33C with u of ~2.3 then gives 80/3.3 ~ 24 ohm so 2 in parallell give ~12 ohm.Note that the plate resitance and Gm varies with current so 80ohm is only valid for currents around 500mA, for higher currents Gm increases and Rp decrease. I run 6C33C at peak currents of 2.5A and at this current Rp is substantially lower, about 40ohms.
Regards Hans
Follow Ups:
Thanks. Just out of curiosity, where do you set the steady-state bias of your 6C33Cs, i.e., what plate voltage and current? Are you using an SE OTL output stage, too?
Hi,No I don't use a single ended or even class A OTL as I think it is practically a hopeless idea to get any useful output power. My OTL works in class AB as do all other OTL amps with reasonable output power, (the idle current is a fraction of the peak current). I use around 200mA idle current with 2 6C33C and get 25W output power in 8 ohm with excellent reliability, I have used the same output tubes now more than 6 years without any noticeable changes in bias point or DC offset. Principal schematic here http://www.tubetvr.com/otl.html
Regards Hans
Hi Hans,Not all OTLs have to be class AB. Its all in the biasing and plate voltage!
The Futterman 'totem pole' style output is push-pull and does not qualify as a Single-Ended OTL. Of course, once rid of the transformer there is no argument any longer for Single-Ended operation either, since Single-Ended amplifiers get their 'magic' from the lack of hysterisis loss in the output transformer. Once done with that, push pull can have the same 'magic' (low level detail).
Best Regards
-Ralph
I expected to get a comment from you Ralph regarding class of operation. As I don't expect you to change your mind regarding if your amplifiers are class A or not, let just say that there is some debate regarding this issue as you are well aware of.I am well aware of that the Futterman circuit is not a single ended one and I for myself don't really care for single ended amplifiers be it OTL or not, I can not see what the "magic" is about.
Regards Hans
You are right- I hear about this one often! However it always comes down to the same thing- our power tubes don't go into cutoff (which is easy to measure BTW) unless the amplifier is driven into clipping. That seems to be the easiest way to distinguish Class A.*Any* OTL can be class A, and our amps were/are not the only ones ever made either. Like I said, its all a matter of plate voltage and biasing. Many manufacturers have chosen not to go class A because of the heat and I concede that the heat has been a marketing problem for us (people often assume that our amps run hot because they are OTLs, which is not true, it runs hot because it is class A).
Some are unhappy with us because we have an A2 window in our operation (like Fisher did back in the 50s). The 6AS7G begins to develop grid current before the grid goes positive with respect to the cathode. So we designed our driver circuit to accomodate this. But Class A2 is still Class A by all the textbooks I have seen.
You might try it. You will have to lower your plate voltage somewhat to avoid burning up the tubes when you increase the bias. The difference in sound is instantly audible- Good Luck!
All the Best,
-Ralph
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