Don't even think about it :-) The KT88 and 6A3 are completely different animals, electrically as well as mechanically.
Have you tried one?or just a wild guess.I would never try any such substitution, for fear of damaging the amplifier, including the possibilities of damaging both its output transformers and its power transformer.
A 6A3 is a very low power 4-pin directly heated triode. It has 3 elements -- a filament/cathode, a control grid, and a plate.
A KT88 is a much higher powered indirectly heated beam power tube. It has 8 pins, providing connections to a filament, a separate cathode, a control grid, a screen grid, beam forming elements, and a plate.
The two tubes have numerous electrical parameters that are highly dissimilar. There are so many differences that I would see no point in taking the time to analyze their consequences. And in any event a schematic and other design information for the specific amplifier being used would be necessary to properly do so.
IMO substituting a KT88 for a 6A3 would at best, if you're very lucky, result in poor sonics. At worst it would result in smoke and fire. My guess is that the result would be somewhere in between, probably a damaged amp.
Rleff, thanks :-)
10-20-15: Charles1dadOne would think so, Charles, but evidently not in this case.
Realdeal, in addition to the architectural differences I cited between the two tubes, with one being a directly heated triode and the other being an indirectly heated beam power tetrode (sometimes referred to as a beam power pentode), you might examine and compare the numbers in the datasheets for the two tubes. Which I did before posting.
You might consider, for example, the numbers for filament current, transconductance, plate resistance, and typical operating voltages and currents for the plates and grid(s) of each tube, among other numbers. As well as the graphs that will be included in a comprehensive datasheet.
Wild guesses? Yes, I did make a couple. In addition to guessing that the result of the substitution would likely be worse than poor sonics, I optimistically made a wild guess that depending on the design of the specific amplifier the result may very well NOT be smoke and fire.
Al's very thorough and thoughtful response is based purely on technical considerations. I feel he is giving you very wise advice, if you want to proceed with this experiment of course you are free to do so. In my opinion these two tubes are about as different as one could imagine. If you do decide to continue this experiment let us all know how it turns out. The analogy would be comparing a Harley Davidson motorcycle to a Dodge Caravan minivan, they're just simply different.
Best of luck,
Found the 10-19 post below on the "Designer Hall of Fame" thread started in '08. Now if we only knew OV, might have a clue what's going on with this line of inquiry.
OV,who designed tube rectified OTL,using DHT tube as an OPT or any of the 7ac based tubes.Make it possible for kt66/88 or any 7ac based tube directly plugged into a 4 pin socket of a 6A3 single ended amp without amplifier mod.
Realdeal (Threads | Answers | This Thread)
Thanks, Ghosthouse. Perhaps in addition to clarifying who OV is, Realdeal can provide a link to detailed information about that amplifier. I find it hard to imagine, to put it mildly, that a practical audio power amplifier using a single-ended 6A3 as its output tube could be implemented as an OTL (i.e., without an output transformer).
Charles, Rleff, Onhwy61, thanks also :-)
There is a difference between "wild" and educated guesses.
Knowing Al and his background and interests and having read many of his posts, I can assure you his guesses are among the best educated you will find.
Since you asked the question, I'll assume you do not know the answer for certain either.
So there you go.
If you find out anything better or more definitive, please share here.
Whatever you do or not, be safe in doing it.
In the broadest sense, this is a valid question in that it is theoretically possible that the amp would work. But, I don't quite understand what would be the purpose and what benefit one hopes to derive from doing this. Yes, by some miracle it might turn out that the KT88 will sound wonderful as a drop in replacement, but that would truly be a miracle (I hardly expect a KT88 to sound good even in a KT88 amp).
The reason the posters in this thread are merely speculating is because no one has attempted the experiment; no one has seen a reason to even make the attempt because it appears to be a bad idea, with only a hope and prayer of a positive outcome.
Back when we were in grade school, my sister thought it was so funny that some school science projects were based on questions or hypotheses that seemed to inquire into areas where the answer had no use or real interest; this question is the audio equivalent of the "Can guppies live in milk?" question she found so amusing.
Great post, Larry, as always! And I think that your "can guppies live in milk" analogy captures the essence of this discussion perfectly.
Mapman, thanks also. And thanks to Bill (Brownsfan) as well, for the best chuckle of the day :-)
I'm wondering, though, whether there may be more to this thread than initially meets the eye. I note that the OP has chosen not to answer any of the four questions that have been presented to him in the thread thus far. Including who OV may be, whom he nominated in the "Designer Hall of Fame" thread, and who allegedly designed an amp utilizing a single-ended 6A3 as its output tube in an OTL configuration (which cannot possibly work in a reasonable manner in conjunction with a speaker load that is even remotely conventional, as can be shown via some simple calculations), and into which a KT88 can allegedly be directly substituted via a socket adapter.
In that regard, and given the lack of clarification about that amp and its designer, I can't help but also wonder about the fact that among the many positive feedback comments RealDeal has received for sales he has conducted here, there are several in which it can be seen that his first name begins with the letter "O."
Al seems to me to be very knowledgeable and sincerely tries to help. He is also obviously generous with both his time and his knowledge. Whether you considered his responses to be helpful a courteous "thank you" for his taking time to try to help was a more proper reply.
I think too that as some others have said, no one seems to have tried what you are considering so it sounds like you'll have to conduct your own experiment.