@Chris Hornbeck
Many thanks for this very helpful explanation.
My deep and extensive reading on this subject has not shed this sort of illumination.
Appreciated.
Many thanks for this very helpful explanation.
My deep and extensive reading on this subject has not shed this sort of illumination.
Appreciated.
It has been several months since I invested in a matched quartet of these Chinese-made SHUGUANG KT88-98 tubes.
I promised to report on them and frankly, they were pretty cheap, so I wasn't expecting too much.
Boy, was I in for a surprise!
These are excellent tubes. Extended in both bass and treble, detailed and still retaining a bit of the 6550 mid-range warmth - if perhaps not quite as lush.
What I have discovered is that each different tube I try in our beloved BEWITCH hot-rod, requires some tweaking to bring out the best in them.
That is to say, each different tube type has its own favourite operating point. It seems no two are the same.
After more than 25 different experiments, using 6550 and KT88 tubes and loads of varying operating points - I have a few dozen spreadsheets, where I have recorded every iteration and every operating point - I have come to the conclusion that there is no such thing as good and bad sounding tubes.
As long as they are healthy tubes, I now see that the circuit can be tweaked to accommodate any suitable tube.
For these KT88-98, we now run the BEWITCH with reduced Gain - using 4 x 6SN7 preamp tubes (no high-gain 6SL7 tubes in circuit, anymore) - offset by much-reduced Global Negative Feedback.
(Nod and thanks to @TonyTecson...)
We now run (small and stubby) Russian NOS 5U4C rectifier tubes which poke out considerably more power than the original Chinese 5Z4P tubes - and it suits these KT88-98s.
The (shared) cathode bias resistor remains at 250R and we only run in triode. I assume that we are getting 15W-18W per channel - which is more than enough around our way.
After much trial and error, I think we will be staying with this configuration, moving forward.
I promised to report on them and frankly, they were pretty cheap, so I wasn't expecting too much.
Boy, was I in for a surprise!
These are excellent tubes. Extended in both bass and treble, detailed and still retaining a bit of the 6550 mid-range warmth - if perhaps not quite as lush.
What I have discovered is that each different tube I try in our beloved BEWITCH hot-rod, requires some tweaking to bring out the best in them.
That is to say, each different tube type has its own favourite operating point. It seems no two are the same.
After more than 25 different experiments, using 6550 and KT88 tubes and loads of varying operating points - I have a few dozen spreadsheets, where I have recorded every iteration and every operating point - I have come to the conclusion that there is no such thing as good and bad sounding tubes.
As long as they are healthy tubes, I now see that the circuit can be tweaked to accommodate any suitable tube.
For these KT88-98, we now run the BEWITCH with reduced Gain - using 4 x 6SN7 preamp tubes (no high-gain 6SL7 tubes in circuit, anymore) - offset by much-reduced Global Negative Feedback.
(Nod and thanks to @TonyTecson...)
We now run (small and stubby) Russian NOS 5U4C rectifier tubes which poke out considerably more power than the original Chinese 5Z4P tubes - and it suits these KT88-98s.
The (shared) cathode bias resistor remains at 250R and we only run in triode. I assume that we are getting 15W-18W per channel - which is more than enough around our way.
After much trial and error, I think we will be staying with this configuration, moving forward.
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Yup. I bought the same tubes - the cheapest quad I could find.
Frankly, I don't even remember if it was eBay or Aliexpress where I found them, but at the tube a quad was 180$CAD shipped.
I've been using them in an amp designed around the supplemental datasheel for the GEC KT88.
Cathode biased 60W amp that idles the tubes at ~40W each.
They are what they say they are: KT88 tubes. But unlike most cheap tubes, these ones last more than 6 months.
Now if only I could find a buyer for the amp they are in...
Frankly, I don't even remember if it was eBay or Aliexpress where I found them, but at the tube a quad was 180$CAD shipped.
I've been using them in an amp designed around the supplemental datasheel for the GEC KT88.
Cathode biased 60W amp that idles the tubes at ~40W each.
They are what they say they are: KT88 tubes. But unlike most cheap tubes, these ones last more than 6 months.
Now if only I could find a buyer for the amp they are in...
