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In Reply to: RE: A tube journey with lesson learned, Re; KT88 & KT150 posted by Mr_bill2 on January 30, 2024 at 20:11:11
They would have to change the entire design of their amps to do that.
The screen grids on sweep tubes are no higher +ve than 175-200V.
FYI both David Berning (EA-230) and Paravicini used sweep tubes often with screen grid drive.
They are a class act in linearity and longevity, something which any Chinese or Russian device won't even come close to.
FYI.
I have seen Quicksilver make glaring elementary errors in their amps which resulted in the immolation of large nunbers of the 8417 valves in those based units - totally unnecessarily it turns out.
I can only imagine they never read the original docs from Westinghouse or the tendency for large audio signal tubes to self bias and runaway.
The sweep tubes have HUGE cathodes which take lots of heater current.
They are capable of delivering over 1 amp as a result and lots of them still won't start glowing red until you start pushing anode diss to well over 80W-90W.
Just saying.
Follow Ups:
And the Berning designed Audionics BA150 was rated at 150 watts with similar tubes although that rating was with only 100 volts AC. The amp put out 225 watts at higher AC. I also know from a friend who was around David Berning at that time(he made David's first preamp) and he told me the BA150 prototype was turned on in 1072 and finally turned off in 1977 with no measurable wear on the EL-509 output tubes.
Another user of similar tubes(EL-519) was Melos who made monoblocks with 4 output tubes rated conservatively at 400 watts. And I should add the tubes(both Melos and David Berning) were screen drive resulting in output curves that were triode like.
The problem though with high output from these tubes is that the screen drive demanded high current drivers meaning you needed solid state drivers for maximum output. The EA230 was a second generation BA150 with a tube driver stage resulting in about half the output of the BA150 since a tube driver stage was limited in current drive.
"screen drive demanded high current drivers meaning you needed solid state drivers for maximum output."
nothing at all wrong with mosfets for driving grids at high voltage swings.
Here in front of me I have a pair of 1949 made AB2 amps. wiht originally 807 pairs in. I converted it to KT8 but seemed to make no difference to the sound.
The G1 drive which can push it to +18V positive comes from 6SN7 cathode follower.
Setting the bias properly for that is quite hard and the 6SN7 gets very hot.
If you have 300V aligned grids like in the 807 it's impossible to run screen drive.
The only reason why sweep tubes work, is because the tolerances and build quality are legendary, combined with a 1.5A-2A heater with monster cathodes to make relatively low impedance devices.
This makes the g1-g2 gain totally different and the curves far more linear.
There are also winding advantages with transformers with 1-2K anode loads + low voltages, like used in Geloso 100W amps and the hi gain EL520 range.
The KT66 and KT88 series have had their day,- that was in Reeves 100, 200W amps of the 1970s-80s.
They still work in Guitar amps as do the 6L6GC.
That's where they should continue to live and die, so who cares when they blow up!
Your comment about screen drive and transformers reminded me that the Melos 400 amp had a damping factor of 20 meaning it had very small interaction with speaker impedances that cause frequency response variations that almost all tube amps have. They never published a review but I saw Atkinson's graph of the Melos 400 frequency response with the simulated speaker load he uses in his tests and there was minimal variation from flat.
I had these many years ago. Nice amps, took up lots of room but looked beautiful and sounded like triode amps should. Only thing, a real pain in the ass was the screw driver needed to get the cage off. Sold them, miss them, wish I still had them even today.
Usual thing isn't it.
If you check you will find ALL valve amps measure FR and distortion at 1W into a resistive load (although you won't see it admitted).
I always measure mine at -2dB from maximum load with both sine and square wave into a genuine inductive/capacitive load (a speaker array) at all frequencies from 40hz-25khz..
(it's LOUD!).
One thing that immediately came across was the very astonishing difference in measured waveforms across the load depending on the amount of negative feedback.
That tells you that the obsession with damping factor is manifest rubbish.
It also tells you current drive (which has v high damping factor) can well resolve speaker driver reactions at medium to high frequencies - but of course NOT such stuff as ESL capacitive loads.
Driving into resistive loads (dummy 50W resistor mounted on a large heatsink), tells you sweet zilch,especially at 1W which is why this entire industry is full of fakery and BS.
A 5W amp running at 1W with a small transformer and cathode bias+loop NFB is a very different animal from a 100W amp running a large transformer at 1W..and mainly running CFB.
..never mind the very large differences in magnetic flux and reactivity on some of the older classics like UTC LS or Partridge compared with modern stuff with the much harsher sounding winding and core technologies.
It's why Partridge based British stuff, Citation, Scott or Fisher or certain Bogen have "that sound" which you can't do today.
FYI I use Scott OPT in my modern PPP amp, and they are fantastic.
Trancendar cloned them but they are gone now.
Both measurements and listeners agreed on them.
Clean v low IMD and v low THD.
Measure good - sound good.
ie.
Very far from the muffled distorting rubbish that's knocked out by people like Jadis for megabucks...
I know David. Use to own his TF-10AH along with the EA-2100. Wish I still had them. Back when I lived in Northern Virginia. At his home once for repairs, he at that time was running very old Magneplanar tympani 1-d screens. One of the nicest guys I've ever met. Along with Don Garber. A pity his passing.Thanks for the memory.
Edits: 01/31/24 01/31/24 01/31/24
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