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KT88 PP UL Design Help Needed

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#21
Chuck Hollander is a name that comes to mind on that . It's very complicated . I did tell Sid off about it . It is in place of a big choke . The ripple was I guess about 15 V . The 37 V dynamic range of the LM317 copes with that . Problem is it must not swing further . Transient can kill it . I speculated that if the transformer was split somehow it only need see the ripple and not need the protection . LM317 is fast enough for a class A if wondering . It is a bit like making LP's from CD as sometimes happens this idea . Slightly wrong yet it works .
 
#22
For 100W output from a pair of KT88's the data sheet gives a -75V (Vg1). Should I use this voltage or half of this voltage on the plate I vs E plots? The individual lines shown do not include -75V. Do I need a different plot? -37.5V gives a current up above 200mA this seems really high. If I extrapolate a line at -75V I would get a current of 0±mA at 560Va which seems too low. From what I see B+=560, Eg1=-58.6 to give Ia=50mA???

JW
 
#24
Listening #111 | Stereophile.com

Didn't find what you wanted will ask friends . It says here hum was very good . No idea why he did it . Sid was a musician . It must have sounded better .He used Quad ESL 63 's if wondering .

I wasn't in the slightest bit interested in valves as we call them at the time ( about 1999 I think ) . I met Sid through a friend and had virtually no idea who he was . He talked me into believing capacitors mattered as he had just discovered it himself . I knew before him about it . I said nothing . He had a child s fascination about things . He wasn't high flung you know . He said he always needed other engineers to help .
 
#26
nigel pearson said:
I can never get my head around them . I was taught by Sid Smith the designer of Marantz amps . He gave up . He agreed my way works . Very nice man was Sid . Sid used LM317 for HT smoothing . It was arranged to work at 400 V and was a famous modification . I always would use cathode bias as it adjusts the KT88 as it ages . Sid always triode connected his amps at home . Sowter in UK make transformers about as good as SIds .
Click to expand...
I had Art Audio Concerto triode/UL kt88 with variable feedback and cathode bias. One of their early units made in GB-terrible workmanship ,excellent sound I regret I did not copy schematic and manual .It was the best sounding KT88 PP amp I heard to date.
 
#28
jwsplatjw said:
For 100W output from a pair of KT88's the data sheet gives a -75V (Vg1). Should I use this voltage or half of this voltage on the plate I vs E plots? The individual lines shown do not include -75V. Do I need a different plot? -37.5V gives a current up above 200mA this seems really high. If I extrapolate a line at -75V I would get a current of 0±mA at 560Va which seems too low. From what I see B+=560, Eg1=-58.6 to give Ia=50mA???

JW
Click to expand...

With todays KT88's, keep Iquies low; or they won't last long at 560V and the business gets costly.

Decades ago, KT88 UL circuits were about that showed Iquies 60mA at 600V+, a non-survivable grilling for todays tubes. That's the reason I went to parallel push pull.

richy
 
#30
This forum is great, my learning has taken leaps since I posted my questions.

I have decided that yes, 560 V is too high and am looking now at 460-480V depending on what I use for a driver tube, if I can use the same B+ for both it reduces the complexity and the part count. It also means I can use the 21st century Maida regulator. Tom has already done the heavy lifting and is selling kits.

So now I need to get my driver up to snuff. At present I have a single 6SN7 drawn as a LTP which will take a 3.5±V up to 124V or about 35 times. It has been suggested to me that I will need more gain when I add the NFB of between 14-20 dB. So now I am considering a pair of 6SN7's wired as a cascode LTP. Can I calculate the gain by finding the gain for a cascode then divide by two for putting it in a LTP?
 
#32
ECC 81 and EF184 are not often thought of to drive KT88 ( 6550 ) . They are possible options . EF184 in Long tail pair perhaps . I don't get involved in PP so out of my depth after that . Look at Quad 11 and see if it can be scaled up . Over to someone who would know on that .

I am looking to buy some KT 88 . I don't want to pay silly money and would use EL156 if I did . JJ is out I think .

http://www.hifitubes.nl/weblog/wp-content/telefunken-el156.pdf
 
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#33
New Quad II forty by TdP which I was checking after it was blowing tubes runs them at 390-400V 100mA in cathode bias circuit . You need Penta Labs Kt88's with ~50W dissipation rate to run in High Power Circuit. Don't forget to confront your design with well regarded circuits. Personally I'd try Allen Wright (may he rest in peace) PP-2 amp.
 
#36
KT 88 tester

K6KHP.jpg


I think this would work OK as a tester ? In a wood or plastic enclosure as it is not isolated . Various valves could be tested and data kept .

For USA I guess a capacitor fed doubler would give the same ?

Put a high grade input signal transformer on it and you would have an amplifier . Safety will be OK then if careful . NE 5534 op amp or boosted OPA 604 would drive it . I have tried something like this in the past built for me by my brother . I felt a mains transformer offered advantages sonically .

The 6V is a switch mode PSU for simplicity ( 6.3 V ) . The -6V bias switch is to lower the KT 88 into a lower bias . That would I guess make an ideal standby switch if an ideal voltage was found ( run from a comparator perhaps with a 1 hour flip flop action ) ? Please improve this idea if you can .

Stontronics 6V 2A Desktop Power Supply Unit T2319ST
 
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