Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Balticnoiseforge on May 12, 2023, 05:54:11 am
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Iam about to embark on a 200w x4 kt88 build, with a pretty standard JTM plexi/laney supergroup/bassman preamp.
Will use a 2.25k OT that is not ultra linear, and have looked around abit for a suitable LTP phase inverter to drive the quad and also with ability to add a pre phase inverter master.
And yeah, trying to keep it sonically close to a normal LTP.
I have looked abit on the Marshall Major, but having the NFB pre tonestack seems like it could cause some problems for certain settings?
Also looked at Laney 200LB with a similar arrangement but the NFB is after the tonestack.
But both those amplifiers are 60-70s, is there any more amplifiers driving four KT88 where they have some different type of arrangement that would work better? ( Iam no designer. )
I know the hiwatts fixed bias PI is out there, but may not sound so great with the kind of preamps i mentioned?
So if anyone have suggestions at amplifier schematics to look at or such id appreciate it, will have ~410vdc to feed the phase inverter.
So if anyone know some, or aware of re fined versions of the Marshall Major styled ones is appreciate to see it. ( Iam aware most Marshall Major schematics are not correct when it comes to values. )
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The Hiwatt 400W uses KT88 and a different driver approach (that seems interesting)
(INPUT 1 and INPUT 2 are the PI output branch)
(https://el34world.com/Forum/index.php?action=dlattach;topic=30300.0;attach=105866;image)
Franco
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Sunn 2000 or Marshall Major plans also work. Another is a HiFi plan; the the Genalex 400W plan specifies 5 pairs of KT88 for 400W - Just cut the # of pairs to 2 from 5.
--Pete
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That Laney plan does not provide a negative bias to the output tubes as drawn.
--Pete
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That Laney plan does not provide a negative bias to the output tubes as drawn.
--Pete
No, both the laney and Marshall Major has some issues with schematics, such as Marshall majors 270r resistors and such - which i assume is ment to be 270k.
Also for your previous response about Marshall Major, would not the Laney with a few modifications work better due to the NFB being post EQ? Also there a 500k master could be used after the treble?
But is there not any more recent options? Or improved variant of that PI circuit?
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Yes, but those amps are driving x6 kt88, looked abit on Ampeg and also hiwatt 400, but hiwatt 400 isnt that bias/tube setup more for the right impendence of x6 kt88 rather than drive? Also looked at other amps that uses cathode followers after their driver stage. But would not that be quite overkill for x4 kt88 at ~200w?
Or is there any way to get a normal LTP to have enough drive using tubes like 12bh7?
I find the Marshall Major way interesting but having the NFB pre tonestack seems like it could cause issues on certain settings?
Which gets to my point, several options for 400w amplifiers, but for ~200w only i found is the major and laney 200.
Using a hiwatt fixed bias feels like a safe bet, but unsure if it would sound good with the old jtm/supergroup/bassman preamp.
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... would not the Laney with a few modifications work better due to the NFB being post EQ?
Don't know exactly what you mean, the Tone Control MUST be out of the NFB Loop
Franco
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> the Tone Control MUST be out of the NFB Loop
A lot of recent designers put it inside. I do not know what they are smoking.
These VINTAGE schematics DO show tone inside NFB, so it isn't just the new kids?
https://www.drtube.com/schematics/marshall/200w.gif
https://www.drtube.com/schematics/marshall/1967u.gif
https://www.drtube.com/schematics/marshall/1978u.gif
I wish, if people want to talk amp design, that the schematics could be LINKED IN THE THREAD. Asking everybody else to search for and sort-out information already on the OP's table is......
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>
https://www.drtube.com/schematics/marshall/200w.gif
https://www.drtube.com/schematics/marshall/1967u.gif
https://www.drtube.com/schematics/marshall/1978u.gif
Thanks for sharing the first schematic, is first Marshall Major ive seen that looks like it been drawn like it should be, ie not having it jumping a across cathode before LTP arrangement etc.
What I ment about having the NFB pre tonestack was that i read it could cause issues on certain settings, and even more if adding a resonance control for it - i never owned one, or built a amplifier with NFB pre tonestack so maybe that is just hearsay and me missundserstanding it? Therafor i thought it would be better having it post tonestack? But maybe iam just way off with that?
Again iam no designer, i just like to clone and build amplifiers - have built a few from scratch but they are still from "templates" of old" or built upon circuits i understand.