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PPP Mono Power Amp 140W (G. Haas, 4xKT88 in use) why only short Life Time ?
PPP Mono Power Amp 140W (G. Haas, 4xKT88 in use) why only short Life Time ?
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Old 16th January 2019, 05:45 PM   #1
tiefbassuebertr is offline tiefbassuebertr  Germany
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Default PPP Mono Power Amp 140W (G. Haas, 4xKT88 in use) why only short Life Time ?

This amplifier was published in the German electronic magazine ELRAD so as in this book: "High End mit Röhren" - go to
High-End mit Rohren. Referenzschaltungen zum Nachbauen: Amazon.de: Gerhard Haas: Bucher

A friend of mine had this power amplifier built by an electronics engineer and stated from the beginning that the life time of KT88 is only a bit longer than a year. Idle current is 40mA and anode voltage aproximately 490VDC.

Schematic is on page 171 from the attached pdf's in German - go to post #2.

I remember a website where the same circuit was published with tweaking notes to extend the life time of that tubes. Unfortunately I don't find it currently. Maybe one of the member will give me some hints or know this website.

Thank you very much.

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Last edited by tiefbassuebertr; 16th January 2019 at 05:53 PM.
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Old 16th January 2019, 07:12 PM   #2
pcan is offline pcan  Italy
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PPP Mono Power Amp 140W (G. Haas, 4xKT88 in use) why only short Life Time ?
According to the drawing at page 172, the output tubes are clustered in close proximity. If the amplifier has been built following this drawing, it needs to be in a ventilated place. Excessive temperature will shorten tube life.
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Old 17th January 2019, 01:11 AM   #3
banat is offline banat  Serbia
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IMO KT88 Rg1 value is to high (220K) , I will reduce those grid resistors value to be not more than 100K ,but in the same time values of four coupling capacitors need to be raised from 0,22uF to about 1uF,each one to be with 1KV= or more voltage rating ,also check for correct nominal 6,3V filament voltage , and check for bad solder joints on those pcb`s ,specially around tube sockets pins .
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Old 17th January 2019, 01:26 AM   #4
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PPP Mono Power Amp 140W (G. Haas, 4xKT88 in use) why only short Life Time ?
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Old 17th January 2019, 01:49 AM   #5
TonyTecson is offline TonyTecson  Philippines
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PPP Mono Power Amp 140W (G. Haas, 4xKT88 in use) why only short Life Time ?
Quote:
Originally Posted by tiefbassuebertr View Post
This amplifier was published in the German electronic magazine ELRAD so as in this book: "High End mit Röhren" - go to
High-End mit Rohren. Referenzschaltungen zum Nachbauen: Amazon.de: Gerhard Haas: Bucher

A friend of mine had this power amplifier built by an electronics engineer and stated from the beginning that the life time of KT88 is only a bit longer than a year. Idle current is 40mA and anode voltage aproximately 490VDC.

Schematic is on page 171 from the attached pdf's in German - go to post #2.

I remember a website where the same circuit was published with tweaking notes to extend the life time of that tubes. Unfortunately I don't find it currently. Maybe one of the member will give me some hints or know this website.

Thank you very much.

Copyrighted material removed by moderation

some pictures of the built up amp will surely help, right now we have no clue as to how that amp was built in real life so we can not really help...
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Old 17th January 2019, 10:53 AM   #6
Kay Pirinha is offline Kay Pirinha  Germany
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Deleted, was nonsense!
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Old 17th January 2019, 12:48 PM   #7
tiefbassuebertr is offline tiefbassuebertr  Germany
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Quote:
Originally Posted by TonyTecson View Post
some pictures of the built up amp will surely help, right now we have no clue as to how that amp was built in real life so we can not really help...
The built amp was exactly according the images of the attached PDF. Because this attachment was deleted, I will make pictures with camera next time. Why do you think, you can help, if you check that pictures ?
Online isn't any image of the version with 4xKT88 mono block, unfortunately.
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Old 17th January 2019, 11:51 PM   #8
TonyTecson is offline TonyTecson  Philippines
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PPP Mono Power Amp 140W (G. Haas, 4xKT88 in use) why only short Life Time ?
well a picture of the innards and the underbelly would be nice...
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Old 18th January 2019, 01:20 AM   #9
multi is offline multi  Australia
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What KT88 are you using?
I built 200 watt amp using 4 x 6550's Tungsol, UL for a friend 560 volts 50 Ma
Max on Grid is 47k for current production fixed bias KT88 & 6550

Tubes last for years.
Are you sure the amp is stable not oscillating ??
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Old 18th January 2019, 10:32 AM   #10
Kay Pirinha is offline Kay Pirinha  Germany
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Is this the amplifier for whom G. Haas claims an output power of 250 watts? With a rather small (w.r.t 250 watts) MD102B core output transformer?
Best regards!
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