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Ebay Tube Kit 6P3P
Ebay Tube Kit 6P3P
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Old 8th November 2017, 09:42 PM   #1
wdecho is offline wdecho  United States
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Ebay Tube Kit 6P3P
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DIY Tube Amplifier Kit 6P3P + 6N8P Single Ended Tube Power Amplifier Kit | eBay

It went together pretty straight forward but with 2 feedback networks I did have oscillation. I eliminated the one from the OPT to driver tube and decreased the value of the resistor going from output tube to driver tube to have adequate feedback for this pentode amplifier and this cured the problem. I do not understand why there are two feedback circuits and may someone can fill me in on why. Upon receiving the kit I was pleasantly surprised to find a NOS Russian 5U4U rectifier tube and NOS Russian 6H8C for the driver tubes. The output tubes were Chinese 6P3P and they did sound OK but I swapped them for some NOS Russian 6P3S's with an improvement in sound. I am very impressed with the quality of the OPT's for such a very inexpensive kit. Stainless steel chassis with holes already drilled, decent iron,tubes and Chinese parts for $192. shipped. As far as sound, very good with nice bass. I measured 4.5 watts at clipping with a nice looking square wave at 1Kz. A touch of ringing I am not too concerned about at the moment. The square wave falls apart above 10K hz which can be expected with inexpensive OPT but the sine waves look good from 20hz to 20K hz. The 4.5 watts is far stronger than the figure. I do own very efficient horns but even with my 85db test speakers the amplifier had authority and I could not detect clipping at a very high volume in my kitchen. Very satisfied with the value.
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Old 9th November 2017, 07:09 AM   #2
artosalo is offline artosalo  Finland
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Quote:
It went together pretty straight forward but with 2 feedback networks I did have oscillation. I eliminated the one from the OPT to driver tube and decreased the value of the resistor going from output tube to driver tube to have adequate feedback for this pentode amplifier and this cured the problem.
Did you check that the polarity of the output transformer is correct?
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Old 9th November 2017, 10:50 AM   #3
wdecho is offline wdecho  United States
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Ebay Tube Kit 6P3P
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Originally Posted by artosalo View Post
Did you check that the polarity of the output transformer is correct?
Yes, but thanks for suggestion. There are blue and red primary wires. Red to +, blue to plate was my assumption if they are color coded as all OPT's I have wired before. I have built pentode amplifiers before with only the plate to plate feedback circuits and they have always sound fine with only one circuit and this build does as well. I lowered the 1.5M ohms resistor to 470K ohms and oscillation stopped and it sounds very good, in fact excellent for less than $200. I did compare the sound to my SET 45 tube amplifier and naturally the 45 did sound better but then the price for building it with good iron was nearing $1000. Also the SET 45 sounds better than most of the many amplifiers I own.

Last edited by wdecho; 9th November 2017 at 11:00 AM.
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Old 9th November 2017, 11:16 AM   #4
ballpencil is offline ballpencil  Indonesia
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44V on the cascode's upper tube grid, 190v on the OP tube g2.. I wonder how did they end up with the operating points. It would be fun to recalculate all operating points and hear the difference, if any.
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Old 9th November 2017, 11:19 AM   #5
Mission720 is offline Mission720  United Kingdom
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Hi - do you have schematics for the circuit please?
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Old 9th November 2017, 11:56 AM   #6
wdecho is offline wdecho  United States
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Ebay Tube Kit 6P3P
Improvements, opinions, suggestions of circuit welcomed. This was a fun project to occupy my retirement time. Great starting point for anyone interested in a pentode build. Again, I eliminated the feedback circuit consisting of the 2K ohm resistor and changed the 1.5M ohm resistor to 470K ohm.
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Last edited by wdecho; 9th November 2017 at 12:00 PM.
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Old 9th November 2017, 12:23 PM   #7
e_fortier is offline e_fortier  Canada
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Since no UL tap on OPT, have you tried triode strapped ?

BR
Eric
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Old 9th November 2017, 12:27 PM   #8
20to20 is offline 20to20  United States
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Quote:
Originally Posted by wdecho View Post
Yes, but thanks for suggestion. There are blue and red primary wires. Red to +, blue to plate was my assumption if they are color coded as all OPT's I have wired before. I have built pentode amplifiers before with only the plate to plate feedback circuits and they have always sound fine with only one circuit and this build does as well. I lowered the 1.5M ohms resistor to 470K ohms and oscillation stopped and it sounds very good, in fact excellent for less than $200. I did compare the sound to my SET 45 tube amplifier and naturally the 45 did sound better but then the price for building it with good iron was nearing $1000. Also the SET 45 sounds better than most of the many amplifiers I own.
Was the oscillation a screetchy squeel?
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Old 9th November 2017, 12:28 PM   #9
wdecho is offline wdecho  United States
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Ebay Tube Kit 6P3P
"Since no UL tap on OPT, have you tried triode strapped ?"

No I have not but it may be something I may try at a later date. I have a number of triode strapped amplifiers and this is one reason I decided to do another pentode amplifier especially for the price. I have to admit it changed my opinion of pentode amplifiers.
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Old 9th November 2017, 12:29 PM   #10
wdecho is offline wdecho  United States
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Originally Posted by 20to20 View Post
Was the oscillation a screetchy squeel?
Yes, awful sounding, no music.
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