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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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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?
 
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.
 
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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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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?
 
"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.
 
OK, that is a perfect example of having the GNFB polarity backward. You can't assume any color coding will give you the correct polarity. I'd put the FB components back in place and then swap the tranny leads on the primary side to reverse the polarity of the tranny. You could also do the swap at the speaker leads as long as you make sure the FB connection stays on the (+) side. Make sure the (-) is chassis grounded.
 
:up::up::up:
OK, that is a perfect example of having the GNFB polarity backward. You can't assume any color coding will give you the correct polarity. I'd put the FB components back in place and then swap the tranny leads on the primary side to reverse the polarity of the tranny. You could also do the swap at the speaker leads as long as you make sure the FB connection stays on the (+) side. Make sure the (-) is chassis grounded.

:up:
 
Thanks for the response. Do you think the sound would be better with both feedback networks or left as is?

You have to consider what electronic effect your changes have made. By eliminating the 2K GNFB primary resistor circuit you opened the parallel connection it had to the driver cathode, and the connection to ground through the output transformer, and that changes the overall cathode resistance. When you lowered the plate to plate 1.5M resistor you may have increased the voltage to the driver plate if there is no coupling cap to block it. Those changes will have moved the operating point of the driver and the input impedance to the output tube grid. The only way to judge how the amp was designed to sound is to get it working according the the builder's concept. Then tear it up.
 
OK, that is a perfect example of having the GNFB polarity backward. You can't assume any color coding will give you the correct polarity. I'd put the FB components back in place and then swap the tranny leads on the primary side to reverse the polarity of the tranny. You could also do the swap at the speaker leads as long as you make sure the FB connection stays on the (+) side. Make sure the (-) is chassis grounded.

Much thanks, problem, riddle solved. I reversed the wires on the primary side of the OPT transformers and installed both feedback networks with proper resistance and it is now as the designer intended. Short listening with a few songs and it sounds very good.
 
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