tony's 6LU8 push-pull power amp build

[Tony]

Hi all,

inspired by this article on the net, 6LU8 Sweep-AMP i decided to build myself one, i have ordered about 700 pcs of these $1 tubes from Roy and Dale Rogalski of Vacuum Tubes Radio Tubes - 5,000 different tubes in stock - Over 10 million tubes! now it's time to put them to good use:



the 6LU8 is like a 6V6 and 1/2 of 12AT7 placed in s single 12 pin compactron envelope, i wanted to use the 21LR8 at first but then the novar sockets had issues, they were loose, i found out that they were intended for tubes like the PL504 which had thicker pins.....oh well.....here is the link for the 6LU8: http://frank.pocnet.net/sheets/123/6/6LU8.pdf

so i designed all the irons needed, the power transformers, filter choke and output transformers, the 2 output transformers used Edrel's RM20 cores which like his more expensive Z11's had the same 0.35mm thikness, Deeco Irons were then used for the power transformers and filter choke, deeco iron has a thickness of 0.5mm....

pics of the irons and some parts:




the power supply used the 3DG4 tube,http://frank.pocnet.net/sheets/093/3/3DG4.pdf it is likewise a hybrid rectifier using solid-state bridge, a toggle selector switch is used to select ss only rectification if desired.....the advntage of a hybrid is that being a full bridge rectifier transformer heating is way much lower than if staright full-wave is used using tube rectifier only......design and construction of the power transformer then becomes easier and cheaper....


construction pics:
















testing:

i used another set of speakers to test whether the polarity of the output transformer was correct, there is a 50-50 chance you will get it, but upon power up motorboating happened so i had to power down and reverse the plate lead connections to the right one.....after that, the amps was dead quiet, no hum, no hiss no noise.....i tested the amp and found out that the sensitivity was quite low, so i hooked up the 7N7 preamp to get a more decent volume.....later on i found out that i connected 470ohms feedback resistor instead of 4.7k so gain was really low....

[Miniwatt]

Very nice By the looks of it you have quite some building experience.

[Tony]

Originally Posted by Miniwatt

Very nice By the looks of it you have quite some building experience.

thanks......i have quite extensive building experience with solid state amps....but it was a p-p 6EM5 that was my first tube build in the 70's.....i am back to building tube amps....

[Miniwatt]

I could never catch up with you: I was born in the 70's..

[mastodon]

Cool project! Those TV tubes are nice; imagine that: a stereo PP amp with just four tubes. Even though it's usually not easy to find the output iron (I wasn't surprised to see you made custom designs), they're nice to tinker with.

But let me get this straight: you got 700 of these tubes? So we can expect a manufacturing output of another 174 of those amplifiers you showcase here?

[Tony]

Originally Posted by mastodon

Cool project! Those TV tubes are nice; imagine that: a stereo PP amp with just four tubes. Even though it's usually not easy to find the output iron (I wasn't surprised to see you made custom designs), they're nice to tinker with.

But let me get this straight: you got 700 of these tubes? So we can expect a manufacturing output of another 174 of those amplifiers you showcase here?

thanks to YvesM, i used his design in winding the output transformer, it has around 10k primary anode to anode resistance to 6 ohms secondary....

those 700 tubes are not all same types, although i have around 40 or so 6LU8 and 21LR8 tubes to play around with......the rest are tv horizontal sweep tubes, damper diodes, and odd voltage double triodes, pentodes etc., etc..

i am not scared of non-6 volt tubes, i can make traffos for then as needed....


so far i have finished 2 albums listening to this amp, first is Beatles' "Rubber Soul" my all time favorite, and Norah Jones' "com away with me." .......very enjoyable listening....

[mastodon]

Ah great, I'll be looking forward to more projects using odd tubes! I like them weird TV tubes.