6LU8 Push-pull power amp and other builds

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Fri Sep 12, 2014 2:13 pm

alexg wrote:
tony wrote:my 832 pp amp lives.....
no global negative feedback...
plate B+ is a bit high at 405 volts, screen at 180 volts...
a slight red plate after 30 minutes so i am going to lose some plate B+
i reckon, about 9 watts per plate ought to do it...
how is the sound? i like it.... ;)
this amp comes with a phono stage built in....

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in all my builds i implement a central ground point

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Hi Tony, what kind of connector did you use for the 832 top connectors?

I have a bunch of 832s and played with them before.

Thanks.


ceramic caps from china....http://www.ebay.com/itm/8-Piece-New-Cer ... 0534528029
meron din kay zhan sheng...http://www.ebay.com/itm/829-829B-832-GU ... 0875374326
pero yung sa akin pinabile ko kay Edrel ng magtungo sya sa China...

Alex, the owner was quite pleased with the amp, bass was fabuluos according to him....
just goes to show how these tubes are under rated... ;)
i am coming up with the big brother version using the 829b or the 815......
Last edited by tony on Sun Sep 14, 2014 8:34 am, edited 1 time in total.
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Sat Sep 13, 2014 10:01 am

from 6AV5 to KT88....this will be the final configuration for this amp....triode mode of course,
with the KT88, the power traffo and OPT are best utilized...
kasi with the 6AV5, hindi man lang uminit yung power traffo... ;)

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Sun Sep 14, 2014 7:21 pm

preliminary sound test....

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G2 is supplied 380volts thru a mosfet regulator,
plates are dissipating about 26 watts each so that
class A window is large, about 12 watts of class A is available...
no signs of stress to tubes and traffos are running cool to the touch...
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Mon Oct 06, 2014 10:35 am

now playing......EL34 parallel push-pull inspired my Marantz 9 monos...
uses the A431 OPT copy done by Rey Luis.....
input is a 6J32/EF86 pentode, then the 6RRH8 driving a 6CG7 cathode followers....

now tweaking to resolve some minor issues.....
preamp is a CCDA 6H8C/6SN7 fed from my MD player playing Bread songs...

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Thu Oct 09, 2014 12:38 pm

a pair of Bendix 7403 power tubes, originally used in radar systems,will see action in my kt-88 set...

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Mon Oct 13, 2014 12:23 pm

the other amp in another color...

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Tue Nov 11, 2014 6:29 pm

6C33C push-pull amplifier
More than a year in the making now.......
Design goal is no global negative feedback, about 50 watts of power output and moderate voltage gain.
Building starts with the power transformers, output transformers, filter choke is dual bay CMC type...
Needless to say, all irons used in this build was hand made by me....
Irons.......
Power transformer...1 ¾ inch center leg stacked to 3 inches, core rating of 800VA...
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Filter choke....1 inch center leg stacked to 1 ¾ inch...
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Output Transformers .... 1 ½ inch center leg stacked to 2 inches,
core material used Edrel's RM18, gage 29 laminations...
800 ohm anode to anode primary, to 2, 4, 8 ohm secondary....
a tertiary winding wound bifilar used as cathode feedback winding..
i doubt if there is a commercial traffo fitting these specs...
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transformers are hung out to dry after a bath in insulating varnish....
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finished transformers with end bells......
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More to come later....
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Mon Nov 17, 2014 7:31 pm

6C33 preheating and initial burn-in....
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Fri Nov 21, 2014 8:46 am

remember this SET, a triode wired 6AV5?
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and draws this much power?
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well, Gerry Sta Maria gave me these pair of tubes, these Bendix hard glass 7403 types were intended for radar application...
this tube has a 45 sec warmup time...http://frank.pocnet.net/sheets/131/7/7403.pdf
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looking at plate curves, mu is around 10, better than a KT88, with g1-g2 mu of 8....
plate resistance is about 667 so a 2.5k OPT fits very well
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in action...
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power draw...better than the KT88....kaya na nyang painitin yung power traffo... ;)
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Sun Nov 23, 2014 9:34 am

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Sun Nov 23, 2014 10:04 am

this amp has been with me for about 2 years now...
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i have changed the original power traffo rated to 250 watts to a bigger one,
this time it can handle more power and therefore runs cooler...

there is an issue, the sound goes soft after two hours of use...so on for more tweaking.... ;)
looking at the 6CL6 data sheet, recommended grid leak resistor for fixed bias was 100k,
i had 1 meg installed, no wonder then that bias tend to drift with time...
so i installed 150k grid leaks, while this made the amp load the preamp more and
had to crank the volume all the way up, this stabilized the input tube, operating points are steady now...
i used red leds to bias the cathode of the 6CL6...
and with about 200 volts at the plate, there is more than enough drive to push the EL34 grids around,
the EL34 are running 20 volt cathodes, so 200 volts being much much greater than 20 volts, there is more than enough drive...
will have to rearrange input volume controls still...
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Wed Dec 03, 2014 5:54 pm

8165 in triode mode.....coming soon....

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Thu Dec 25, 2014 10:45 am

7403 update.....
improving upon the issues i was having with the input stage...
i decided to replace the 6J32/EF86 with the 12at7 in an aikido configuration....
B+ of 400 volts and 2mA.....
this is going to be a final na for this amp...
lessons learned here will be used in my next SET builds, the 4D32 SET...

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this is the circuit for those of you who want to try....
cathode followers used to isolate the input aikido and make it do its "thing"...
not bothered by the miller of the output tube....
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without fear, i can definitely say that the 7403 is a poor mans' 300B, very good all around....
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Thu Jan 01, 2015 9:03 am

uso talaga ang give aways....
sa akin me nagregalo ng isang pares ng GU-50 tubes with sockets.....
a fellow tube diy'er gave those to me, tinutulungan ko syang magbuo ng sarile nyan mga amps...
another tube to try hopefully this year.......;)
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Sat Jan 10, 2015 7:00 pm

intermission please, this is a solid state amp designed by Danny Eulogio,
a Pinoy engineer based in Seattle, Washington and in collaboration with Dandy F. Menor of
Baguio came up with a board design, they now sell kits of good quality parts and very low cost..

some progress report...

resistors are lifted off board by about 1/16 inch so as air can circulate freely..
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one board almost done, used shrink tube to physicaly couple input ltp trannies..
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two boards done....
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with a heatsink, both board assemlies will share a common heatsink..
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Tue Jan 20, 2015 9:38 am

natapos na yung retreat ni padre Jorge....pwede na ulit...
tony's redline...
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Thu Feb 05, 2015 10:51 am

DIY line conditioner housed in an Alexan HC841-01 plastic casing...

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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Thu Feb 05, 2015 12:43 pm

newly made power and OPT's for this years' projects...
front line is all power traffos, except the two 3/4inch ones which are opts...
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Wed Feb 11, 2015 10:19 am

just as we were beginning to enjoy this amp, one tube bacame gassy.... ;(
so what to do?
well, i replaced to octal socket to septar 7 pin socket once more,
this amp has gone full circle, from a failed 829b set to a 5898....http://www.r-type.org/exhib/aaa0001.htm

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the difference between the 829 and 5894 tubes are:
1. 5894 has a single cathode, this makes the transconductance identical for both plates
since there is only one source of cathode currents unlike in the 829b which had two cathodes tied together..
2. the 829b has 30 watts of plate dissipation per plate compared to just 20 watts for the 5894
3. the plate pins are thicker on the 5894 than with the 829b....

some are of the opinion that these two tubes are interchangeable, this maybe
but i caution you that plate caps once used in the 5894 will not be usable for the 829b anymore
as it would have become loose...

after rewiring...
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power draw is around 110 watts, very similar to the 7403....
operating point is 300 volts plate at 80mA cathode, for about 12 watts per plate...
this is way below the 40 watt limit for this tube...
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in the dark....
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Re: 6LU8 Push-pull power amp and other builds

Postby tony » Tue Feb 17, 2015 7:18 pm

beware of fakes....fakes tubes are everywhere....another reason why i like teeve toobs.....
http://www.jacmusic.com/nos/images/Fakes/index.htm
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