• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

first build 6em7 and it works

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hi all,
wanted to share my first scratch built tube amp with the list. I have been reading off and on for a bit and last month started to build a 6em7 amp based on Gary Kaufman's design.

I designed an SS power supply and got it within 2 volts of his voltages. Box is 1/2 inch birch ply with a little enlayed detail. Top plate is steel from a computer filler rack panel. All parts are low cost and nothing special. OT are edcor and PS tranny is big hammand 270fx 550ct thing.

The thing sound nice - clear high and a good detailed bass, which I like. very rewarding project, but man building amps requires so many skills, woodworker, metal worker, designer, artist and engineer. very rewarding.

here are some pics - still need to dress wires and clean up a few things inside.

tubes are the best!

Jeff in Oakland CA
 

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Hi Jeff.

I built the same amp but with the tube ps. It really does sound
nice doesn't it? I found much different voltage levels by plugging
in different 6em7's. This did not affect the sound as far as I can
tell, but maybe I don't have "golden ears".

I now use it for rear speakers in home theater, and another
6em7 amp of my own design for the front and for music. here is
that circuit.



Wish I could show my creations but I don't have a digital camera.


Cheers. Rolf.
 
Hi.

I just hooked up a headphone jack to my Kaufman 6em7 amp.
Never been much into phones before, but it sounds awesome!
Just played Dire Straights first album on vinyl. With speakers the
drums never sounded quite right on this album and I always
thought it might be the amps very low power output. through
phones the drums are much cleaner sounding.

Rolf.
 
Real nice... What OPT did you end up with?

This amp was my first attempt at building tube amps also. I've ventured into building many other "simple" types of SE amps but alway come back to this design. Its something about this tube that is magic to my ears. Too bad it outputs so little power...

rman, I'm tempted to try your design since you mentioned that it sounded pretty much the same but with more power. I just never got the time. Someday...
 

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rman, I'm tempted to try your design since you mentioned that it sounded pretty much the same but with more power.

Cool see thru design. I used Edcor XSE15-8-5k outputs. Since I was Happy with how the first one turned out I used them on the next one again.

The second amp was inspired by John Broskie's arcticle

http://www.tubecad.com/articles_2001/Accordion_Amplifier

I used Garry kaufman's power supply with a higher voltage transformer, and a second filament transformer so I could reference the tube that has the Cathode follower and the upper output triode, heater to a higher votage.

Jeff, are those the transformers you used too?
 
Rman;

I just took a closer look at your accordion design. What an immensely clever way to deal with the B+ mismatch in the 6EM7! What is the purpose of the diode on the CF? Makes me want to spring the quarter of a C note to pick up a quad of those.

Do you find the CF necessary to have enough drive for both tubes or is it a because it was there thing? Did you consider paralleling the two high mu triodes instead?

You getting about 4 watts out of it?

mike
 
Bruce,

I've made the numbers easier to see. they were quite small.


Mike,
The diode normally does nothing, it is there to prevent the grid
of the CF from ever getting more positive than the Cathode,
which could happen if the input tube became disconnected or
failed. as for the CF, that came from another of Broskies articles.
http://www.tubecad.com/articles_2001/Inv_Dist_Cancellation/index.html

If nothing else it keeps the current through the input stage
constant which can't be a bad thing. I never thought about
parelleling them. From what I understand now, this is done to
reduce noise. This doesn't seem to be much of an issue with
these tubes. But maybe It would be a good idea for the extra
curent. I read somewhere that this is more important than low
output impeadeance for driving the output tubes, and after all there are two of them. If you protype this amp, why not try
both ways and see if there is any difference.

From my crude measurments, I belive it puts out about 2.2 watts,
as opposed to about 1.2 from the single tube version,but my PS didn't quite make it to the target 400 volts, only about
375. Output impeadeance is about 2 to 3 ohms. My test
equipment is crude, I can't even afford a decent set of scope probes.

If any of you build this amp please let me know. I am no tube
guru, more of a newbie, and this is my first original design, but
I do think it sounds very good.

Cheers,
Rolf.
 

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Another thought.

More power can be had from these tubes. I biased the outputs
at -18 volts on the grid to get the loadlines to look as linear as
I could. but to get as much voltage swing as possible they should
have -35 volts on the grid. Fred Nachbaur did this for his low cost
set project.
 
I took another look at Fred's design. I saw that a long time ago but that was before I knew anything about the EM7 series. He looses some of his biasing advantage to a rather high transformer impedance (almost 10k with 8 ohm load).

BTW I mistakenly referred to a plate voltage mismatch in an earlier post. I misread the data sheet and thought the max Va for the output triode was 150V which is not the case. My bad... but it is still a clever design. :D

mike
 
Hi Folks,

have not looked at this thread for a bit - listening to my 6em7 SE..still discovering stuff I never heard before on music. SS just does not have the detail to compete.

question about the PP 6em7 amps. What is the overall current draw on the PS? With the SE and just two tubes I had to get about 100 to 110 ma. with pp design would it be 200ma or is
the current less because of the pp design?

I like the tube for its price and dual use design. I plan on a 6t4 line amp next, but a pp 6em7 is on the list to build.

best,
Jeff
 
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