The "$250" Amp? - 6LU8 Compactron SE, UL
- Thread starter Wigwam Jones
- Start date
GOYA
Super Member
Since I finally got an oscilloscope, I wanted to do some testing on this amp.
Square wave at 1kHz and 1V:
I increased input voltage on a sine wave until I saw clipping.
Clipping occurred at just above 2.1 Vrms.
At 2.1 Vrms the output was measured at 8.3 Vrms into an 8Ω load.
RMS power calculates to 8.6 watts.
I have no idea what the distortion was at that output but I have no doubt it was very high.
Square wave at 1kHz and 1V:
I increased input voltage on a sine wave until I saw clipping.
Clipping occurred at just above 2.1 Vrms.
At 2.1 Vrms the output was measured at 8.3 Vrms into an 8Ω load.
RMS power calculates to 8.6 watts.
I have no idea what the distortion was at that output but I have no doubt it was very high.
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Same thing Goya said, no problem and no changes needed.
Evan110185
New Member
is there any reason why no one has done this yet? I guess no one wants to spend $200 in OTs for just a compactron?
beofservice
New Member
Adding volume control
There are a couple ways to do it. One simple way is to install a 100k audio taper pot between the input RCA jacks and the first section of the tube.
I suggest you search the web for single ended amp schematics that use a volume pot to see how the volume control is wired. Note that the functionality does not just add resistance into the incoming signal, but rather shunts off some of the signal to ground.
There are a couple ways to do it. One simple way is to install a 100k audio taper pot between the input RCA jacks and the first section of the tube.
I suggest you search the web for single ended amp schematics that use a volume pot to see how the volume control is wired. Note that the functionality does not just add resistance into the incoming signal, but rather shunts off some of the signal to ground.
donprice
Wound up workin' at a gas station....
My apologies in advance for only skimming the first and last few pages...
Is this project suitable as a first electronics DIY project? I'm tube curious...
No is a perfectly acceptable answer.
Thanks,
Don
Edit - most of my DIY experience is with woodworking and limited speaker builds (indignia and multiple ewave variants). Limited soldering experience and no test equipment beyond speaker building stuff (multimeter, R-L-C meter, test mics). As an engineer I believe I can do about anything DIY...even if it costs twice as much and takes three times as long to do it.
Is this project suitable as a first electronics DIY project? I'm tube curious...
No is a perfectly acceptable answer.
Thanks,
Don
Edit - most of my DIY experience is with woodworking and limited speaker builds (indignia and multiple ewave variants). Limited soldering experience and no test equipment beyond speaker building stuff (multimeter, R-L-C meter, test mics). As an engineer I believe I can do about anything DIY...even if it costs twice as much and takes three times as long to do it.
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Evan110185
New Member
My apologies in advance for only skimming the first and last few pages...
Is this project suitable as a first electronics DIY project? I'm tube curious...
No is a perfectly acceptable answer.
Thanks,
Don
Edit - most of my DIY experience is with woodworking and limited speaker builds (indignia and multiple ewave variants). Limited soldering experience and no test equipment beyond speaker building stuff (multimeter, R-L-C meter, test mics). As an engineer I believe I can do about anything DIY...even if it costs twice as much and takes three times as long to do it.
This was my first major electronics project and it turned out great. I spent many hours studying this thread, the parts list, and the wiring diagram. Also spent some time studying electronics basics/safety, which was especially helpful after i got my first shock.
bobrown14
Super Member
My apologies in advance for only skimming the first and last few pages...
Is this project suitable as a first electronics DIY project? I'm tube curious...
No is a perfectly acceptable answer.
Thanks,
Don
Edit - most of my DIY experience is with woodworking and limited speaker builds (indignia and multiple ewave variants). Limited soldering experience and no test equipment beyond speaker building stuff (multimeter, R-L-C meter, test mics). As an engineer I believe I can do about anything DIY...even if it costs twice as much and takes three times as long to do it.
Yes this is a good DIY project. Hardest part for me building from scratch is case building and wire layout. That said I enjoy it more than working with PCB boards. That's just me tho. I got started fixing/upgrading speakers so you're on a good track to take it to the next level.
Go for it, you'll have plenty of help right here if/when you need it. No stupid questions. You need to get comfortable with high voltage, or should I say a healthy respect. I'm never totally comfortable with +300v. Bleeder resistors are your friends and one hand in the pocket after first power up.
Cheers,
Bob
HatchetJack
Chillin' The Most
Moved question to another thread
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whoaru99
Epic Member
The two things I'm specifically wondering about are
1. Changing OPTs for dedicated headphone use.
Putting 300 ohms on an 8 ohm OPT will certainly change the impedance on the plates. IOW (maybe better stated as AFAIK) 5K load per the design would now be something on the order of 187K. Not good, presumably.
So, was eyeing up the Edcor 15K / 600 OPT which I think would bring the plates back to 7.5K load using the 300 ohm cans.
Is that the right approach?
If so, what other components would need to be altered and what values might those be?
2. In addition to above, if wanted triode mode what additional changes necessary? I can probably wade through the connections to do that, but if other component values need to change I don't know.
1. Changing OPTs for dedicated headphone use.
Putting 300 ohms on an 8 ohm OPT will certainly change the impedance on the plates. IOW (maybe better stated as AFAIK) 5K load per the design would now be something on the order of 187K. Not good, presumably.
So, was eyeing up the Edcor 15K / 600 OPT which I think would bring the plates back to 7.5K load using the 300 ohm cans.
Is that the right approach?
If so, what other components would need to be altered and what values might those be?
2. In addition to above, if wanted triode mode what additional changes necessary? I can probably wade through the connections to do that, but if other component values need to change I don't know.
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Point to point
I just came across this thread and thoroughly enjoyed it. I'm new to any type of amp building. I would like to try building this amp. My question is: are there any photos (besides page 66) that would give me a "point to point" look? Schematics are becoming easier for me, but I still would feel more confidant seeing a pictorial shot.
Fire away, we'll answer what we can..
I just came across this thread and thoroughly enjoyed it. I'm new to any type of amp building. I would like to try building this amp. My question is: are there any photos (besides page 66) that would give me a "point to point" look? Schematics are becoming easier for me, but I still would feel more confidant seeing a pictorial shot.
beofservice
New Member
I've since taken mine apart so no pics. Even if I had some I'm not sure it would help. The pin numbers on the sockets are always hard to read in person let alone from a picture.
Building an amplifier is all about unifying 2 documents - the schematic and the tube diagram (pin-out). On my very first amp here is what I did:
Printed out the schematic and the tube pin-out diagram. Then I wrote pin numbers onto the schematic. Then I created a drawing using paper and colored pencils where I represented the tube sockets (with pins) and the other components spread out on a sheet of paper the size of the chassis I was planning to use. I drew in every resistor and capacitor, and decided on paper where I was going to put terminal connection strips, etc. After that I ordered the parts that I didn't have already and assembled the amplifier.
If you are going to do a breadboard version first you do not need to be as organized nor plan ahead as much. Just get a piece of plywood or MDF and go at it. Either way, and obviously, you have to make sure you get the filament wires hooked up to the filament pins, the high voltage DC (B+) gets hooked up to the right pins, ground resistor hooked up to the right pins, etc. Not terribly difficult. If there is a volume control that can be a little tricky but nothing major. You can do it!
Building an amplifier is all about unifying 2 documents - the schematic and the tube diagram (pin-out). On my very first amp here is what I did:
Printed out the schematic and the tube pin-out diagram. Then I wrote pin numbers onto the schematic. Then I created a drawing using paper and colored pencils where I represented the tube sockets (with pins) and the other components spread out on a sheet of paper the size of the chassis I was planning to use. I drew in every resistor and capacitor, and decided on paper where I was going to put terminal connection strips, etc. After that I ordered the parts that I didn't have already and assembled the amplifier.
If you are going to do a breadboard version first you do not need to be as organized nor plan ahead as much. Just get a piece of plywood or MDF and go at it. Either way, and obviously, you have to make sure you get the filament wires hooked up to the filament pins, the high voltage DC (B+) gets hooked up to the right pins, ground resistor hooked up to the right pins, etc. Not terribly difficult. If there is a volume control that can be a little tricky but nothing major. You can do it!
beofservice,
thank you for your advice. I will advance using your layout suggestions. I can see how it will help me with the schematic.
The parts list doubles everything except the tubes and power transformer. Is this because I will be mirroring this single channel? If so, why not double the tubes? I'm having trouble completely wrapping my head around everything.
thank you for your advice. I will advance using your layout suggestions. I can see how it will help me with the schematic.
The parts list doubles everything except the tubes and power transformer. Is this because I will be mirroring this single channel? If so, why not double the tubes? I'm having trouble completely wrapping my head around everything.