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The "$250" Amp? - 6LU8 Compactron SE, UL

Discussion in 'DIY' started by Wigwam Jones, May 10, 2008.

  1. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    I am constructing the Compactron amp. I've started with the power supply. I have all my wires and capacitors hooked up, but not soldered. I have not added the 0.47uf capacitors since the parts list said they were not necessary....are they?

    I will be grounding this unit using the "Star Grounding" technique. Using the schematic from November of 2008 I see the first ground coming off of the center tap then the ground (center tab) continues until it hooks up with the B+....is this correct? I planned on having the B+ go to a transfer strip (with the positive at one end and the center tap ground at the other). Then hooking up my audio channel. This leads to another set of questions.....most importantly is the power transformer strong enough to handle two channels?

    Thanks,
    Mike
     

    Attached Files:

  2. beofservice

    beofservice New Member

    Messages:
    8
    The ground and B+ can never connect with each other directly, but only through a load of some type like a tube, resistors, capacitors (sort of, they are not actually a load) etc. In this PS schematic the capacitors are the only thing between B+ and ground. Note that capacitors block DC, which is why you can connect the B+ to ground with a capacitor without causing a short circuit. The .47 are not required. I'm not familiar with the specs of the transformer, but it was probably intended to serve 2 channels.

    Please be very careful. As I hope you know, high voltage can be fatal.
     
    Last edited: Feb 9, 2014
  3. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Thank you for the heads up-----I'll be energizing the unit through an isolated transformer using only one hand while standing on a rubber mat. This question will show you how little I really know.....if the capacitors are blocking DC, then why was the rectifier tube generating DC?
     
  4. beofservice

    beofservice New Member

    Messages:
    8
    DC is what makes the tubes work and enables them to amplify. The purpose of the PS is to convert the house current (120 VAC) into the B+ (300 VDC). In layman's terms, the DC goes out through the part of the PS schematic marked B+ to the output transformer and the tubes, through the tubes and through the cathode resistors. Then the circuit connects to "ground" at that point. The audio part of the circuit is the "load" I was talking about.

    The capacitors in the PS are there to smooth the DC that is coming out of the rectifier in bursts (again, layman's terms). The capacitors absorb (charge) some of the DC during the bursts, then give it off (discharge) in between bursts which evens out the flow of DC. This is called "reducing ripple".

    There are much better and more technical explanations all over the internet...

    Make sure your isolation transformer and/or your amp are fused. About 2A should work as the fuse for the amp. That way if you make a big mistake you probably won't destroy your transformers because the fuse will blow.

    Also, I see that the note next to the B+ says "L&R" I assume that means left and right channels.
     
  5. jupitersspot

    jupitersspot Member

    Messages:
    51
    Location:
    Oregon - west coast USA
    Supply transformer

    I'm getting the pieces together to make this amp (hey, 5 years late to the party is about my speed!). I like the Antek toroidal transformers, but they seem to be out of stock for the AS-3T275. However they do have the AS-3T325 which appears to be very similar but for the higher output voltage. The heater taps seem identical: 4A of 6.3v. It's on the other winding that things differ. Rather than .5A at 275v the AS-3T325 provides .45A at 325v -- both still rated at 300VA.

    I think that the 325v vs. 275v is not too meaningful since I'll just be using it to get B+ at around 300v. I don't know whether the current difference is anything to worry about. I don't know if the current delivered by the 6LU8 tubes to the output transformers is limited by the Antek's current capacities (i.e. the 6LU8s can pass more current than the Antek can supply) or rather the Antek has more current capacity than the tubes can pass along to the output transformers.

    Any opinions on whether the higher voltage but lower current Antek would be a reasonable power supply transformer?
     
    Last edited: Mar 14, 2014
  6. BOfService

    BOfService New Member

    Messages:
    2
    Location:
    Atlanta
    Having a transformer that can produce more current than what is required helps make the power supply "stiff" and well regulated. Either .45a or .5a are well over double what your amp will consume so either works in that regard.

    You may be underestimating the difficulty in getting 300vdc from the power supply though. The PS design makes 300v using the 275v transformer. The same design is going to produce approximately 350vdc if you start with 325vac.

    There are various strategies for producing a lower output voltage from a given transformer, but they often involve a significant redesign of the PS. That is all do-able but may be beyond your abilities or ambition. Far easier would be to purchase a transformer that provides 275vac out. Hammond and Edcor probably offer such as do others I'm sure. They do not need to be toroids.

    Gary
     
  7. Kegger

    Kegger Anything can be S "MODed" Super Mod

    Messages:
    11,958
    Location:
    Michigan,warren
    Bofs is pretty much right on.. :)

    But yes you could use that tranny for the B+ by using a smaller input cap then the 6.5uf spec'd.
    Something like 2-4uf there should put you in the ballpark of an usable voltage between 300-340v.
     
  8. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Hello,
    I’m having a problem getting power to the audio section. When my amp is turned off I have 120 to 124 volts ac to the switch. Neutral is zero volts, however when I turn the toggle switch to the on position I get no volts of ac forward to my primary 115v hookup. I do get voltage on the neutral of 124 volts ac…..this now lights up my indicator light (125vac), I get voltage to the rectifier tube (which lights) and heaters on the 6LU8’s (which don’t light). I get no voltage to the 10uf capacitor (7uf on schematic) and no voltage to the first choke (the choke has only two black wires which I assume can be hooked up either way).

    Any thoughts?
     
  9. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Here’s my compactron amp. Front and back. I haven’t been successful in getting the amp to work (yet)——open somewhere. I’ll put it on the shelf for a couple of months then try troubleshooting it again. Since this is my first attempt I can’t see the forrest for the trees. All and all a very good learning experience.
     

    Attached Files:

  10. Kegger

    Kegger Anything can be S "MODed" Super Mod

    Messages:
    11,958
    Location:
    Michigan,warren
    A few detailed pictures of the bottom of the amp might help us troubleshoot with you.

    But it does Look pretty cool though.. :)
     
  11. beofservice

    beofservice New Member

    Messages:
    8
    When you switch your power switch on you get no voltage after the switch? I suspect you are not measuring correctly.

    Since your filaments all seem to be working you must be getting ac to the transformer. If you are getting no DC to the capacitors you probably have the rectifier wired incorrectly.

    As another poster suggested, a good clear picture of the underside would be helpful.
     
  12. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Hello,
    Please don't laugh too loud when you look under the hood. One photo shows the main wiring and the other shows my grounding.

    Before I hooked up the audio, I did receive power through to the B+. Trying to measure the voltage I would blow the fuse. After hooking up the audio I lost voltage (after the switch) however the indicator light would light as well as all the heaters.

    Concerning the transformer: Gray is my hot and white my neutral, I used yellow after the initial hookup with white neutral. I took the red leads to pins 4 and 5 of the rectifier tube with the yellows going to pins 7 and 2. Pins 1 and 8 I braided and took to the positive side of the B+(I have 1 and 8 going separately not attached at 1 and 8 as shown on the schematic). The center tap going along the negative side until hooking up with the B+.

    I apologize for the different colors as the only 600V wire I had was blue and yellow.

    Thanks for looking,
    Mike
     

    Attached Files:

  13. sled108

    sled108 Super Member

    I think I see your Problem...Why do you have (4) 6LU8s? Are you trying to run this as Dual Mono?
     
  14. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Actually, in my thinking, I was trying to run it as 2-channel/stereo.
     
  15. BOfService

    BOfService New Member

    Messages:
    2
    Location:
    Atlanta
    Need another pic that shows all 4 output tube sockets

    Can't quite see the the entire sockets in the picture. Couple thoughts though:

    Your chassis looks great from the top. You have bought some nice "iron" and done a nice job installing the transformers. The amp should sound good once you get it wired up correctly.

    Are those single ended transformers or push-pull? If they are single ended then you only need 2 6LU8 tubes total. The triode "tube", and the pentode output "tube" are both together inside one physical glass tube. The amp design (schematic diagrahm) as seen here is for a single ended amplifier. If that is what you are building then you can leave 2 of the tubes sockets mounted but not hooked up.

    http://www.audiokarma.org/forums/attachment.php?attachmentid=434372&stc=1&d=1370290155

    If you are building a push-pull design please provide a link to the schematic.

    Assuming a single ended amplifier, for the 2 tubes that will be used the wire connections need to be exactly as shown in the schematic.

    1) The 300v B+ needs to go through the output transformer (OP), then to pin 4.
    2) The ultralinear tap from the OP needs to go through a 1.2k resistor, then to pin 8.
    3) In between pin 2 and pins 6 or 7 is connected a 2.7k resistoe and a .33uf capacitor. There are several other components also connecting between the B+ and pin 2.
    4) Pin 9 connects to the 680-750 ohm resistor and the 220uf/.33uf capacitors, which connect to ground.
    5) The filaments wires need to be connected to both tubes at pins 1,12.
    6) the input from the RCAs connects through a 510 ohm resistor to pin 10
    7) Pin 11 connects to ground via a 620 ohm resistor.

    Looks to me like several of these things are not correct, but hard to tell from the photo. I have some concerns about your choke and capacitor wiring too, so it would be a good idea to check those too.

    I hardly need to mention that those "flying" connections where wires connect to resistors that are just floating in the air have got to go. Completely unsafe. Don't do that. 300v can stop your heart.

    Once you have corrected and double checked your connections try it again.

    Gary
     
  16. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Gary, thank you for your time.

    It is a single ended amplifier. My (uneducated thought process) was to double up the output for more wattage. I have a Magnavox 88-02-00 that has four tubes thus my poorly thought out approach.

    I took all the parts from the bom on this thread: http://www.audiokarma.org/forums/sho...&postcount=708

    I started to disconnect one set of tubes. This should allow space between the existing transfer strips to place the loose resistors. My connections were as you stated, but doubled up----would this, in itself cause the problem?

    Can I double up the OP Ultralinear tap as well as the connections coming from the OP transformers? Otherwise I would only need one OP transformer.

    Once I repair my work I'll upload some new photos from the interior.

    Mike
     
  17. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Gary,
    Thank you for your time and input.

    It is a single ended amplifier. I took everything from the parts list on page one of this thread.

    I had everything hooked up as the schematic had shown, however, I doubled up for each pair of tubes. There in might be my problem. I disconnected two of the tubes and started connecting one set within the transfer strips. I still have some resistors to move.

    I still don't get power. I have power of 125 before the switch with nothing after the switch....as it should be. When I turn the switch on I get no power after or before the switch, but I get power to the rectifier tube and all the heaters (including the indicator light). I've attached a file of the transformer and photo of the rectifier tube with power supply. Could the transformer be bad? I've attached the spec's for the transformer, as I could have hooked it up wrong.

    Again your help is much appreciated,
    Mike
     

    Attached Files:

  18. RDLAlaska

    RDLAlaska New Member

    Messages:
    1
    Unsafe wiring

    Mike, Have you made any progress on your amp? From the pictures I can see incorrect wiring. The pictures are hard to see for sure, but it looks like the most DANGEROUS is one black primary wire from your transformer is going to the caps on the right. If you have not fixed this. Do NOT plug in amp. Take more pictures without glare and close-up of transformer and rectifier wiring. The red wire with the yellow stripe is the center tap for the high voltage secondary wiring. This wire needs to be grounded, but only after disconnecting the black primary wire from the caps. This black primary wire goes to the common side of the 120 volt source connector. Make sure your primary circuit only goes from the 120v hot to fuse to power switch to one black primary from second black primary to 120v common side of your source. This keeps the primary isolated from the secondary as it should be. I will be traveling and working a lot the rest of this summer. I will check back when I can. Be very careful.

    Richard
     
  19. waggs098

    waggs098 Well-Known Member

    Messages:
    997
    Location:
    Amherst, NE
    Nice to see people so willing to help.
     
  20. Toerau

    Toerau New Member

    Messages:
    39
    Location:
    Paris, KY
    Richard,
    Thank you for your input. I'm away from my amp as I reside during the summer months in Kentucky. Winter time I'm in California where I do my experimental building. When I return to California I will certainly return to this amp and follow your direction (with care as I know how dangerous direct current is) and keep you informed of my progress.
    Mike
     

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