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RCA console amp RS-199C

Discussion in 'Tube Audio' started by HBrown, Mar 19, 2017.

  1. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,523
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    Middletown,New York
    What is the frequency? The spike at the end of the square is unusual. Can you post a pic of the signal from the phone?
     

     

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  2. HBrown

    HBrown Active Member

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    I did look at the input signal, but didn't get a pic of the input signal itself. The input signal was set at 1kHz. The spikiness at the ends of the square seem to be present in the signal itself straight from the phone. The downward sloping as Gadget said indicated low frequency cut was not present in the input signal.

    When you get the scope set to really see the input signal by itself with good resolution it is obvious it's not a really high quality signal. I'm going to lay my hands on a real function generator. I haven't ordered one yet, (I hit several shops and flea markets yesterday looking for one with no luck).

    I moved the amplifier into the house for a listening session last night. I'm going to put it back on the bench and hook it back up and test some more. I'll try to get some more pics of the signals.
     
  3. HBrown

    HBrown Active Member

    Messages:
    325
    Location:
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    Hey everyone- I'm still gathering my parts to move the amplifier guts from the old nasty chassis to a new one. I have the new chassis, new sockets, input jacks, binding posts, and am just waiting on hook up wire and a few odds and ends.

    But, now I went to listen to the amp and it has developed a hum seemingly from one power-on cycle to the next. I believe it to be a 60 Hz hum. It is volume knob setting independent, primarily one channel, and continuous if the amp is turned on. If I power it up with the 6FQ7 removed there is no hum. The coupling caps, doubler caps, and most of the resistors are only a few weeks old, but something has let go. It has been sounding pretty nice. What would be my most likely culprit? I want to sort it before I tear it all apart to re-chassis this thing. I hope it's not my 6FQ7 tube itself.
     
    Last edited: May 17, 2018
  4. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,523
    Location:
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    It may just be a bad connection, as the breadboard is temporary. If it was working the way you like it, I would just rebuild it on the new chassis.If you suspect the tube,switch it with the other one.
     
  5. HBrown

    HBrown Active Member

    Messages:
    325
    Location:
    South Georgia
    I spent a little time checking connections/soldering and found no problems. I don't have a spare 6QF7 to try. I'll have to order one if that turns out to be it. Swapped out coupling caps and that wasn't it. Checked resistors all around the 6QF7. Being just one channel has me thinking not power supply. It's such a rats nest in the experimental state it's in.
     
  6. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,523
    Location:
    Middletown,New York
    Maybe an rca jack or Cable problem?
     
    Last edited: May 18, 2018

     

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  7. HBrown

    HBrown Active Member

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    It still hums even with no connections to the input jacks.

    The amp still works fine on both channels- it just has the loud hum overlaying the right channel only.
     
  8. HBrown

    HBrown Active Member

    Messages:
    325
    Location:
    South Georgia
    Anyone have a suggestion as to how to identify the offending component?
     
  9. BinaryMike

    BinaryMike Pelagic EE Subscriber

    You could start by comparing AC signal voltages between channels at each stage with the input jacks shorted. There should be a major difference at some point, with the greater voltage exposing where the hum first appears in the offending channel. Check DC voltages in the preceding stage when you find that point.
     
  10. HBrown

    HBrown Active Member

    Messages:
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    Location:
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    I'll try that Mike- so short both input jacks and I suppose put my multimeter's black lead to the ground bus and then with red lead probe points along both channel's signal path looking for the differences, AC & DC? I know it will depend on many things, but how large a difference might be outside of component tolerances? Reason I ask is I have done some casual probing already and noticed a little difference here and there, but probably within 2%-8% or so.

    I'll go get serious about it and see what I find.
     
  11. HBrown

    HBrown Active Member

    Messages:
    325
    Location:
    South Georgia
    I'm still going to try and get it running right again before moving components but I got a start on mounting my tube sockets.

    In case anyone does like me and finds the knockout punches set at harbor freight tools- leave it there.

    I bought them because it said 3/4 on the package and dumbass me didn't check it before slugging out a hole. It's sized for a 3/4" conduit hub- the hole it makes is much bigger- too big for the tube socket. Darnit. I was excited to use it and made my first hole- then discovered my error. It's the hole on the far right. I had to cut a thin aluminum washer to sandwich it to center the tube in the oversized hole. Now that tube will sit .030" lower than the others. I had to hack the other 3 holes with large shop equipment. I got it done, but its a little rougher hole finish than I had in mind. Guess I should have bought the step drill instead of those darn knockouts. IMG_20180519_212902581.jpg
     
    Last edited: May 19, 2018

     

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  12. HBrown

    HBrown Active Member

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    Can anyone tell me the probable best orientation for the transformers to minimize interference? I plan on centering the power tranny towards the backside and the outputs to either side of it.
     
  13. BinaryMike

    BinaryMike Pelagic EE Subscriber

    The axis of the winding on each OPT should be oriented at 90 degrees relative to the axis of the power transformer winding.
     
  14. HBrown

    HBrown Active Member

    Messages:
    325
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    How does this look from an interferance perspective? I plan on AC line in, fuse, power supply/caps to be directly under main transformer.
     

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  15. gadget73

    gadget73 junk junkie Subscriber

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    Should be fine, but that is an absolutely massive power transformer for a 3 wpc amplifier. Not necessarily a bad thing.
     
  16. HBrown

    HBrown Active Member

    Messages:
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    Location:
    South Georgia
    Gadget, thanks for taking a look and commenting. Ha ha, yes, it is a healthy transformer. It actually deflects the top surface of that cheap aluminum chassis a few thousanths from just the weight of it. Being as this is my first effort at assembling an amp like this and doing the chassis/fab work I didn't want to spend much on a heavy duty chassis only to make mistakes and hack it to bits. I've assembled a few tools to do this sort of chassis work and found out what works and doesn't work now. The next one will go smoother.

    The transformer doesn't even get very warm on this thing running for hours. The power supply on this amp is stout even in my rats nest breadboard original effort. After I got my gain from adding the 12AU7 it has pretty good volume and authority- much more than my other 6BQ5 SET amps that have tube rectifiers/chokes. It's got all the ass I need from a small tube rig. If I wasn't kind of impressed with it, I wouldn't be building it into a new chassis. (Well, and to get rid of that nightmare cadminum plated chassis)
     
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  17. HBrown

    HBrown Active Member

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    A shot of the back side so far. I'm waiting on a few new terminal strips to come in the mail to mount the doubler caps etc. IMG_20180520_210913880.jpg
     
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  18. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,566
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    maybe attach some small channel underneath for bracing. Screw the transformer mounts through it, and a screw at each end close to the side wall. That should stiffen it up considerably.
     
  19. HBrown

    HBrown Active Member

    Messages:
    325
    Location:
    South Georgia
    Gadget, that should work . Maybe a few small 1/4 X 1/4 X 1/16 aluminum angles from the hobby shop bolted crossways underneath like you said. Easy enough to do here, and plenty of room underneath. However, it's really not so bad as it is, I just wouldn't want to drop it or otherwise slam it down hard. That would surely cause some damage to this thin chassis. Heck, it wasn't but $13 so I'm not inclined to complain too loudly.
     
  20. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,566
    Location:
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    I've been accused of over-engineering things. Nothing too strong ever broke :) Its also easier to do that stuff when there are no wires in the way vs doing it later.

    I do have a couple of things that the top deck has some deformation from rough handling / bad packaging in the past. Transformers are a big chunk of mass, and that Newton guy was something of a jerk when he came up with that inertia thing. Thats actually part of why I elected to not rebuild my two big mono amps into a dual mono arrangement. The chassis I had wasn't really robust enough to handle the weight of 4 big transformers. That, and it would have been too heavy to easily deal with. I think if I'd dropped it, the chassis would have just folded up like a wet paper hat.
     

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