RCA console amp RS-199C

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

  1. HBrown

    HBrown Active Member

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    321
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    South Georgia
    Ever have a night where it just ain't meant to be? This an' that was keeping me from the shop earlier- then I slipped and almost busted my butt running from house to my shop in the rain. Then, get some tunes going, and the iron fired up and make a bit of progress- grab the iron to make the next joint and it's cold. No amount of fiddling or replacing the tip fixed it. Dang soldering station has taken a dump. Not in the mood to tear apart a soldering station and troubleshoot it, I just shut down the shop and came back inside. I'll grab a $7 walmart soldering iron tomorrow. I used them for 30 years before buying a temp controlled station. Probably got 5 of them around, but couldn't lay a hand on one if my life depended on it. No biggie- just one of them days!

    At least the local classic rock station is on point tonight.
     
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  2. HBrown

    HBrown Active Member

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    Ok! Well, I got all the parts transferred over, and working on the new chassis. The hum I was experiencing was my 6FQ7 tube going out. I tried a different one, and no more hum. The amp is pretty quiet, no hum at all. Surprising, as whacked up as the underside wiring turned out- geeze, I don't see how you guys do that awesome looking wiring!

    I ran a large ground bus in a loop from negative output terminal across to the other negative terminal, then tied to (and helped supported by) a single chassis point solder lug that the power supply is tied to. All I can say is there it is, and it works with no hum or noise. The only complaint at all is when my volume control nears about 3/4 volume I am getting break-up or distortion. It's not really a problem, as it's plenty loud before reaching that point, but the issue is there nonetheless. I pretty closely followed the schematic I posted near the end of this thread. Thoughts? Suggestions? IMG_20180605_215359103.jpg IMG_20180605_215126798.jpg IMG_20180605_215207222.jpg
     
    Last edited: Jun 12, 2018
  3. gadget73

    gadget73 junk junkie Subscriber

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    Possible you're just over-driving it. These are only good for ~3 watts of output power.
     
  4. HBrown

    HBrown Active Member

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    Location:
    South Georgia
    Gadget, that's a very likely possibility. The output is pretty darn good for a 6BQ5 SE.

    I'm wondering what would happen if I threw a pair of the inexpensive 8 ohm AES output transformers as spec'ced for the DG-SE1 at it. I realize I would have to revisit the feedback etc.. but it may help to match the outputs to my load. Thoughts?
     
    Last edited: Jun 12, 2018
  5. HBrown

    HBrown Active Member

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    321
    Location:
    South Georgia
    Well, this amp I built was literally put together by taking an existing design that was working, but not so well, changing and using R and feedback values suggested by others, and copying a gain stage with associated component values from other sources. I chose things that seemed like it would work- based on staring at similar schematics and with a good bit of experimenting with resistors and cap values it does work- and actually sounds good.

    Now, I would like to learn how to evaluate what I have got, and if need be, make some intelligent changes.

    For the 6BQ5, using the Robinette online calculator, I plugged in my plate to cathode voltage (291V), V drop across the cathode resistor (7.33V), and my cathode resistance (140 Ohm) and found that my 6BQ5 total plate current is 49.5 mA and plate dissipation is considerably beyond the 12 W limit at 14.4 W.

    Now, the tubes arent redplating, and dont seem to be excessively hot or anything. The amp sounds good until I drive it beyond around 3/4 of the volume pot, then it does get distorted sounding- but it is putting out very good volume and good open sound quality for a 6BQ5 before that point is reached.

    Do those numbers look feasible to the veteran guys here? Do I need to change my cathode resistor to a higher ohm value to try and bring dissipation down to 12 W?

    I recognize that what I really need to do is start at the first stage and evaluate everything thru the amp, draw load lines etc... but I'm not to a point knowledgewise where I can do that yet.
     
    Last edited: Jun 13, 2018
  6. gadget73

    gadget73 junk junkie Subscriber

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    A quick look at Dave's re-work of the Magnavox 8600 indicates he's running the 6bq5 tubes at 47.5ma for a pdiss of 11.8. Bumping the resistor to 150-160 ohms probably wouldn't be an awful idea.

    http://audiokarma.org/forums/index....-series-a-lot-more.665735/page-6#post-8885716

    has the schematic for that just in case you want to compare to yours. Different transformers will want different feedback stuff of course but honestly single ended EL84 amps are not complex enough to make any of them horribly different from each other.

    On the other hand, a peek at the official RCA tube data book tells me 65ma cathode current and 300v plate and screen produce 12w plate dissipation and 2 watts screen dissipation as the max allowed under design-center. Precisely how they manage that math I don't know, .065 * 300 = 19.5 plate + screen watts according to my calculator.

    Given those conditions, what you're running at isn't excessive but its not going to be the easiest on tubes either.
     
    Last edited: Jun 13, 2018

     

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  7. primosounds

    primosounds Parallel single ended EL84 ,EDCOR OPT

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    Hey HB, your wiring is not bad at all. After doing or building you get the knack for how to layout wiring and components. With each subsequent try you will undoubtedly get better. I really like the way you did the ground bus going from the speaker posts and tied at the middle. I find that having 2 stages/tubes before the power tube to be somewhat unusual for a SE el84 amp. Most amps only have a single driver tube. The el84 being an 'easy' tube to drive to full power.
     
  8. BinaryMike

    BinaryMike Pelagic EE Subscriber

    The backstory is that this approach preserves the original feedback network along with the original output transformers, making it unnecessary to re-engineer for good closed-loop performance.
     
  9. HBrown

    HBrown Active Member

    Messages:
    321
    Location:
    South Georgia
    Hey, thanks for the encouragement on my wiring. It got a little '3D' the further I got with it. When I get it at a level I'm happy with function wise, I'm going to rewire and add a few more terminal strips for wire/component support and safety.

    I wasnt particularly trying to make it more complicated than neccessary- but as a tube newbie fumbling around with it, nothing I was trying with the original 6FQ7 or a 6DT8 driver was driving the output where it needed to be (where I knew it could be- based on other SE 6BQ5 amps I've seen) for me to be able to really effectively use only a CD player, or my phone as input sources. I also wanted a volume control on the amp to more easily control my sources without having to drag around a separate pre-amp. So, the logical thing was to hang another gain stage in there to supply the needed gain and accomodate the volume control.

    With the extra gain stage, the gain problem is solved and I am now getting all the gain the 6BQ5 outputs can stand (or maybe all the 6FQ7 can stand coming from the 12AU7) I seem to have reached the single 6BQ5 power limitations- but, with efficient speakers, that has proven to be plenty enough volume and sound quality for my needs out of this little rig.

    Now, as a tube newbie, I'm trying to more fully understand what I've built, and maybe improve it a little from here.

    If I had to guess, I'd say the two gain stages are slightly overloading the 6BQ5 at the higher volume settings. Plus, I'm using the stock RCA outputs that from what I gather, are for 4 Ohm speaker load. I have ordered a set of the inexpensive 8 Ohm output transformers from AES as spec'ed for the DG-SE1 to try and correct the output load. I'm going to toss those on there and attempt to address the feedback etc. to match them and then see where I'm at.

    I think a little tweaking from here is all thats needed, but I'm bumping against my knowledge level and being able to understand and do what would help it.
     
    Last edited: Jun 14, 2018 at 5:21 AM

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