New Project: 'Junk Parts' Amp build - Voltage / Bias question

Discussion in 'Tube Audio' started by Wharfcreek, Jan 10, 2019.

  1. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    I shall give that a shot. Will be interesting to see how close the 'calculation' comes to what the actual PS voltage is. One question: Is the 5AR4 really going to drop 35 volts? I was under the impression that this tube dropped significantly less than that. Also, will capacitor value in uf make any difference in this? My experience has been that in using 30 and 47 uf caps in the PS, that voltages got pretty high. However, when I dropped those values some, and like I mentioned above, used a 10uf right off the rectifier pin 8...... I found voltages within the PS dropped measurably....by like 15 or so volts as I recall. TSD
     

     

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  2. kward

    kward AK Subscriber Subscriber

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    Voltage drop at rectifier depends on the current. I'm going from memory. It might be a bit less

    My experience is the DC on the B+ doesnt change much if first cap is like 22 uF or larger. Depends on current though.
     
  3. kward

    kward AK Subscriber Subscriber

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    By the way my moto amp uses a 3V rectifier -- a 3DG4, and its voltage drop is right around 15V at the current demanded from my amp, but I believe the 5AR4 drops more than the 3DG4 under similar conditions.
     
    Last edited: Jan 11, 2019
  4. gadget73

    gadget73 junk junkie Subscriber

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    I really don't like tapping the output transformers after a resistor. The more impedance you add to the supply, the more things sag and that does nothing good for power output or distortion. OPT comes straight off the first cap, anything else can go where it needs to. If I have a choice about it, I try to not have the screens after a really big resistor either. Maybe tap that after the choke.
     
  5. 6DZ7

    6DZ7 Super Member

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    Y'all, et al. C'm on....
     
  6. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

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    I just noticed that the Channel Master also uses a 12AU7 in front of a pair of 12AX7s. I think a lot has to do with the strength of your source signal. If its a CD or similar 2v source you might just be able to eliminate that first tube. Lots of amps just use two 12AX7s. I'm listening to a Fisher SA-16 using iTunes and it seems to work fine with that setup. The Maggies and others do fine too.
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    The 12AU7 stage is to make up for the signal loss in the tone controls.
     
  8. kward

    kward AK Subscriber Subscriber

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    Something on your mind, 6?

    I'd be okay with it as long as the resistor is small. Even a small resistor (say 47 ohms) kills ripple down quite a bit if used in conjunction with fairly large filter cap after it; I'd prefer a choke though, instead of a plain old resistor.

    I know--ripple (mostly) cancels in a balanced stage if tubes are reasonably well matched. In any case it will be less of an issue if EFB is used, since EFB will track the voltage sag and auto adjust the bias accordingly. Thought I heard Wharf mention maybe using EFB on this amp if things work out...

    .
     
  9. knockbill

    knockbill Addicted Member

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    Beautiful job, as usual Scott!!! Really like that blue... Thanks for the thread Tom,,, more lessons to be gained here!
     
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  10. 6DZ7

    6DZ7 Super Member

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    If WC sticks to the parts list specs there's 140mA to use. No sweat for the rectifier. If 30uF > choke 56R > 20uF = B+ to handle a 70W amp we all know and love, then just give me a dozen ea. 33uF, C-354, 47uf caps for any dozen EL-84 amps.
     
  11. FlaCharlie

    FlaCharlie AK Subscriber Subscriber

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    Maybe I'm just a simpleton but I have no idea what your point is. And what's a C-354?
     

     

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  12. s-petersen

    s-petersen Scott Subscriber

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    C-354 is a Dynaco choke
     
  13. Shadowdog

    Shadowdog Super Member

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    If you want less sag in a PS , a Choke from what gadget73 and a few others are saying you want one with with lower resistance , like close to 20 ohms, and Edcor has them for reasonable $.
     
  14. 6DZ7

    6DZ7 Super Member

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    Dynaco's little basic @1.5H, @60R, 200mA choke. The chokes in common Magnavox amps have similar specs. They are inexpensive, small, fit anywhere. I just refer to them because they're common and are maybe a bit higher value than other common 1H low R chokes.

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    Last edited: Jan 11, 2019
  15. Shadowdog

    Shadowdog Super Member

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  16. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    OK, good thoughts here on the choke. But, as indicated about the ST-70 and the C-354 choke, I believe those Dynaco folks figured that the DC power supply was OK with this choke in line. The Hammond 156R has nearly the same specs as the C-354 with the same 50 ish ohms of DC resistance, and 1.5 H induction. The 354 is I believe rated at roughly the same 200 ma of current as the 156. So, I'm thinking that since I have a 156 here, that's what I'll use. Also, I might add that the OTs in this build are perhaps a bit less demanding than those in other EL84 builds.....and certainly less so than those in the ST-70. As this amp isn't intended to be anything other than a 'functional daily drive / work horse' type unit.... I think that 156 will be fine. There's still a chance that I'll need a dropping resistor some place in the PS as well. So, as all this is still a 'work in process'.... I think I'll have to do some 'punting' down the road. WC
     

     

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  17. gadget73

    gadget73 junk junkie Subscriber

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    A pair of EL84 at full scream at typical datasheet conditions is good for about 115ma of plate + screen current, so 230ma for a stereo amp. Add a few for the driver and you're at ~235ma at full output power. Idle you're at 160ma for the output plus a few for the driver, call it 165 and its likely not far off.

    Likely that EFB can reduce that though. Both of my EL84-type amps with EFB run at fairly low current. The one seems happiest at 23ma per-pair idle current. The Magnavox I think runs 44ma per-pair. Not a clue what they do at full output, I don't believe I ever measured it.
     
  18. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    Thain, I took a rather different approach to what I thought might end up being the total current level in this amp. I simply took a look at an ST-35 schematic, noted the cathode resistor in that amp was a 95 ohm value, then looked at the voltage across it....which was listed as being 13.5 V. I simply did the math to determine that this was about 142 ma of idle current draw on the outputs. Like you said, adding a few more ma for the driver tubes, and I put the thing at still being under 150ma of total current. I then looked at the PA-774 power transformer itself and it is only rated at 180ma of DC current 'rectified'. As you noted, something like EFB might actually reduce this total current even more, particularly at full output. But, again, the OTs in the amp I'm building are only rated at 15 W each, so I believe that running the EL84s at full tilt is somewhat out of the realm of possibility when using these output transformers. If I understand it correctly, they're simply not up to that level of current draw. So I figure the output section in this project is going to have to be somewhat 'under-built' in order to use these OTs in conjunction with the EL-84 tubes. In so doing, I don't think I'll have any problem using that 156 Hammond choke. I've seen plenty of old amps running at about 12 Watts / side......and they sound damn good! That's all I'm hoping for with this. BTW, I also found a choke that I'd used before, a TRiad C-24X....which is 1H at 240ma and 50 ohms. I just ordered 3 of them from Arrow....... all 3 incl. shipping came to under $30. They too work well......and now I'll probably use one in this build.
     
    Last edited: Jan 16, 2019
  19. 6DZ7

    6DZ7 Super Member

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    No, bro. Use a 5R4 in that baby and it's a good Hail Mary to the end zone. Get the GY version, ST shape. 2A heater. You don't have to use those ugly ol' tater mashers, just the straight GY.
     
    Last edited: Jan 16, 2019
  20. Wharfcreek

    Wharfcreek AK Subscriber Subscriber

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    6, I have a couple of those, as well as a couple more on order. For some reason I believe I recall buying a pair as they related to a set of Heathkit amps I'd planned to rebuild. On a sadder note, those amps came in a 'swap' deal with our dear departed AK member AudioDon (RIP). I do plan to get to those one day!! Anyway, I recall that rectifier is a 'directly heated' unit like the 5U4, right? So, not quite as beneficial in the 'slow-start' category as the 5AR4. But, about a 67 volt drop in that tube IIRC, similar to the 5U4, but with a 1A reduction in heater current. Not a bad consideration if needed. I suppose it's a lot better than another resistor in the PS circuit.
     

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