• Sweet! - Paul Joppa 10:48:35 06/21/04 (8)
  In Reply to: Simple 84 posted by Gordon Rankin on June 18, 2004 at 10:44:59:

  I love the simplicity of a one-tuber "spud" amp. I've often thought that a single EL-84 would be nice, except that it needs a little too much drive. The step-up is a great solution!

  I see it is drawn as a pure pentode amp. Looks to me like it could be wired with a switch, triode - ultralinear - pentode. Gordon, any comments on this choice?

  The only risk I see is that the 5v winding on the PGP8.1 is rated 1.25 amps, and the 5Y3 draws 2.0 amps. Maybe Mike (once he is back from vacation) can answer how far this winding can be safely "pushed"? I know he is generally conservative in his ratings. If not, the lowest current 5 volt tube I know of is the 5W4 at 1.50 amps. In 6 volt rectifiers there are 0.6 amp tubes, for a total of 2.12 amps from the 6v winding - that's only 6% over the 2A rating.

  The other solution would be silicon diodes, which I like but I recognize that not everyone does. In this case, they would raise the voltage quite a bit, perhaps too much. Even the 6X5 has a fairly high perveance and will raise the voltage some.
  
  

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  • Re: Sweet! - Eli Duttman 07:03:25 06/22/04 (0)
    In Reply to: Sweet! posted by Paul Joppa on June 21, 2004 at 10:48:35:

    PJ,

    Perhaps a 7Z4 energized by the "5" V. is "the answer". The 7Z4's heater draws 900 mA. Given the capability of the series connected "2.5" V. windings, at a 900 mA. draw, obtaining the minimum 5.67 V. needed seems possible. Gordon's remarks about the windings being 2.75 V. is encouraging.
    
    Eli D.
    
    

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  • PGP 8.1 current ratings - mqracing 19:26:23 06/21/04 (0)
    In Reply to: Sweet! posted by Paul Joppa on June 21, 2004 at 10:48:35:

    Hi folks:

    a review of the engineering folder for the 8.1 indicates the following;

    there are two separate 2.5 volt windings each is rated at 1.25Arms. In sereies you would have 5 volts at 1.25Arms. In parallel you would have 2.5 volts at 2.5Arms.

    the 6.3 volt winding is rated at 2.5 Arms.

    now... the question is could you draw say 2 amps rms from the 5 volt winding (the two 2.5 volt windings in series)? The answer is... you'd have to try it and check several things. first check and see what your loaded voltage is with a 2 amp draw on this winding. If the voltage drop is not too great then your ok on this front. Second... would be to check the temperature rise of the unit... does it make it run hot?

    It is unlikely that running this one winding harder than the design center will cause much grief. Largely because it makes up a relatively small percentage of the total VA rating of the transformer.

    But... it is a condition that the designer/engineer would have to evaluate in his specific application for it's suitability.

    cheers,

    MSL

    
    
    

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  • Re: Sweet!- some answers - Gordon Rankin 13:46:59 06/21/04 (5)
    In Reply to: Sweet! posted by Paul Joppa on June 21, 2004 at 10:48:35:

    Paul,

    Actually the PGP8 original was 5@2A and then on the 8.1 these where upped in wire size (or down in gauge however you want to say it) so actually it is higher than 2A. That is why the 2.5V's are always 2.75Vac @ 2.5A's.

    Also remember much of a power transformers specifications fall on the total power rating. In this configurations max on each winding is 6@2 (actually PG8 was 2.5A), 5@2 and HV at 125ma ac. So we are well below the design area and I think the 8.1 in this usage will be loafing.

    I was always taught that ultalinear in SET was a poor choice. Since it is not balanced it can change the effectiviness of the output transformer because of the DC draw and the change in DC draw depending on output power. Triode wired pentodes to me is a total waste of time, might as well just use a triode.

    I think this is a great starter amp for anyone looking for a great little project.

    Thanks
    Gordon
    J. Gordon Rankin
    
    

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    • Re: Sweet!- some answers - Paul Joppa 16:08:37 06/22/04 (0)
      In Reply to: Re: Sweet!- some answers posted by Gordon Rankin on June 21, 2004 at 13:46:59:

      Thanks for the feedback.

      Mike has posted on the PGP; looks like however you cut it there is plenty of filament power.

      I agree about triode-wired pentodes, if you are determined to do a triode. Your info on the sonics of ultralinear vs pentode is a valuable addition, I have not heard that before. I guess I have a soft spot in my heart (or head?!) for an ultralinear EL-84 because it was the first amp I designed and built. I loved it, but it's been a long time - over forty years ago; I was probably 15 or 16. I certainly have not done any comparative listening since then, at least any controlled comparison. I may have to build this amp just to hear the differences.
      
      

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    • Yeah - but SE UL really kicks ass in a guitar amp! (nt) - Jim Dowdy 22:11:06 06/21/04 (2)
      In Reply to: Re: Sweet!- some answers posted by Gordon Rankin on June 21, 2004 at 13:46:59:

      nt
      
      

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      • In tests only 1 out of 10 ejoyed it - Gordon Rankin 09:59:06 06/22/04 (1)
        In Reply to: Yeah - but SE UL really kicks ass in a guitar amp! (nt) posted by Jim Dowdy on June 21, 2004 at 22:11:06:

        Jim,

        If you really like distorted then that may be true. But most people who tried it when I did it, didn't care for it.

        Gordon
        J. Gordon Rankin
        
        

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        • Well, there were only two of us... - Jim Dowdy 14:23:14 06/22/04 (0)
          In Reply to: In tests only 1 out of 10 ejoyed it posted by Gordon Rankin on June 22, 2004 at 09:59:06:

          but, every time we flipped the switch from UL to triode mode, we just looked at each other and said, NAH!!!

          :-)

          Output tube was an Eimac 4-125A tetrode.
          
          

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    • Re: Sweet!- some answers - GSI 16:09:10 06/21/04 (0)
      In Reply to: Re: Sweet!- some answers posted by Gordon Rankin on June 21, 2004 at 13:46:59:

      What is output impedance (or damping factor) for this pentode output stage with 5k trans on 8 ohm tap?? With only 1-3dB NFB I think output impedance is very high. Internal impedance for EL84 pentode tube is about 38 kohm. This transformed (lowered) 625 times in output trans and we have about 60 ohms output impedance without NFB. So in the end we need at least 20dB of negative feedback to reach even 6 ohm output impedance. I'm calculating right or..?

      
      Girius
      
      

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• Re: Simple 84 - mqracing 13:18:22 06/18/04 (2)
  In Reply to: Simple 84 posted by Gordon Rankin on June 18, 2004 at 10:44:59:

  Hi Gordon:

  thanks for the Simple 84 schematic. I hope you don't mind a question or two.

  :::If you have a cap coupled preamp make sure the cap is at least 4uF or above, tranny a better method.::::

  Is the 4uf cap on the output of the preamp to prevent dc from getting to the EXO-173? And is the size is dictated (or necessary) to maintain bandwidth in the 173 down to some low freq level (example; 10hz)?

  
  Could this cap be placed "in front" of the 173... if for example someone had a preamp but did not want to go inside it and change anything?

  ::::You could make this an integrated by subbing in a Transformer Vol control with +6dB of gain.::::

  speaking of gain. won't this amp require some additional gain (i.e., an active preamp) or use of a TVC set up for gain?

  If you went straight in with a cd output... say it was two volts and then the 173 steps up that voltage by a factor of two... you would only have four volts to drive the grid of the EL-84 and wouldn't get full output (would ya)?

  And to keep costs down (plus I'm not sold on TVC's)... what if you had an additional (optional) gain stage in front of the EXO-173... say for guys with passive pre's or guys who would want to come off of their cd player output only (with perhaps a vol control on the amp side)....

  
  Again... neat... it's been a long while since i've seen a really neat looking EL84 series fed SE circuit...

  I remember at the first VSAC hearing the Sun Audio 6V6 SE amp... and preferring it sound wise to their 2A3 SE amp.

  thanks again,

  MSL

  
  
  
  

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  • Re: Simple 84- some answers - Gordon Rankin 13:43:15 06/18/04 (1)
    In Reply to: Re: Simple 84 posted by MQracing on June 18, 2004 at 13:18:22:

    Mikey,

    In regards to the cap, if people are cap coupling then they proabably have at least 3-4 which would be fine. No one is leaving a HV dc path to an RCA so in almost all cases this would be ok. I do know a tube maker who only uses 0.5uf considering that the input to the amp a high 100K. For them they would have to up the output cap.

    ::::You could make this an integrated by subbing in a Transformer Vol control with +6dB of gain.::::

    speaking of gain. won't this amp require some additional gain (i.e., an active preamp) or use of a TVC set up for gain?

    ~~~~~~~~ 4VAC is the maximum input needed! That's why it soo cool!

    If you went straight in with a cd output... say it was two volts and then the 173 steps up that voltage by a factor of two... you would only have four volts to drive the grid of the EL-84 and wouldn't get full output (would ya)?

    YEP!

    And to keep costs down (plus I'm not sold on TVC's)... what if you had an additional (optional) gain stage in front of the EXO-173... say for guys with passive pre's or guys who would want to come off of their cd player output only (with perhaps a vol control on the amp side)....

    You could do what you want or better yet get a digital source with a good output and vol control and you would be better off.

    THE LESS STAGES THE BETTER I ALWAYS SAY.

    There is a beer with my name on it somewhere!

    Thanks
    Gordon

    
    J. Gordon Rankin
    
    

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    • Re: Simple 84- some answers - Paul Joppa 17:40:58 06/18/04 (0)
      In Reply to: Re: Simple 84- some answers posted by Gordon Rankin on June 18, 2004 at 13:43:15:

      > > I do know a tube maker who only uses 0.5uf ...

      That could be me; it's what's in my phono preamp. :^)

      One way to increase the input impedance might be to get Mike to stack the -173 with mu-metal.
      
      

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• That looks pretty elegant Gordon! - Bas Horneman 13:07:49 06/18/04 (0)
  In Reply to: Simple 84 posted by Gordon Rankin on June 18, 2004 at 10:44:59:

  I must have looked at it for about a minute before I noticed that it does not have a driver tube!

  Cheers,
  Bas
  
  

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