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Packard Bell DPA30 vs DPA20?

Discussion in 'Tube Audio' started by ferninando, Mar 26, 2015.

  1. ferninando

    ferninando Lunatic Member

    Messages:
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    whats the diff between a DPA30 and DPA 20 Packead Bell consol amp?
    Anybody know?
    Thanks.
     

     

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  2. century tek

    century tek Super Member

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    I have seen them both but can't remember which is which. I think one runs four 6BQ5 tubes push/pull each channel, and the other one runs only two 6BQ5's single ended each channel.

    Don't quote me on that 'cause now I'm going to have to look them up.
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    its 10 better obviously :)

    honestly, don't know.
     
  4. cademan

    cademan Addicted Member

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    The DPA 20 & 30 have the same tube line-up. Four 6BQ5 tubes, two for each channel in push pull. I own the DPA 20 and it has the tuner preamp which has to be powered from the power amplifier via umbilical connection.

    Both pics are from e-bay. No affiliation!

    First pic shows the DPA 30. It looks to be the same as the DPA 20 but has a different power transformer. Possibly this model didn't come with a tuner/preamp.

    2nd pic is the DPA 20. Not my pic but I have the exact same amplifier.
     

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    Last edited: Mar 27, 2015
  5. ferninando

    ferninando Lunatic Member

    Messages:
    12,467
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    one is ultralinear the other is not, is all I know. What makes it ultralinear.?
     
  6. cademan

    cademan Addicted Member

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    8,986
    If you are referring to the amps on the bay, the one listed as ultralinea is actually not ultralinear. The guy probably doesn't know what he's talking about so be aware. You can look at the underside pics and see that it uses regular push/pull, plate to plate with center tap output transformers.

    Packard Bell never made an ultralinear amplifier that I am aware of.

    From Wikipedai. No affiliation!

    "A pentode or tetrode vacuum-tube (valve) configured as a common-cathode amplifier (where the output signal appears on the plate) may be operated as:

    a pentode or tetrode, in which the screen-grid is connected to a stable DC voltage so there are no signal variations on the screen-grid (i.e. the screen-grid has 0% of the plate's output signal impressed on it), or
    a triode, in which the screen-grid is connected to the plate (i.e. the screen-grid has 100% of the plate's output signal voltage impressed on it), or
    a blend of triode and pentode, in which the screen-grid has a percentage (between 0% and 100%) of the plate's output signal impressed on it. This is the basis of the distributed load circuit, and is usually achieved by incorporating a suitable "tap" on the primary winding of the output transformer that the vacuum-tube (valve) is connected to.
    The impression of any portion of the output signal onto the screen-grid can be seen as a form of feedback, which alters the behaviour of the electron stream passing from cathode to anode".
     
    Last edited: Mar 27, 2015

     

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  7. ferninando

    ferninando Lunatic Member

    Messages:
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    yes cadman, those are the ones. only visable diff is the pwr trannys.
    Thanks for clearing that up.
     
  8. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    As long as we're on the subject, has anyone deciphered Bob Carver's mod on these that makes them $uper good?
     
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  9. ferninando

    ferninando Lunatic Member

    Messages:
    12,467
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    I have a friend who has one , will try to get info from his.
     
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  10. jaymanaa

    jaymanaa AK Subscriber Subscriber

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    Those look like Mullard output tubes, with very low hours. Not much nickel on the glass yet, just enough to tell the tale.
     
  11. biscuithead

    biscuithead Me likes the eargasm retroplasm... Subscriber

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    Have you or anyone else done anymore digging into the DPA-20, the DPA-30, or the Baby Silver Seven?

    I have read the thread on "DC restoration" and it kind goes all over.... without a conclusion.....
     

     

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  12. USA Soccer

    USA Soccer Well-Known Member

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    Bump I am curious about these...
     
  13. biscuithead

    biscuithead Me likes the eargasm retroplasm... Subscriber

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    I have been learning alot... I have 4 of these, and I have a pair that have potted transformers that are a delight! I have only got them "limping along" but they sound as good as the Dennis Had amps I've owned...
     
  14. biscuithead

    biscuithead Me likes the eargasm retroplasm... Subscriber

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    Only difference in the 20vs30 is the 20 has another winding on the power transformer for the tunerpreamp. The 20 power transformer is potted and when not used to power the tuner, stays a lot cooler... you know the possibilities
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    so 3 years later :)

    Lets drop some schematics in here just for simplicity. Images linked from here

    Help me pick an amp design: P-P EL84 with 12ax7 driver

    stock schematic (the 5w zener at the lower part of the schematic is NOT stock)

    [​IMG]
    [​IMG]

    Bob Carver version

    [​IMG]


    The only significant change is the bias system. Stock is a cathode resistor, Bob used a zener diode to effectively make it fixed bias. It should have better distortion performance since the bias point will not shift as you pass more signal through the amp, and it should bump the power output as well. Using EFB instead of the zener diode will do very similar things and make it a little easier to adjust the bias. I see its running higher voltages, guessing it has a different rectifier but I've not seen that portion of the schematic. WAG, its probably now a solid state rectifier and originally 5U4GB ? That would be about the 70v difference.

    The HF stability stuff is tweaked, I see bigger coupling caps, the bias point on the voltage amp section is changed. Overall its a good amp design to begin with, and as long as the transformers are capable it should perform well. Direct coupled split load inverters were used on a ton of things.

    **edit** fixed the image
     
    Last edited: Aug 22, 2018
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  16. Leland47

    Leland47 AK Subscriber Subscriber

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    I copied a Silver Seven Auction....back when. I'll try to post these screen shot in order...but not guaranteeing anything. I've got 2 of the 30's. Plus the 30s have those OPT terminals sticking out of the top of the transformers don't they? (and the 20s don't?) So, I think different OPTs.

    I hope this doesn't break any rules. Mods please take this down if it is a problem.
    And if Bob Carver sees this, Thanks for the information on this amp.
     

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  17. century tek

    century tek Super Member

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    He never really tells you what you want to know. Maybe a trade mark infringement.........I don't know. :dunno:
     
  18. Leland47

    Leland47 AK Subscriber Subscriber

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    Also, I think there is a 30-1, 30-2 and a 30-3. But don't know the differences.

    Here's a couple more schematics.

    index copy.png index-1 copy.jpg index copy.jpg
     
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  19. gadget73

    gadget73 junk junkie Subscriber

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    honestly I'm more than a little tempted to try the zener diode thing on my Fisher 600. I can't EFB it since it uses two tubes as the cathode resistor, but I can strap a zener across it to keep the bias fixed. Maybe I'll throw a diode on my next parts order and clip it in as an experiment. At worst, it will be a horrible failure and we'll never speak of this again.
     
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  20. biscuithead

    biscuithead Me likes the eargasm retroplasm... Subscriber

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    Thanks for the screen shots!

    I have a 30 also. The outputs on the 20s just have the wires tucked into the last layer of lacquered paper and ran under chassis to the term strip.... then each side (R&L) has a feed ran to that center screw term.

    Then on my pair the two screws are jumped (coupled) to result in paralleling the two channels into a double power (and additive THD) . My Dpa20s were found in the same Rpc3 console, effectively being used as mono- blocks . I also have the tags advertising it as a 60watt unit...

    I haven't ohmed out the 20 vs 30 opts yet but they appear the same and have similar stampings (ink not pressed) .
    Having a 20 next to a 30, just by appearance and inference, the 20s look to be the higher cost build.
     
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