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D&A 12 tube phantom arcs over on high

Discussion in 'Amplifiers' started by Staybolt, Apr 13, 2018.

  1. Staybolt

    Staybolt Well-Known Member

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    It works fine on low but on high it arcs over and pops and cracks... I changed some tubes and tested all of them before i put them all back in. And yes, i know the dangers of AC tube amps.....


     

  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Would you be able to post some pictures of the inside of the amp, top and bottom? It might help us spot a problem.
     
  3. Staybolt

    Staybolt Well-Known Member

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    I will later. Kinda busy for the next few days.
     
  4. 357magnum

    357magnum Well-Known Member

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    Lately Iv'e been seeing a lot of posts on D&A 10 & 12 tubes it brings back memories my nephew brought my "10" to a local cb/friend/tech since they have both past .Tech's family had no idea where my amp was , Hmm ! I sure miss them all especially my nephew who passed @ 19 out of stupidity ! Yup you all know what it was . But regardless of my BS , Great Amp !
     
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  5. kopcicle

    kopcicle Well-Known Member

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    Just arcs ? no explosions ?

    :)
     
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  6. Staybolt

    Staybolt Well-Known Member

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    Lol no explosions but the caps look original. Gonna replace them. My amp is at a friends that said would work on it but hes a mechanic and works 7 days a week so im gonna get it back.
     
  7. Staybolt

    Staybolt Well-Known Member

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    They are not bad. Lots of them around here. Sadly a lot of guys ruin them running dual final rigs into them....
     
  8. nomadradio

    nomadradio Analog Retentive

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    The 12-tube Phantom routinely destroys the High/Low relay.

    All the later 10-tube Phantom amplifiers had only two relays. One between the coax sockets on the rear panel, and one for switching on the high voltage to the tubes.

    The older 12-tube version is different. It has a third relay used to select High/Low mode.

    In "Low" side, the RF output comes directly from the four driver tubes. The eight final tubes remain on standby. That third relay is located under the deck between the driver tubes' sockets and the final tubes' sockets. It serves to route the power from the driver tubes directly to the antenna for Low side, or route it into the eight final tubes for high side.

    Just one problem. The black plastic insulating block that supports the contact points will only tolerate RF at low voltages.

    Low, as in 50 ohms. That relay works just fine to switch the input and output between the coax socket. The relay between the coax sockets almost never fails from arc breakdown.

    But the RF passing through the High/Low relay is operating at the high RF voltages feeding out of the four driver tubes.

    This high-voltage (high-impedance) RF will break down the black plastic insulator on the High/Low relay after 15, 20 or more years. And this amplifier is well over 40 years old.

    But that's what I suspect is going on. Your High/Low relay is breaking down, causing arcs across the insulation between the contact points.

    I suspect that this is one reason the Phantom was redesigned around 1974 or 1975, to eliminate that third relay and deal with this weak spot in the 12-tube design.

    I have my suspicions about other reasons for going to 10 tubes, but Ed Dulaney is no longer around to ask what he was thinking.

    73
     
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  9. kopcicle

    kopcicle Well-Known Member

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    Well in support of , support of , support of, sort of ...
    I grabbed that file ages ago (here or from central or from misfits or ...)
    So for Ed , Central , Nomad, the OP and anyone that trips across one of these in mostly complete condition I just uploaded it ....

    So I can't remember if Nomad and I had this conversation here or it's the remnants of a bad dream. But that many (beam power pentodes) tubes and all the "tank" capacitance ends up in the tubes with little left over for tune. The Load variable wasn't much more than an old 225pf broadcast variable found in old stereo receivers and the tune wasn't much more than a single moving plate sandwiched between two fixed plates. From memory, the blocking cap was a pair of 500pf doorknobs in parallel and the coil was about 5 turns, 1 1/4" dia , 3/16" copper tube.

    The power supply is of particular interest . The bleeder resistance is placed across the entire capacitance and not the individual caps. This can have curious and exciting consequences.

    For those of you that can and do the math please educate the unwashed masses. For those of you that don't do the math please listen to those that do. As I recall it's around 35pf (or more) per tube. This gets more than a little "squirrelly" at radio frequencies when you parallel more than 4 tubes. I'm sure I'm not explaining this correctly or sufficiently enough for the digital volt ohm meter with a screw driver crowd (no offense intended) and not to the exacting standards of the professionals here. (remember , I'm a hack) I do know that any attempts to quiet, modify, improve, re-design the tuned circuits of the 12 (or 10) tube Phantom were futile. Believe me I tried when I thought I was Marconi's gift to radio frequency amplification and "knew better" than all these somebodies what to do. Well I didn't , and still don't , and as a matter of fact won't. Only sparks, arcs, catastrophic destruction and tears will ensue.

    Have you checked tube depot for current prices of 6MJ6/6LQ6 ? How about ebay ? Have you considered the use of those transformers paralleled and doubled into either a choke or cap input ? I'm suggesting selling the glass and or iron to those that don't know any better and moving on.

    Ever seen what happens to a top fuel car between rounds ?
    Imagine what happens to to a D&A 12 tube Phantom between 10 minute key downs . The comparison isn't as big a stretch as you might think.

    I'd keep the case intact and eventually stick something more well behaved in it just for nostalgia .

    ...we now return you to our regular thread drift
     

    Attached Files:

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  10. 357magnum

    357magnum Well-Known Member

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    Kop , No reference to the 12 tube but I've spent many years working on Drag Cars , it started with Gene Snow " The Snow Man " back in 1971 @ The Summer National's in NJ . But I toned it down a bit sold my Bracket Car ( 10.90's ) and worked for a local family on Super Street and Pro Mod . If you know about NHRA we got more in common than radio's !o_O:ROFLMAO:
     
  11. kopcicle

    kopcicle Well-Known Member

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    B/Gas Opel GT "Generic racing" 1979-'80 278/331 CID
    C/SM '67 Nova "Novocaine" 3.45" stroke +.400" deck BBC
     
  12. 357magnum

    357magnum Well-Known Member

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    Kop , A/gas , 48 Anglia BBC with a stick , that 4 speed didn't work well I broke the shifter going into second and if you know Old school Anglia's you sat in the middle guess what I hit in my anatomy when the shifter broke ? Didn't feel good for sure !lol I don't want to tie this thread up with our memories I'll have to pm you . PS I have articles written about me in National Dragster .So I'm done talking about race cars on this thread out respect , I just got carried away . Sorry Staybolt !
     
  13. Onelasttime

    Onelasttime Well-Known Member

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    No way would I want to re-tube that! Does that use a high voltage transformer or was just a diode board and 110V going into the diode board?

    If it actualy has a big transformer then a HV board I would seriously consider re-tubing to maybe some Russian tubes or at the very least something that was designed to amplifier RF and something with ceramic and steel and something with heat sink on the top of the tube. Maybe 4CX150 or 4CX250. I would imagine 4c 4CX250 would be a better way to go than 12 TV sweep tubes! Sweep tube can cost more or be equal in cost to the 4CX250's and like tubes. Those tubes have great multiplication factory and need very little input drive!
     
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  14. kopcicle

    kopcicle Well-Known Member

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    The 4CX250 requires much more HV plus a screen supply. Grounded grid isn't impossible but problematic. Grid driven through a swamped input is much easier.

    The EU equivalent is the EL/PL 509/519 beam power pentode. These work well with the available 1K HV. There are many articles on their use throughout web as there still exists a 400w power restriction on "the" Island.
    https://pa0fri.home.xs4all.nl/Lineairs/Frinear400/frinear400eng.htm

    Any of the horizontal deflection amplifier tube (sweep, Novar, beam power pentode) based amplifiers are a relic consigned to the past and thankfully so.

    I'll repeat that the proper disposition of any of these death traps is to part them out completely and sell any good tubes.
    The Iron will always come in handy. The rest is scrap.
     
  15. Onelasttime

    Onelasttime Well-Known Member

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    The EL509/519 are still rather expensive. I have a few and on top of that shipping often cost's as much or more than the cost of the tube itself. European Countries and Canada have insane shipping rates!

    Yes I was just tossing out the idea of a real hf tube in general due to bang for your buck and useful life cycle.
     

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