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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    Help on Mods original Dynaco ST-70 with VTA board/PAS3X

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    Pigboat

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    Join date : 2020-05-07

    Help on Mods original Dynaco ST-70 with VTA board/PAS3X Empty Help on Mods original Dynaco ST-70 with VTA board/PAS3X

    Post by Pigboat on Sun May 17, 2020 12:02 pm

    Just aquired

    Dynaco ST70 with VTA board
    Dynaco Pas3X

    Obviously Mods have been made.
    What do you see??
    What do they need??
    Thanks for input.

    Help on Mods original Dynaco ST-70 with VTA board/PAS3X 20200513 Help on Mods original Dynaco ST-70 with VTA board/PAS3X 20200514
    Help on Mods original Dynaco ST-70 with VTA board/PAS3X 20200515 Help on Mods original Dynaco ST-70 with VTA board/PAS3X 20200517
    Help on Mods original Dynaco ST-70 with VTA board/PAS3X Unname10
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    Pigboat

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    Help on Mods original Dynaco ST-70 with VTA board/PAS3X Empty Chirp Chirp

    Post by Pigboat on Mon May 18, 2020 11:19 pm

    Must be wrong Forum to request feedback
    Thanks Sleep
    peterh
    peterh

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    Post by peterh on Tue May 19, 2020 2:45 am

    The question is vague.
    Yes, the devices has been altered, a st70 has added tubed4hifi board, a pas with
    randomly changed caps.
    But all this is best visible if you compare with the manual(s)
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    Pigboat

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    Post by Pigboat on Tue May 19, 2020 10:37 am

    [quote="peterh"]The question is vague.
    Yes, the devices has been altered, a st70 has added tubed4hifi board, a pas with
    randomly changed caps.

    Duh, no kidding.
    Your help is "Vague" Basically "Look it up"

    Since this is a "Dynaco" site, I didn't think I had to spell it out.  
    So I'll try to answer my vague inquiry.
    I see a

    ST70 w/VTA kit build
    VTA Kit, Bob Latino or DynaKit Parts ? or other.
    Bias test points on board ? Read 180 ?
    proper transformer? Acceptable tubes ?
    Needs to address surface pitting. how ?
    Diodes out of takeoff socket, just power on lights ?
    Volume control. Sonically good ?
    Wiring birds nest acceptible?
    Heat has anybody added heat sinks ?
    Suggestions.

    PAS3X Factory build
    1 old, 1 new Board ?
    "randomly changed caps" Known Mod ??
    Updated volume control, "random" ??
    Updated power supply, OK ? or update and eliminate rec tube and cap ?
    Sonic controls still in circuit, OK, or eliminate, is centering really "Flat" ??
    Cluster F tubes, suggestions ?
    Is cover required for shielding, or can I let the breath ?
    Suggestions

    Sorry about "vague" inquiry. Just trying to start a conversation.
    Thought I could get at least a "Nice" rig.
    peterh
    peterh

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    Post by peterh on Tue May 19, 2020 11:33 am

    Pigboat,
    sorry if you was offended, it was not intentional.

    As you now have some questions i can answer some.
    As for PAS ( it is a pas3X by the way)
    Q. "Sonic controls still in circuit, OK, or eliminate, is centering really "Flat" ??"
    No need to modify, if tonecontrol pots are centered the tonecontrol is out of the circuit.
    Q. "Is cover required for shielding, or can I let the breath ?"
    No it will hum if you remove cover. Besides it consumes 16w so there is no need for breathing.
    Q. "Updated power supply, OK ? or update and eliminate rec tube and cap ?"
    No if it does not hum it's ok. Sometime in the future you could install i power supply board
    that replaces the B+ cancap and the filament caps and rectifiers ( those are already replaced)
    One could eliminate 12x4 , some power boards have un4007 diodes for that purpose, but sonically
    is no difference.

    As for the st70 i have no comments
    PeterCapo
    PeterCapo

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    Help on Mods original Dynaco ST-70 with VTA board/PAS3X Empty Re: Help on Mods original Dynaco ST-70 with VTA board/PAS3X

    Post by PeterCapo on Tue May 19, 2020 11:52 am

    Pigboat wrote: ...
    Duh, no kidding.
    Your help is "Vague" Basically "Look it up"

    Since this is a "Dynaco" site, I didn't think I had to spell it out. ...

    The best way to learn about your gear is to get a hold of complete documentation and study it.  Forum discussion can augment that.  Specific questions are helpful.

    Your VTA Stereo 70 looks like it may have a 5U4GB rectifier tube, in which case the amp will make something less than full power.  Ordinarily, this amplifier takes a 5AR4/GZ34 rectifier tube, which would allow it to make full power output.

    However, changes might have been made that require the use of a 5U4 rectifier.  It’s not uncommon for used gear to have some kind of homegrown modification that is undocumented.  The wiring otherwise looks like it leaves something to be desired.  If it holds steady bias and sounds good to you, then leave it be.  If not, then I’d be tempted to do a complete teardown and rewire according to the original build manuals.  If you want to switch to a 5AR4/GZ34, I’d suggest bringing it up on a variac and see what the voltages are on the test nodes given in its build manuals.

    The PAS appears to have significant parts upgrades.  I’d get a hold of a PAS-3X manual with the original pictorial wiring diagrams and go through the setup and adjustment sections as well as the voltage chart on the right-hand side of Page 11.  The existing parts upgrades should render this PAS-3X a pretty good sounding preamp, if you like a clean, all-tube sound.
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    rjpjnk

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    Post by rjpjnk on Thu May 21, 2020 1:13 am

    The most important mod is to remove the lock washer that fell on the line stage board in the PAS before it shorts something out Wink
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    tuberadios69

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    Help on Mods original Dynaco ST-70 with VTA board/PAS3X Empty Bias supply

    Post by tuberadios69 on Mon Jun 08, 2020 10:34 pm

    It appears that the selenium rectifiers are still in the bias supply of the PAS 3X.  Those should be replaced with diodes.  Should be instructions on here in other posts or, on Audiokarma.  The wonder caps on the two big caps under the green selenium rectifiers are apparently being used as bypass caps.  I don't think those are necessary.  Look at the date code on the two big caps and if they are over 15 years old, replace them.  i use Sprague Atom caps (called/branded "Vishay" now?).  The yellow caps running to the two tone controls are ok and a recommended mod.  

    As far as the Stereo 70 goes, if it is biasing correctly, I'd be tempted to leave it alone.  You might want to check the voltages on the output tubes for proper values as noted on the Stereo 70 assembly manual (which is available free from numerous sources including hiengine.com).  I can't see where diodes have replaced the selenium rectifier but perhaps it is simply hidden in the pic.  I assume the LED's sticking out of the front octal sockets are for setting the bias.  Perhaps someone else can chime in here regarding proper settings.  By the way, your comment was kinda snarky when you finally added all the questions you wanted answered.  You should have asked those up front.  Just sayin".
    GreggW
    GreggW

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    Post by GreggW on Tue Jun 09, 2020 12:27 am

    Rectifier tube could be a 5V3A or 5V3. If a 5V3, those draw 3.8 amps filament current. The 5V3A draws 3 amps, which the transformer should handle with no issue. 5V3As are almost always on eBay for $10-20. Somewhere on the forum is a rectifier comparison chart.
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    Pigboat

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    Post by Pigboat on Mon Aug 24, 2020 2:02 pm

    "By the way, your comment was kinda snarky when you finally added all the questions you wanted answered.  You should have asked those up front.  Just sayin".[/quote]

    Thanks for pointing out what you saw.

    My question, By the way, was "what do you see" Not what do I see. Just sayin.

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