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    Help on Mods ST-70 VTA/PAS3X

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    Pigboat

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    Join date : 2020-05-07

    Help on Mods  ST-70 VTA/PAS3X Empty Help on Mods ST-70 VTA/PAS3X

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

    Just aquired

    Dynaco ST70 VTA
    Dynaco Pas3X

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

    Help on Mods  ST-70 VTA/PAS3X 20200513 Help on Mods  ST-70 VTA/PAS3X 20200514
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    Pigboat

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    Join date : 2020-05-07

    Help on Mods  ST-70 VTA/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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    Join date : 2012-12-25
    Location : gothenburg, sweden

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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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    Join date : 2008-12-05

    Help on Mods  ST-70 VTA/PAS3X Empty Re: Help on Mods ST-70 VTA/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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