The Dynaco Tube Audio Forum

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


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    caferacer59

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    Post by caferacer59 on Fri May 22, 2020 9:02 am

    great forum and i hope to learn a lot. My name is Jerry and I have gotten back into 2 channel HiFI after a long hiatus. Long Story short, i picked up a Dynaco ST-70 and a PAS 3 a couple of weeks ago and know nothing about them really, however, I am pretty thrilled with the sound. I do not have fancy stuff and i was running a Pioneer sx-750 with a pair of KG-4's that I have had forever. Sensitive Speakers and I like them however, they could get fatiguing with the SX-750 and those Horns. My local record store friend sold me the Dynaco stuff very inexpensively because he said they worked but that was about it. Now that being said, here are some dumb questions, the biasing sockets on the front of the amp have been removed, does that mean that inside the amp is a self biasing board ? It sounds good, the balance is nice and tone also, everything on the pass 3 is set to neutral and frankly aren't needed at all. I have just bought a quad of gold lion KT77's and want to pop them in to see what characteristics change. Is there any problem with doing so, I am a little concerned that I don't understand if the mod on my st -70 presents any danger of taking the perfectly working el 34's out and popping the new tubes right in.

    I will say that the amp and pre, rolled off the highs on my KG4s perfectly, my sources are an Akai 201d rtr, a music hall 1.5 and a sonus zp-120, all sources sound great.

    I am not a tech, but I am chasing good sound, this stuff is getting me damn close.

    Best regards,

    Jerry
    peterh
    peterh

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    Post by peterh on Fri May 22, 2020 9:50 am

    caferacer59 wrote:great forum and i hope to learn a lot.  My name is Jerry and I have gotten back into 2 channel HiFI after a long hiatus. Long Story short, i picked up a Dynaco ST-70 and a PAS 3 a couple of weeks ago and know nothing about them really, however, I am pretty thrilled with the sound.  I do not have fancy stuff and i was running a Pioneer sx-750 with a pair of KG-4's that I have had forever.  Sensitive Speakers and I like them however, they could get fatiguing with the SX-750 and those Horns.  My local record store friend sold me the Dynaco stuff very inexpensively because he said they worked but that was about it.  Now that being said, here are some dumb questions, the biasing sockets on the front of the amp have been removed, does that mean that inside the amp is a self biasing board ?  It sounds good, the balance is nice and tone also, everything on the pass 3 is set to neutral and frankly aren't needed at all.  I have just bought a quad of gold lion KT77's and want to pop them in to see what characteristics change.  Is there any problem with doing so, I am a little concerned that I don't understand if the mod on my st -70 presents any danger of taking the perfectly working el 34's out and popping the new tubes right in.

    I will say that the amp and pre, rolled off the highs on my KG4s perfectly, my sources are an Akai 201d rtr, a music hall 1.5 and a sonus zp-120, all sources sound great.

    I am not a tech, but I am chasing good sound, this stuff is getting me damn close.  

    Best regards,

    Jerry
    It was a very stupid thing of previous owner to remove the fron octal sockets, it's here
    one measure bias, something you MUST do from time to time, and especially when
    installing new tubes.

    Self biased ? Unlikely. You must open up and compare with the manual.

    Is this stupid owner have removed the sockets, is there any other external measuring points ?
    if not you need to turn the amp on it's side and open it to be able to measure bias on
    the output tube sockets.

    I recommend you get the manual ( can be found at this site ) and start examining
    what the (stupid?) previous owner has done more. It should have been proper documentation
    with the amp that could clear up this issues. If not i suggest you locate a technician  that
    can help you restore the amp.

    Sorry to deliver bad news but i think it's better to know then sat in the dark.


    Last edited by peterh on Fri May 22, 2020 9:52 am; edited 1 time in total (Reason for editing : Spelling)
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    caferacer59

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    Post by caferacer59 on Fri May 22, 2020 9:56 am

    Thanks, I am not sure this is answering my inquiry directly. It sounds very good. I assume if the autobias is working correctly then the octal sockets are superfluous. But again thanks for your response. I have a tech, but it aint broke. Bill Evans Sunday at the village vanguard sounds sublime.
    peterh
    peterh

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    Post by peterh on Fri May 22, 2020 10:18 am

    caferacer59 wrote:Thanks, I am not sure this is answering my inquiry directly.  It sounds very good.  I assume if the autobias is working correctly then the octal sockets are superfluous.   But again thanks for your response.  I have a tech, but it aint broke.   Bill Evans Sunday at the village vanguard sounds sublime.
    Are you sure it has an autobias card ? Or are you guessing ?
    In that case you should have documentation.
    If there is no autobias there is a danger to destroy both tubes and amp if you ignore bias adjustment. Have it checked asap !
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    caferacer59

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    Post by caferacer59 on Fri May 22, 2020 10:48 am

    I am in the office but I googled some images of autobias cards, and I am pretty sure from memory, it looks similar, but thank you , when I get home i will confirm.
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    Dave_in_Va

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    Post by Dave_in_Va on Fri May 22, 2020 11:08 am

    If you're going to swap in some different power tubes, make sure you clean and re-tension each socket.
    PeterCapo
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    Post by PeterCapo on Fri May 22, 2020 5:42 pm

    Sorry, but I am not clear - what reason do you think there is some kind of auto-bias board installed?  Because the two octal sockets on the face have been removed?  Do you have any other reason to think there is an auto-bias installed?

    I think the best way to establish the configuration of your Stereo 70 is to post a series of photographs showing the things you are talking about, as well as the rest of the top side and also the inside underneath with the bottom cover removed. We'll look forward to seeing your photos.
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    caferacer59

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    Post by caferacer59 on Fri May 22, 2020 6:07 pm

    Here are a couple pics
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    caferacer59

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    Post by caferacer59 on Fri May 22, 2020 6:09 pm

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    PeterCapo
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    Post by PeterCapo on Fri May 22, 2020 6:55 pm

    More photos might be helpful.  The original bias pots are gone, though there appear to be [what may be] two miniature trim-pots on the PC board that might be used for biasing or, perhaps more likely, for balancing the phase inverter with the aid of lab equipment.

    I can tell you that this is not a Dynaco PC board; it no longer has the original Dynaco driver circuit.  This is a modified amplifier, and without pertinent documentation there isn't much I can say about it.  Without more photos and without documentation, I don't know if it has some kind of autobias circuit or if it needs manual biasing.  Autobiasing circuits for the Stereo 70 are pretty rare, though.  Maybe someone else will recognize the board and offer some comments.
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    caferacer59

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    Post by caferacer59 on Fri May 22, 2020 8:12 pm

    Thanks. I’ll get more pics. All I can say is I have had it for a month and under heavy use it sounds really nice. A big step up from my ss amp and it was no slouch either.
    PeterCapo
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    Post by PeterCapo on Fri May 22, 2020 8:18 pm

    Tubes are great.
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    caferacer59

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    Post by caferacer59 on Fri May 22, 2020 8:20 pm

    Especially with my kg4s. My buddies were very impressed with the sound stage and detail. I’m just not sure what I got here
    PeterCapo
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    Post by PeterCapo on Fri May 22, 2020 9:07 pm

    Here's a copy of the original Dynaco Stereo 70 manual. There should be enough here for you to notice some differences: https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf
    peterh
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    Post by peterh on Sat May 23, 2020 4:39 am

    caferacer59 wrote:Here are a couple pics
    Any autobias board is located inside, you have to open the bottom to show this.

    What we have seen so far shows that this is an ex-dynaco , it has very little in
    common with a dynaco amp. Unless you have got proper documentation you are on
    your own here. Sorry.
    You could contact the seller and ask for documentation.

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    Post by PeterCapo Yesterday at 3:49 pm

    Sometimes a name, in the same foil used for the circuit traces, is visible right on the bottom side of the PC board. If you can get a name off of the bottom side of the board, you'll have something to Google.

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