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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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    VTA ST-70 problem

    Chris A
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    Chris A


    Posts : 13
    Join date : 2019-10-15
    • Post n°1

    VTA ST-70 problem Empty VTA ST-70 problem

    Post by Chris A Mon Dec 18, 2023 9:55 am

    Hi All,

    A curious problem with my Latino ST-70...

    I switch on and no sound in either channel. All four main tubes are glowing. Of the three tubes on the driver board, the left one is glowing but the middle and the right tubes are not.

    This has happened three times - the first after the amplifier was moved and the latter two after cleaning the amp by using a lightly damp cloth to remove the dust on the case's surface. I was doing my best to avoid touching the tubes, but I suppose it's possible that I'm inadvertently disturbing something, (eg moving a tube slightly). On each occasion, I carefully remove and replace each tube, but this does not bring back the sound. The way I get the sound back is to leave the amp on and the sound returns after an hour or so. On checking the two tubes that weren't working, they both are now warm and glowing.

    Can anyone offer any suggestions as to what's going on here? Could I have a dry joint somewhere? Can anyone help with which connection points I should be looking at that produces the symptoms I'm describing?

    Many thanks.

    A merry Christmas to all and wishing for a peaceful New Year all around the world (fat chance but no harm in wishing).

    Chris A, London.
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°2

    VTA ST-70 problem Empty Re: VTA ST-70 problem

    Post by Bob Latino Mon Dec 18, 2023 1:59 pm

    Chris A wrote:Hi All,

    A curious problem with my Latino ST-70...

    I switch on and no sound in either channel. All four main tubes are glowing. Of the three tubes on the driver board, the left one is glowing but the middle and the right tubes are not.

    This has happened three times - the first after the amplifier was moved and the latter two after cleaning the amp by using a lightly damp cloth to remove the dust on the case's surface. I was doing my best to avoid touching the tubes, but I suppose it's possible that I'm inadvertently disturbing something, (eg moving a tube slightly). On each occasion, I carefully remove and replace each tube, but this does not bring back the sound. The way I get the sound back is to leave the amp on and the sound returns after an hour or so. On checking the two tubes that weren't working, they both are now warm and glowing.

    Can anyone offer any suggestions as to what's going on here? Could I have a dry joint somewhere? Can anyone help with which connection points I should be looking at that produces the symptoms I'm describing?

    Many thanks.

    A merry Christmas to all and wishing for a peaceful New Year all around the world (fat chance but no harm in wishing).

    Chris A, London.

    Hi Chris,

    If the driver tubes finally light up after an hour, it is probably a bad solder connection on the filament lines that feed that channel.

    1. Resolder the two "FIL" eyelets on the driver board on the channel that won't light up until about an hour.

    2. Also resolder pins 2 and 7 on the front output tube socket on the channel that has trouble lighting up the driver tubes. Pins 2 and 7 on the FRONT two output tubes feed the "FIL" eyelets on the driver board.

    See below ..

    Bob

    VTA ST-70 problem ST-70-pictorial-CCS-SIDEWAYS-2-TEMP

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    Chris A
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    Chris A


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    • Post n°3

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    Post by Chris A Tue Dec 19, 2023 7:04 am

    Hi Bob,

    Good to hear from you and many thanks for your help.

    I'll have a go at the amp over the Christmas holidays and will report back.

    Regards,

    Chris
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    Chris A
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    Chris A


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    Post by Chris A Thu Dec 28, 2023 10:57 am

    Hi again All,

    Earlier today I opened the amp up and checked the likeliest locations detailed by Bob as to where this bad joint might be and Pin 8 of V3 (the one that joins Pin 7 to the 10 Ohm bias resistor) was found to be the culprit. How satisfying! So I re-soldered it.

    While I was in there I carefully re-tightened the input phono connectors and the speaker terminals to the chassis (or maybe not as carefully as I thought...).

    When I switched on, no sound. All four output tubes light up, as do the three smaller tubes on the VTA board, but the rectifier tube is dead. I tried another but it made no difference.

    I'm going to have a look back over the posts on this forum to see whether I can get some clues, but if anyone has any ideas on what to test for or where to look that would be much appreciated.

    Regards,

    Chris
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    Chris A
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    Chris A


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    Post by Chris A Fri Dec 29, 2023 8:20 am

    An update to yesterday...

    The rectifier tube is a Mullard GZ33. I have two spares, both new. On trying the spares the result was the same: the tube didn't light up and there was no sound. On substituting for a Sovtek 5AR4, that tube lit up and the sound returned. So on the plus side, I have a working amp again.

    However, I can't understand why three GZ33s don't work and the 5AR4 does. I don't believe all three GZ33s have failed. Does anyone have any idea of what's going on here?
    Any help appreciated.

    Regards,

    Chris
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    Cubdriver
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    • Post n°6

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    Post by Cubdriver Mon Jan 01, 2024 8:31 pm

    Chris -

    The GZ33 and 5AR4 have the same pinout, so that isn't the problem.  I gather that the amplifier was working using the GZ33 previously?  It does seem strange that three GZ33s aren't working, but the 5AR4 is.

    I'd first check the GZ33 filaments for continuity using an ohmmeter - they are between pins 2 and 8 on the tube.  When viewed from the bottom (pins facing you) with the key at 12:00, the pins are numbered clockwise from the key - pin 2 will be the second pin to the right of the key, and pin 8 will be the one immediately to the left of the key.  The filaments should be relatively low resistance - if the read open the tubes are bad.

    Secondly, are the GZ33 pins thinner than those of the 5AR4?  Are they clean, and not oxidized?  If they're not making good contact with the terminals of the socket, the tube may not heat properly.  I see quoted heater draws of between 2.8 and 3A for the GZ33, and only 1.9A for the 5AR4, so it'll be more sensitive to a poor connection.

    -Pat

    Edit to add - check the solder connections at the rectifier socket, too - one or more of those making a poor connection could be the culprit, too.
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    Chris A
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    Chris A


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    Post by Chris A Wed Jan 03, 2024 9:03 am

    Hi Cubdriver,

    Many thanks for your reply. I will carry out checks as per your suggestions.

    Regards,

    Chris
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