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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


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    Vintage VTA Deluxe Stereo 70 Questions

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    pickindoug


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    Join date : 2022-12-04

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    Post by pickindoug Sun Dec 04, 2022 10:08 am

    Hello all, I recently picked up 3 modded Stereo 70.  All were mods from the 1980-1990 era.  One is a Sutherland, one a GSI (with 8417 in triode mode) and the topic of the moment, a VTA with the deluxe board from 1990.  Having some trouble getting this unit to operate correctly.  The previous owner and modder added a 1K 10 watt resistor between the rectifier tube and the 470 uf cap stack.  I don't have a proper schematic, but it appears he was trying to limit surge on startup, as 235 uf would definitely hurt a GZ34.  B+ at pins 3/4 of the EL34 is only 390, and with the bias scheme as he set it up, I barely got to 40 mv across the 10 ohm resistors.  He had the bias resistors at 12k - bias pot - 30k - ground.  Was measuring -42v on the lowest setting of the bias pot at pin 5 of the EL34.  Changed the 30k to 15k and was able to get the reading across the cathode resistor to about 100 mv, and the voltage at pin 5 was down to ~ -29.  Put a 22k in place of the 12k to reposition the voltage divider and while it got better, I was still in the 200 mv range for max bias, and began to notice a slight burning smell.  With each bias attempt I also checked a 1kHz sine wave and in each case the sine wave was stepped mid wave.  Max power at clipping was anemic at best.  I suspect the prior owner took this amp on an unfinished journey.  I don't have any documentation to help me return this to the intended state of mod, so I would be grateful for any comments from the group.  Have tried several times to load a pic of the chassis.  Hopefully it displays.  If not, I will try to add. https://i.servimg.com/u/f74/20/46/71/27/pxl_2011.jpg


    Last edited by pickindoug on Sun Dec 04, 2022 10:10 am; edited 1 time in total (Reason for editing : adding photo)
    peterh
    peterh


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    Location : gothenburg, sweden

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    Post by peterh Sun Dec 04, 2022 11:05 am

    pickindoug wrote:Hello all, I recently picked up 3 modded Stereo 70.  All were mods from the 1980-1990 era.  One is a Sutherland, one a GSI (with 8417 in triode mode) and the topic of the moment, a VTA with the deluxe board from 1990.  Having some trouble getting this unit to operate correctly.  The previous owner and modder added a 1K 10 watt resistor between the rectifier tube and the 470 uf cap stack.  I don't have a proper schematic, but it appears he was trying to limit surge on startup, as 235 uf would definitely hurt a GZ34.  B+ at pins 3/4 of the EL34 is only 390, and with the bias scheme as he set it up, I barely got to 40 mv across the 10 ohm resistors.  He had the bias resistors at 12k - bias pot - 30k - ground.  Was measuring -42v on the lowest setting of the bias pot at pin 5 of the EL34.  Changed the 30k to 15k and was able to get the reading across the cathode resistor to about 100 mv, and the voltage at pin 5 was down to ~ -29.  Put a 22k in place of the 12k to reposition the voltage divider and while it got better, I was still in the 200 mv range for max bias, and began to notice a slight burning smell.  With each bias attempt I also checked a 1kHz sine wave and in each case the sine wave was stepped mid wave.  Max power at clipping was anemic at best.  I suspect the prior owner took this amp on an unfinished journey.  I don't have any documentation to help me return this to the intended state of mod, so I would be grateful for any comments from the group.  Have tried several times to load a pic of the chassis.  Hopefully it displays.  If not, I will try to add.  https://i.servimg.com/u/f74/20/46/71/27/pxl_2011.jpg
    If this was mine i would scratch everything except for the transformers and build myself
    an Dynaco ST70 . Dynapartskit.com has a suitable circuit board. Trying to resurrect an unknown
    lacking documentation and still try to get the performance of an original is very hard.

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    pickindoug


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    Post by pickindoug Sun Dec 04, 2022 11:15 am

    Appreciate your thoughts. Since I have 4 stereo 70s, one factory original and the 3 modded versions, I'd like to make each one correct based on the intent at the time, then evaluate. For example, the Sutherland version was far better than I expected. The GSI is on the bench now. I can always return them to stock or try a different mod. It's just money and time.
    tubes4hifi
    tubes4hifi
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    Post by tubes4hifi Fri Dec 09, 2022 4:26 pm

    for the VTA70 amp, refer to the wiring diagram
    https://www.tubes4hifi.com/VTA70wiringdiagram3.jpg
    the cap on the output of the rectifier tube should be no larger than 40uF MAX.
    Then thru the choke (which is about 50 ohms) and then at least 100uF but not more than 400uF MAX.
    ( our upgraded quad cap is 40-80-30-20uF )
    The bias pots on the board (not the original Dyna ST70) are 1K-50Kpot-27K to ground.
    That gives a normal bias range of around -30 to -60vdc. Bias should be set to 40-50ma which is 0.40 to 0.50vdc across ten ohms from cathode to ground.

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    pickindoug


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    Post by pickindoug Fri Dec 09, 2022 5:44 pm

    Thanks for the diagram. It confirms my suspicion regarding the errant power supply mod.
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    pickindoug


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    Post by pickindoug Sat Dec 17, 2022 6:08 pm

    Thanks admin - I'm back into this amp now, and realize the 90's era VTA board I have is a deluxe version with high value filter caps mounted instead of the original can as shown in the link you sent. Documentation is sketchy. What I have here shows the GZ34 thru a 50ohm resistor into 235 uf (2-470 in series). No way that was going to work, but that's what the sketch with the original VTA mod board shows. Perhaps they intended to use a plug in SS rect, but it's not on the materials list.

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    MechEngVic
    MechEngVic


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    Post by MechEngVic Sun Jan 01, 2023 3:17 am

    Interesting! Keep us posted on your progress!

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    pickindoug


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    Post by pickindoug Mon Jan 02, 2023 4:29 pm

    Final comments on this one. I left the high value filter stack in place - 235 uf - choke - 100 uf, and fronted that with a double stack of 1N4007 diodes into the 50 ohm resistor. Added a CL 80 on the AC line to slow things down a little, dialed in the bias, and it sounded quite good. The square wave performance was quite good on the bench, and the listening tests left me with the notion that this amp will be in rotation for a while. I start with initial impressions, but it takes me weeks to listen to enough music to really evaluate an amp. Appreciate the input from the group.

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