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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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    ST70 el34 bias low

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    directmetalmonster


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    Post by directmetalmonster Sat Oct 23, 2021 3:34 pm

    Hey everyone,

    First time posting, also a first time dynaco user. I am a noob from a tech/electrical standpoint for transparency haha. This is my first dynaco st 70 which was built brand new by Kenny Russell from Funtastic Vintronics. It's a beautiful piece. Anyway, I have been running the amp and it has been performing beautifully since I received it back in April of this year. I decided to do some tube rolling and installed a matched quad of mullard reissue el34's purchased from upscale audio. When I went to bias I noticed the reading was quite low at .40v and was told by the builder that it should be adjusted to between 1.4-1.56 vdc for each side but the bias pot maxes out at 1.2 vdc both sides. I take it thar this is not normal? I understand some tubes have different biases but kinda at a loss of understanding tbh. Hopefully some more experienced minds can shed some insights?

    Thanks in advance

    Justin
    Bob Latino
    Bob Latino
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    Post by Bob Latino Sat Oct 23, 2021 6:46 pm

    directmetalmonster wrote:Hey everyone,

    First time posting, also a first time dynaco user. I am a noob from a tech/electrical standpoint for transparency haha. This is my first dynaco st 70 which was built brand new by Kenny Russell from Funtastic Vintronics. It's a beautiful piece. Anyway, I have been running the amp and it has been performing beautifully since I received it back in April of this year. I decided to do some tube rolling and installed a matched quad of mullard reissue el34's purchased from upscale audio. When I went to bias I noticed the reading was quite low at .40v and was told by the builder that it should be adjusted to between 1.4-1.56 vdc for each side but the bias pot maxes out at 1.2 vdc both sides. I take it thar this is not normal? I understand some tubes have different biases but kinda at a loss of understanding tbh. Hopefully some more experienced minds can shed some insights?

    Thanks in advance

    Justin

    Justin,

    Post a photo of the inside wiring of your amp so we can be sure how the amp is wired. Check the sticky above on "How to Post a Photo" on the is forum. The photo must be hosted on another web site. Postimages.org is a good site to host your photos.

    Bob

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    directmetalmonster


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    Post by directmetalmonster Sat Oct 23, 2021 7:13 pm

    Ok Bob, thanks for the reply. I have inserted these 2 photos in this post, one is of the entire bottom wiring along with a close up of the bias pots if that helps.


    https://postimg.cc/gallery/r8WkTkm
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    directmetalmonster


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    Post by directmetalmonster Mon Oct 25, 2021 11:45 am

    Hey Bob,

    Were you able to take a look at the wiring photos?

    Thanks!
    Bob Latino
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    Post by Bob Latino Mon Oct 25, 2021 2:18 pm

    directmetalmonster wrote:Hey Bob,

    Were you able to take a look at the wiring photos?

    Thanks!

    I looked at the photos and saw that the selenium rectifier had been replaced with a diode. A diode has more "forward voltage" than a selenium rectifier. As such, the two 10K resistors on the seven lug terminal strip should be replaced with about 5K resistors. This should allow you to get the bias within the proper range of 1.25 to 1.56 VDC on each of the two pairs of output tubes.

    Bob


    ST70 el34 bias low 685373-BC-8-ACF-4-A6-A-9-B97-E9885-B61-D90-E

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    directmetalmonster


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    Post by directmetalmonster Mon Oct 25, 2021 2:30 pm

    Thank You for your insight Bob. Would these ( the two gray resistors with stripes in between the bigger blue capacitors be considered the "bias resistors" then?

    I must also mention that the original NessTone el34's were able to be biased at 1.4vdc with room to increase voltage when the amp arrived 7 months ago. I presumed the low bias rating I took a few days ago was because they had become weak and could no longer be biased at that same 1.4vdc reading, would this be a correct assumption? I definitely put more than 1000 hours on the tubes.

    As I have the mullards currently biased somewhat "cold" at less than 1vdc per pair, just wanted to know if it's still safe to continue to do so if I don't end up having a tech change those resistors out in the immediate future. As it sounds quite nice and dynamic, I believe this will help to prolong the life of the tubes even more so?

    Cheers,
    Justin
    Bob Latino
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    Post by Bob Latino Mon Oct 25, 2021 4:54 pm

    Hi Justin,

    Yes .. Those gray resistors are the two resistors to change out. Try two 5K three watt in place of those two 10K 3 watt resistors and see if that gets you to the 1.40 - 1.56 VDC range in your bias measurement ? See red circle around those two resistors below.

    Those two output tube pairs should be biased at about 1.25 VDC for each pair. This will give a bias current of about 40 milliamps for each output tube.

    Bob

    ST70 el34 bias low 685373-BC-8-ACF-4-A6-A-9-B97-E9885-B61-D90-E-RED

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    tubes4hifi
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    Post by tubes4hifi Wed Oct 27, 2021 12:27 pm

    looks like a very nice build, but at first view I didn't see any cathode resistors to measure the bias, but now I see a black resistor on each output pair.
    That should be a 15 ohm resistor, you may want to check those and make sure. If so, then as Bob said, set bias about 1.25vdc per channel.
    Dynaco ran the bias way too high on those amps and the tubes wear out faster at higher bias.

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    directmetalmonster


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    Post by directmetalmonster Wed Oct 27, 2021 1:36 pm

    tubes4hifi wrote:looks like a very nice build, but at first view I didn't see any cathode resistors to measure the bias, but now I see a black resistor on each output pair.
    That should be a 15 ohm resistor, you may want to check those and make sure.   If so, then as Bob said, set bias about 1.25vdc per channel.
    Dynaco ran the bias way too high on those amps and the tubes wear out faster at higher bias.

    Yeah those are black 15.6ohm resistors. Right now the bias can only max out at .93vdc per channel with these mullard reissues el34's and my nos mullard blackburn rectifier. However when I had my gold lion rectifier in, it would max out at 1.2vdc per pair.

    I also did hear some very light buzzing coming from the amp itself, not from the speakers. You would only hear it if you stuck your ear right over the amp. Can't tell if it's from the transformer or what.

    tubes4hifi
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    Post by tubes4hifi Thu Oct 28, 2021 12:47 pm

    so likely the old rectifier is worn out, since you can get higher bias with the newer one.
    The buzz is probably mechanical vibration of the power transformer, you can buy some rubber bushings or grommets and place those between the transformer and the chassis.

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