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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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    ST70 el34 bias low

    directmetalmonster
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    directmetalmonster


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    • Post n°1

    ST70 el34 bias low Empty ST70 el34 bias low

    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
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    Bob Latino
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    • Post n°2

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

    Hey Bob,

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

    Thanks!
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    • Post n°5

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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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    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
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    Bob Latino
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    • Post n°7

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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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    Roy Mottram
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    Post by Roy Mottram 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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    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.

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    Post by Roy Mottram 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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    Post by directmetalmonster Sat Dec 09, 2023 3:23 am

    Hey everyone,

    It's been a minute since I last posted but wanted to give an update. I recently took the st70 to a very reputable tech here in NYC and discovered the culprit for the low bias was a bad can capacitor made by CE sold through dynakit parts. It was probably part of a bad batch but my tech said the quality is very low. As the quality of caps used are high esr with a low rating of 2k hours. He left the original can in for aesthetics but bypassed it underneath with a terminal strip connecting 3 (12k rated) Nichicon low esr 500v 22uF caps and a 47uF 600V cap. This solved the bias issue as the bad can cap was causing a major leakage of b+. It also solved the hum/buzz issue too.

    https://i.postimg.cc/hj0kCQnS/IMG-1469.jpg

    https://i.postimg.cc/NMYM9XbY/RNI-Films-IMG-4-A2-E0-AFC-E550-4892-9-B53-4-EDBFD65-D060.jpg

    I currently have a fresh pair of EH 6CA7s running stable at 1.2V per side.

    Another question, perhaps Bob would be able to shed some light, but as my dynaco is not a VTA model, it does utilize the new PA-060 Transformer from triode. Would it be possible to use a GZ33 or GZ37 rectifier in place of the gz34? I was looking at purchasing either purely based on aesthetics and sound quality. But have heard it could cause a drop in voltage and that it would draw more current on start up.

    If anyone could give a definitive yes or no that would be greatly appreciated.

    cheers!

    Justin
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    • Post n°12

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    Post by Bob Latino Sat Dec 09, 2023 10:23 am

    directmetalmonster wrote:

    Another question, perhaps Bob would be able to shed some light, but as my dynaco is not a VTA model, it does utilize the new PA-060 Transformer from triode. Would it be possible to use a GZ33 or GZ37 rectifier in place of the gz34? I was looking at purchasing either purely based on aesthetics and sound quality. But have heard it could cause a drop in voltage and that it would draw more current on start up.

    If anyone could give a definitive yes or no that would be greatly appreciated.

    cheers!

    Justin

    Hi Justin,

    Yes - Since your amp has an upgraded power transformer, you could use a GZ33 or GZ37 as the rectifier in your amp. Both of these tubes were made by Mullard even though they are usually in a white box and have a "Valve Electronic" marking on the top flap of the box. Sometimes a GZ33 will say CV5745 on the top of the box instead of GZ33. The only downside to these tubes is the price. The going rate for one is now is usually in the $100 to $150 range.

    Bob

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    Post by directmetalmonster Sat Dec 09, 2023 12:43 pm

    Bob Latino wrote:
    directmetalmonster wrote:

    Another question, perhaps Bob would be able to shed some light, but as my dynaco is not a VTA model, it does utilize the new PA-060 Transformer from triode. Would it be possible to use a GZ33 or GZ37 rectifier in place of the gz34? I was looking at purchasing either purely based on aesthetics and sound quality. But have heard it could cause a drop in voltage and that it would draw more current on start up.

    If anyone could give a definitive yes or no that would be greatly appreciated.

    cheers!

    Justin

    Hi Justin,

    Yes - Since your amp has an upgraded power transformer, you could use a GZ33 or GZ37 as the rectifier in your amp. Both of these tubes were made by Mullard even though they are usually in a white box and have a "Valve Electronic" marking on the top flap of the box. Sometimes a GZ33 will say CV5745 on the top of the box instead of GZ33. The only downside to these tubes is the price. The going rate for one is now is usually in the $100 to $150 range.

    Bob

    Thank you Bob! Would both tubes be a soft start like a gz34 and do both the 33 and 37 drop the same amount of voltage or does one do more than the other?

    thanks again!

    Justin
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    • Post n°14

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    Post by Bob Latino Sat Dec 09, 2023 12:57 pm

    Yes - both the GZ33 and the GZ37 takes about 10 - 15 seconds after turn on to start giving high voltage. I have used both GZ33 and GZ37 myself and both seem about the same in regards to the level of DC high voltage they create.

    Bob

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