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    Too High Bias ST-70

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    Kennybugs

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    Post by Kennybugs on Sun May 31, 2020 5:44 pm

    Hi, just joined this forum. This is my first post. I just bought a ST-70 off the bay. It is a later model with Z326 output  transformers and smaller or shorter power transformer. It looks all original right down to the original diode (which I need to replace).

    I bought it with no tubes. I have two RCA black plate 7199's NOS, Mullard 5AR4/GZ34 and 4 new Tung Sol EL34B's.

    Hooked up a set of speakers to it. Slowly brought it up on a Variac and checked the power transformer voltages with no tubes in it and it checked fine at 117 volts. I then brought it up on a Variac with the rectifier tube in and starting at 50 volts and then 70, then 90 and stopped at 117 volts. Waiting about 15 minutes each step. All seemed fine. Turned it off and pulled the rectifier tube and put the rest of the tubes in and checked the filament heater voltages again. Checked fine. Put the rectifier tube in and turned it back on. Measured all the voltages as per the build manual and they all measured good.

    I set the bias to 1.56 on both sides. A little bit of jumping around but seemed fine. Hooked it up to a PAS-3 and FM-3 I have had for like 15 years. It didn't sound the best at first but after a couple hours started to sound better. The next day it sounded like the day before when I first fired it up, but not too long after started sounding good. The tubes are burning in and the amp is starting to sound really good.

    I checked the bias again after a few hours running and it was 1.91 on one side and 1.93 on the other side. OK, just adjust the bias again. But it won't go below 1.89 on either side. But the bias is rock stable. I suspect the new tubes are drawing too much current. That sound right?

    How should I go about fixing the problem? I want to leave this as stock as possible. I thought when I fix the problem I would add the diode then. Can I change the 15.6 ohm resistors for 10 ohm to fix the problem. Then would I have to set bias to 1.00?

    Or should I get a set of output tubes with lower amp draw?

    Thank you, Kenny
    peterh
    peterh

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    Post by peterh on Sun May 31, 2020 6:00 pm

    Kennybugs wrote:Hi, just joined this forum. This is my first post. I just bought a ST-70 off the bay. It is a later model with Z326 output  transformers and smaller or shorter power transformer. It looks all original right down to the original diode (which I need to replace).

    I bought it with no tubes. I have two RCA black plate 7199's NOS, Mullard 5AR4/GZ34 and 4 new Tung Sol EL34B's.

    Hooked up a set of speakers to it. Slowly brought it up on a Variac and checked the power transformer voltages with no tubes in it and it checked fine at 117 volts. I then brought it up on a Variac with the rectifier tube in and starting at 50 volts and then 70, then 90 and stopped at 117 volts. Waiting about 15 minutes each step. All seemed fine. Turned it off and pulled the rectifier tube and put the rest of the tubes in and checked the filament heater voltages again. Checked fine. Put the rectifier tube in and turned it back on. Measured all the voltages as per the build manual and they all measured good.

    I set the bias to 1.56 on both sides. A little bit of jumping around but seemed fine. Hooked it up to a PAS-3 and FM-3 I have had for like 15 years. It didn't sound the best at first but after a couple hours started to sound better. The next day it sounded like the day before when I first fired it up, but not too long after started sounding good. The tubes are burning in and the amp is starting to sound really good.

    I checked the bias again after a few hours running and it was 1.91 on one side and 1.93 on the other side. OK, just adjust the bias again. But it won't go below 1.89 on either side. But the bias is rock stable. I suspect the new tubes are drawing too much current. That sound right?

    How should I go about fixing the problem? I want to leave this as stock as possible. I thought when I fix the problem I would add the diode then. Can I change the 15.6 ohm resistors for 10 ohm to fix the problem. Then would I have to set bias to 1.00?

    Or should I get a set of output tubes with lower amp draw?

    Thank you, Kenny

    I suggest you replace the Se (bias rectifier).
    The simplest way is to mount an 1n4007 diode on the Se tabs ( leave the
    Se as solder stand it won't do any harm for now).

    You might even get the too little bias with the 1n4007 diode, in that case you
    need to adjust the 10K resistor ( the one between the 2 50 uF caps in the bias
    circuit) to 15k

    The above gives you a quick fix, the permanent solution is to
    get a bias kit from dynakitparts.com it will include all needed parts.
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    Big Harry

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    Post by Big Harry on Sun May 31, 2020 6:01 pm

    I had the same problems with my two ST70's before I replaced the driver boards with VTA boards a couple of years ago. On one amp I replaced the output tubes which solved the problem and on the other I replaced the selenium rectifier in the bias circuit with a 1N4007 silicon diode and I also replaced the caps in the bias which fixed that amp. If the amp hasn't been recapped you might want to consider doing that as that can eliminate problems later on.


    Last edited by Big Harry on Sun May 31, 2020 6:03 pm; edited 1 time in total (Reason for editing : words left out)
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    Kennybugs

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    Post by Kennybugs on Sun May 31, 2020 9:53 pm

    Thank you Peterh and Big Harry. It looks like I may just rebuild the whole amp and also upgrade it. I just got done rebuilding two Heathkit W5M's. They turned out good. I put a bucking transformer in each one inside the chassis and also put in a high voltage delay board made for a Dynaco ST-70. Put in all new resistors and capacitors. Had to buy new output transformers because one measured high resistance. I used Classic Tone 40-18080 Hi Fi style transformers. Put in RCA 12au7's black plates, RCA 5R4 rectifier's and Tung Sol 7581A output tubes. They sound good. But the ST-70 sounds darn good also even old and stock.  I remember a guy trying to sell me one back in the mid 70's. I really liked it, but didn't buy it.

    Thank you both, Kenny
    PeterCapo
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    Post by PeterCapo on Sun May 31, 2020 10:07 pm

    If you want to completely rebuild them, you can.  But IMO it isn't necessarily necessary.  I second the suggestion to get the bias rebuild kit from dynakitparts.com https://www.dynakitparts.com/shop/st-70-bias-circuit-kit/

    Also, dirty or loose contacts in the five octal sockets are notorious for causing problems.  Suggest cleaning and retensioning them, especially contacts #5 of the four EL34 sockets, which is where the bias voltage is applied.  99% isopropyl, interdental brushes and compressed air do a good job of cleaning them.  Retensioning them can be accomplished with a small flat blade screwdriver or small awl-like tool.
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    Kennybugs

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    Post by Kennybugs on Sun May 31, 2020 11:11 pm

    PeterCapo, I will clean the sockets and tighten them like you suggest. If that doesn't work I will change out the resistor like Peterh suggests. And if it still doesn't work change out the caps like Big Harry said. Then decide if I want to rebuild or upgrade it. But it will be a little while before I can do that. It just sounds so nice now. And is quiet. And has good solid bass and pretty nice highs. But man that power transformer sure gets hot. But I think it is OK and isn't getting too hot.

    Kenny
    PeterCapo
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    Post by PeterCapo on Sun May 31, 2020 11:17 pm

    Normal for it to get really hot. As long as you use the 3A fuse, it should be fine.

    For a general checkup, I'd suggest making use of the reference voltages listed on the same page as the power supply schematic, in the original manual, on the page immediately before page 6: https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf
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    Kennybugs

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    Post by Kennybugs on Sun May 31, 2020 11:39 pm


    That's the page and test points I used. It measured real close to those voltages.
    peterh
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    Post by peterh on Mon Jun 01, 2020 2:21 am

    Kennybugs wrote:Thank you Peterh and Big Harry. It looks like I may just rebuild the whole amp and also upgrade it. I just got done rebuilding two Heathkit W5M's. They turned out good. I put a bucking transformer in each one inside the chassis and also put in a high voltage delay board made for a Dynaco ST-70. Put in all new resistors and capacitors. Had to buy new output transformers because one measured high resistance. I used Classic Tone 40-18080 Hi Fi style transformers. Put in RCA 12au7's black plates, RCA 5R4 rectifier's and Tung Sol 7581A output tubes. They sound good. But the ST-70 sounds darn good also even old and stock.  I remember a guy trying to sell me one back in the mid 70's. I really liked it, but didn't buy it.

    Thank you both, Kenny
    No! It's not time for any large scale "rebuild". It's time to fix the
    problem you have, which seems to be a insufficient bias.

    When this is repaired, use it for a while so you know it's working ok. Then
    it's up to you if you want to use the solder iron.
    peterh
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    Post by peterh on Mon Jun 01, 2020 2:24 am

    Kennybugs wrote:PeterCapo, I will clean the sockets and tighten them like you suggest. If that doesn't work I will change out the resistor like Peterh suggests. And if it still doesn't work change out the caps like Big Harry said. Then decide if I want to rebuild or upgrade it. But it will be a little while before I can do that. It just sounds so nice now. And is quiet. And has good solid bass and pretty nice highs. But man that power transformer sure gets hot. But I think it is OK and isn't getting too hot.

    Kenny

    No again, you should replace the Se rectifier , replace with a Si diode, an
    1n4007 will do just fine. Then you _might_ need to change a resistor to
    be able have the bias adjustment range suit all tubes.

    Don't replace items at random , you risk to get into a morass of problems.
    peterh
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    Post by peterh on Mon Jun 01, 2020 2:27 am

    Kennybugs wrote:
    That's the page and test points I used. It measured real close to those voltages.

    Unfortently there is no figures of the bias (-) voltage. The weak point is the
    rectifier itself, they tend to go "blank" when they fail. Parallelling with a
    Si diode will show if this is your problem.

    Why is the bias voltage omitted ? Noone knows, but this might have been
    considered a "fail-proof area" where no problem is expected ever. I do not
    have a working st-70 thus i cannot tip of what voltage to expect.
    PeterCapo
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    Post by PeterCapo on Mon Jun 01, 2020 6:50 am

    That's good advice from peterh.

    The bias voltage shows up in the chart as -32 VDC on any EL34 socket pins 5 and 6, no?  Below the chart, it also shows reference voltages of 50 VAC on bias rectifier cathode and -65 VDC on bias rectifier anode.  All are referenced to 117 VAC across PT primary.  With a 120 VAC PT, they would be referenced to 120 VAC across PT primary.
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    Post by Kennybugs on Mon Jun 01, 2020 7:15 am

    OK, I'll replace the diode first. I was going to replace one part at a time. It's just I have the resistor so I was going to do it first, but I'll get the diode and do it first.

    Once I get it running right then I'll play it for awhile. Probably just leave it like it is then.

    Kenny
    PeterCapo
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    Post by PeterCapo on Mon Jun 01, 2020 7:17 am

    Hey, if it ain't broke, don't fix it Wink
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    Big Harry

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    Post by Big Harry on Mon Jun 01, 2020 8:31 am

    Both of my ST70's sounded good before I replaced the driver boards. My issue with the original boards was the 7199 tubes and their scarcity. I have a good supply of 12AU7's so the change to the VTA boards made sense to me and their cost is reasonable. I still recommend replacing the caps and any out of tolerance resistors on the original boards as the amps are now somewhere close to 50 years old and if they haven't reached the end of their service life, they will in the near future.
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    Post by PeterCapo on Mon Jun 01, 2020 8:39 am

    Big Harry wrote:Both of my ST70's sounded good before I replaced the driver boards. My issue with the original boards was the 7199 tubes and their scarcity. I have a good supply of 12AU7's so the change to the VTA boards made sense to me and their cost is reasonable. I still recommend replacing the caps and any out of tolerance resistors on the original boards as the amps are now somewhere close to 50 years old and if they haven't reached the end of their service life, they will in the near future.  

    For anyone who may not be aware, adapters are available to substitute the inexpensive and readily available 6GH8A and also 6U8A in place of the 7199 https://www.dynakitparts.com/shop/9-pin-socket-adapter/

    The original carbon composition resistors could be okay.  The reason why they are sometimes said to be out-of-tolerance is because they are hygroscopic, which probably contributes to de-soldering heat throwing them out of tolerance.  Leaving them alone may be the best approach.  The key is the reference voltage values found in the manual.  As long as you're close to those values, and if the bias is reasonably stable, and if it sounds good without hissing or humming or other strange noises, then it should be all set.


    Last edited by PeterCapo on Wed Jun 03, 2020 10:21 pm; edited 1 time in total (Reason for editing : Clarification)
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    Post by peterh on Mon Jun 01, 2020 9:22 am

    Big Harry wrote:Both of my ST70's sounded good before I replaced the driver boards. My issue with the original boards was the 7199 tubes and their scarcity. I have a good supply of 12AU7's so the change to the VTA boards made sense to me and their cost is reasonable. I still recommend replacing the caps and any out of tolerance resistors on the original boards as the amps are now somewhere close to 50 years old and if they haven't reached the end of their service life, they will in the near future.  
    The original caps and resistors have shown their reliability and will not need
    replacement unless one of them is found broken.
    Most Resistors can be measured in place ,like most caps. None needs to be
    removed for sanity check.

    Most of the trouble st70 owners get comes from wild replacements and improvements !

    Things that need change is EL34 and 5AR4, they are "burned out" after a few
    1000h and should be replaced.

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    Post by Kennybugs on Mon Jun 01, 2020 11:29 am

    Thanks for all the tips and help. Will let everybody know what was bad or needed changed and how it goes when I get the parts and fix it.
    tubes4hifi
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    Post by tubes4hifi on Wed Jun 03, 2020 8:49 pm

    the bias value has absolutely nothing to do with the driver board (why has nobody posted this yet?)
    First, replace the selenium diode with a new 1N4007 that costs 10c (well, might be $8 with shipping).
    Then, with NO tubes in the amp, turn on and see if you can adjust the bias pots to read from around -25 to -50vdc on each output tube pin 5.
    If that works, the bias voltage and the bias pots are good, if not, well replace the bias pots, or better yet, buy the VTA70 board
    and fix ALL of the problems instantly and have amp that sounds 10X as good as anything using a 7199 tube.
    Likely the final problem is the output tubes are worn out.
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    Post by Kennybugs on Wed Jun 03, 2020 9:21 pm

    Thanks for your reply. The tubes are new. I have diodes coming. I also bought some CL-80 surgistors. I did replace the 10k resistor with a 15k resistor but the bias went up. So I paralleled the 10k and 15k to make 6k and that worked. But i want to replace the diode and see if that fixes the problem with the 10k in place.

    I will do what you suggest when I get the diode and measure the voltage at pin 5.

    Kenny
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    Post by PeterCapo on Wed Jun 03, 2020 10:10 pm

    tubes4hifi wrote:the bias value has absolutely nothing to do with the driver board (why has nobody posted this yet?) ...
    While I am not quite sure what you might be thinking of here, I believe some parts on the driver board could affect the bias.  For instance, the coupling caps could be leaking.  Also, there are the 270KΩ resistor pairs at E6 and E21 where the bias voltage is applied to the PC board on its way to the EL34s.  If one or more of these decades old, hygroscopic carbon comp resistors are unstable, then you've got another possible source of a bias problem from the PC board.
    tubes4hifi wrote: … If that works, the bias voltage and the bias pots are good, if not, well replace the bias pots, or better yet, buy the VTA70 board and fix ALL of the problems instantly and have amp that sounds 10X as good as anything using a 7199 tube. …
    The pride you take in your work is appreciated, Roy.  But if what you say is true, then your circuit must also sound 10X better than Cary tube amps, McIntosh tube amps, and Parasound Halo amps – because I have had them all here and have been impressed with how well my Stereo 70 (original Dynaco topology) kept up with them.  Of course, personal sonic preference is subjective and as a principle must be applied equitably.
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    Post by tubes4hifi on Wed Jun 03, 2020 11:37 pm

    Peter,
    yes, and I know you haven't ever bought any VTA products so you don't have a clue how good they are,
    unlike 90% of the other people who are on this forum
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    Post by PeterCapo on Wed Jun 03, 2020 11:58 pm

    tubes4hifi wrote:Peter,
    yes, and I know you haven't ever bought any VTA products so you don't have a clue how good they are,
    unlike 90% of the other people who are on this forum

    How good they are, as in 10X better than any of the other amps I mentioned? Do I need to buy a VTA product to form an opinion as to the likelihood of this, or are my years as an experienced audiophile enough?

    Do you officially publish your complete specs so everyone can see how objectively superior the VTA circuit is to many other well-regarded products, then? Not talking about measurements reported by your customers. I mean your own formal, published statement as to the complete performance specifications of the VTA circuit? Apologies if I missed it, but I don't recall seeing it on your website.

    Actually, I have your cap board in my PAS, and it has been working out fine. I also have a non-functioning VTA Stereo 70 I recently acquired locally but have not yet had time to troubleshoot. But this is kind of beside the point.
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    Post by tubes4hifi on Thu Jun 04, 2020 12:17 am

    published specs are nearly worthless, as most any piece of "hi-fi" gear has specs so good that they way below listening thresholds.
    I'll just say that Bob and I consistently (and this means like every week of every year for the past 15-20 years)
    get emails from customers saying that our equipment is so far superior to others they have owned and heard.
    This includes McIntosh, Audio Research, Cary, Pass Labs, and others, most of which cost 3-10x as much for equivalent gear.
    I'm not talking about joe-blow customers with a $2000 system, I'm talking about customers with $50K+ systems.
    Good enough for me and our customers !!
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    Post by PeterCapo on Thu Jun 04, 2020 12:44 am

    tubes4hifi wrote:published specs are nearly worthless, as most any piece of "hi-fi" gear has specs so good that they way below listening thresholds.
    I'll just say that Bob and I consistently (and this means like every week of every year for the past 15-20 years)
    get emails from customers saying that our equipment is so far superior to others they have owned and heard.
    This includes McIntosh, Audio Research, Cary, Pass Labs, and others, most of which cost 3-10x as much for equivalent gear.
    I'm not talking about joe-blow customers with a $2000 system, I'm talking about customers with $50K+ systems.
    Good enough for me and our customers !!

    Well, I'll have to respectfully disagree with your statement that formally published specs are nearly worthless.  Performance specifications, verified by disinterested independent lab tests, have long been an integral part of knowing what you're getting - a transparent assurance for those who prefer to be informed consumers.  Apart from this, it's pretty hard to know how to respond to the rather extreme claims you have made over time.

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