Original Dynaco st70 High bias one side

Hi all- I started dissecting the amp to try to lower the noise floor, was slight hum in both channel speakers.. Tubes used to bias fine. It appears I did something or some coincidence occurred...

While working on this i changed out the selenium for a diode haven't changed resistors yet didn't want to add more variables to problem.


One side bias's fine- .. Other side shoot up to close to 3v dc very quick and im qwuick to turn it off... can't get it lower.. Tried swapping tube sides problem stay on the same side so I know its not the tubes..

Rectifier 425 v on both tags that are suppose to have dc

Cap can
square 275
Triangle 350
Unmarked 420
Semi circle 425

Getting -40v on pin 5-6 on good side
-35 v pin 5-6 on bad side

any help greatly appreciated. thank you

appologizes for mispelling typing on tablet w no keyboard and its quite frustrating!

Add the bypass resistors on the Bias Supply, and you will be fine. The silicon diode has far less voltage drop than the OEM selenium device, so you will not be able to bias correctly unless you make the change.

So two things- problem started before I replaced the selenium, it’s actually the reason I did replace it.. also- after my first post I went ahead and parallels’d a 10 and 20k resistor in proper positions- really no change.. one side can bias proper- the other shoots up the lowest it goes is 2.8

Is that hum you are getting 60 hz or 120 hz?
What is your wallplate voltage?
Have you replaced the bias caps?
Check the 0.10 uF bias caps.
Switch the power tubes - does the problem stay or follow?

Otherwise, all your voltages are within reason. 'Cept for the ■, which is way low.  Add this to the low hum you are getting and I suspect that your quad-cap is starting to fail-short. If the hum is 120 hz, that is almost a certainty. Does it overheat at all?

So the hum or noise was when the amp was operating and I was trying to improve.. now I can’t because of this bias issue which I need to address first

No. As your bias issue may be related to the hum. What happens first is based on a more thorough and more comprehensive diagnosis. Before going into detail, what is your level of expertise, and what is your level of tooling?

I am guessing you have a VOM.
Do you have an ESR meter?

I feel like o shouldn’t of brought up the hum.. before taking it apart it was biasing fine.. the hum/ or noise was very low- had to be real close to the speaker to hear it.. but maybe I’m wrong and it is related.

I’m a hobbyist- have vom ecr Meter plenty of tools extra parts

Few other things- last year I replaced the driver board w a new one. Same schematic except wires for 6gh8a tubes.. fitted it with new parts orange caps etc...


I replaced can cap a few years ago.. last year one of the sections blew.. and I just put a large cap underneath instead of replacing the cap and it’s been working. That’s the cap that’s connected to the rectifier

could the two 50uf bias caps be causing this? Those are original.. I just thought if that was the case it would be affecting both channels

tubefan9 wrote:could the two 50uf bias caps be causing this? Those are original.. I just thought if that was the case it would be affecting both channels

Well yes, both channels.
No, the observed phenomena would not necessarily be identical in each channel.

One of the great weaknesses (of several) in the OEM Laurent design is that the bias pots serve as voltage dividers in this application, from a single voltage source. When the source is well-behaved, the pots will adjust nicely. If ths source should be producing too much voltage, one side might bias correctly, but the other side will run high. To anyone who has biased a significant number of these beasts, this will be obvious as it is a constant adjustment until both sides are correct. Taking one side, only to the "correct" 1.56 V, and then not checking the other is not going to work out so well. In the OEM design, the selenium diode was part of that overall circuit, inserting a silicon diode throws that out. Follow this thought:
a) The voltage was already drifting too high - any number of reasons - but the selenium diode was simultaneously increasing in internal resistance - which is, by the way, what they do.
b) So, correct bias was possible.
c) All of a sudden the the selenium diode goes away, a silicon diode is in its place.
d) *POOF*, correct bias is no longer possible.

I will make one very basic assumption about the unit in question: That the tubes are all good and verified as such. NOTE: Good or bad tubes would have little or nothing to do with hum or bias unless internal shorts or other obvious failures obtained. And that would be counter-intuitive to the narrative so far. Not impossible, but unlikely.

Accordingly I will make two suggestions to Tubefan9:

a) Replace the bias capacitors. Minimum 47 uf, maximum 60 uF, minimum 80 VDC. 105 C. Example: https://www.mouser.com/ProductDetail/Vishay-BC-Components/MAL211990518E3?qs=sGAEpiMZZMu3dWSqd4Tl0AYVZSvceoL0OhTm97d%2fefw%3d

b) Check, and consider replacing, all sections of the Can Cap. Either with an aftermarket board (suggested), or with a new 4-section cap - most authentic, but costly.

Ok I’ll try that out. I need to order those 50uf caps I think but I have stand ins for the cap can that will stand out but be good enough for testing. Thanks for the advice.

Ok to use 2- pairs of 100uf 100v caps in series since that’s what I have on hand?

Ok- tried swapping the caps and got same results.. yes tubes are good

One thing to point out- I noticed the voltage on the square section was low too.. so I replaced that part with a 30uf 500v cap I had laying around and it still has that lower voltage.. so doesn’t appear to be the cap causing that.

One thing that has been on my mind reading the threads about the replacement of the selenium rectifier with a silicon diode. This circuit generates a -ve bias voltage and the lower forward voltage drop of the silicon diode should increase the -ve voltage. As a result the bias voltage available to the grids should be more negative.

The result of this would of course alter the adjustment range of the bias circuit which should be capable of biasing the output tubes more into their cut off region and the measured voltage across the cathode resistor should be lower not higher..... I assume the cathode resistor(s) are still ok?

I know the ST120 is slightly different but I get about -75v available form the bias circuit and my output tubes run at approx -60v on the grids. Scale this accordingly for the lower output ST70.

Whilst you're investigating the bias circuit you could leave out the rectifier tube so that nothing gets cooked. Your lower B+ voltage "may" be due to one channel drawing excessive current so get the bias voltages even first. Since the bias supply is identical between left and right you should get the same range.

With no HT supply do you get the same voltage range on all the output tube grids? Just a though in case you have minor leakage in the grid coupling cap. One side of this cap sits at a high +ve voltage from the driver tube and the other at the -ve potential from the bias circuit.... Does your replacement driver board setup use the original Dynaco combined bias circuit and common cathode resistor?

Once you've sorted that out you can get back to hum tracing....

If you can post a photo of the underside of your ST70 then maybe one of the Eagle eyed peeps on here might spot something.

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PeterCapo wrote:Try working your way back from the channel that has the bias problem.  IOW, start with the two sockets for the bad channel - clean and retension the contacts with extra attention to contact #5.  Clean thoroughly with interdental brushes and at least 91% isopropyl.  Blow out the contacts with compressed air before they dry.  

The socket contacts are shaped like the letter "C"  Retension by pressing gently on the top and/or bottom of the "C" but do not press on the side of the "C" directly opposite the split in the "C" if you see what I mean.  See to it that you don't close the "C" too much.  Take some Dramamine first.

Then, reflow the solder connection to contacts #5 on both sockets.

If that doesn't help, trace the circuit back from contacts#5 and reflow solder along the way.  Use some paste or liquid flux to enable a good reflow.

I've been considering replacing all the original tube sockets either way... I'll do these suggestions tonight.
If I do replace the sockets - any advice on which fit properly or work best from here - tubesandmore 8pin sockets

When I get home later I'll take more measurements and pictures to post.
Thanks everyone for their help so far!

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tubefan9 wrote:
If that doesn't help, trace the circuit back from contacts#5 and reflow solder along the way.  Use some paste or liquid flux to enable a good reflow.

I've been considering replacing all the original tube sockets either way... I'll do these suggestions tonight.
If I do replace the sockets - any advice on which fit properly or work best from here - tubesandmore 8pin sockets

When I get home later I'll take more measurements and pictures to post.
Thanks everyone for their help so far![/quote]

Peter C. is steering you in the right direction.

Pretty much all tube sockets, whether ceramic (my preference), Bakelite or other plastic use tin-plated bronze contacts. When-new, these are, in theory, spring bronze. However after repeated heating to 250F+, that tempering gets reduced. So, pretty much any socket needs to be cleaned and tightened after a few repetitions of tube changes. I keep a set of dental picks for the process, as well as tiny spiral brushes that leave no residue. DO NOT use any abrasives anywhere near these sockets. If a 'scraping' is absolutely necessary, a wooden (hardwood) toothpick is the roughest thing you should use. Nothing that creates marl (not the clay).

On compressed air - unless you have a filter-dryer on your compressor - not the home-made stuff. This can have a LOT of water in it if you are not careful. I keep a couple of these around:

https://www.amazon.com/FIRSTINFO-Aluminum-Pneumatic-Refillable-Pressure/dp/B00YF7Q1TI/ref=pd_lpo_vtph_263_lp_t_2?_encoding=UTF8&psc=1&refRID=P98ATYSXV5R1MDYBPREY

One is always full of high-proof isopropyl alcohol, the other with whatever the need requires. So when it comes to cleaning stuff, I can rinse it down, for cheap.

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