ST-120 - Debug / Repair Left Rear 20vdc

I have for about two - three years now been using my ST-120 build



Rectifier tuber started with Genalex U77 GZ34
It died about eight months ago and replaced with Tung-sol 5AR4


Last week I powered on and no sound.

1) Checked bias and 0vdc on all four. So figured it was rectifier tube again.
2) Replaced with another new 5AR4 .. still 0vdc. Does that means this new 5AR4 is bad??? Hmm.
3) Having purchased a solid state rectifier for debug: T-SSR01 I installed that and have voltage.. and as expected.. it was hot
a) adjusted bias back down on tubes to be range .5 for Left Front, Right Front, and Right Rear but.. odd.. Left Rear was at 20.x So way over range
b) I fliped Right and Left tubes and Left Rear still was showing 20. I turned down bias adjustment all the way down all went down to .1 but Left Rear stayed 20.x
4) Played low level 20db for 3 min then got a weird sound .. screeching... about 40-70DB and turn off right away.

So I don't think it is rectifier. Not main tube

Reviewed steps from build guide. But not sure debug beyond what I have done

Suggestions?

penguinpages wrote:

I have for about two - three years now been using my ST-120 build



Rectifier tuber started with  Genalex  U77 GZ34
It died about eight months ago and replaced with Tung-sol 5AR4


Last week I powered on and no sound.

1) Checked bias and 0vdc on all four.  So figured it was rectifier tube again.  
2) Replaced with another new 5AR4 .. still 0vdc.  Does that means this new 5AR4 is bad??? Hmm.
3) Having purchased a solid state rectifier for debug: T-SSR01 I installed that and have voltage.. and as expected.. it was hot
    a) adjusted bias back down on tubes to be range .5 for Left Front, Right Front, and Right Rear but..  odd.. Left Rear was  at 20.x  So way over range
    b) I fliped Right and Left tubes and  Left Rear still was showing 20.  I turned down bias adjustment all the way down all went down to .1  but  Left Rear stayed 20.x
4) Played low level 20db for 3 min then got a weird sound .. screeching... about 40-70DB and turn off right away.

So I don't think it is rectifier.  Not main tube

Reviewed steps from build guide.  But not sure debug beyond what I have done

Suggestions?

the 10ohm resistor has blown due to a flashover in the power tube. Junk this tube and replace the resistor. It should be a film resistor preferably "flameproof" not larger then the old one.
This also killed the rectifier tube.

Are other tubes swapped into this location also "junk"

AKA.. swaped in all four at one point in time to that spot to baseline.

penguinpages wrote:

Are other tubes swapped into this location also "junk"  

AKA.. swaped in all four at one point in time to that spot to baseline.  

The tube that caused the "flashover" should not be used as it has a high probability to
do this again.
If you can replace it with something that mates the remaining thats good. Otherwise
it's recommended to get a new matched pair for that side.

Thanks for the response.


https://www.dynakitparts.com/shop/metal-film-resistors-1-w/

1watt 10ohm.

I assume this is the part I am looking for.


Is this a part / issue that happens often? Is there a root cause or thing to watch for to avoid this?

penguinpages wrote:
Thanks for the response.


https://www.dynakitparts.com/shop/metal-film-resistors-1-w/  

1watt 10ohm.

I assume this is the part I am looking for.


Is this a part / issue that happens often?   Is there a root cause or thing to watch for to avoid this?

the root cause is a power tube gone bad.

Have a few of these resistors, this will shorten down time if it happens again.
Replacing power tubes periodically or whenever bias fails to keep stable will prevent most of these
occurrences.

Note that the 10ohm resistor acts a fast fuse to protect output transformer and other parts. It should be small enough to "burn away" fast

Thanks for explaining and .. your wisdom to know root cause.




Without a tube tester....

1) Can I assume that any / all of the tubes inserted in that Left Rear slot with the damaged part are suspect? Or.. as you noted this is almost like a fuse, and so tubes I put in to "baseline" are ok as this is fail circuit open? Reason I am asking is that the multimeter bias voltage was 20+ (vs expected .2-.5 ) and so did I damage tubes is the question.
2) I also assume that the upstream rectifier tube I used as a test is also fried. It was brand new, but all voltage on bias ports show 0. Vs when I use the solid state one , I see Voltage and can bias (all but Left rear which sits 20+).

It is helpful to me if I can learn a bit how to root cause and fish for self. And build out some debug path logic to test with.

1) IF no glow: check fuse.
2) If fuse checks to have continuity: did you plug it in .. check AC. If AC.. 110-126,... oh boy.. likely transformer, are they warming up?
3) If glow but bias voltage 0 then... likely rectifier tube. Replace and reset bias all the way down and bring up / test.
4) If bias 20+ range, then, check resistor on that tube slot is damaged, (if not visible check with multimeter via ???) and.. consider that tube bad , ?AND rectifier is also bad?
5) If bias does not hold steady for range +-.05 from baseline of .5, then consider that main tube bad.. Replace in pairs
6) If all above does not work, have a beer, and post to forum for help


Thanks,

Personally i would start with a fresh quartet of matched KT88/6550. If one of your old gone bad risk is
that the other may follow. New tubes are always _new_ !

ECC82 tubes (or equivalents ) are ok until they make nasty noise or hum.

Turn down bias pots to minimum, connect speakers install all tubes and power up. Start adjusting
bias to 0.5V

Place your favorite record on the player, listen ! When record is finished check/readjust bias.

Enjoy the music.!

I have four nice new KT88 that I was going to move to.. so that is solid

But do I trust my "new" rectifier that shows 0vdc on bias contacts.. Or.. its now FUBAR and or not worth risking.. So order new one.

Pen, as always, Peter is spot on. I have not had a rect tube fail since;

1- Run the amp directly from a GFCI that latches down for a commercial power fail, (even a flicker).
2- VARIAC set at 117 vac . Dale

I need to validate but ...  I think there is something wrong.


The post was:

"...10ohm resistor has blown due to a flashover in the power tube...."


Picture I posted showing the blow out seemed to align with that  story.


Color bands on that resistor: Brown Black Black Brown


I ordered METAL FILM RESISTORS 1 W - 10 kOhm 1 Watt


But (as shown in below picture where I placed next to it to compare:   Its bands are:   Red Yellow Black Brown





I am no expert.. and most likely wrong or miss understanding but this does not match:


I need to validate but ...  I think there is something wrong.


The post was:

"...10ohm resistor has blown due to a flashover in the power tube...."


Picture I posted showing the blow out seemed to align with that  story.


Color bands on that resistor: Brown Black Black Brown


I ordered METAL FILM RESISTORS 1 W - 10 kOhm 1 Watt


But (as shown in below picture where I placed next to it to compare:   Its bands are:   Red Yellow Black Brown

https://i.servimg.com/u/f48/20/33/91/19/tranis11.png


I am no expert.. and most likely wrong or miss understanding but this does not match:



https://i.servimg.com/u/f48/20/33/91/19/damage10.png


BTW.. the reason it blew I think I also have root cause.  the ground nut it binds to was loose. So my guess this poor contact created issue.  Other posts were good so must have just not tightened it sufficiently.


Just want to make sure before I desolder and solder in new one...




BTW.. the reason it blew I think I also have root cause.  the ground nut it binds to was loose. So my guess this poor contact created issue.  Other posts were good so must have just not tightened it sufficiently.


Just want to make sure before I desolder and solder in new one...

penguinpages wrote:I need to validate but ...  I think there is something wrong.


The post was:

"...10ohm resistor has blown due to a flashover in the power tube...."


Picture I posted showing the blow out seemed to align with that  story.


Color bands on that resistor: Brown Black Black Brown


I ordered METAL FILM RESISTORS 1 W - 10 kOhm 1 Watt


But (as shown in below picture where I placed next to it to compare:   Its bands are:   Red Yellow Black Brown





I am no expert.. and most likely wrong or miss understanding but this does not match:


I need to validate but ...  I think there is something wrong.


The post was:

"...10ohm resistor has blown due to a flashover in the power tube...."


Picture I posted showing the blow out seemed to align with that  story.


Color bands on that resistor: Brown Black Black Brown


I ordered METAL FILM RESISTORS 1 W - 10 kOhm 1 Watt


But (as shown in below picture where I placed next to it to compare:   Its bands are:   Red Yellow Black Brown

https://i.servimg.com/u/f48/20/33/91/19/tranis11.png


I am no expert.. and most likely wrong or miss understanding but this does not match:



https://i.servimg.com/u/f48/20/33/91/19/damage10.png


BTW.. the reason it blew I think I also have root cause.  the ground nut it binds to was loose. So my guess this poor contact created issue.  Other posts were good so must have just not tightened it sufficiently.


Just want to make sure before I desolder and solder in new one...




BTW.. the reason it blew I think I also have root cause.  the ground nut it binds to was loose. So my guess this poor contact created issue.  Other posts were good so must have just not tightened it sufficiently.


Just want to make sure before I desolder and solder in new one...

Yes, you got the wrong resistors. You got 10 kohm ( 10.000 ohm) but should have 10 ohm ( 10 )

Loose grounding screws are bad. Make sure all of them are tighted.

Back at this post.


I did not realize the unit I built for Tube Amp was / is a http://www.tubes4hifi.com/ ST-120

The site offers full kits but not parts such as to order replacement transisotrs.

I went out and googled around for "Hifi audio transistor brands" and came across notes about "UXCell" being used for many tube amps... which is funny because those were the ones I first ordered and returned due to "size difference" in concern about quality..


https://www.amazon.com/dp/B07PPTS1BG?psc=1&ref=ppx_yo2ov_dt_b_product_details





As you can see its significantly smaller in wire guage and size. Does size matter .. some say it does..


But would you resolder all four so they are consistent, or, is this truly a commodity item and not in signal path of significance.

Again... thanks for the input.