Hum/Hiss/Static out of Left Channel (Speaker)

Again, I appreciate the warm welcome on this site.  So a little about my situation- I have an original Dynaco ST-70 and PAS 3X.  Both are in great shape with the typical maintenance/replacement of 2 non-Mullard EL34s (Sovtek and EH) and NOS RCA (Japanese) GZ34 on the ST and replacement of the 12X4 on the PAS.  I have well over several hundred hours on these two components over 10+ years.  I have seen previous posts on trying to remedy via swapping tubes and have tried that without avail.  Thinking about replacing the resistors from peterh post, but was hoping it was as easy as replacing all original 12ax7 tubes. It sounds as if these rarely fail. I note on the PAS manual it could be isolated in the volume section (PC5) section.  Although I have soldering irons, I am concerned about my own ability to do this without cold soldering. Thanks, Chip

Chipster1969 wrote:Again, I appreciate the warm welcome on this site.  So a little about my situation- I have an original Dynaco ST-70 and PAS 3X.  Both are in great shape with the typical maintenance/replacement of 2 non-Mullard EL34s (Sovtek and EH) and NOS RCA (Japanese) GZ34 on the ST and replacement of the 12X4 on the PAS.  I have well over several hundred hours on these two components over 10+ years.  I have seen previous posts on trying to remedy via swapping tubes and have tried that without avail.  Thinking about replacing the resistors from peterh post, but was hoping it was as easy as replacing all original 12ax7 tubes. It sounds as if these rarely fail. I note on the PAS manual it could be isolated in the volume section (PC5) section.  Although I have soldering irons, I am concerned about my own ability to do this without cold soldering. Thanks, Chip

Try to specify the problem. Is it hum ( low freq tone ) or hiss ( like rain ) ?
Is it only in the st70 ( pas 3 powered off ) or does it come when the pas is active ?
If it comes with the PAS, is it on all inputs or only on phono ?

And finally , what resistors ?

So there has always been a slight hum from the ST70 without the PAS hooked up and running directly to my CD player. With the PAS hooked up, I get the additional hiss (almost like radio station distortion)and occasional pops. It is present on all inputs and is more pronounced on initial powering up and when increasing volume (gain)? As far as the resistors I am referring to stevethemaestro post from Jul 2020 post- you had recommended replacing two plate resistors on the line board. I appreciate the quick response.

Chipster1969 wrote:So there has always been a slight hum from the ST70 without the PAS hooked up and running directly to my CD player.  With the PAS hooked up, I get the additional hiss (almost like radio station distortion)and occasional pops. It is present on all inputs and is more pronounced on initial powering up and when increasing volume (gain)? As far as the resistors I am referring to stevethemaestro post from Jul 2020 post- you had recommended replacing two plate resistors on the line board.  I appreciate the quick response.

To start with the hiss , yes the plate resistors ( 470k and less probable 100k) on the line board is a good guess.

As for the hum in the st70, is it present if a pair of shorting plugs are inserted instead of the PAS ?. If unequal right / left, will it move if the 7199 is swapped between the channels ?

So with the plate resistors I (or someone) will need to be re-soldered. With that being said, what is a good video to watch on soldering as I have only attempted once.

I will definitely try the "shorting plugs"; not familiar with them. I will try swapping the 7199s. Thanks peterh

Chipster1969 wrote:So with the plate resistors I (or someone) will need to be re-soldered. With that being said, what is a good video to watch on soldering as I have only attempted once.

I will definitely try the "shorting plugs"; not familiar with them.  I will try swapping the 7199s. Thanks peterh

The plate resistors should be replaced ! Use 1/2 w metal or carbon film.

Will do- is there a recommended video on soldering for dummies? lol

Chipster1969 wrote:Will do- is there a recommended video on soldering for dummies? lol

Try google.  But a very importent detail : use leaded solder ( 60/40 ) or (63/37)

The old resistors are easises removed by cutting both sides and then gently picking the
leads out while heating them with the solder iron. Then you need to open the hole with either
a de-soldering pump or a cupper strip.
See https://youtu.be/pA6wFV9NLSw  for the usage of a de-soldering pump

Hmm. this is better : https://youtu.be/kd9FJgZE77I

Note that keeping the board steady is important , and sucking fast before the solder solidifies.
For professional work one can use a iron with a hole in the middle connected to a vacuum pump, that begins to be expensive.

Solder wick is an alternative, it will however not clean withing the hole as well as a pump.

Thank you! Wish me luck

Chipster1969 wrote:Thank you! Wish me luck

To add an idea;
on these types of boards i usually cut the component, heat the solderpad in order to gently remove the component leg and then heat again and use the desolder pump to clean the hole in the board. Best is to such from the component side, that way one can suck at the same time as the iron heats. It also in my opinion is gentler to the board to such thus way.

These original boards are fragile. (Modern "through the hole plated" fiberglass boards can stand more abuse.) But even if the copper islands loosens one can rebuild the path with pieces of wire soldered on the component leg and the remaining copper.

Don't use a solder iron that is heated to cope with lead-less solder, they will easily destroy the glue between board and copper.

I appreciate answering my previous question "hiss/hum" on my PAS-3X several months ago.  The situation has been resolved and the sound is fantastic!
My new question is whether I need to re-set the bias for a "paired quad" set of new Electro Harmonix EL34s for my ST70. Thank you, Chip

Chipster1969 wrote:I appreciate answering my previous question "hiss/hum" on my PAS-3X several months ago.  The situation has been resolved and the sound is fantastic!
My new question is whether I need to re-set the bias for a "paired quad" set of new Electro Harmonix EL34s for my ST70. Thank you, Chip

After changing power tubes bias should always be checked / adjusted. Even better is to
turn bias adjustment pot at the end that has lowest current when new tubes are installed ( highest negative grid voltage), that way there is no risk of current run-away before bias is adjusted.
Or did a misunderstand the question ?

So is there a step by step process to this? What direction am I turning the screws- clockwise or counterclockwise? Thanks

Chipster1969 wrote:So is there a step by step process to this?  What direction am I turning the screws- clockwise or counterclockwise? Thanks

start the amp with current tubes, attatch voltmeter at the bias adjustment taps and turn
the bias adjustment pot. The direction that reduces the voltage is correct.
Then change tubes, power on and after several minutes start adjusting bias as per the manual.

Thank you again peterh!

One final question peterh- does the amp need to be hooked up to the speakers and pre-amp for me to do that, or can I simply plug the unit into the wall and power it up?

Chipster1969 wrote:One final question peterh- does the amp need to be hooked up to the speakers and pre-amp for me to do that, or can I simply plug the unit into the wall and power it up?

ALWAYS have a load connected to the speaker outputs !! It can be your speakers or it could be a suitable power resistor ( 8 ohm > 10w )
The inputs might be open or connected, accurate bias adjustment however needs a "no signal" .

Thank you.