Dynaco ST-70 Buzzing in Left Channel

Hi folks!

I was lucky enough to find a completely untouched ST-70 (along with a PAS 3) from the original owner. When I slowly brought it up to life, I noticed a buzzing sound coming out of the left channel. I'm looking for some guidance from you Dynaco gurus.

Preface: I have some experience with guitar tube amps (changed filter caps, setting bias, checking values, etc.), so I know the principles of how a tube amp works; however, I'm not the greatest at troubleshooting.

Here's what I did so far to isolate the issue:

1. Cleaned all tube sockets and pins
2. Removed ALL tubes and turned it on. - dead quiet.
3. Plugged the rectifier - dead quiet.
4. Plugged in EL34s and 7199 in the right channel - Very slight hum.
5. Removed the right channel tubes and inserted the left channel EL34s and 7199 - Big buzz.
6. Swapped right and left tubes to rule out a bad tube - buzzing still happening in the left channel.
7. Checked bias on both channels and adjusted to 1.56v. - No issues.

I realize that I'll have to recap the entire thing sooner rather than later, but I'd love to know what's wrong with the amp before starting to replace parts.

Would anyone be able to steer me towards my next step(s)?

Thanks a million!

Blue Apple wrote:Hi folks!

I was lucky enough to find a completely untouched ST-70 (along with a PAS 3) from the original owner.  When I slowly brought it up to life, I noticed a buzzing sound coming out of the left channel.  I'm looking for some guidance from you Dynaco gurus.  

Preface:  I have some experience with guitar tube amps (changed filter caps, setting bias, checking values, etc.), so I know the principles of how a tube amp works; however, I'm not the greatest at troubleshooting.

Here's what I did so far to isolate the issue:

1. Cleaned all tube sockets and pins
2. Removed ALL tubes and turned it on. - dead quiet.
3. Plugged the rectifier - dead quiet.
4. Plugged in EL34s and 7199 in the right channel - Very slight hum.
5. Removed the right channel tubes and inserted the left channel EL34s and 7199 - Big buzz.
6. Swapped right and left tubes to rule out a bad tube - buzzing still happening in the left channel.
7. Checked bias on both channels and adjusted to 1.56v. - No issues.

I realize that I'll have to recap the entire thing sooner rather than later, but I'd love to know what's wrong with the amp before starting to replace parts.

Would anyone be able to steer me towards my next step(s)?

Thanks a million!

Does it buzz if you remove the 7199 ?

peterh wrote:

Blue Apple wrote:Hi folks!

I was lucky enough to find a completely untouched ST-70 (along with a PAS 3) from the original owner.  When I slowly brought it up to life, I noticed a buzzing sound coming out of the left channel.  I'm looking for some guidance from you Dynaco gurus.  

Preface:  I have some experience with guitar tube amps (changed filter caps, setting bias, checking values, etc.), so I know the principles of how a tube amp works; however, I'm not the greatest at troubleshooting.

Here's what I did so far to isolate the issue:

1. Cleaned all tube sockets and pins
2. Removed ALL tubes and turned it on. - dead quiet.
3. Plugged the rectifier - dead quiet.
4. Plugged in EL34s and 7199 in the right channel - Very slight hum.
5. Removed the right channel tubes and inserted the left channel EL34s and 7199 - Big buzz.
6. Swapped right and left tubes to rule out a bad tube - buzzing still happening in the left channel.
7. Checked bias on both channels and adjusted to 1.56v. - No issues.

I realize that I'll have to recap the entire thing sooner rather than later, but I'd love to know what's wrong with the amp before starting to replace parts.

Would anyone be able to steer me towards my next step(s)?

Thanks a million!

Does it buzz if you remove the 7199 ?

That's a negative! Dead quiet without the 7199 in the left channel.

Blue Apple wrote:

peterh wrote:

Blue Apple wrote:Hi folks!

I was lucky enough to find a completely untouched ST-70 (along with a PAS 3) from the original owner.  When I slowly brought it up to life, I noticed a buzzing sound coming out of the left channel.  I'm looking for some guidance from you Dynaco gurus.  

Preface:  I have some experience with guitar tube amps (changed filter caps, setting bias, checking values, etc.), so I know the principles of how a tube amp works; however, I'm not the greatest at troubleshooting.

Here's what I did so far to isolate the issue:

1. Cleaned all tube sockets and pins
2. Removed ALL tubes and turned it on. - dead quiet.
3. Plugged the rectifier - dead quiet.
4. Plugged in EL34s and 7199 in the right channel - Very slight hum.
5. Removed the right channel tubes and inserted the left channel EL34s and 7199 - Big buzz.
6. Swapped right and left tubes to rule out a bad tube - buzzing still happening in the left channel.
7. Checked bias on both channels and adjusted to 1.56v. - No issues.

I realize that I'll have to recap the entire thing sooner rather than later, but I'd love to know what's wrong with the amp before starting to replace parts.

Would anyone be able to steer me towards my next step(s)?

Thanks a million!

Does it buzz if you remove the 7199 ?

That's a negative!  Dead quiet without the 7199 in the left channel.

Then it seems to be heated <> cathode leakage in the 7199.
Replace. Either with a new 7199 if possible or any of the replacements ( 6u8 6bl8 etc) plus an
adapter.

Thanks for the note, Perterh!

I should have added that the noise doesn't follow the 7199. It stays in the left channel.

While looking around, I found the video below. They called the "buzzing" AC hum. This is pretty much the exact sound I'm getting out of the left channel. They fixed it by changing the cap can.

Would the "AC hum" caused by a leaky cap disappear when removing the 7199?

Does not follow the 7199 ? But still one side only ? Then the can cap is ok , it's used for
both channels.
The 7199 is feed from two different circuits of the mains transformer. Both are "floating" AND
blocket for AC by a small duel capacitor ( 0.02 ) mounted on a terminal strip, connected by
2 wires green/yellow and brown yellow. You could solder another pair of caps ( 20nf or thereabout)
across this cap .
The problem with the 7199 comes from a rather hich voltage between the cathode (T) and filament. By time this erodes the insulation acurrent ( AC) is starting to creep. By floating the filaments
as done in the ST70 one hopes that the voltage is higher and this destructive effect wont occur.
By some reason you might have lower DC on one of these ( the side that hubs if my guess is right)

If that is not the source, check for loose ground connections.

Many thanks for the info!

I'll double-check my grounds and see if adding that capacitor on the left channel removes the hum.

Just to be sure, we're talking about this double cap, correct?  And I can just add two 20nf in parallel with the originals to see if it takes care of the issue?

Yes, this is the cap in question.

Before anything, measure the voltage on the 2 windings at power. It should be a DC .

Well, it turns out that the hum was coming from a broken mono/stereo switch. I noticed the hum becoming increasingly louder as I hovered my hands over it. I moved it a bit while keeping it in the stereo setting, and the hum completely stopped.

It ran and played beautifully for about 10 minutes before blowing the fuse. I guess it's time to start the restoration.

Thanks a million for your help, Peterh!