2x original Dynaco ST-70's with VTA board with continued issues after service

Hi All,

I have 2 original Dynaco ST-70's.

1. I bought as a good working item along with a full Dynaco setup as my first real tube stereo.  It already had the VTA kit on it, so is really the only Dynaco I know.

2. I bought a 2nd-hand stock ST-70 and I challenged myself to do the VTA mod on it.  I can follow directions and solder, but am truly a novice when it comes to circuitry.  Bob helped me out with a few outstanding questions and was super supportive--as it seems he always is.

After the upgrade on #2, I hooked these up to my system using them to power 2 sets of speakers--which I swapped out a bunch of different times until I found a system I liked.  Over time (a matter of months), i noticed that #2 was starting to exhibit a loud 60 cycle hum.  It wasn't consistent at first, but after a while made it unusable.  At the exact same time, #1 started to have sporadic white / pink noise out of the left channel and a little quieter noise out of the right.  Upon closer listen, I could also hear a 60-cycle hum in addition to that.  The noise had gotten very loud and also made it unusable.  

At this point, I needed help with troubleshooting.  I have normally brought my gear to Deltronics locally (Suburbs location).  They have always been very nice, but I find that I bring my gear back to them multiple times under warranty as the initial fix rarely was 100%.  Vintage gear is finicky, so I get it.  I realize that I want an expert to review these.  I had noticed that tube4hifi offered repair service if I covered the shipping at a very reasonable bench rate.  Although I knew that after shipping two of these items both ways + repair would likely mean that these items are no longer worth what I have into them, but I want to keep them and I want them to be perfect--because when they are functioning, I love the sound.

I talked with Roy and sent them over to him to troubleshoot > fix.  He was super communicative and helpful.  He noticed some obvious issues:
Most tubes need to be replaced (not surprising as I hadn't done this yet on either), some cold solder points and I had messed up part of the wiring on #2.  He fixed them up in a week and shipped them back.  

While I was waiting, I decided to bite the bullet and buy 2 new sets of matching tubes just to have everything pristine when they got back. (Mullard EL34, Tung-Sol GZ34 / 5AR4).  I installed them, biased and connected to my rig.  

Turned them on and instantly noticed that the hum and the noise was there on both items.  Literally, nothing had changed!  I sat on it as I was about to explode.  After all the effort and $700 in repairs and tubes, I was listening to the same damaged amps.  I replaced the speaker wire, removed the preamp - no change.  I swapped out the power conditioner, brought them to a different room and a different set of speakers, no change.  No input is connected at all, so it definitely is an issue with the amps or power.  I am thinking less and less that the house's power or other environmental things are leading to the issue as they both have the same, yet different issues.  And, I don't notice anything like this on my other gear.

I contacted Roy and he was also surprised--he actually recommended that I come here to look for help.  The noise is loud and instantly obvious and he said that they were both 100% silent in his shop before they returned them to me.  I trust that he is the expert and that all was tested before the return.  I just have no idea what to do now.

Pic of Oscilloscope (iPhone) and vid of white noise below

Any ideas for obvious troubleshooting? What can you recommend?

I appreciate any expertise you can provide.  (like I said, I am a novice w/ rudimentary skills + testing gear)

I bought some overpriced shorting RCA plugs for the INPUTS of my VTA-120. Generally useless, but nice for when I want to ensure that there is NO noise on the inputs of my VTA-120. If I am curious about a noise, I put on the shorting plugs and crank the amp up. Usually I am greeted with silence.

Thanks for the ideas. Had some shorting plugs around, but unfortunately (or fortunately) no change in the signal.

What are "shorting plugs on unused RCA inputs on the pre" ?? Dale

Sounds like it may be the input tubes - try switching them around. The center tube, as a voltage amplifier, is most suspect....

Did you find the cause of the noise yet? The spectrum you posted looked like it was showing 60 and 120 Hz hum, but I listened to your video and that is definitely not hum. It is a wide band hiss I am hearing, and it is much louder in the left channel. (Note, this could be due to which driver you had you mic closest too. I assume the midrange horn? Any 60/120 Hz hum will not be present here due to the crossovers. You would need to record again directly at the woofer to capture the hum) You mentioned that you changed power tubes, but did you change the small tubes? This sort of noise can definitely be due to a defective small signal tube.

rjpjnk wrote:Did you find the cause of the noise yet? The spectrum you posted looked like it was showing 60 and 120 Hz hum, but I listened to your video and that is definitely not hum. It is a wide band hiss I am hearing, and it is much louder in the left channel. (Note, this could be due to which driver you had you mic closest too. I assume the midrange horn? Any 60/120 Hz hum will not be present here due to the crossovers. You would need to record again directly at the woofer to capture the hum) You mentioned that you changed power tubes, but did you change the small tubes? This sort of noise can definitely be due to a defective small signal tube.

I haven't yet. I purchased the VTA kit w/ new tubes last year, so would've been surprised that those were already bad. That being said, I started to move them around and did notice some weird things. On #2, I swapped out tubes and got some really loud pops which prevented me from further testing. I got rid of all my 2nd rate speakers for testing, so was worried about damaging the heresy's.

And I have tendency to get analysis paralysis and can't make up my mind on replacement tubes. I will end up buying two more sets and just replace them all just to eliminate that as a possibility.

Who has a good suggestion on replacement input tubes? I've read so many different forum posts, but really just need something clear and reliable. I have been buying through tubedepot.

Thanks for all your thoughts and suggestions!

Any tubes should be fine. I would go cheap for now to see if it is the problem. If you like tubedepot get these JJs. I have them in one of my amps and they work great.

https://tubedepot.com/products/jj-ecc82-12au7-preamp-vacuum-tube

So, I purchased a pack of JJ's for testing.  This appears to have fixed one of the amps (thank you!).  The other now just has continued buzz in the right channel.  

The fact that one amp is now pretty much silent and the other has horrendous buzz points to a technical issue w/ that amp and not a environmental house issue.  

The video below shows me turning it on w/ the new tubes for the second time.  the first time had a bunch of loud pops initially that scared my dog out of the room. Very loud noise out of right channel only now.  

I'll take your ideas for continued troubleshooting!

Is the hum with the input disconnected and shorted? Not too clear from your pictures as to whether you have one amp connected to one speaker and the other to the other.

Is the hum on both channels of the affected amp? If so then, given that it sounds a lot like 120hz (full wave rectified), I'd be looking at supply smoothing cap or grounding to it.

I would have thought that a heater cathode leak in one of the valves would have given 60hz as would an unsmoothed bias supply.

The faulty amp is shorted and connected to both speakers and only am only experiencing the loud hum in the right channel. Thanks!

Maybe the problem is mechanical. My ST70, when it was still stock, developed a hum in one channel. I found if I put a little pressure on the 7199 on that channel the noise went away. I floated the power transformer on some rugger grommets and had no more problems. When I modded it I used the replacement power transformer with the taller lamination stack and still floated it on the grommets. I built mine with the 6SN7 board and had no problems with noise.
Also maybe your amp is too close to something else that might be inducing the noise? Try moving the amp several feet away from where you usually have it.

So my gmail inbox got this email from the forum. Now I've spent 15 minutes trying to get on here to offer some quick advice to the fellow member. So what exactly IS a "basket'? And who comes up with these terms? Why can't I simply log in to answer a simple question??

NOW, all pissing and moaning aside: Looking at the pictures provided,iIt looks like the filter CAN might not be soldered to the chassis

Good luck,
Jack

Any time you experience loud pops, especially when moving a tube in its socket or flexing a circuit board you are dealing with a loose connection somewhere. I would start by using Deoxit and cleaning the input mono/stereo switch followed by cleaning the tube sockets. Also loosen and re-tighten the circuit board mounting screws and nuts. Do a quick check after each cleaning and tightening operation to see if the unit is still exhibiting the same problem.

Sometimes solder connections can be at fault. If not enough heat was used to get good flux flow and solder melting during the soldering process. I have solved many intermittent or troublesome problems with amplifiers and radio receivers over the years simply by going through circuits one at a time and re-flowing solder connections. Recheck operation after each circuit area has been re-soldered. Years ago I even ran into some tube sockets for PC boards which had nickle-tin plating that separated from the base metal underneath! You never know what you are going to encounter. Some of these can be tricky to find, but persevere! Always remember to try something different if what you are currently trying changes nothing. It often helps to stop and walk away from a unit for a while and then come back to it another day. A clear mind and rested and relaxed body and a new day can make a difference.

I recently have had several new-in-box JJ tubes (12AX7) that were defective. One had developed a crack in its base and lost vacuum. Another one had vacuum and the heater was operating, but the emission output from both sections of the tube was almost non-existent. I recently began to use new release TungSol 12AX7 and 12AU7 tubes - so far so good. I may look for some NOS GE, RCA or Sylvania tubes as back-ups.

Best wishes!

Joe KC5LY

Hello all:

From the underside picture I can see a few things that catch my attention. As they say a picture help troubleshooting from afar....

C354 Choke
This choke looks like it burned. The dark black resin is a telltale sign that this part may be shorted, and even leaking DC onto the chassis.

De-solder the wires of the C354 Choke and measure the DCR (DC Resistance).

It should measure around 60 Ohms. I am measuring a C354 now with my Fluke 87V VOM and the reading is 60.6 Ohms.

If your Choke measures differently, or even is shorted, you can measure from each wire to the metal shroud. Your VOM should measure an Open Circuit for DC. If your VOM shows any measurable resistance, replace the C354 Choke.

Tube Rectifier

I can see two diodes on your rectifier socket. This is fine, and some folks like to make this modification. I would remove these diodes and let the 5AR4/GZ34 do it's job without any help.

Power Supply

If the engine room is not working correctly, the boat will not move well.
I would, one you have the amp setup with all of the necessary precautions (HIGH DC VOLTAGES) measure the DC voltages starting with connecting Resistive Dummy Loads to your speaker terminals to avoid the noise.

Start working your way from the rectifier tube, then move to the Power Supply capacitor. Make sure that the values of the Power Supply resistors are correct.

You should refer to the Dynaco Manual for approximate voltages.

If all of your DC voltages are correct, then make sure that each Power Tube is drawing the proper DC quiescent current. The board you have may use current sensing resistors.

You can install these temporarily and check that each EL34 is drawing the correct DC current. EL34's dislike anything over 50 milliamps.

Maximum Plate Dissipation

Measure the Plate to Cathode Voltage across the Power Tube and then multiply this voltage by the current through the tube.

For the EL34, the maximum plate dissipation is 25 watts; P=VI

Make sure the the EL34 tubes are operating under 25 watts with the amp idling.

If all of these check out, then you likely have a wiring error somewhere.

If you have a signal generator, you can trace a 1 KHz Signal into the RCA Jacks and carefully trace the sine wave from Input to Output.

Hope this helps you out!

www.shermanaudio.com

Rich Sherman
shermanr@prw.net

Hello,

I noticed that the -FB wire from the right channel speaker jacks, is right up against the frame of the filter choke.
This may inject 120hz buzz into the phase inverter. Move it away a little from the choke and see if that helps.

Best,

Tommy J

Also, make sure that all the trannies, including the filter choke bolts 'n' nuts are tight.
I've found that if they're not, this sometimes creates weird hum / noise issues.

Best,

Tommy J