Weird ST-70 distortion problem

I recently acquired an ST-70 on eBay - my first venture into tube equipment. After installing a new driver board, replacing all of the capacitors and putting in a new set of output tubes I made some modifications to the original amp. These included moving the power switch to the front where the left preamp connector had been, adding a rear stereo mini jack and connecting it to a DPDT switch where the stereo/mono switch used to be (The front inputs are connected to the other side of the switch) and added a volume control where the right preamp connector had been.

Everything seemed to be working fine. I had my PC's Creative Labs Audigy sound card connected to the rear stereo mini jack and I would play a bluetooth receiver and a CD player by swapping cables on the front inputs. The majority of the time I listen to music through the PC. Here's where the weirdness comes in. When I play streamed music on the PC, i.e. Pandora, everything works fine. However, if I play MP3s or CDs through the PC, after about an hour of playing fine, the music gets real distorted. Sometimes when I pause the music for 20 - 30 secs and restart it everything will be okay. One time I didn't pause the music because I wanted to see if it would clear up on its own. It didn't - in fact, after a minute or so the rectifier tube starting glowing a bright orange. I was afraid the tube would get so hot the glass would shatter so I immediately turned it off. Amazingly, the rectifier tube doesn't appear to be damaged as it is working as it did before.

The rear jack is connected to the front switch with a shielded cable (Mogami W2947) with the shield grounded on both ends. I have not had any audible hum on this input. Again, everything, including the PC, plays fine through the front input and streamed music plays fine through the rear input. I connected a different PC to the rear input and it seems to play okay so the problem appears to be some incompatibility between the Creative Labs sound card and the amp. I've been using the Creative Labs board for years with my Denon solid state amp and have never had a problem.

My theory is that when playing back higher "quality" music (I consider MP3 and CD to be higher "quality" than streamed), the Creative Labs board generates more harmonics and these harmonics aren't being suppressed enough by the shielded cable and then cause a problem in the amp. I'm not sure if I should be trying to do more shielding on the cable or if there is some mod I can do on the amp to prevent the problem. I'm hoping someone on this forum can provide some insight.

I hope you have a way of checking your idle current (bias). Those preamp power sockets on the front also serve as test points for the bias of your output tubes.

ST-70 has no capacitor at the input. A DC offset will distort the signal, and maybe your soundcard
can create a dc offset.
A Cap in series will prove or reject this theory.

First thing I'd do is adjust the soundcard output level (volume) ... should be easy enough in the control panel. Possibly just overdriving the amp's input.

Or get an outboard DAC. I get the best results with my Maverick down three or four notches from full output, the amp also turned down some, jRiver Media Center at 95%. and full system volume out of the HTPC.

The inter-relatedness of all things ... oooOOOooohmmmmmmmmmmm ...

I also had a problem with the center channel of my oPPo SACD overdriving a Carver Cube ... simple enough fix. Just cut an RCA in half and wired in a pot ...



Ya like the custom case? Next time I'll have to remember to clean the applesauce out first.

I also think a small decoupling cap in series with the input should fix this.

corndog71 wrote:I hope you have a way of checking your idle current (bias).  Those preamp power sockets on the front also serve as test points for the bias of your output tubes.

I connected the bias test points to an unused eyelet on the driver board and check it there. Eventually, I will add banana jacks for test probes in place of the 4 ohm speaker outputs since I only need the 8 ohm output.

sKiZo wrote:First thing I'd do is adjust the soundcard output level (volume) ... should be easy enough in the control panel. Possibly just overdriving the amp's input.

Or get an outboard DAC. I get the best results with my Maverick down three or four notches from full output, the amp also turned down some, jRiver Media Center at 95%. and full system volume out of the HTPC.

The inter-relatedness of all things ... oooOOOooohmmmmmmmmmmm ...

I also had a problem with the center channel of my oPPo SACD overdriving a Carver Cube ... simple enough fix. Just cut an RCA in half and wired in a pot ...



Ya like the custom case? Next time I'll have to remember to clean the applesauce out first.

I've already experimented with various output level settings. Didn't make a difference.

bluerondo wrote:I also think a small decoupling cap in series with the input should fix this.

Since I made my first post I did another mod to the amp. I used metal tape to tape the shielded cable connecting the rear input jack to the front input selector switch to the chassis. It made a big difference. I was able to play MP3s and CDs for 4 straight hours without any distortion. That's more than twice as long as before. After 4 hrs. I got some distortion that lasted about 2 seconds then went away on its own. I played another 55 minutes without any problem then started getting a crackling sound. I could still hear the music okay but with the noise on top of it. Reminded me of how it sounds when playing a dirty record. When I paused the music the noise also stopped so it wasn't just random amp noise.

Adding a small cap to the inputs also occurred to me but I was thinking of putting it from the input to ground at the mini jack connector. I thought a 100 pf cap might kill the harmful high frequency harmonics without having any negative effects on the audio signal. What is your opinion of doing this?

gabe wrote:

bluerondo wrote:I also think a small decoupling cap in series with the input should fix this.

Since I made my first post I did another mod to the amp.  I used metal tape to tape the shielded cable connecting the rear input jack to the front input selector switch to the chassis.  It made a big difference.  I was able to play MP3s and CDs for 4 straight hours without any distortion.  That's more than twice as long as before.  After 4 hrs. I got some distortion that lasted about 2 seconds then went away on its own.  I played another 55 minutes without any problem then started getting a crackling sound.  I could still hear the music okay but with the noise on top of it.  Reminded me of how it sounds when playing a dirty record.  When I paused the music the noise also stopped so it wasn't just random amp noise.

Adding a small cap to the inputs also occurred to me but I was thinking of putting it from the input to ground at the mini jack connector.  I thought a 100 pf cap might kill the harmful high frequency harmonics without having any negative effects on the audio signal.  What is your opinion of doing this?

A small cap to ground is another thing , it will generally prevent higher frequencys entering the amp.
What i (and other) suggested was a cap IN SERIES with the input, this as st-70 has the first
grid DC-coupled to the input, and DC offset in the signal will adversely affect the sound.

peterh wrote:

gabe wrote:

bluerondo wrote:I also think a small decoupling cap in series with the input should fix this.

Since I made my first post I did another mod to the amp.  I used metal tape to tape the shielded cable connecting the rear input jack to the front input selector switch to the chassis.  It made a big difference.  I was able to play MP3s and CDs for 4 straight hours without any distortion.  That's more than twice as long as before.  After 4 hrs. I got some distortion that lasted about 2 seconds then went away on its own.  I played another 55 minutes without any problem then started getting a crackling sound.  I could still hear the music okay but with the noise on top of it.  Reminded me of how it sounds when playing a dirty record.  When I paused the music the noise also stopped so it wasn't just random amp noise.

Adding a small cap to the inputs also occurred to me but I was thinking of putting it from the input to ground at the mini jack connector.  I thought a 100 pf cap might kill the harmful high frequency harmonics without having any negative effects on the audio signal.  What is your opinion of doing this?

A small cap to ground is another thing , it will generally prevent higher frequencys entering the amp.
What i (and other) suggested was a cap IN SERIES with the input, this as st-70 has the first
grid DC-coupled to the input, and DC offset in the signal will adversely  affect the sound.

Preventing higher frequencies from entering the amp was my intention since I believed that to be the cause of my original problem. I wasn't aware of the problem with the DC offset. Thanks for the heads up. The metal tape I added seems to have fixed the original problem. I'm not sure if the new symptoms I'm seeing are related to the original problem or something new. What is your opinion?

I assume your suggestion to add a series capacitor was in response to my original problem. Would you also recommend doing that in light of what I'm seeing now? If so, what value of capacitor would you recommend and does it matter which type (mica, ceramic disk, etc.)?

gabe wrote:

peterh wrote:

gabe wrote:

bluerondo wrote:I also think a small decoupling cap in series with the input should fix this.

Since I made my first post I did another mod to the amp.  I used metal tape to tape the shielded cable connecting the rear input jack to the front input selector switch to the chassis.  It made a big difference.  I was able to play MP3s and CDs for 4 straight hours without any distortion.  That's more than twice as long as before.  After 4 hrs. I got some distortion that lasted about 2 seconds then went away on its own.  I played another 55 minutes without any problem then started getting a crackling sound.  I could still hear the music okay but with the noise on top of it.  Reminded me of how it sounds when playing a dirty record.  When I paused the music the noise also stopped so it wasn't just random amp noise.

Adding a small cap to the inputs also occurred to me but I was thinking of putting it from the input to ground at the mini jack connector.  I thought a 100 pf cap might kill the harmful high frequency harmonics without having any negative effects on the audio signal.  What is your opinion of doing this?

A small cap to ground is another thing , it will generally prevent higher frequencys entering the amp.
What i (and other) suggested was a cap IN SERIES with the input, this as st-70 has the first
grid DC-coupled to the input, and DC offset in the signal will adversely  affect the sound.

Preventing higher frequencies from entering the amp was my intention since I believed that to be the cause of my original problem.  I wasn't aware of the problem with the DC offset.  Thanks for the heads up. The metal tape I added seems to have fixed the original problem.  I'm not sure if the new symptoms I'm seeing are related to the original problem or something new.  What is your opinion?

I assume your suggestion to add a series capacitor was in response to my original problem.  Would you also recommend doing that in light of what I'm seeing now?  If so, what value of capacitor would you recommend and does it matter which type (mica, ceramic disk, etc.)?

A folie type ( polyproylen , paper ) 0.22 uF would do. Any voltage will do as we do not expect more then a few volts
This could be mounted on the rca itself, elongated with a wire insulated with crimp.

I wonder why the various boards available has left this out ( or at least a place to mount it)

One _should_ design stuff like these with the assumption : expect input to be worst case, expect output to
be shorted and/or open, and then design accordingly.

Your friendly neighborhood Radio Shack will sell you a non-polarized cap for $1-2 each that would work perfectly in this application. Any small value 0.1uF to 1.0uF low voltage (>10V) cap would be fine. Do you have a DVM to check the output of your sound card for DC content? Have you tried using another source like a CD or DVD player or tuner?