Distortion and Hum ???

I'd been having distorted sound issues with my ST-120 for a while now and finally got the chance to try and figure out the cause and am not sure why it started and have I really fixed it.

Succinct:
Can a tube cause distorted sound? Can a tube cause hum?


Quick read:
At one point I began to hear my ST-120 sound was distorting at times. (??) Thought I noticed some distortion even at lower volume levels between 10:00 to 2:00 on the attenuator. The sound seemed to have rougher edges on the vocals; more so with a four or more member band. It sounded OK with acoustic guitar and vocals, yet seemed overwhelmed with complex passages and I could hear distortion.  Electric guitar, drums...

Then while troubleshooting the distortion issue a hum developed in both channels even with volume down and no input whatsoever. Going back to three RCA clear top's and a Sovtek 5AR4 seem to have resolved the issue of hum and distortion.

Here are the details of my "troubleshooting". I am an absolute beginner with electronics. So I now wonder if the combination of replacing tubes with the Sovtek GZ34 and RCA clear top's resolved the hum problem. If you are brave, read my detailed best memory of what I did to decide if it was a tube issue or something more serious.


The long of it:
In my ST-120 I had KT-120's, RCA 12AU7 Clear top's and Mullard GZ34.

To troubleshoot I by passed the line stage preamplifier and used just a Schiit Mani phonostage.

I reverted back to the tubes that came with the kit except for the Mullard GZ34 - 6550 power tubes and the GE 12AU7's.

Still thought I heard distortion even at moderate levels.

Went through a shell-game swapping between the driver board 12AU7's. First a used GE 12AU7 by Mullard, then a new 12AU7 Mullard in the center slot, Sometimes it seemed better, other times I thought I still heard distortion.

Swapped the Mullard GZ34 rectifier for the Chinese 5AR4; it arced, and the left side new Mullard 12AU7 driver board tube flashed. I powered off. Fuse survived. Tossed out the left 12AU7. Put the Mullard GZ34 back in.

Again I reverted to GE's that came with the amp. Seemed OK, although now I began to hear a hum. The hum occurred with the volume down all the way and with no input.

So now I'm troubleshooting hum and no longer the distortion issue.

I went through another round of tube swapping, and still hum.

I got three new Sovtek 12AU7W tubes. Still hum.

Swapped in Tung-Sol KT120's. The  Mullard GZ34 arced pretty badly, and the fuse blew. (I think I did not give the amp enough time before I power cycled. ?)

I replaced the Mullard GZ34 with a Sovtek 5AR4. The 6550's went back in as well. Waited 10 minutes and it powered on. Still hum.

Replaced the GE 12AU7's with the Sovtek 12AU7W's. Still hum.

Replaced the Sovtek's back to the RCA Clear Tops. Hum barely audible now.   (Normal?)

Tubes now used are the 6550 / RCA Clear tops and Sovtek 5AR4.

WHEW!

I'm not a tech like some of the people here but I've found that most tube amp problems are tube related and when you start messing with rolling tubes you must clean and re-tension your tube sockets.

I'm sure that early on in my VTA ST70 ownership I lost an NOS Mullard 5AR4 and one of my NOS Mullard EL 34's because I was messing around swapping tubes in and out without cleaning and especially, re-tensioning the sockets.

OK... one does what one can with what one has. And you seem to have gotten things down to a dull roar. But, a few questions:

a) Is/was the hum 60 Hz or 120 Hz? 60Hz is typical of incipient rectifier failure as AC is getting past it. 120 Hz suggests inadequate filter caps, or, again, incipient failure of same.

b) Distortion may be due to many issues, including poor rectification or poor filtration. Various small-signal tubes will amplify sections of frequencies in various ways, so distortion, hum presence, or not, and so forth will vary.

c) I expect that if my rampant speculation is correct, and you used 'the very best' tubes (but for the rectifier), that would give the worst results in terms of distortion or hum. From the other angle, I think the tubes in place are slightly deaf, so you are not less distortion and little hum.

I do not know how skilled you are at diagnostics, nor do I know what tools you have or how comfortable you might be around B+ voltages. But, the first thing I would test for is residual AC off the filter caps. If you measure more than a very few fractions of a VAC, you have a problem. And as you are well over 500 VDC on that line, you need to be exceedingly careful on how you measure, with what you measure and that you do not 'cross' anything.

If you live anywhere near 19027, I would be glad to have a look - same to HMMmmmmmm - gratis.

Peter W. wrote:OK... one does what one can with what one has. And you seem to have gotten things down to a dull roar. But, a few questions:

a) Is/was the hum 60 Hz or 120 Hz?  60Hz is typical of incipient rectifier failure as AC is getting past it. 120 Hz suggests inadequate filter caps, or, again, incipient failure of same.

b) Distortion may be due to many issues, including poor rectification or poor filtration. Various small-signal tubes will amplify sections of frequencies in various ways, so distortion, hum presence, or not, and so forth will vary.

c) I expect that if my rampant speculation is correct, and you used 'the very best' tubes (but for the rectifier), that would give the worst results in terms of distortion or hum. From the other angle, I think the tubes in place are slightly deaf, so you are not less distortion and little hum.

I do not know how skilled you are at diagnostics, nor do I know what tools you have or how comfortable you might be around B+ voltages. But, the first thing I would test for is residual AC off the filter caps. If you measure more than a very few fractions of a VAC, you have a problem. And as you are well over 500 VDC on that line, you need to be exceedingly careful on how you measure, with what you measure and that you do not 'cross' anything.

If you live anywhere near 19027, I would be glad to have a look - same to HMMmmmmmm - gratis.

A) Because I heard the hum mostly from the mid range driver I will guess it was 120Hz

C) Do you mean a so-so rectifier will cause hum? I don't understand what is meant that the tubes in place are "deaf".

I consider myself an "assembler" and not a technician. I do have a multi-meter and am willing test around high voltage. I just need directions as to what, and where to test. Shoot...I'm 4+ hours from your zip.

Thanks!

Please note the interpolations.

A) Because I heard the hum mostly from the mid range driver I will guess it was 120Hz

That is a good guess. 60 Hz hum is quite distinct and not typical of audio stuff - mostly SS and mostly solid-state rectifiers.

C) Do you mean a so-so rectifier will cause hum? I don't understand what is meant that the tubes in place are "deaf".

  /\/\/\/\/\/\/\/\ becomes /\/\/\/\/\/\  In the former case, you hear 60 Hz, in the latter case 120 Hz. So, knowing the frequency of the hum helps with diagnosis. And here is where a frequency counter can remove all doubt.  

I consider myself an "assembler" and not a technician. I do have a multi-meter and am willing test around high voltage. I just need directions as to what, and where to test. Shoot...I'm 4+ hours from your zip.

The first thing you want to test is for AC-over-DC on the B+ side. So, if you have a capable auto-ranging VOM (such as a Fluke, B&K or similar) put it on the ACV Setting and test all the DCV test points. If you get anything over a very small couple of fractions of a volt read-out on AC, your filter caps are on their way out. IF you get significant ACV on the rectifier output, then your rectifier is on its way out. And, of course, it could be both.

This is the time for a very good tube checker. This could give you a quality test rather than just an emissions test.

On tube 'deafness' - An amplifier handles all frequencies fed to it irrespective of source. However the response curve of any given small-signal tube varies considerably from tube-to-tube both new and as they age. "Considerably" is a weasel-word in this case, as the variations are not usually sufficient to matter *at all* in most cases. But in some cases, older tubes may be a little deaf at certain parts of the spectrum relative to other parts - tubes may also favor as well as be deaf. Hence "bright" or "neutral" or "dull" or "better at the low/high/whatever end". In other words, they vary.

Hope this helps. But the bottom line is that hum is seldom "typical".  

Thanks!

[/quote]

From a rank amateur's POV:
Output tubes rarely induce hum or distortion, in and of themselves. They either work or they don't, although before failure they will sound a bit dodgy. They're basically a GIGO device.
Signal tubes can drive you crazy.
I'd follow PeterW's advice and be sure to check for AC leaks, if you know what you're doing. Wear lead gloves around that 500-volt stuff and do your prodding with a wooden or plastic probe.
A properly functioning VTA amp should be quiet as a mouse fart, even with your ear right up next to the speakers.

Thanks for the insight on tube deafness! I expect to be able to test for voltage leakage sometime over the next week and will post my findings.

A big issue for me is I tend to "permanently" install my gear with careful cabling that is hidden. Then it becomes a small project undoing the installation for repairs. This week I'm getting a new cabinet for my gear that will make it much easier to move components. Ahhh....

jimmeq wrote:Thanks for the insight on tube deafness! I expect to be able to test for voltage leakage sometime over the next week and will post my findings.

A big issue for me is I tend to "permanently" install my gear with careful cabling that is hidden. Then it becomes a small project undoing the installation for repairs. This week I'm getting a new cabinet for my gear that will make it much easier to move components. Ahhh....

I have the same problem, jimmeq. Stereo's in a beautiful walnut cabinet but every time I move it away from the wall to get at something in the back, plugs fall out and I end up collecting dust-bunnies having to deal with lying on the floor until wife gets home and pulls me upright.
But it's *such* a pretty cabinet!

The short:
Following Peter W's's suggestion of checking voltages the ST-120 went to the bench. The voltages measured as they should, no issue there. I think the distorted sound was caused by the one or two wires connecting to the circuit board that were cold that I discovered and re soldered. The hum is caused by a brand new Tung Sol 12AU7WA. The original tubes supplied work fine. All is well!


....and long of it.
After putting most of the original tubes in the ST-120, (exception- rectifier tube) disconnecting, and moving it to a new console table the ST-120 was silent as it should be. I feel certain I heard a distinct hum before so I thought maybe I had a grounding issue that was causing the ST-120 to hum? Still thought I was hearing some distortion though it was playing.

Then one day it was absolutely silent and not the good silence when it's operating. No sound whatsoever. All tubes were glowing. I went through the standard troubleshooting and it was definitely the ST. Since I was going to check voltages as had been suggested, and had not used the amp for a few weeks I checked the capacitors first and they each met specification. Good!

So to be 100% certain there was no sound I turned it on. No sound. Then it hit me. The rectifier tube. It was a Sovtek which normally has a bright glow near the bottom. The top had a glow but the bottom was not glowing which is what caused me to think it might be bad.

After replacing the rectifier tube with a spare I had, the ST-120 began to play. After increasing the volume, the rectifier tube flashed and I powered down as quickly as possible. I have one more rectifier tube left but am now combing though the wiring and solder points to see if there is a short.

I found a couple of points where solder dripped. Trimmed them and while placing the board back on its screw foundations a wire broke loose. Obviously a cold solder point. And then another became loosened. (I'd blame it on shoddy workmanship but I was the assembler. Red-faced embarrassed.) Re-soldered, and checked several other wires and all was solid.

I put the original Sino rectifier tube in and powered on; the sound was wonderful! YES!!! I placed the ST-120 back into my system and had a great listening session! The raspy sound was gone, hard edges on vocals were smooth again and I was able to play some rock pretty loudly. "Song Of The Wind" from Santana's Caravanasari CD sounded the best it has ever sounded on the ST-120! Even from the beginning a year ago, this song has just not sounded right.

A few days passed and I thought I'd try the Tung Sol 12AU7WA's I got to sort out the hum issue. Powered on and hmmmmmmm. The hum returned! Powered down, went back to the original 12AU7's and the ST-120 was silent as it should be.

So I have gathered that the distortion/raspy/hard edge was cold solder points, and the hum is a bad 12AU7WA. Finally!, all is right with the world again.....

I'll try the Weber after all my rectifier tubes are used... I already bought a couple of inexpensive tubes in addition to the Sino that is in service. I'm learning...slowly and surely. Thanks!

Jim, forgive me if this was already asked and answered...just trying to help. Did you do the "yellow sheet mod"?

https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod

Yes, the yellow sheet mod was done. The AC is not problematic at my house so I removed the mod because one of the diodes went bad. My understanding is the mod is mostly needed for areas where the AC is prone to quick on/off/on cycles. (Of course now that I write this...)

Quote Jim McShane: "The diodes simply rectify the AC wave and therefore the rectifier tube never sees the large peak inverse voltage that it would normally see. Without the diodes the rectifier tube could be exposed to the difference between the B+ volts (say 450 volts) and the most negative AC voltage it sees when the sine wave is at its most negative point. That depends on the secondary voltage of the trafo, but 350 volts would not be uncommon. So the rectifier tube sees a net difference of 450 plus 350, or 800 volts. This was more than some of the current production GZ34s could take without flashing/arcing. With the rectifiers ahead of the rectifier tube they never see any of the AC negative voltage swing, they only see pulsating DC. So the maximum difference is under 450 volts (or whatever the B+ voltage is in that particular amp). This made it possible for the Sovtek and Shuguang/Chinese 5AR4s to survive. Fortunately the recently released new Tung-Sol and Genalex reissue rectos (and I suspect the new Mullard reissue as well) are much better tubes."

jimmeq wrote:Yes, the yellow sheet mod was done. The AC is not problematic at my house so I removed the mod because one of the diodes went bad. My understanding is the mod is mostly needed for areas where the AC is prone to quick on/off/on cycles. (Of course now that I write this...)  ;)

You are - wrong !
The diodes will limit the back-voltage the tube rectifier is exposed to. They won't save you
of power is turned off and back on ( nothing will !)
Limitation of back-voltage might save a rectifier tube from flashing , and possibky prolong it's life.

One of these are helpful



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peterh wrote:

jimmeq wrote:Yes, the yellow sheet mod was done. The AC is not problematic at my house so I removed the mod because one of the diodes went bad. My understanding is the mod is mostly needed for areas where the AC is prone to quick on/off/on cycles. (Of course now that I write this...)  

You are - wrong !
The diodes will limit the back-voltage the tube rectifier is exposed to. They won't save you
of power is turned off and back on ( nothing will !)
Limitation of back-voltage might save a rectifier tube from flashing , and possibky prolong it's life.

What causes back-voltage?

jimmeq wrote:

peterh wrote:

jimmeq wrote:Yes, the yellow sheet mod was done. The AC is not problematic at my house so I removed the mod because one of the diodes went bad. My understanding is the mod is mostly needed for areas where the AC is prone to quick on/off/on cycles. (Of course now that I write this...)  ;)

You are - wrong !
The diodes will limit the back-voltage the tube rectifier is exposed to. They won't save you
of power is turned off and back on ( nothing will !)
Limitation of back-voltage might save a rectifier tube from flashing , and possibky prolong it's life.

What causes back-voltage?

The AC from the transformer alternates between + 400V and -400V . The -400 is "back-voltage" as
the rectifier has to block it , in fact the rectifier has 800V ( or more ) across it. A series diode
will block some of this, thus reducing the reverse voltage.