ST-120 Issues

I've already emailed Bob about this but I thought you guys might be able to help me out as well. I decided to upgrade the caps on my driver board with Russian caps, and the procedure was a simple swap...To my surprise however, after about a minute of having the amp on, the filament in the back left tube glowed very brightly and was red plating! I switched the amp off as soon as I could, then began checking all the connections made to that particular socket. I also measured the voltages out of each socket on pins 5 and 6, which turned out to be in spec, as well as confirming the 10 ohm measurement from biaset to ground. I then proceed to plug all the power tubes back into their sockets. After about a minute of power up, the troubled tube no longer red plated but loud distorted and crackling noises came out of the left speaker even with the volume all the way down. The bias measured good and stable for all tubes except that back left tube, which measured about 0.200V and about 18.5mA. Well below spec. Strange... I tapped each tube to see if any of them had gone microphonic and interestingly enough every tube I tapped resulted in a loud distorted ringing noise, but isolated to the left channel. I'm not really sure what has caused this and I hope someone can help me out! Thanks!

Could just be bad luck and the tube protested to the upheaval... But if you can post a clear photo of your work then the smart eyes on here might just notice something.Given the microphony on the left channel i'd carefully check your work. Does the bias inequality follow the output tubes if you swap them over? For the tube with the low standing current, can this be altered using the bais pot? What is the voltage on the grid.

For reference with Sovtec 6550:-
With an HT of 509V on mine (5AR4) the grids are at about -60V for 50mA.
With an HT of 540V on mine (SS Rect) the grids are at about -65V for 50mA.

As has been said before, running a tube amp without the tubes in is not a good idea due to the increased HT. But in your case, if it were my amp, i'd leave the left pair of output tubes out and power up for a short period whilst monitoring the grid voltages. These should be as above (approx) and alterable using the bias pot. Any low voltage or even positive voltage will indicate that maybe one of your PIO caps is leaky.

Don't run for too long in this state, but in reality it's not far from what happens when the amp is first switched on and the 5AR4 comes up before the output tubes...

Steve

thermionicvinyl wrote:I've already emailed Bob about this but I thought you guys might be able to help me out as well. I decided to upgrade the caps on my driver board with Russian caps, and the procedure was a simple swap...To my surprise however, after about a minute of having the amp on, the filament in the back left tube glowed very brightly and was red plating! I switched the amp off as soon as I could, then began checking all the connections made to that particular socket. I also measured the voltages out of each socket on pins 5 and 6, which turned out to be in spec, as well as confirming the 10 ohm measurement from biaset to ground. I then proceed to plug all the power tubes back into their sockets. After about a minute of power up, the troubled tube no longer red plated but loud distorted and crackling noises came out of the left speaker even with the volume all the way down. The bias measured good and stable for all tubes except that back left tube, which measured about 0.200V and about 18.5mA. Well below spec. Strange... I tapped each tube to see if any of them had gone microphonic and interestingly enough every tube I tapped resulted in a loud distorted ringing noise, but isolated to the left channel. I'm not really sure what has caused this and I hope someone can help me out! Thanks!

The "russian caps" have a metal enclosure. If they will contact anything it might
remove the affected tubes bias.
The caps should be wrapped in insulating material ( tape, crimp whatever) to prevent this.

The affected tube is junk. Remove and get at least a matched pair to replace.

A couple of things right off the bat:

a) This is why I am suspicious of "boutique" caps. Unless one is capable of screening and testing such caps "at home", one really never knows what one will get. Further to this, the entire issue of counterfeit parts comes into play when they are foreign and from a 'troubled' source. Even honorable local sellers of parts can be fooled.

b) Nor would I "junk" the red-plated tube without further testing. On the other hand I have a GM-type tube tester, so that attitude may be because I have the means to verify the quality of a tube.

With that in mind, you want to re-check all resistances, verify all connections, looking for cold-solders, broken traces, whatever.

Clean and tighten the octal sockets - much discussion on how already, here.
And, were it me, I would test out-of-circuit, each and every one of those PIO caps. If they vary-from-spec by more than a percent-or-two (the limits of your meter) and from each other, the same - junk them and start over with a good-quality film cap.

If you retain the PIO caps, as-noted, insulate them. Use heat-shrink or similar, not just black tape.

If you do not have a tube-tester, you can look for shorts, anyway, on the frazzled tube. Do at least that.

Best of luck with it!

being its only one tube,my money is on a tube pin, thats going by what you've said,
maybe some skank got in there when you pulled them out to do the job initially.
or a pin got loosened.

Some good points... Verify metal cap enclosures doesn't touch anything as the DC bias voltage supply circuits are directly below where the coupling caps mount. Also re-tension/clean the power tube pins to ensure they make solid contact. But as mentioned may just be a coincidence

All of the above. Good time to also take real close look at the board and snip any long tails that might be shorting either to the chassis or the Russian PIOs (if that's what you're using).

And yah ... tubes don't take too kindly to red plating. Yours is probably toast, or at least significantly compromised ...

I'd also take a close look at the tube connections on that socket, including the bias resistor to ground. Not uncommon for those to work, get jostled about during a wiring session, and go fubar.

As far as insulation on the metal jackets go, I figure I probably went a bit overboard here, but they ARE kind of purty they are ...

good point about the tube possibly going south,have you tried switching sides with the tubes. 1 at a time, to verify the tubes are not subject.

I decided to try swapping the tubes around to see if the problem “follows” the tube or remains that that socket. After switching left back and right back tubes, the very bias low voltage and amperage was now over on the right back socket whereas before it was in the back left. I checked the back left socket (the original problem area) just to make sure and the bias for that tube was well within spec. I haven’t tried opening the amp yet but I wanted to know what you guys thought of this.

thermionicvinyl wrote:I decided to try swapping the tubes around to see if the problem “follows” the tube or remains that that socket. After switching left back and right back tubes, the very bias low voltage and amperage was now over on the right back socket whereas before it was in the back left. I checked the back left socket (the original problem area)  just to make sure and the bias for that tube was well within spec. I haven’t tried opening the amp yet but I wanted to know what you guys thought of this.

Seems like the tube is well-and-truly toast. That is usually the case when the problem follows the tube.

Simple solution - but before willy-nilly installing a new pair of tubes, do check _EVERYTHING ELSE_ first. You do not want to toast a brand new tube.

Checked everything, capacitors looked and measured well, resoldered suspect connections, and cleaned tubes/sockets. The problem of very low bias still follows that particular tubes but the crackle is gone. It seems to be running much warmer than the other 3 even with the bias pot turned all the way down. I have replacement tubes on that way.

thermionicvinyl wrote:Checked everything, capacitors looked and measured well, resoldered suspect connections, and cleaned tubes/sockets. The problem of very low bias still follows that particular tubes but the crackle is gone. It seems to be running much warmer than the other 3 even with the bias pot turned all the way down. I have replacement tubes on that way.

Congratulations on a successful diagnosis and appropriate solution!

Haha! Now if you follow me over here: https://dynacotubeaudio.forumotion.com/t3768-ph-16-issues#35884

I’m trying to fixed something else as well!