VTA ST120 Bias Adjustment

Some more insight to tube matching...

http://www.ramlabs-musicreference.com/virtues.html

I discovered what appears to be some arching beween the quad cap section three terminal where it connects to the 4700 ohm resistor and the chassis. There was black discoloration on the chassis beneath that connection. I bent the terminal and resistor up a bit to provide more clearance and put a piece of electrical tape beneath it on the chassis. Now the amp biases more easily and holds steady.

After replacing all of the 10 ohm 2 watt bias resistors at least once over the past five months I'm hoping I have solved and fixed this issue.

Dang it anyway jim! I swear that after I replaced the last bias resistor and made more space between the quad cap terminal and the chassis my biases were more steady and easier to adjust but tonight during the first couple minutes of warmup I heard the dreaded pop and crackle out of the left channel and suspect I have blown yet another bias resistor.

On my day off this Friday I will replace the bad bias resistor and check all voltages paying close attention to those on the rectifier pins.

This recurring problem is driving me nuts!!!

So Troy recommends that I put 10 ohm 5 watt resistors in place of the 10 ohm 2 watts.

Any opinions on this?

The 2W rating on those resistors are meant to be sacrificial ... they act as fuses to protect the circuit. Not sure as upping the rating is such a good idea as you could end up with a more expensive failure.

If all the connections are good and you're getting good test voltages otherwise, I'd still suspect a bad bias pot that's allowing the circuit to go intermittent. I had one go bad and take out a resistor and red plate the tube. Replaced all of them and haven't had a problem since.

sKiZo wrote:The 2W rating on those resistors are meant to be sacrificial ... they act as fuses to protect the circuit. Not sure as upping the rating is such a good idea as you could end up with a more expensive failure.

If all the connections are good and you're getting good test voltages otherwise, I'd still suspect a bad bias pot that's allowing the circuit to go intermittent. I had one go bad and take out a resistor and red plate the tube. Replaced all of them and haven't had a problem since.

Or possibly a flashing power tube -- or possibly some transients/disturbances that creates a huge voltage peak in the preamp-amp chain -- or just dropping the needle with volume turned up.

Grounding together all audio gear and possibly a linefilter on the common powerbar could reduce sensitivity to voltage spikes and disturmaces from outside.

Another source could be short power interrupts, especially if the amp is "improved" with larger caps. Then the bias supply might sink faster then the B+ and this might be lethal!

I agree with sKiZu here, the bias resistors will last forever unless something goes wrong. Then they are supposed to disappear in a cloud of smoke, and this faster then a fuse would do.

Regarding sKiZo's point about possible issues with the bias pots, since this has happened on all four bias resistors (i.e., all four pots and output tubes), I doubt that I have four bad bias pots.

I have two sets of KT88s and I have cycled four different rectifier tubes through the amp during this issue so I think I can rule out bad tubes.

I'll check solder connections on the board and probably install 10 ohm 3 watt resistors.

If this continues I'll start thinking about sending the amp in for service.

guentherj2 wrote:Regarding sKiZo's point about possible issues with the bias pots, since this has happened on all four bias resistors (i.e., all four pots and output tubes), I doubt that I have four bad bias pots.

I have two sets of KT88s and I have cycled four different rectifier tubes through the amp during this issue so I think I can rule out bad tubes.

I'll check solder connections on the board and probably install 10 ohm 3 watt resistors.

If this continues I'll start thinking about sending the amp in for service.

Since postage is a bit on the heavy side for these things, I'd just pull the boards and send them to Troy.

deepee99 wrote:

guentherj2 wrote:Regarding sKiZo's point about possible issues with the bias pots, since this has happened on all four bias resistors (i.e., all four pots and output tubes), I doubt that I have four bad bias pots.

I have two sets of KT88s and I have cycled four different rectifier tubes through the amp during this issue so I think I can rule out bad tubes.

I'll check solder connections on the board and probably install 10 ohm 3 watt resistors.

If this continues I'll start thinking about sending the amp in for service.

Since postage is a bit on the heavy side for these things, I'd just pull the boards and send them to Troy.

I think, deepee's idea makes good sense, after you have looked at all the solder joints on the rest of the amp and convinced yourself that they all look up to snuff.

Thanks for your 2 cents Elrick. Now you should come over and enjoy the Fisher system. No issues with it!

Maybe after I finish up with some more Funkadelic through my Mark III's.

Where is a good price for kt 120's ?

I probably paid a bit more buying from the Tube Store, but they did a nice job of matching them ...

http://www.thetubestore.com/Tubes/KT88-Tube-Types/Tung-Sol-KT120

Reminds me of a fun conversation I had after purchase about the stickers they put on the base.

So, how do I get these silly stickers off the tubes?

The stickers aren't silly!

OK then ... how do I get these silly tubes off the stickers??

Anyway ... the secret is heat. Take a blow drier to them and they peel right off. Those shiny chrome bases aren't meant to be covered.

I think I'd order them from here:

http://www.mcshanedesign.net/tubes.htm

Or here:

http://tctubes.com/Tung-Sol-KT120-tubes.aspx

Both seem to do an excellent job of burning in and matching power tubes.

Quality of testing and matching is more important than the final price.

I see McShane's got the KT120's for $190 a quad ... not too shabby! TC's price is even better.

I'm looking for a reliable set of backups - the Psvane KT88's are drifting all over the place and won't hold bias. Bummer. I see the Genelex KT88 reissues aren't much more than the KT120's and are highly regarded ... Hmmmm ...

They're drifting already?  Wow, that didn't take long.  A few hundred hours of use, maybe?

I have a quad of KT-120s that I got from The Tube Store.  So far so good, but they have probably less than 100 hours on them.  Would have ordered them from Mcshane or TC Tubes, but The Tube Store had a good sale going on at the time.  Oh, and the stickers on the base fall off naturally from use (heat).

Bob,

The poster of this thread having problems with the resistors blowing has another thread on the other forum...  http://www.audiokarma.org/forums/showthread.php?t=589646&page=3

The person is reaching out for help on the issues he has had since he got his ST120 amp up and running 8 months ago.  Would you normally accept a return on an ST120 when it has these types of issues right out of the gate?  Feel bad for the guy spending a grand and having this happen...  He isn't even using the amp anymore having gave up and now runs his older Fisher.

Pics would help.  I wonder if he just got a bad run of tubes.  That's happened to me back when I had an ST70.  I went through several Sovtek and JJ 5AR4 tubes before settling on Ruby selected 5AR4.  I didn't have problems with those.  The cheap EH EL34s I had were not only boring sounding but ended up either red-plating within a few months or getting zapped by my failing rectifier tubes.  I was using those cheap ceramic sockets too which can also cause problems.  Every other wire and connection was solid.  Cheap tubes and bad tubes and bad sockets caused me all sorts of headaches.

I used the octal sockets that came with my ST120 kit and so far have had no issues.  I used all Belton sockets for my ST35 and really like the serious grip they have on the tube pins.  When I build another ST70 I will use all Belton sockets.

He mentioned swapping in other KT88 and GZ34 tubes on "The other forum"

PS - I'm not trying to start any trouble here... I just feel for someone having this happen right out of the gate on a 1K$ amp.

mantha3 wrote:He mentioned swapping in other KT88 and GZ34 tubes on "The other forum"

PS - I'm not trying to start any trouble here...  I just feel for someone having this happen right out of the gate on a 1K$ amp.  

If it's not the tubes then he missed something, or his soldering skills need improving, or he has a bad part(s). Simply going over every connection can eliminate most wiring problems. Beyond wiring problems that leaves the tubes. I don't know where he got his tubes from but for all we know they could all be random and terribly mismatched.

I can appreciate how intimidating this can be to a noob. The point is with patience and tenacity he should be able to solve this. It is after all a kit.

corndog71 wrote:

mantha3 wrote:He mentioned swapping in other KT88 and GZ34 tubes on "The other forum"

PS - I'm not trying to start any trouble here...  I just feel for someone having this happen right out of the gate on a 1K$ amp.  

If it's not the tubes then he missed something, or his soldering skills need improving, or he has a bad part(s).  Simply going over every connection can eliminate most wiring problems.  Beyond wiring problems that leaves the tubes.  I don't know where he got his tubes from but for all we know they could all be random and terribly mismatched.

I can appreciate how intimidating this can be to a noob.  The point is with patience and tenacity he should be able to solve this.  It is after all a kit.  

He had the amp built by someone. I agree with you on that thought. I said to the amp owner this advice:

When you build an ST120 you have excellent build instructions from Bob. I built mine.

I would flip that amp over and just review each step from start. weak solder spot? stuff like that. If you really just go step by step and review each step I think you could be done in 1 hour or 2. I bet you spot something. Bob also mentions in the build instructions some details where you want resistor leads as short as possible.

If you build an ST120 right it should not have this issue and work so long it still runs 40 years later like the ST70s we see running today that are 40-50 years old

Tough situation... Did the amp builder make an error? Is some part in the amp not in spec and throwing things off...?

Thanks for the empathy mantha3. I have two sets of KT88s - the Sovtek set I got from Bob with the amp and a set of NOS Svetlanas. I've rolled the 5AR4s (a new Gold Lion, a NOS Sovtek and a NOS Mullard. I just got a new Tung Sol 5AR4 from Tube Depot and will put that in when I think I'm good to go. I am doing what you suggested - going through and inspecting all the build and solder connections even though I got the amp built. From what I understand, if a KT88 arcs it is likely to do that again so if I blow more resistors I will pull that tube, mark it and replace it along with the bias resistor. Yes, this is frustrating since I got the amp built and have been dealing with replacing bias resistors since I've had it. I hope that eventually it will work for a while without problems.

as has already been pointed out, and I speak from first hand experience, it is usually a bad solder joint.
Tube circuitry by nature uses high voltages, so is more sensitive to bad soldering than conventional SS circuitry.....in most cases   
Although my problem/bad solder joint was not as severe as yours, I had a low level hum, turns out the ground connection to the VTA was bad.
A good thing about fault finding, you learn a lot.
Good luck, I'm sure you'll be up and running and enjoying awesome music in no time!

OK - lets put this thread to bed. I sent my amp back to the builder and expect to get it back in working condition. I have had good feecback from Bob and the builder throughout this problem and hoped that I would have been able to solve it. Thank you all for your input. The Fisher X100B is a reliable stanby so I can continue appreciating my music in the meantime - although not quite as satisfying as the ST120..