New Member, New ST120!

Hi,

After several years of back and forth I'm finally dipping my toes in the tube ocean.

Purchased a VTA ST120 kit from Bob last week. Read all of the reviews all over the net on tube amp kits--Bob's is by far cream of the crop.

Since I've never owned a tube unit, I decided to start out with Bob's optional tube set Sovtek 6550WE, 3 12AU7, upgraded caps, 21 step attenuator  and and a gz43/5AR4 rectifier. My plan is to run the unit without a pre-amp since my only source is a Bryston DAC and it puts out a fair amount of voltage from the unbalanced outputs.

So anyhow, got the boxes Wednesday afternoon and spent 3 days of hard work (2 assembling the kit and painting the chassis; 1 day debugging).

I had a couple of early problems...first I mis-wired the rectifier tube which caused it to arc when i fired the unit up  Sorted that out with Bob's help in 10 minutes.

Then I checked for bias voltage--discovered I was shorting two low-voltage wires on the driver board to chassis (again, with Bob's help).

Cleaned that up, checked bias voltages (all good), inserted output tubes and fired the unit up to set bias and....rectifier tube arcs a second time . That took a little longer but sorted that out after a couple of hours of help from Bob's east coast tech Bill...(another great guy--thanks!)

Anyhow finally ready to roll at about 6PM last night. Set initial bias, played music for two hours, went to dinner (left the unit on), came back about 2 hours later and the unit was dead (blown fuse).  .

Sooooo this morning I rechecked all of the voltages, installed a fresh rectifier tube (thinking the first rectifier tube may have gone south after all of the punishment I meted out during the diagnostic / initial start-up runs), installed a new fuse, re-biased the tubes (drum roll please.......)

So far so good. Been humming along at moderate volume for the last 2 hours without issue.

Keeping my fingers crossed that the rectifier tube was the issue. However if another fuse pops with the new rectifier tube, Bob suggested I recheck solder joints while I'm cleaning up the rats nest I created when I undid all of the tie wraps during the debugging phase--Bob suggested that heat expansion can cause a poor solder connection to weaken to the point of failure and given this is my first project of this sort I don't doubt my soldering skills are still suspect.

Quite an adventure, more to come (including pictures if I can ever figure that out...)

Oh yea, I have to mention that Bob is the most customer friendly, supportive guy--I used to run a very large customer support center for a very large company, so I get customer service/support. Bob (and Bill) get 5 STARS all around for putting the customer first!!!

Jay

Jay . Welcome . Bob and all the folks here are spot on. I would not , however, leave a tube amp on
and go leave it. As our buddy (loosely put) DeePee has said "don't turn your back on dem bitches" . Dale

Dale Stevens wrote:Jay . Welcome .   Bob and all the folks here are spot on.  I would not , however, leave a tube amp on
       and go leave it.  As our buddy (loosely put) DeePee has said "don't turn your back on  dem bitches" .    Dale              

Haha, understand--I'll shut it down next time we do a pizza run

It has been running continuously for over 6 hours today without issue, so I'm cautiously optimistic that swapping out the first rectifier (which arc'd twice) addressed the problem!

Cheers and happy Labor Day to all!

jsl1234, One thing you need to know about the 5AR4 tube is that once it starts arcing for whatever reason, it will most likely do it again soon.  Arcing damages the tube immediately. So, whether it was due to faulty wiring or just age, it needs to be replaced.  When it arcs, it will blow the fuse.

You should keep a 5U4GB tube around for when the 5AR4 fails until you get another 5AR4..  You can get NOS for cheap on Ebay.
You'll need to re-bias, but otherwise the amp will function normally.

You might just decide to keep using the 5U4GB because it is a much more robust tube that is more reliable than the 5AR4.
The only drawback is that you will have a slightly lower B+. Unless you run the system at window rattling levels, you will likely not notice a difference.

I've never really thought about tube amps being like the women in my life but i can see the similarities now that deepee put everything into the proper perspective.

And its true. When you are giving them 100% of your undivided attention Rectifiers can get really hot and sensuous...and when you take your eyes (and ears) off of them to focus on something they get all sparky and flare up and go all evil on you.

Tubes4ever wrote:jsl1234, One thing you need to know about the 5AR4 tube is that once it starts arcing for whatever reason, it will most likely do it again soon.  Arcing damages the tube immediately. So, whether it was due to faulty wiring or just age, it needs to be replaced.  When it arcs, it will blow the fuse.

You should keep a 5U4GB tube around for when the 5AR4 fails until you get another 5AR4..  You can get NOS for cheap on Ebay.
You'll need to re-bias, but otherwise the amp will function normally.

You might just decide to keep using the 5U4GB because it is a much more robust tube that is more reliable than the 5AR4.
The only drawback is that you will have a slightly lower B+.  Unless you run the system at window rattling levels, you will likely not notice a difference.

Thanks for the tip. I had already run out and purchased 2 spare JJ GZ34 / 5AR4 at a local music shop here in Richmond knowing I was going to need them I figure I'll run through those for however long they last (hopefully a while).

What do you think of the solid-state cooper cap? It seems like a much more reliable alternative altogether (i did not howver install the circuit delay option so not sure I can through in the SS unit without the modification.)

Quick question to verify I have my facts right regarding needing a preamp (or not in this case).

My only source to the amp is my Bryston DAC. According to the Byston spec the DAC outputs 2.3v unbalanced with 100 ohm impedance. As near as I can tell the input sensitivity of the amp is 1.4 volts to reach peak output (60 watts).

So I guess adding a preamp will not effect the 'loudness' of the amp, only drive it to peak sooner (say 12 o'clock on the volume control vs maybe closer to 3 o'clock.....?)

jsl1234 wrote:What do you think of the solid-state cooper cap? It seems like a much more reliable alternative altogether (i did not howver install the circuit delay option so not sure I can through in the SS unit without the modification.)

They are more reliable, but you lose the nice glow of the rectifier tube.  I prefer only tubes to be the active components on my tube amp. But others here use the Weber Copper Cap and are happy with it.

You could also perform the "Yellow sheet mod" which is a sticky on the forum.  This involves soldering two diodes and moving two wires around on the rectifier socket.  This will greatly reduce the arcing risk with the 5AR4.  However, in this case the diodes are doing the actual rectification and the tube is simply acting to drop the B+ to its normal level.  I prefer my rectifier tubes to do the actual rectification, but that's just me.

As for needing a preamp, you don't really need one as what you have puts out more than enough voltage to drive the amp to full output.

Just started hearing a very low level scratching noise out of one channel.
Volume control doesn’t alter this very low level noise. Reseated all cables, shut off the source DAC, definitely the AMP.
Unit recently fully rewired by Bill when he installed a new SCM module for me and the tubes less than 100 hours.
Have not swapped speaker cables or output tubes to see if noise follows, is that the right approach to triage this?

Thanks in advance....

peterh wrote:

jsl1234 wrote:Just started hearing a very low level scratching noise out of one channel.
Volume control doesn’t alter this very low level noise. Reseated all cables, shut off the source DAC, definitely the AMP.
Unit recently fully rewired by Bill when he installed a new SCM module for me and the tubes less than 100 hours.
Have not swapped speaker cables or output tubes to see if noise follows, is that the right approach to triage this?

Thanks in advance....

No.
I would start with :
- turn off amp  wait a minute
- switch the two outer 12au7 tubes ( or ecc82 if you like )
- turn on. if scratch chnged side, get new 12au7 tubes,
if sound disappeard : bad luch. But it will come back some day
if sound stays on same side, :  swich leftmost 12au7 with middle one ( after power off)

Report back please.

Yes, What Peterh said. Also, consider that a lame tube in your pre-amp can produce the same racket, regardless of volume setting. I would fire up the power amp with the pre-amp OFF but still connected. That will help isolate the issue. If the amp running by itself is making noise, now you'l know. Double-check by powering-off the main amp(s) for a minute or two, turn the pre-amp on, let it warm up, then re-start the power amp. If you're hearing racket now and you weren't with just the amp on, the problem is likely a bad preamp tube.

Thanks. I just swapped the 12au7 left/right and and the same side is still crackles a bit. Next step is swap middle with left (do I have to put them in original position before swapping middle to left)?

BTW: I don’t run a preamp—I run straight out of my Bryston DAC to the amp. Will report back in a hour if the noise moves, is still there, etc...

jsl1234 wrote:Thanks. I just swapped the 12au7 left/right and and the same side is still crackles a bit. Next step is swap middle with left (do I have to put them in original position before swapping middle to left)?

BTW: I don’t run a preamp—I run straight out of my Bryston DAC to the amp. Will report back in a hour if the noise moves, is still there, etc...

Next step would be to clean and retension your sockets . . .

Well. Swapped the center with the right and the problem still persists.....

jsl1234 wrote:Well. Swapped the center with the right and the problem still persists.....

Then let's hope it's just a socket problem. If not, then it's high-voltage time.

peterh wrote:....Did you buy it assembled ? Then return to maker for diagnosis and repair. If
you made it yourself, start checking all you solder joints, check that no
components is making contacts , and look for suspicious color changes.
Poking around with a wooden stick with amp running might cause sound, this
indicates that the problem is where you poke. But this is starting to be
dangerous. Maybe external help is needed.

Be careful!

Bill reworked / rewired it for me after I did the first build re: My first try at this resulted in a rats nest of wiring plus a blown SCM cap. So I sent it to him to repair. Was working fine for the past two or three weeks.

I may be imaging this but very early this AM I ran it Triode mode for the first time just to see what it sounded like. Then shut it off after an hour, threw it back in Pentode mode where I normally run it. Not sure if that is when’s the noise started but I never noticed it before this morning..

jsl1234 wrote:

peterh wrote:....Did you buy it assembled ? Then return to maker for diagnosis and repair. If
you made it yourself, start checking all you solder joints, check that no
components is making contacts , and look for suspicious color changes.
Poking around with a wooden stick with amp running might cause sound, this
indicates that the problem is where you poke. But this is starting to be
dangerous. Maybe external help is needed.

Be careful!

Bill reworked / rewired it for me after I did the first build re: My first try at this resulted in a rats nest of wiring plus a blown SCM cap. So I sent it to him to repair. Was working fine for the past two or three weeks.

I may be imaging this but very early this AM I ran it Triode mode for the first time just to see what it sounded like. Then shut it off after an hour, threw it back in Pentode mode where I normally run it. Not sure if that is when’s the noise started but I never noticed it before this morning..

That pendtode-triode switch carries a hell of a lot of juice, which is why Bob L. recommends cranking the volume way down before toggling it. Others recommend powering things completely off before doing so. I'd certainly re-flow the solder on those big screw lugs on the switch with a soldering gun. Highly unlikely, but the switch contact(s) could be defective, too.

Have resoldered the input tube socket per Bill and also pentode switch per deepee99...

Will run for a while and see what shapes up and report back tomorrow.

Thanks for all of the advice.

Jay

Well, hard to say.

I resoldered the input tube socket last night, per Bill, plus checked the inside for any funky color on caps and resistors.

Reseated all the tubes in their original position, fired it up (all late last night).

noise came back and stayed there.

Shut it down to go to bed.

fired it up this morning but reversed the speaker wiring, just to make sure the sound followed the output terminals and didn’t stay with the speaker.

Scratching noise did follow the output, but after the amp warmed up for 10 minutes or so the noise abated!

Shut the amp down to head to the office.

Came home from the office about 2PM, put the speaker wires back in their correct position, let the amp warm up for minutes and blasted some Tom Petty and other nonsense just to drive the amp.

quiet as a church mouse from what I can tell all afternoon so far, even before it fully warmed up.

Just shut it down for now, I’ll fire it up again later tonight and watch closely, to see if the noise comes straight away after turn on and/or fades as the amp heats up.

maybe the noise is simply gone for now.

maybe it was a very dirty power line last night?

It’s a tube amp, more to come I’m sure....

Does it make sense to swap power tube positions first?

jsl1234 wrote:Does it make sense to swap power tube positions first?

Yes, swap tubes, see if the problem follows the tube(s) or is specific to the socket. Just be careful, retention, etc., etc.

peterh wrote:

jsl1234 wrote:Does it make sense to swap power tube positions first?

no
that would involve rebiasing all of them.

But using the triode/UL switches might be an idea as the problem started at the
time they were used.

I had a very similar problem and swapping the power tubes fixed the issue. Probably was a socket connection issue, but the problem was resolved. Rebiasing will not have an effect on whether or not the noise issue exists.