vta st-120 question

Hi all. I'm wondering if I have an issue with my vta st-120. What is happening is that when I turn it on and as it is warming up, I get a startling amount of static and clicks from the speakers. After a minute or so the static stops. It plays fine but the clicks and static is startling and frazzling my nerves ha, ha. Is it normal to hear the static as the tubes are warming up? It's done this from the first time I turned it on about 3 weeks ago. I did replace the center driver tube (because it flashed when I first turned it on after about a week or so) and one KT88 as Bob seemed to think it was shorted because I couldn't set the bias. The bias is fine on all tubes now but would like some info on the crackling on warm up. I have also noticed as well that all 4 KT88's are wobbly in their sockets but I think I still have good pin contact because if I wiggle the tubes while it's playing it has no effect. Are KT88's inheritly loose fitting? Thanks for your time...Cheers! Leigh

Thanks Bob for the quick response. Originally I only had a crackle from the right channel and when I replaces the KT88 and the center tube and replaced the 10ohm resister the crackle went away temperarily. I concluded that the problem probably came from the shorted KT88. Last night when I turned it on the crackle was back in both channels this time. Thinking back now I did need to use extra pressure putting the new center tube in. Maybe I cracked a solder joint then. I will check that center tube for cold solder joints as well as the other locations you mentioned and post a reply. I thought that since I had removed the KT88's a few times that I may have stretched the pin openings a bit. One thing does come to mind about the rectifier tube and that is, that the tube doesn't want to seat fully in the socket. I was affraid to try and seat it further as I was afraid I would break the tube in my hand. I'm sure I have a good connection on that tube though but will take a closer look. Thanks again...Leigh

Hi Bob. It's been a couple of days since I've had a chance to look at the amp. Last night I checked for cold solder joints and I reheated all tube sockets as needed. Basically I have inspected all the areas you mentioned except replacing the rectifier tube (I do have a spare). When I turned it back on last night there was a very slight crackling in the left speaker but not much. I listened for 45 minutes or so and then heard a faint popping in the left speaker. I turned the music down and it was a quite loud pop, pop, pop sound from the left channel. Maybe every two seconds or so. I shut it off for the evening and tried 3 new jj's and then turned it back on. There was a faint hum (which I've never heard before) and then crackling noise coming from both speakers louder than I have ever heard. My plan was to double check the bias on the kt88's but I was afraid to leave it turned on the crackling was so loud. Before opening it up I did wiggle the driver tubes to see if the static changed but it seemed to have no affect. I checked it tonight with no input to the amp and the stepped pot set as low as it would go. Would it damage the amp if I removed the driver tubes and then turned it back on to see if the crackling has gone? I don't really know what else to check other than removing the vta board and scrutinizing it further. Sorry this is "long winded" but I wanted to explain as much as I could. Any advice would be appreciated from anyone on the forum. Cheers! Leigh

I popped out all 3 driver tubes and turned it back on. A small but definitely noticeable amount of crackle from both speakers. Can bias still be checked without the driver tubes? Thanks...Leigh

All KT88's biased and stable at.550 volts but still a minor crackle from both speakers that sounds exactly the same from both speakers. Ok its been on for 10 minutes or so and now no noise from either speaker. Could a choke cause that?

In went the left and right driver tubes. Still the same crackle from both channels which I expected. After warm up it seems to subside and then every 30 seconds or so I get a crackle from both speakers. I've wiggled the kt88's and rectifier tube, tapped the quad cap and various other parts and no change. Seems the more it runs the crackle subsides which makes me think something is warming up and stopping the noise. Bob, I must be frying your mind right now. Sorry about that! Tomorrow I will insert the center tube and await the excitement! Work prevails now so must sleep! Any suggestions from any forum members would be greatly appreciated. Thanks..Leigh

I can point you to an affordable LCR meter that really works well. Having previously been burned with an LCR meter that wouldn't measure inductance,
I bought an Elenco LCM-1950 from Parts Express. It's the one they advertise as their best seller. I measured some inductors and caps of known value and
it is dead accurate. I can't vouch for the other capacitance meters they sell because I haven't tried them, but for $95, this one works.

Thanks Bob and ramon68, I will check into a capacitor tester. In the mean time I am going to look closer at the solder joints around the caps and tube sockets. I haven't yet turned it back on with the center tube installed because I'm pretty sure I know what the result will be. I've built so many kits over the last 40 years with great success but this one is tinkering with my head haha. Mind you my eyes aren't what the used to be either. Bob, am I safe to assume that because the driver tubes were taken out on last test that possibly it's not the driver board that's causing the problem? I was thinking today at work that maybe it was the quad cap at fault but wondered why. Thanks for everyone's help and I know it will be up and running in no time! I will keep you posted. Cheers!....Leigh

Bob.

I opened up the amp last night for another look around and still couldn't see any issues so decided to fire it up while open and see if by tapping any caps etcetera I could pin point the problem. I was using a plastic nut driver so I was well insulated. I put the output tubes and rectifier in but left the driver tubes out. Fired it up to the same familiar crackle in both speakers as well as a popping in the left channel. What I did see was that every time I heard a pop I could see an arc on the vta board between pins 1-2 or 8-9 (not exactly sure which pin is 1 and which pin is 9. Regardless it is the trace between the pins at about the 7 oclock position when looking inside the amp with the back of the amp closest to me. One leg of the .149uf upgrade capacitor is also connected to the same trace. There is no solder bridge between the two pins (as I checked with an ohm meter) and the solder joints look fine but it is right at the thinnest part of the trace that seems to run between them that arcs. I should be getting my LCR meter tomorrow (Wed) but thought I would run this past you. Could the .149uf cap be at fault here possibly? Thanks..Leigh

Thanks Bob I will let you know what type of readings I get. I've never had the need to check capacitors before. Is it ok to leave them in the circuit when checking them or must they be removed? Also if I find it to be bad will the amp work if I remove it. I ask only because I'm not sure if these caps are "extra" ones or if they are "replacing" what would have come with the amp otherwise. Thanks again.....Leigh

Thanks Bob and don't worry it will be off. I have been looking at pics of other peoples st-120's and I notice that on my amp, the diode near the bottom of the vta board is actually mounted on the bottom (underside, inside the amp). I'm thinking I read something wrong or interpretted the instructions incorrectly. When looking in the amp with the transformers closest to me the band on the diode is on the left side (same side of the amp as the time delay board would be on if installed). Maybe the instructions when saying "bottom" in big letters did not mean the underside of the board. Yikes!

Ok then, it seems I read the instructions properly as it "is" mounted on the bottom and soldered on the top.

Hi Bob. Finally got my capacitor meter and checked the coupling caps and all were close to the .149 written on the caps. I tried checking the quad cap in the circuit but was getting high readings around the 200uf (give or take) mark on 3 of the 4 tabs (20, 40 and 80...30 was about 80uf). I assume because it was still in the circuit and I was also reading other components but not sure. The filtering caps checked out fine as did all the other caps on the vta board. I unsoldered the wires connecting the vta board to the amp in case something was touching ground etc and it looked as though one wire may have been touching a tube pin on the left driver tube causing the popping sound I heard, as well as the arc between the two pins. I did remove one leg of the coupling cap that went to the left channel to see if the value changed with the meter and for some reason forgot to resolder it in place when I was finished. I fired the amp up and noticed a small, quick arc across what I'm sure was the 47uf cap on the left channel. Quickly shut it off and saw the leg of the coupling cap not connected. I resoldered the coupling cap. The 47uf cap looked fine in shape and size so didn't think much of it. The arc was very small and only happened once. I fired it up again and the crackling was gone in the speakers (yeh) as well as the popping in the left channel but a hum appeared in both channels that seems to be at the 120hz range. It comes on as the amp warms up and about 3 or 4 minutes later the right channel goes silent but in the left channel it's still there and quite loud. I played music for about 15 minutes and although it sounds fine I can still here a hum in the left speaker although not quite as loud but definitely noticeable for everyone in the room to comment on it! Did I fry the 47uf cap on the left channel maybe? The amp has now been on for about 30 minutes and the right channel is still quiet but the left channel is still humming. Anyone's input is appreciated. Cheers...Leigh

Just an update. The amp has been on for about an hour and the hum in the left channel has decreased some but is still audible. Thanks...Leigh

Bob. No I did not get the tubes from you, mine are from the the Tube Store locally. I purchased Shuguang KT88's and 3 JJ 12au7's as well as a couple of JJ GZ34's. Last night I did some tube swapping and here is the breakdown:
- Replaced the center tube with an ElectroHarmonix..no change in hum. Left channel loud hum and right channel noticeable.
- changed right tube with left tube (both JJ's)..much better. Right channel silent and left channel noticeably less hum but a small buzz.
- changed center tube with jj 12au7..right channel dead silent. Left channel very minor.
- after 10 minutes of warming up the amp was nearly dead quiet with very minor hum on left channel.
- replaced the center tube with a 12au7 groove tube..pretty much the same as above with maybe a little less hum on left channel.
- played music for maybe another 45 min and all hum was pretty much gone (livable).
- turned amp off.

Fast forward to this morning.

- turned amp on and had hum the same as when I turned it on last night.
- played music for about an hour and pretty much all the hum is gone except a bit in the left channel.

- 12au7's in the amp as I left it last night are. -JJ's on the left and right and groove tube in the center.

I'm beginning to think that the amp warming up has had more to do with the hum being reduced than rolling the tubes last night. Originally when I had all my static noise and pops in the left channel there was no hum at all. After I got the arc across the 47uf and shortened the wires going to the vta board the static was gone and the hum appeard. Would a damaged 47uf cap on the left channel cause hum? I was going to replace it and see but around here I can only seem to find a 47uf cap rated for 100 volts so that's no good. I can live the hum after it warms up but would like to get to the bottom of it as I feel I'm nearly there! Any thoughts from anyone? Cheers..Leigh

No I haven't tried resoldering the cap but when I looked at it the solder joints looked fine. It can't hurt to reheat the connection though. I have resoldered all the tube pins on all three sockets but will look at them again also. Would it damage the amp to pull all three driver tubes and then turn it on to see if the hum is gone, or the left and right or just the center tube etc... I'm thinking that if the hum is still there without the 12au7's in the circuit it may mean the hum is being caused from somewhere else. Thanks Bob...Leigh

Ha, no I won't, just the 12au7's. Will let you know what I find out. Something worth mentioning though. I had the amp turned off for a couple of hours and just turned it back on a few minutes ago. No hum at all from the right channel now...just the left!