tube amp diagnosis

[TTA]

my new to me 2004 svt av is causing drama again. but the electrical type.

at first i would play and after 20 or so minutes would get a loud sudden “snap” through the speakers now and then. maybe every few minutes. it was associated with a tube flash (did not identify which). I figured arcing.

i did some easy stuff like swap the preamp tubes and it was fine. It was looked at and cold solder joints identified and fixed. (details unknown)

so the snapping is now gone but after 20 or so minutes of playing i had two things:

- very quiet “fax” like fuzz that would come and go. taping the amp could stop, alter or start it.”
- after ~20 minutes, volume reduction. first brief for maybe 10s, then normal, but it kept returning and for longer.

i looked on the back and had two green lights with the right bank solid red. left bank would flash red whenever i played. play note, goes red, stop playing , no left red light.

turned the amp off, let it cool a bit and restarted. began with two green lights, but after 30s the right bank began to flash red, and after a minute of so was solid red. right bank would go red whenever i played, and volume quickly reduced.

my (limited experience) take is we have either another cold solder joint, or we just have an aging tube (all circa 2007 or so, but who knows when put in, i was told new, matching and low hours).

before making a run to the tec again, i was thinking of swapping tube trios to see if the red light followed or not (hint at bad tube vs connection).

thoughts?

 

[beans-on-toast]

The bias LED’s are only meaningful at idle with no input. Ignore what you see when playing.

Try swapping trios and see what the bias does at idle. Identify the correct tubes in the trio.

A bad tube is a possibility. So are bad solder joints. Check the power tube socket tension. Inspect the sockets for cracks and oxidation on the socket contacts. Power tube socket solder joints should be reflowed. There is a technical bulletin on models that do not have octal reinforcing standoffs under the octal socket mounts. Bad components could also be a possibility.

 

[TTA]

The bias LED’s are only meaningful at idle with no input. Ignore what you see when playing.

Try swapping trios and see what the bias does at idle. Identify the correct tubes in the trio.

A bad tube is a possibility. So are bad solder joints. Check the power tube socket tension. Inspect the sockets for cracks and oxidation on the socket contacts. Power tube socket solder joints should be reflowed. There is a technical bulletin on models that do not have octal reinforcing standoffs under the octal socket mounts. Bad components could also be a possibility.

thanks . my next step was to swap sets to see if the red light follows. my understanding is the 3 closest to the rear panel are represented with the right led’s.

 

[agedhorse]

If your amp was recently worked on and is still under service warranty, ask the tech to look at it again to see if maybe they missed something. Otherwise, if you screw with it you run the risk of the tech saying you broke it in your attempt to fix it yourself and now they want nothing to do with it or any warranty.

This could be caused by a LOT of different problems...

 

[elgranluis]

when I bought my svt (MTI, supposedly as close to the original as possible) , it was running on 5 tubes without any sign of malfunction.

If you can make the issue come and go by tapping the amp , it is indeed a physical issue. Sometimes we “fix “
an issue (like solder joints) and rule it out because it’s already fixed. But sometimes they weren’t fixed right in the first place.

The way that I or any tech that I consider competent would handle this , is to power up the amp and tap the individual xonponents until I find the source of noise. No noise ? Play the amp , tap components , rinse and repeat.

In an unrelated topic , there’s a “show ignored content Button “. Like , what is the purpose of ignoring someone of you end up reading their crap , but fortunately it comes with the caption “you are ignoring this content.”

 

[Wasnex]

AFAIK you have an SVT AV which is a special edition CL.

Bias controls and indicators should look like this.

A red bias indicator indicates one or more tubes is over current.

The output section is push/pull. Push has three 6550 tubes and pull has three 6550 tubes.

Basically the bias indicators have two thresholds.

1. Green uses AND logic. Green turns on when all three tubes in the circuit have >15mA of current.
2. Red us OR logic. Red turns on if one or more tubes in the circuit have >30mA
3. If you get red without green, at least one tube is over current and one tube is under current. Good chance one tube is totally nonop.

When a 6550 arcs it can put on a light show. The screen resistors in SVTs are designed to act as fuses. If you have red without green, there is a good chance that the screen resistor on the tube that arced is blown.

The arcing could have occurred from a worn out tube. Another possibility is the pins in the tube sockets need to be retensioned or the amp has some other sort of intermittent connection.

I think it would be a good idea to take the amp to a qualified tech.

 

[coreyfyfe]

thanks . my next step was to swap sets to see if the red light follows. my understanding is the 3 closest to the rear panel are represented with the right led’s.

The tube sets are arranged in triangle shapes. I found these reference images to show the groupings

 

[TTA]

If your amp was recently worked on and is still under service warranty, ask the tech to look at it again to see if maybe they missed something. Otherwise, if you screw with it you run the risk of the tech saying you broke it in your attempt to fix it yourself and now they want nothing to do with it or any warranty.

This could be caused by a LOT of different problems...

yeah this is very fair. i think there is a combo of problems. my theory is a cold solder maybe caused arcing which has now weakened a tube. maybe a bit of a snowball of issues.

 

[TTA]

AFAIK you have an SVT AV which is a special edition CL.

Bias controls and indicators should look like this.
View attachment 7443325
A red bias indicator indicates one or more tubes is over current.

The output section is push/pull. Push has three 6550 tubes and pull has three 6550 tubes.

Basically the bias indicators have two thresholds.

1. Green uses AND logic. Green turns on when all three tubes in the circuit have >15mA of current.
2. Red us OR logic. Red turns on if one or more tubes in the circuit have >30mA
3. If you get red without green, at least one tube is over current and one tube is under current. Good chance one tube is totally nonop.

When a 6550 arcs it can put on a light show. The screen resistors in SVTs are designed to act as fuses. If you have red without green, there is a good chance that the screen resistor on the tube that arced is blown.

The arcing could have occurred from a worn out tube. Another possibility is the pins in the tube sockets need to be retensioned or the amp has some other sort of intermittent connection.

I think it would be a good idea to take the amp to a qualified tech.

yeah it is going back (again).

i am getting two green lights always. just right bank starts flashing red also as it warms up, then stays full on, and the right red begins to flash. but both green always on.

amp apparently had a shut off event, fixed by a preamp tube, but it has passed hands and i suspect it has had a hard life (electrical) and might have aquired some stressed components.

from what i gather it may just need new tubes also. But i don't want to put in new tubes to have them damaged.

 

[TTA]

The tube sets are arranged in triangle shapes. I found these reference images to show the groupings

View attachment 7443354

View attachment 7443353View attachment 7443354View attachment 7443353

thanks. good reason for me to not touch as the internet is full of misinformation.

 

[JamesGrant]

If I were to bet I’d be thinking loose connection/cracked solder joint. Not component failure - works when at a certain thermal condition or physical pressure.
What I would do:
Take out tubes and clean sockets and tube legs. Take out board and reflow all the solder joints. I appreciate that’s not for everyone - but easy for me. Quick and free.
Work safely do all you can to discharge caps safely - disconnect from mains, switch amp on, connected to speaker load (output load may be isolated when power off so that’s no guarantee). Leave off overnight, measure voltage across caps with multi meter, use power resistors to drain them. Ground the groundplane of the board before you start work on the board, etc etc etc.

 

[6thplanet]

very quiet “fax” like fuzz that would come and go.

how close is your cell phone during this?

 

[TTA]

when I bought my svt (MTI, supposedly as close to the original as possible) , it was running on 5 tubes without any sign of malfunction.

If you can make the issue come and go by tapping the amp , it is indeed a physical issue. Sometimes we “fix “
an issue (like solder joints) and rule it out because it’s already fixed. But sometimes they weren’t fixed right in the first place.

The way that I or any tech that I consider competent would handle this , is to power up the amp and tap the individual xonponents until I find the source of noise. No noise ? Play the amp , tap components , rinse and repeat.

In an unrelated topic , there’s a “show ignored content Button “. Like , what is the purpose of ignoring someone of you end up reading their crap , but fortunately it comes with the caption “you are ignoring this content.”

tempting to tap tubes, and to swap trios (easy stuff and i am a curious type) but also wise to leave to warranty work (to limit chance of finger pointing).

I have yet to meet someone able to get under my skin to require a block/ignore. But i am also a part timer here (and probably a great candidate to ignore). i just pick topics i am interested in and move on if i realize i am wrong when i do click. And i embrace conflicting views. i think the latter makes me somewhat rare these days.

If I were to bet I’d be thinking loose connection/cracked solder joint. Not component failure - works when at a certain thermal condition or physical pressure.
What I would do:
Take out tubes and clean sockets and tube legs. Take out board and reflow all the solder joints. I appreciate that’s not for everyone - but easy for me. Quick and free.
Work safely do all you can to discharge caps safely - disconnect from mains, switch amp on, connected to speaker load (output load may be isolated when power off so that’s no guarantee). Leave off overnight, measure voltage across caps with multi meter, use power resistors to drain them. Ground the groundplane of the board before you start work on the board, etc etc etc.

I think when i retire and have more free time i will learn more of such things. I would love to have the ability/confidence. and one day i may need to do it.

 

[JamesGrant]

I’m no ‘mend it Mark’ but I used to work for an A/V company in their workshop doing repairs on pro audio gear. To be honest I always felt I got lucky when the things worked after repairing anything complicated! Handy with a scope, meters etc. good at reading schematics and careful testing and documentation at suitable probe points - I suppose this gets you a long way there for ‘component level’ repair. I always felt out of my depth if I couldn’t understand what the circuit was doing - which was the vast majority of repairs I did! I feel the anxiety coming back to me now!!

 

[Geri O]

how close is your cell phone during this?

We had a young lady at church who kept saying her IEM pack was bad and making “funny fax-y noises that came and went” in her ears.

I didn’t want to be the guy that would have to admit I looked at her backside and saw the pack and her phone shoved in the same rear back pocket. (I’m old enough to be her grandfather, so there was that….)

But I did…..

Problem solved. Thankfully, she, nor anyone else made anything of it…..

 

[agedhorse]

If I were to bet I’d be thinking loose connection/cracked solder joint. Not component failure - works when at a certain thermal condition or physical pressure.
What I would do:
Take out tubes and clean sockets and tube legs. Take out board and reflow all the solder joints. I appreciate that’s not for everyone - but easy for me. Quick and free.
Work safely do all you can to discharge caps safely - disconnect from mains, switch amp on, connected to speaker load (output load may be isolated when power off so that’s no guarantee). Leave off overnight, measure voltage across caps with multi meter, use power resistors to drain them. Ground the groundplane of the board before you start work on the board, etc etc etc.

Since there is no “ground plane” on that board, the repair attempt won’t be off to a good start. Then there’s the issue of creating new problems rather than identifying the cause of the symptoms and repairing the actual problem.

In some of the service manuals I wrote over the years, I used the following quote from the first service manager I worked under (while I was studying for my engineering degree).

“If it’s not broken, don’t break it. If it is broken, don’t break it worse”

 

[TTA]

how close is your cell phone during this?

great question as i noted it does influence the background noise if i get close. it always remains a metre away otherwise. but it may be a red herring for the noise. the volume loss and red led on the right bank once warmed up is likely the real issue. will test more ro see if my cell influences at a distance

 

[Five or Six]

I've seen guys just throw parts at motorcycle's without resolve. Then, they bring it to me to fix. I could have saved them a bunch of money spent and more headache. Then I have to fix what they did wrong along with find / fix the real problem at a fraction of the cost they threw at the bike. Take it to the tech. It's faster, easier on you, and they know what to look for. Good luck.

 

[TTA]

I've seen guys just throw parts at motorcycle's without resolve. Then, they bring it to me to fix. I could have saved them a bunch of money spent and more headache. Then I have to fix what they did wrong along with find / fix the real problem at a fraction of the cost they threw at the bike. Take it to the tech. It's faster, easier on you, and they know what to look for. Good luck.

yeah 100%. my take was swapping tubes is low time, low effort, and would differentiate between tube problem vs electrical problem depending on the red light following the tube or not.

I figure if we dont know how to swap tubes than we have bigger issues. plus diagnosis is fun.

Imho the key is not doing things randomly, rather having logic and using first principal understanding.

balance that with warranty risks.

you must know the book: “Zen and the Art of Motorcycle Maintenance”

 

[rickdog]

It's worth noting that the SVT runs the output tubes right around the maximum plate voltage rating of some current production 6550s.

My SVT-CL had a similar problem, which I suspect was a set of tubes (Tung-Sol reissues) I had put in it that couldn't handle it - I gave up on them after one, then another failed (arced internally while idle, not playing). I ended up having to replace a screen resistor after the second tube arced, and put in a set of JJ KT88s. No further trouble with them - but I rarely use that amp, so who knows?

Output tubes "circa 2007" would make me suspicious - but at the price of output tubes today you don't want to buy a set just to try that theory Take it back to your tech.