Blackline SVT repair

[Low End Curse]

Hi all, first post here.

So I recently reacquired my first SVT and the guy I had sold it to hasn’t used it in at least 10 years. I’ll try not to be long winded, but offer as much detail as I can.

When I got it back, it would not power on at all, checked the fuse and it was a blown 2 amp. I replaced it with a proper 10 amp and it then powered on fine. I let it warm up for a while and when I flipped the standby off the power tubes glowed like lightbulbs (I didn’t see which tubes or if it was all of them, but it was BRIGHT) and there was a loud hum. Flipped it off right away and opened up the amp. All the electrolytics are original and resistors R6, R46 and R47 look suspect.

I don’t have the money to have a tech look at it right now, so I’m looking for advice or references to troubleshoot the issue myself. I’ve worked on amps before, have all the necessary equipment, and done a few cap jobs, but not sure where to start with this one.
Thanks in advance!

 

[BogeyBass]

I would stabilize the bias so it stops red plating. And yes replace the suspect resistors since they most likely drifted.
Replace all the screen resistors. And raise the values to lower screen voltage. 220 to 330 ohms

Do general maintenance like clean sockets pots etc etc. And touch up solder connections on the sockets and super inspect all the boards and touch up any further bad connections.

Check cathode resistors for drifted values or just make the world scream bloody murder and replace them all lol muah hah hah. Same with bias cap I'll assume it's wasted.

If one or few tubes is red plating it could be passive devices related to that socket or bad tubes. If they all red plate then the bias is not stable

 

[Low End Curse]

I would stabilize the bias so it stops red plating. And yes replace the suspect resistors since they most likely drifted.
Replace all the screen resistors. And raise the values to lower screen voltage. 220 to 330 ohms

Do general maintenance like clean sockets pots etc etc. And touch up solder connections on the sockets and super inspect all the boards and touch up any further bad connections.

Check cathode resistors for drifted values or just make the world scream bloody murder and replace them all lol muah hah hah. Same with bias cap I'll assume it's wasted.

If one or few tubes is red plating it could be passive devices related to that socket or bad tubes. If they all red plate then the bias is not stable

Thanks! Sounds like a good plan.

 

[Wasnex]

Hi all, first post here.

So I recently reacquired my first SVT and the guy I had sold it to hasn’t used it in at least 10 years. I’ll try not to be long winded, but offer as much detail as I can.

When I got it back, it would not power on at all, checked the fuse and it was a blown 2 amp. I replaced it with a proper 10 amp and it then powered on fine. I let it warm up for a while and when I flipped the standby off the power tubes glowed like lightbulbs (I didn’t see which tubes or if it was all of them, but it was BRIGHT) and there was a loud hum. Flipped it off right away and opened up the amp. All the electrolytics are original and resistors R6, R46 and R47 look suspect.

I don’t have the money to have a tech look at it right now, so I’m looking for advice or references to troubleshoot the issue myself. I’ve worked on amps before, have all the necessary equipment, and done a few cap jobs, but not sure where to start with this one.
Thanks in advance!

If you don't know how to deal with checking and safely bleeding off the power supply voltages, you should not be poking around in the amp as it has enough juice to easily kill you.

The following is my opinion and YMMV.

R6 is the plate load resistor for the second triode of the PI. R46 sets the amount of feedback from the output transformer to the PI. R46 is a balance/bleeder resistor for node A in the power supply. None of these should impact bias or output tube current. A partially shorted C9B would pull down the bias voltage and let the output tubes run away.

If R47 is open, the high voltage supply, node A, will not bleed off so be careful or you can get shocked with lethal voltage. During normal operation, a small current passes through R47 and R48 to balance the voltage across C10 and C12A. If C10 and C12A need to be replaced, IMHO, its a good idea to also replace R47 and R48.

I would pull the output tubes and check working voltages throughout the amp. Check all the power supply nodes and make sure the voltages are getting to all the pins on the output tubes. While you are in the process, adjust the bias for max negative voltage on pin 5 of the output tubes. It's possible somebody simply set the bias wrong and the resultant high current blew the main fuse. Note: It's fairly common for these amps to blow screen resistors when one of the output tubes is running too hot. The schematic shows working bias at -47V but I believe bias with moderns tubes is probably closer to -55V. Set the voltage as high as it will go for now.

Some amps have a second pigtail wired inline with the cartridge fuse that can become stressed if the main fuse is blown a couple of times.

Replace parts as needed. If all the voltages check good, put the output tubes back in. Put your test leads in the bias test points. When you flip the amp to operate watch the tubes for red plating and check the reading at the bias test points. The reading should be very low since you have maxed the bias out. If the tubes begin red plating turn the amp back to standby for a few seconds and then power it off. Note whether all tubes were read plating or just a few. If just a few tubes were red plating, pull the bad tubes in pairs...one from each side of the push pull circuit...pull one good tube if necessary to maintain balance across the output transformer.

If the tubes are not red plating, adjust the bias for to about 48mV at both test points. Next check the voltage drop across each screen resistor. Careful here as you will be working with lethal voltages again. The idea here is to confirm all six tubes are working. If some of the tubes are not working, it is best to replace them in matched trios. If all six tubes are conducting, adjust the bias the 72mV and you should be good to go.

 

[agedhorse]

I predict this not going well.

Do you have a DMM, scope, a current limiting supply?

How well do you understand tube circuit theory?

Unless you understand this stuff, the tech that ends up working on this will appreciate leaving things alone and not creating additional work.

 

[Low End Curse]

I predict this not going well.

Do you have a DMM, scope, a current limiting supply?

How well do you understand tube circuit theory?

Unless you understand this stuff, the tech that ends up working on this will appreciate leaving things alone and not creating additional work.

I appreciate the concern, I do have a scope, power limiting supply, digital meter, and wave generator and I’ve diagnosed and done cap jobs on a handful of less complex amps (silvertone twin twelve, tremolux, OR120) just never dealt with an SVT. It’s a different beast entirely.

 

[Low End Curse]

I predict this not going well.

Do you have a DMM, scope, a current limiting supply?

How well do you understand tube circuit theory?

Unless you understand this stuff, the tech that ends up working on this will appreciate leaving things alone and not creating additional work.

I should also say, this is the type of advice I’m very open to. I don’t want to cause a bigger problem through being impatient. Waiting until I can afford a tech to repair it properly is still very much an option.

 

[Wasnex]

I should also say, this is the type of advice I’m very open to. I don’t want to cause a bigger problem through being impatient. Waiting until I can afford a tech to repair it properly is still very much an option.

IMHO, If you didn't find my previous post at least a little insulting, probably better to wait . At the very minimum you need to understand the hazards and protocols of working with high voltage, because you will have plenty of exposure poking around in an SVT.

 

[Low End Curse]

IMHO, If you didn't find my previous post at least a little insulting, probably better to wait . At the very minimum you need to understand the hazards and protocols of working with high voltage, because you will have plenty of exposure poking around in an SVT.

Hahahahaha point taken! Thanks. I appreciate the knowledge and honesty. It’s the reason I came here.

 

[agedhorse]

Study the schematic, once you understand how this amp works, and note some specific tricks relating to (very) high plate voltage operation, you will better appreciate some of our cautions. I worked under the developer of the SVT and he was kind enough to discuss some of the hows and whys that lead to the design, and some of the reasons behind some of the oddities in the (unique at the time) architecture.

Be very aware of what you are looking at and probing, there's quite a bit of stored energy (remembering that even though the PSU cap values look small, the energy stored is a function of the SQUARE of the voltage)

 

[Wesley R]

Study the schematic, once you understand how this amp works, and note some specific tricks relating to (very) high plate voltage operation, you will better appreciate some of our cautions. I worked under the developer of the SVT and he was kind enough to discuss some of the hows and whys that lead to the design, and some of the reasons behind some of the oddities in the (unique at the time) architecture.

Be very aware of what you are looking at and probing, there's quite a bit of stored energy (remembering that even though the PSU cap values look small, the energy stored is a function of the SQUARE of the voltage)

An SVT story: Way back in 1979 or so I was as a some musicians friends apartment. On guy had his SVT opened up as was cleaning circuit boards, touching up solder joints and a bunch of stuff. Typically this guy had been a very good tech and fixed or maintained many low and high power amplifiers. When he fired up the SVT there was a huge popping sound, the release of the magic smoke and a hole in a circuit board that I could fit two fingers through.

 

[beans-on-toast]

Some techs discharge amp capacitors by shorting a screwdriver across the terminals. It isn’t such a good idea. It should be done through a resistor that will limit the current while slowly discharging the capacitor.

There’s enough energy in a charged cap to take a bite out the steel. Imagine what this can do to your finger.

 

[Low End Curse]

Some techs discharge amp capacitors by shorting a screwdriver across the terminals. It isn’t such a good idea. It should be done through a resistor that will limit the current while slowly discharging the capacitor.

There’s enough energy in a charged cap to take a bite out the steel. Imagine what this can do to your finger.

View attachment 3106444

I have a proper resistor equipped discharging setup. It was the first thing we made in my electronics class back in high school.

 

[Low End Curse]

Thanks to everyone who replied here, this forum has been an invaluable resource to me. Not sure why it took me so long to become a part of it!

I did some testing last night. C9B is definitely shorted as are C10 and C12A R47 and R48 are both open as well. I ordered the electrolytics kit from Fliptop , along with C1, C3, C4, C5 and C6.

 

[beans-on-toast]

Thanks to everyone who replied here, this forum has been an invaluable resource to me. Not sure why it took me so long to become a part of it!

I did some testing last night. C9B is definitely shorted as are C10 and C12A R47 and R48 are both open as well. I ordered the electrolytics kit from Fliptop , along with C1, C3, C4, C5 and C6.

Don’t forget the one in the preamp. C23, 40uF @ 450 VDC.

I use this one: Invalid Link Removed, 47uF @ 500VDC F&T.

Hopefully, C23 will be in your kit.

 

[Low End Curse]

Don’t forget the one in the preamp. C23, 40uF @ 450 VDC.

I use this one: Invalid Link Removed, 47uF @ 500VDC F&T.

Hopefully, C23 will be in your kit.

Yep, that one is in the kit! Thanks!

 

[Wasnex]

Some techs discharge amp capacitors by shorting a screwdriver across the terminals. It isn’t such a good idea. It should be done through a resistor that will limit the current while slowly discharging the capacitor.

There’s enough energy in a charged cap to take a bite out the steel. Imagine what this can do to your finger.

More importantly, imagine what it could do to your heart.

 

[somebrains]

yikes

 

[basscapes]

Some techs discharge amp capacitors by shorting a screwdriver across the terminals. It isn’t such a good idea. It should be done through a resistor that will limit the current while slowly discharging the capacitor.

There’s enough energy in a charged cap to take a bite out the steel. Imagine what this can do to your finger.

View attachment 3106444

I still have the scar on my left hand from an electrical burn that I got when I opened my V4B and stupidly thought it would be easy to "service" it, way back when I was 17 and knew absolutely NOTHING about tube amps (never mind electronics!).

Now, and an EE Tech, even though I've been into a LOT of tube gear, things like the SVT (and some Audio Research hi-fi power amps) still scare me.

Approach with RESPECT and CAUTION, or prepare to die...

 

[tubedude]

An SVT, and more so the 400PS, take specialized knowledge and experience. That rebuild kit sounds good, but if you see even a little problem post install, turn it over to a tech with SVT experience.