Hi David! Hope all is well...I remember watching this video a while back and since this past December, I had an SVT Classic come in for repair 3 times (same amp) within 4 months for the same problem (different cold tube) and I have to be honest and say that this is a poorly designed amp and a headache to work on. It got to the point where after the third time, the amp came back again 2 weeks ago with 2 holes burnt right through the PCB. I'm really not fond of the 6550's sockets being mounted on a PCB. What I had to end up doing is, eliminate the PCB, cut and mount a metal plate with conventional flanged tube sockets and wire everything in that section point to point in order to put a final stop to the problem...mind you, Ampeg was NO help in giving me support as a technician, and for a replacement PCB. I have customers who have the old SVT's from the early 70's which are a breeze to work on. Hope all is well with you. How's the 1kW audio amplifier coming along? :)
Frank - good to hear from you. Yes, much of the new stuff is junk. On a recent YT video I posted, it was obvious that the metal in the tube sockets is so soft that simply removing the tube wallowed out the pin connections and would cause failure on that tube - that's pretty bad, huh? I sure admire your commitment to fixing the SVT - had to have been a lot of work. I agree that the '70's SVT are easy to work on - unfortunately the 12DW7's and 12BH7's often times get replaced with 12AX7's which is a bad idea. My KW amp works amazing well but I have not had any opportunity to put it to actual work. Maybe something will come along at a local gig where I can allow it to make some real noise :-)) I think the KW amp's best purpose would be voice and PA or lead but probably not bass as the tube plates really heat up under sustained low frequencies.
It's interesting that you mentioned about the cheap metal in today's tube sockets. I've been encountering that same issue quite frequently with newer amps. I came up with a quick fix that has helped so far...I install socket savers. Granted, it brings the tubes down a little lower, but saves from the heat being so close to these cheap sockets. However, when I come across an amp with tight quarters some times, it isn't possible to use them. I've been keeping in touch with Mark up at Amp Experts; he lives about 3 hours north-west of me in Conn. I'm here in New Jersey...Marks a nice guy, we've exchanged schematics on occasion. By the way, if you're ever in need of schematics on something, drop me a line, because I have a huge schematic library in PDF form. I'm also starting to scan my SAMS library a little at a time...a lot of work, but that stuff is hard to come by these days. I'm in the process of building a bias box after seeing the really nice home-brew that you built, however, I'm tossing around a few ideas...one is, I'm building it with dual sockets and cables, but I'm debating whether or not to put meters right in the box plus the binding posts. What's your opinion on it?
Frank Ferraro Commenting on your last paragraph - yes, I think two sockets would be a real advantage as you could easily switch back and forth between two tubes at a time. I am not sure there is any real advantage to measuring the plate, screen and cathode currents - it could be enlightening at times and on some tubes - The one I built works but is labor intensive - also, I have noticed that if I leave one tube in the tester (in a push-pull setup) and actually run dynamic tests, the extra length of wire, etc will affect the outcome so it is only good for static testing under no signal conditions - thanks for the offer on the schematics - I will definitely keep that in mind -
It is function 19 - hit Clear then punch in 19.004 and then SPCL. Function 19 reads power and then the dot and then the impedance, i.e., to read power out of your oscillator with a 600 ohm output, punch in Clear, 19.600 and then SPCL and you can read the mill watt output of your oscillator - I love the 8903.
Hi David! Thanks for sharing this. Really informative! I had one of these come in and your vid was a real time saver! Thank you so much!!! Id love to see your bench setup. Do you have a video on your setup? Or perhaps willing to share some info? Looking for some inspiration to upgrade mine to the next level! Thanks again, your vids are great!! Chris
If the LED's are firing then the problem is in the output section. One of the tubes is not actually working. There are 10ohm resistors on the main board that are also prone to opening up when a tube goes nuclear. If the 220 ohm on one of the tubes is open it's a real good bet that the tube may show heaters and look like it's hot when it is not functioning. The output tube monitoring circuit looks at the draw of each tube and won't let the green LED come on if they are not all working. I have seen the monitoring circuit opamps screw up, but that is really rare. The opening up of the 10 ohm and 220 ohm resistors is pretty normal wear and tear on these Beasties.
Randy Gabbard what’s the wattage of the 220ohm and 10ohm resistors here? And regarding the 10ohm, you said it’s on the main board. Is the larger board on the bottom the one you speak of, or top board where the 220 is? I’m away from the shop where I’m working on one of these. Trying to get the parts ordered and shipped by the time I get back. Thank you and thank you El Paso!
Actually, I never measured anything definitively bad with the ohm meter but the tube was "cold" in a way that appeared to be a lack of screen voltage. It is impossible to work on this amp powered up. I changed the screen resistor (I changed all of six if them) and re-soldered all around the socket and it cured the issue - hopefully forever...
I was never able to measure anything wrong with an ohmmeter but changing the screen resistor and re-soldering the socket fixed it. I am guessing the screen resistor was opening up under load. The amp at the end of the video is an RF amp for 20 meter amateur radio operation. It will go in a cabinet which already has the tuned circuits for the output and the 3000 volt HV PS. I am considering making an AF amp with the 3-400Z tubes - should give me about a kilowatt of output.
when it comes to the way a tube amplifier is designed, what's the biggest difference between the way a vacuum tube hi fidelity amplifier is built versus a vacuum tube stage amplifier?
Unfortunately, one never knows exactly what the plate current is on the new amps with the LED's. I don't remember the exact amount that Ampeg recommends for current on the vintage amps but I know it is pretty low. Do you recall the voltage across the 1 ohm cathode resistors in the vintage SVT's?
nice video, I'd like to know what was the problem with the amp and how you found it in the end also I'm curious about the amp you showed in the end of the video, could you make a whole video about it?
You don't have to remove the main board in order to remove the tube board (maybe you didn't do that) - though it is a nuisance owing to a couple of the cables. We (day job) have a lot of these, and 90% of failures are just tubes. Most of the rest are the 220 Ohm resistors. Occasionally there's an issue with the bias and.or bias monitoring circuit. The bias monitoring circuit looks at all the tubes and won't go green if one of the tubes is too far out from its mates, so no worries about having a hot or cold one. Normally ~260W at onset of clipping (using the eyes on a scope).
HI. I have the SVT. CL. made in USA model. Recently , I have been getting some static noise through my amp. speaker cab. I changed amp heads to eliminate the problem coming from my speaker cab. So that's isolated from the equation. The static noise is definitely from the CL. head while I'm playing. I switched cables and even different electrical outlets.The Static crackling kind of noise is still there. I took off the back screen and blew out some dust with a can of dust off. I took a pencil and lightly tapped the tubes with a pencil. The only time the static happened while doing this was when I tapped lightly on the smaller last tube on the right on the bottom. But it only did it once as I wiggled it a little. I played again and the static remains . Any help or guidance you could give me would be appreciated. Thank you .
+Roy Eraca I forgot to mention that my Power Tubes were replaced by a service center some time ago, but I haven't played this particular CL head in a while. So there aren't all that many hours on these power tubes. I don't believe any other tubes were replaced as I had purchased this used. Thanks again.
Thank you very much for the high quality video, I have a Laney supergroup 200 I want to have gone through but can you post a video about one if you ever come across it? I'm in Michigan, much too far to be able to send it to you for work.
I am pleased you enjoyed the video. I don't take in amplifiers for repair anymore so it is unlikely I will run across your amplifier - but you never know... on rare occasions I work on one fellow's equipment her in El Paso. Thanks for your comments.
Output transformers rarely get very hot unless they are close to hot vacuum tubes or the power transformer so warm is OK. Power transformers can get hot - hot enough that you cannot keep your hand on them and that can be OK also. Just the smallest amount of air blowing over a vacuum tube instrument can make a huge difference in temperature. Vacuum tubes can easily run at 400 degrees F and, as mentioned above, can cause components close to them including the electrolytic cans, to get quite hot also and getting these capacitor cans hot is not good for them. Hope this helps.
I actually didn't know it was music creation amp and don't know how tell tell tube amps one from another because I really don't know a lot about tube amplifiers as only in the last few years has tube amplification become popular again. When I was growing up through my early adult years, tube amps basically didn't exist other then a VERY small number of independent makers most people never heard of.
I have an svt cl that immediately blows the fuse when you switch it off standby. I also noticed the led doesn't turn green like it is supposed to before you switch the standby. Any ideas on the problem.
I would start by pulling all the output tubes - if that stops it from blowing fuses then you could plug two in at a time (one on each side) or even one at a time and try to determine which output tube is shorted - if it blows fuses with no output tubes installed I would assume a power supply problem and you might need to seek a good tech - good luck.
i have a svt cl that has the same problem, right side bias will turn green, but not the left. i just changed the power tubes with a matching 6 pair and same problem. everything sounds fine though. what are the potential risks or continuing to playing it as is? thanks
Probably all that is going to happen is reduced performance. If you have or can get/borrow a infra red (IR) "laser" thermometer like I use, check the temperature of each of the 6550's and I am pretty sure you will find one or more much cooler than the others - not by 10 degrees but by 100 degrees or more - those will be your tubes that are not doing their share.
Question . When looking at the amp from the back, for the 3 preamp phase inverter tubes, is the 12ax7 on the right side or the left? I can’t find this info anywhere!
This has been a lot of years ago so I don't remember off hand but the first tube, I assume the 12AX7, input would be the one with the wires from the input jack. connected to the grid (either pin 2 or pin 7 of the 12AX7 socket) . The last tube would be the one that has capacitors off the plates (pin 1 and pin 6) driving the output tubes. As far as I know, the driver tubes are usually in a line from input to output. Hope this helps. Looking at the schematic at this link , the input 12AX7 is V1. www.tangible-technology.com/schematics/Ampeg/SVT/
@@ElPasoTubeAmps Thanks man! I didnt have it open so was abit hard to tell off the schematic but I think I got it! I actually just tried what I assumed it was, and when I turned on the amp there was a loud hum, but I only turned it on for like 2 sec lol, so I switched them around and the hum left and it worked great lol. Hillbbilly amp teching here lol. So looking at the tubes from the back of the amp it was 12AU7/12AU7/12AX7. Thanks for your help though! Much Appreciated!
Yes, I am certain air flow will help lengthen the life of the tube - it doesn't have to be forced air directly on the tube - even a gentle flow over and around the amplifier will help.
5% THD in the stereo world is intolerable but in the music-creation world it is hardly noticeable and would be called "clean" - even 10% THD is "musical". Fuzz boxes turn the sine wave into a square wave at 50% THD so we don't measure the music creation amps by the same standards as we do HiFi amps. Scanning a music creation amp with an instrument like a HP 8903 from 20 to 20 KHz is ridiculous and hopeless.as the frequency response of these amps is totally off-the-scale.
I think calling them "music creation" amps is just my way of saying it is not a HiFi/Stereo amp. HiFi amps really must have a pretty flat response from 20-20KHz to be considered "HiFi" but guitar amps don't have that level of frequency response but they are perfectly suited for their purpose. HiFi amps sound sterile in "making" music bur not in the playback of music.
I know this is old, but I'm bored. The shame is, there's no craft anymore. No pride in design. Things now are designed to be thrown away and replaced, not repaired. Mass production and labor costs have killed All the giants. Marshall is the only one offering hand true wired amps, and the price is outrageous.
I always enjoy this man's videos. Especially when he's working with the SVTs.
Hi David! Hope all is well...I remember watching this video a while back and since this past December, I had an SVT Classic come in for repair 3 times (same amp) within 4 months for the same problem (different cold tube) and I have to be honest and say that this is a poorly designed amp and a headache to work on. It got to the point where after the third time, the amp came back again 2 weeks ago with 2 holes burnt right through the PCB. I'm really not fond of the 6550's sockets being mounted on a PCB. What I had to end up doing is, eliminate the PCB, cut and mount a metal plate with conventional flanged tube sockets and wire everything in that section point to point in order to put a final stop to the problem...mind you, Ampeg was NO help in giving me support as a technician, and for a replacement PCB. I have customers who have the old SVT's from the early 70's which are a breeze to work on. Hope all is well with you. How's the 1kW audio amplifier coming along? :)
Frank - good to hear from you. Yes, much of the new stuff is junk. On a recent YT video I posted, it was obvious that the metal in the tube sockets is so soft that simply removing the tube wallowed out the pin connections and would cause failure on that tube - that's pretty bad, huh? I sure admire your commitment to fixing the SVT - had to have been a lot of work. I agree that the '70's SVT are easy to work on - unfortunately the 12DW7's and 12BH7's often times get replaced with 12AX7's which is a bad idea. My KW amp works amazing well but I have not had any opportunity to put it to actual work. Maybe something will come along at a local gig where I can allow it to make some real noise :-)) I think the KW amp's best purpose would be voice and PA or lead but probably not bass as the tube plates really heat up under sustained low frequencies.
It's interesting that you mentioned about the cheap metal in today's tube sockets. I've been encountering that same issue quite frequently with newer amps. I came up with a quick fix that has helped so far...I install socket savers. Granted, it brings the tubes down a little lower, but saves from the heat being so close to these cheap sockets. However, when I come across an amp with tight quarters some times, it isn't possible to use them.
I've been keeping in touch with Mark up at Amp Experts; he lives about 3 hours north-west of me in Conn. I'm here in New Jersey...Marks a nice guy, we've exchanged schematics on occasion. By the way, if you're ever in need of schematics on something, drop me a line, because I have a huge schematic library in PDF form. I'm also starting to scan my SAMS library a little at a time...a lot of work, but that stuff is hard to come by these days.
I'm in the process of building a bias box after seeing the really nice home-brew that you built, however, I'm tossing around a few ideas...one is, I'm building it with dual sockets and cables, but I'm debating whether or not to put meters right in the box plus the binding posts. What's your opinion on it?
Frank Ferraro Commenting on your last paragraph - yes, I think two sockets would be a real advantage as you could easily switch back and forth between two tubes at a time. I am not sure there is any real advantage to measuring the plate, screen and cathode currents - it could be enlightening at times and on some tubes - The one I built works but is labor intensive - also, I have noticed that if I leave one tube in the tester (in a push-pull setup) and actually run dynamic tests, the extra length of wire, etc will affect the outcome so it is only good for static testing under no signal conditions - thanks for the offer on the schematics - I will definitely keep that in mind -
Thank you for your kind reply.
It is function 19 - hit Clear then punch in 19.004 and then SPCL. Function 19 reads power and then the dot and then the impedance, i.e., to read power out of your oscillator with a 600 ohm output, punch in Clear, 19.600 and then SPCL and you can read the mill watt output of your oscillator - I love the 8903.
Hi David! Thanks for sharing this. Really informative! I had one of these come in and your vid was a real time saver! Thank you so much!!!
Id love to see your bench setup. Do you have a video on your setup? Or perhaps willing to share some info? Looking for some inspiration to upgrade mine to the next level!
Thanks again, your vids are great!!
Chris
If the LED's are firing then the problem is in the output section. One of the tubes is not actually working. There are 10ohm resistors on the main board that are also prone to opening up when a tube goes nuclear. If the 220 ohm on one of the tubes is open it's a real good bet that the tube may show heaters and look like it's hot when it is not functioning. The output tube monitoring circuit looks at the draw of each tube and won't let the green LED come on if they are not all working. I have seen the monitoring circuit opamps screw up, but that is really rare. The opening up of the 10 ohm and 220 ohm resistors is pretty normal wear and tear on these Beasties.
Randy Gabbard what’s the wattage of the 220ohm and 10ohm resistors here? And regarding the 10ohm, you said it’s on the main board. Is the larger board on the bottom the one you speak of, or top board where the 220 is? I’m away from the shop where I’m working on one of these. Trying to get the parts ordered and shipped by the time I get back. Thank you and thank you El Paso!
Actually, I never measured anything definitively bad with the ohm meter but the tube was "cold" in a way that appeared to be a lack of screen voltage. It is impossible to work on this amp powered up. I changed the screen resistor (I changed all of six if them) and re-soldered all around the socket and it cured the issue - hopefully forever...
I was never able to measure anything wrong with an ohmmeter but changing the screen resistor and re-soldering the socket fixed it. I am guessing the screen resistor was opening up under load. The amp at the end of the video is an RF amp for 20 meter amateur radio operation. It will go in a cabinet which already has the tuned circuits for the output and the 3000 volt HV PS. I am considering making an AF amp with the 3-400Z tubes - should give me about a kilowatt of output.
You're an awesome tech.
sounds nice
when it comes to the way a tube amplifier is designed, what's the biggest difference between the way a vacuum tube hi fidelity amplifier is built versus a vacuum tube stage amplifier?
Unfortunately, one never knows exactly what the plate current is on the new amps with the LED's. I don't remember the exact amount that Ampeg recommends for current on the vintage amps but I know it is pretty low. Do you recall the voltage across the 1 ohm cathode resistors in the vintage SVT's?
nice video, I'd like to know what was the problem with the amp and how you found it in the end
also I'm curious about the amp you showed in the end of the video, could you make a whole video about it?
Good video. Thanks. Was the problem a bad solder joint or the screen resistors?
You don't have to remove the main board in order to remove the tube board (maybe you didn't do that) - though it is a nuisance owing to a couple of the cables. We (day job) have a lot of these, and 90% of failures are just tubes. Most of the rest are the 220 Ohm resistors. Occasionally there's an issue with the bias and.or bias monitoring circuit.
The bias monitoring circuit looks at all the tubes and won't go green if one of the tubes is too far out from its mates, so no worries about having a hot or cold one.
Normally ~260W at onset of clipping (using the eyes on a scope).
How did you get your HP 8903A to read 4OHMS in watts? thanks George
HI.
I have the SVT. CL. made in USA model.
Recently , I have been getting some static noise through my amp. speaker cab.
I changed amp heads to eliminate the problem coming from my speaker cab. So that's isolated from the equation. The static noise is definitely from the CL. head while I'm playing. I switched cables and even different electrical outlets.The Static crackling kind of noise is still there.
I took off the back screen and blew out some dust with a can of dust off.
I took a pencil and lightly tapped the tubes with a pencil.
The only time the static happened while doing this was when I tapped lightly on the smaller last tube on the right on the bottom. But it only did it once as I wiggled it a little.
I played again and the static remains . Any help or guidance you could give me would be appreciated. Thank you .
+Roy Eraca I forgot to mention that my Power Tubes were replaced by a service center some time ago, but I haven't played this particular CL head in a while.
So there aren't all that many hours on these power tubes. I don't believe any other tubes were replaced as I had purchased this used.
Thanks again.
Thank you very much for the high quality video, I have a Laney supergroup 200 I want to have gone through but can you post a video about one if you ever come across it? I'm in Michigan, much too far to be able to send it to you for work.
I am pleased you enjoyed the video. I don't take in amplifiers for repair anymore so it is unlikely I will run across your amplifier - but you never know... on rare occasions I work on one fellow's equipment her in El Paso. Thanks for your comments.
@@ElPasoTubeAmps thanks for the fast reply I hope all is well!
one of my output transformers stay cool to the touch, room temp, the other gets warm, but not HOT. is this normal for one to be cold?
Output transformers rarely get very hot unless they are close to hot vacuum tubes or the power transformer so warm is OK. Power transformers can get hot - hot enough that you cannot keep your hand on them and that can be OK also. Just the smallest amount of air blowing over a vacuum tube instrument can make a huge difference in temperature. Vacuum tubes can easily run at 400 degrees F and, as mentioned above, can cause components close to them including the electrolytic cans, to get quite hot also and getting these capacitor cans hot is not good for them. Hope this helps.
I actually didn't know it was music creation amp and don't know how tell tell tube amps one from another because I really don't know a lot about tube amplifiers as only in the last few years has tube amplification become popular again. When I was growing up through my early adult years, tube amps basically didn't exist other then a VERY small number of independent makers most people never heard of.
I have an svt cl that immediately blows the fuse when you switch it off standby. I also noticed the led doesn't turn green like it is supposed to before you switch the standby. Any ideas on the problem.
I would start by pulling all the output tubes - if that stops it from blowing fuses then you could plug two in at a time (one on each side) or even one at a time and try to determine which output tube is shorted - if it blows fuses with no output tubes installed I would assume a power supply problem and you might need to seek a good tech - good luck.
Hey ive got this same exact issue, did you end up fixing it?
i have a svt cl that has the same problem, right side bias will turn green, but not the left. i just changed the power tubes with a matching 6 pair and same problem. everything sounds fine though. what are the potential risks or continuing to playing it as is? thanks
Probably all that is going to happen is reduced performance. If you have or can get/borrow a infra red (IR) "laser" thermometer like I use, check the temperature of each of the 6550's and I am pretty sure you will find one or more much cooler than the others - not by 10 degrees but by 100 degrees or more - those will be your tubes that are not doing their share.
thanks for the response.
Jesse Briseno did you ever solve the problem? I have a similar issue. Amp works but the red light on the left won't go out.
Question . When looking at the amp from the back, for the 3 preamp phase inverter tubes, is the 12ax7 on the right side or the left? I can’t find this info anywhere!
This has been a lot of years ago so I don't remember off hand but the first tube, I assume the 12AX7, input would be the one with the wires from the input jack. connected to the grid (either pin 2 or pin 7 of the 12AX7 socket) . The last tube would be the one that has capacitors off the plates (pin 1 and pin 6) driving the output tubes. As far as I know, the driver tubes are usually in a line from input to output. Hope this helps. Looking at the schematic at this link , the input 12AX7 is V1. www.tangible-technology.com/schematics/Ampeg/SVT/
@@ElPasoTubeAmps Thanks man! I didnt have it open so was abit hard to tell off the schematic but I think I got it! I actually just tried what I assumed it was, and when I turned on the amp there was a loud hum, but I only turned it on for like 2 sec lol, so I switched them around and the hum left and it worked great lol. Hillbbilly amp teching here lol. So looking at the tubes from the back of the amp it was 12AU7/12AU7/12AX7. Thanks for your help though! Much Appreciated!
do you think airflow for the valves would improve longevity?
Yes, I am certain air flow will help lengthen the life of the tube - it doesn't have to be forced air directly on the tube - even a gentle flow over and around the amplifier will help.
Where can I find how to build one of those bias probes?
I have made a couple of videos on bias probes. Here is one that might help.
ua-cam.com/video/jCSgc3xzy5g/v-deo.html
5% THD in the stereo world is intolerable but in the music-creation world it is hardly noticeable and would be called "clean" - even 10% THD is "musical". Fuzz boxes turn the sine wave into a square wave at 50% THD so we don't measure the music creation amps by the same standards as we do HiFi amps. Scanning a music creation amp with an instrument like a HP 8903 from 20 to 20 KHz is ridiculous and hopeless.as the frequency response of these amps is totally off-the-scale.
All those tubes are out of range.
But I like the presentation
I think calling them "music creation" amps is just my way of saying it is not a HiFi/Stereo amp. HiFi amps really must have a pretty flat response from 20-20KHz to be considered "HiFi" but guitar amps don't have that level of frequency response but they are perfectly suited for their purpose. HiFi amps sound sterile in "making" music bur not in the playback of music.
5 THD is pretty bad for a quality (and probably expensive) tube amplifier, how much do these 300 watt tube amps cost these days anyway?
people say that these are the same as the American cl and they are not.they are not built the same and components are cheaper
i don't buy new anything to do with audio. W
I know this is old, but I'm bored. The shame is, there's no craft anymore. No pride in design. Things now are designed to be thrown away and replaced, not repaired. Mass production and labor costs have killed All the giants. Marshall is the only one offering hand true wired amps, and the price is outrageous.