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6550 Tubes for Ampeg SVT
- Thread starter Slpbass81
- Start date
Ambient temperature factors into an electrolytic capacitor’s service life. Capacitor labels include a nominal capacitance, a tolerance, and a temperature rating. Operating a capacitor at a lower temperature, extends it’s service life. Change in temperature due to ripple current affects capacitor life. So does ambient temperature, humidity, and how the capacitor is used.
Although the tube is rated to run hot, the capacitor can have an extended service life if it is cooler. So if an electrolytic capacitor is in close proximity to a tube that runs hot, cooling helps.
Thermal shock should be avoided. A hot tube being taken out into the cold of winter isn’t good. Same for mechanical shock in general. Avoid bouncing your amp around during transport. Things can come loose. Use common sense.
Although the tube is rated to run hot, the capacitor can have an extended service life if it is cooler. So if an electrolytic capacitor is in close proximity to a tube that runs hot, cooling helps.
Thermal shock should be avoided. A hot tube being taken out into the cold of winter isn’t good. Same for mechanical shock in general. Avoid bouncing your amp around during transport. Things can come loose. Use common sense.
Do you believe this? That a properly biased tube can just wear out to the point where the bias can't control the plate current anymore and it gets into a 'run-away' state and needs to be replaced?
"We should also mention that when it comes to power/output tubes another issue can be red-plating. This is sometimes due to wrong biasing of the amp, which really isn’t a tube problem. But there are times when a properly biased amp has a tube that begins to red-plate. This is a sign that that specific tube is failing and in a “run-away” state were the current can’t be controlled by the bias voltage. A tube like this should be replaced. If not, the amp will eventually blow a fuse or worse damage other parts."
How To Tell If Your Tube Is Bad - www.thetubestore.com
I have come across red plating a handful of times. In my experience it does not happen at "normal" current levels. It typically happens at current levels that exceed the plate dissipation rating. I believe I have seen or read about red plating with tubes a bit below 100% dissipation, but still running rather hot. This may suggest a crappy tube design that doesn't actually meet the plate dissipation spec IMHO. It could also indicate the plate's current capacity has become degraded over time.
Here are three things I know that lead to red plating:
1. The control grid is damaged.
2. Excess gas/contaminants inside the tube. The getter may clear this if it's a new tube. If the glass envelope has a leak, the problem will return.
3. The bias supply and grid network have too much resistance in series with the control grid.
1. If the control grid is damaged, the tube may run away almost immediately. I bought a non-working TE V4 with a blown HT fuse and what appeared to be relatively new 6550s. What I found is the tubes would hold bias as long as I fed the amp a low voltage with a variac. But as I turned the voltage up, at some point (about 90V AC) that tube current broke over and the HT fuse blew. I tried the 6550s in a 100W Marshall, and they would not hold bias there either. A new set of tubes and the V4 worked fine.
I also had a couple of vintage Mullards that were red plating in my Hiwatt DR103. The amp still sounded fined. I did not hear any degradation of the sound, but noticed the glowing plates. The plates on the two bad tubes were just barely red plating, but they were running way hotter than the other two tubes. The tech replaced the two bad tubes and upgraded the bias supply, and the amp has been fine since.
2. & 3. Theoretically there is no current flow through a tubes control grid. But in the real world there tends to be a little bit of current. If the tube is gasey, the control grid heats up and eventually starts to behave like a second cathode. In other words the control grid starts emitting electrons that are pulled to the plate. I believe this can happen if the control grid is misaligned as well.
When the control grid releases electrons to the plate it is referred to as reverse current flow. Reverse current flow can actually pull the voltage of the bias supply down, especially if the the resistance in the bias supply is too high. From what I have seen the current slowly ramps up as the voltage in the bias supply spirals down. At some point the tube transitions fairly quickly to full on. From what I have seen, this final transition takes a few seconds.
What may not be clear is one tube pulling down the bias can cause the other tubes the share the same bias supply to red plate.
AFAIK all tubes are a little gasey and have a small amount of grid current, which is one reason why tubes have a limit for grid to ground resistance. If the amp has too much resistance in the grid network and bias supply, even a good tube can go into thermal runaway and red plate.
As far as red plating indicating a weak tube...this is not 100% accurate. If you have two tubes in parallel and one has weak emissions, it's probably not going to red plate for the simple matter that the cathode can not release enough electrons to overheat the plate. But if the other tube is good, it can red plate if certain circuit conditions are met. For example if the bias too low or their is too much resistance in the grid and/or bias circuits.
This is explained to the best of my knowledge level, but keep in mind I am not an Engineer, so it may not be 100% technically accurate.
Here are three things I know that lead to red plating:
1. The control grid is damaged.
2. Excess gas/contaminants inside the tube. The getter may clear this if it's a new tube. If the glass envelope has a leak, the problem will return.
3. The bias supply and grid network have too much resistance in series with the control grid.
1. If the control grid is damaged, the tube may run away almost immediately. I bought a non-working TE V4 with a blown HT fuse and what appeared to be relatively new 6550s. What I found is the tubes would hold bias as long as I fed the amp a low voltage with a variac. But as I turned the voltage up, at some point (about 90V AC) that tube current broke over and the HT fuse blew. I tried the 6550s in a 100W Marshall, and they would not hold bias there either. A new set of tubes and the V4 worked fine.
I also had a couple of vintage Mullards that were red plating in my Hiwatt DR103. The amp still sounded fined. I did not hear any degradation of the sound, but noticed the glowing plates. The plates on the two bad tubes were just barely red plating, but they were running way hotter than the other two tubes. The tech replaced the two bad tubes and upgraded the bias supply, and the amp has been fine since.
2. & 3. Theoretically there is no current flow through a tubes control grid. But in the real world there tends to be a little bit of current. If the tube is gasey, the control grid heats up and eventually starts to behave like a second cathode. In other words the control grid starts emitting electrons that are pulled to the plate. I believe this can happen if the control grid is misaligned as well.
When the control grid releases electrons to the plate it is referred to as reverse current flow. Reverse current flow can actually pull the voltage of the bias supply down, especially if the the resistance in the bias supply is too high. From what I have seen the current slowly ramps up as the voltage in the bias supply spirals down. At some point the tube transitions fairly quickly to full on. From what I have seen, this final transition takes a few seconds.
What may not be clear is one tube pulling down the bias can cause the other tubes the share the same bias supply to red plate.
AFAIK all tubes are a little gasey and have a small amount of grid current, which is one reason why tubes have a limit for grid to ground resistance. If the amp has too much resistance in the grid network and bias supply, even a good tube can go into thermal runaway and red plate.
As far as red plating indicating a weak tube...this is not 100% accurate. If you have two tubes in parallel and one has weak emissions, it's probably not going to red plate for the simple matter that the cathode can not release enough electrons to overheat the plate. But if the other tube is good, it can red plate if certain circuit conditions are met. For example if the bias too low or their is too much resistance in the grid and/or bias circuits.
This is explained to the best of my knowledge level, but keep in mind I am not an Engineer, so it may not be 100% technically accurate.
- Feb 12, 2006
- 61,045
- 5
- 220,994
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Yes, I do believe this. A runaway condition is most often caused by a defect in the tube or the biasing circuitry. Generally a worn out tube struggles to keep the emissions high enough to bias properly within the normal range of bias that the tube was designed for, and remains closer to cutoff than saturation.
When I first got my V4B going, I was monitoring the bias voltage. As I was playing I never heard a change in sound, but as I stopped for a minute, I heard a little pop sound. I looked over at the amp and the bias voltage had dropped significantly and all four tubes were red plating. I quickly shut it off and found the filter caps in bias ckt had leaked. After replacing them it's working fine, with the same power tubes.
True if the grid was powered by voltage source.
In reality you see grid stoppers, 47k each tube for the SVT schematic.
Backwards the 47k grid stopper connects to low impedance of a cathode follower.
The cathode follower tries to keep biasing voltage consistent at ~-45 Volts.
Once electrons "fly back" from grid to low impedance output of cathode follower, its the 47k grid stopper that determines the current draw.
If the electrons throughout the tube try to pull up the grid bias voltage up to ~0V
45V / 47k = ~1mA
I think this amount of additional current was way too small just to cause enough additional plate current to make any of them red glowing in reality.
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Well, these are general guidelines that shall help to fix issues with tubes.
However not every issue that may happen in real practice may be addressed with those guidelines.
Sometimes a preamp tube may be very "touchy" once you knock on its glass with fingers. Rather than any tube issue its sometimes the schematic design (especially high gain designs) that may give the impression of a microphonic and worn out tube.
At this point it wouldn't make me wonder (nothing at all) if many but still good tubes had been replaced due to this "guideline" issue.
Sometimes it even may help to reduce those wrong thought "microphonic issues" once a wrong tube was placed into a schematic, for example a 12AU7 that is a low gain type where actually a 12AX7 should be.
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Btw, regarding microphonic issues with tubes.
I think I do know one or two things about tubes. Yet me myself I sometimes try very hard to detect a microphonic tube by only tipping them on the glass with my finger nails.
Way to often it happened to me myself that I was sometimes thinking, well, this one might be a worn out one that may suffer on some microphinic issue.
Put it in a brand new one, just the same!
I think I do know one or two things about tubes. Yet me myself I sometimes try very hard to detect a microphonic tube by only tipping them on the glass with my finger nails.
Way to often it happened to me myself that I was sometimes thinking, well, this one might be a worn out one that may suffer on some microphinic issue.
Put it in a brand new one, just the same!
- Feb 12, 2006
- 61,045
- 5
- 220,994
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Thousands and thousands are replaced that are entirely fine IME.
The phase inverter of my Mesa D180 seems super sensitive to mechanical vibration. The chopstick test makes the tube produce a rattle like a bunch of small, thin metal plates clacking together. I am not all that concerned about the chopstick test, but I am concerned if I hear a rattle while I am playing.
The original tube was a Mesa 12AT7. Aside from the rattle and a little bit of hum, I loved the sound of this tube in the amp. Tried a new EH 12AT7, NOS Jan 12ATC7WC, and NOS Mullard CV4024. I noted differences in the tone and dynamic response between these four tubes.
My notes:
The EH and JAN tubes went back in my "new" box with a note. If they rattle in another amp, I'll move them to the "previously new" box.
The Mesa tube went in my previously new box. Some of the tubes in this box were pulled because I preferred the tone of another tube, and some have faults of some sort that do not necessarily make them totally unusable. A few should probably go in the trash, but I'll give them one last chance in another amp.
When I rolled tubes in my Hiwatt DR201, the tube I chose for V2 was an old 7025 pulled from my Showman Reverb because one of the triodes sputters and hisses. The Hiwatt does not use the faulty section and the good section of the tube still sounds fabulous; best of all of the tubes I tried.
Actually I have rehomed quite a few tubes that I pulled from other amps for various reasons.
The last tube I threw away was a dead vintage Telefunken ECC81 that I pulled from the Showman Reverb. I tried it in another amp and it was dead there too
.
When I do the chopstick test on my 87 SVT, the 6C4 produces a similar rattle to the D180's phase inverter. AFAIK, this is a common problem for 6C4s. I don't hear the rattle when playing the amp in it's current location. The rattle was fairly audible when I had the amp stacked on a different set off speakers, but only when I cranked it up a bit. I have a variety of NOS 6C4s, but I'll leave it alone for now.
The original tube was a Mesa 12AT7. Aside from the rattle and a little bit of hum, I loved the sound of this tube in the amp. Tried a new EH 12AT7, NOS Jan 12ATC7WC, and NOS Mullard CV4024. I noted differences in the tone and dynamic response between these four tubes.
My notes:
-EH12AT7—Similar gain and sound to Mesa 12AT7, open/airy…also lots of mechanical rattle.
-JAN 12AT7WC—seemed lower gain, softer, and more compressed. Less hum due to lower gain. Would have kept this tube except it had a slight amount of rattle.
-Mullard CV4024—warm and plenty of gain, but less hum and no rattle. The heater in this tube flashed when I first turned on the power, but it seems fine for now.
Rattle drives me nuts, so the Mullard stayed in the amp, even though I preferred the tone of the other tubes. This is one of those YMMV situations. Another person could easily think the Mullard had the best tone. -JAN 12AT7WC—seemed lower gain, softer, and more compressed. Less hum due to lower gain. Would have kept this tube except it had a slight amount of rattle.
-Mullard CV4024—warm and plenty of gain, but less hum and no rattle. The heater in this tube flashed when I first turned on the power, but it seems fine for now.
The EH and JAN tubes went back in my "new" box with a note. If they rattle in another amp, I'll move them to the "previously new" box.
The Mesa tube went in my previously new box. Some of the tubes in this box were pulled because I preferred the tone of another tube, and some have faults of some sort that do not necessarily make them totally unusable. A few should probably go in the trash, but I'll give them one last chance in another amp.
When I rolled tubes in my Hiwatt DR201, the tube I chose for V2 was an old 7025 pulled from my Showman Reverb because one of the triodes sputters and hisses. The Hiwatt does not use the faulty section and the good section of the tube still sounds fabulous; best of all of the tubes I tried.
Actually I have rehomed quite a few tubes that I pulled from other amps for various reasons.
The last tube I threw away was a dead vintage Telefunken ECC81 that I pulled from the Showman Reverb. I tried it in another amp and it was dead there too
. When I do the chopstick test on my 87 SVT, the 6C4 produces a similar rattle to the D180's phase inverter. AFAIK, this is a common problem for 6C4s. I don't hear the rattle when playing the amp in it's current location. The rattle was fairly audible when I had the amp stacked on a different set off speakers, but only when I cranked it up a bit. I have a variety of NOS 6C4s, but I'll leave it alone for now.
