Many vintage amplifiers and modern low-cost tube amplifiers don't have a HT fuse. A short in the output tube may cause a transformer failure or may damage the components connected to the cathode. Check your amplifer manual or the schematic to see if there is a protection. On the positive side, shorted tubes are rare. On the many amplifiers I have, I only got a shorted output tube once. The most common tube decay telltale sign is a increase of distortion and reduced power. Small voltage gain tubes may also fail by becoming microphonic or noisy.
Edit: I checked your amplifier manual from the manufacturer website, and there is a HT fuse protection indeed: it is called Plate fuse in the manual.
Edit: I checked your amplifier manual from the manufacturer website, and there is a HT fuse protection indeed: it is called Plate fuse in the manual.
Last edited:
It is also not uncommon for other components to die and take the tube with them as well, notably coupling caps. Damage can range from slight to catastrophic.
Another failure mode is in amps designed to run tubes near their limit. This can push ageing valves to fail from a variety of causes, and can trip up some modern production tubes which don't always meet the quality standards of previous generations.
I doubt your Prima Luna amp should concern you in this regard however [emoji16]
Another failure mode is in amps designed to run tubes near their limit. This can push ageing valves to fail from a variety of causes, and can trip up some modern production tubes which don't always meet the quality standards of previous generations.
I doubt your Prima Luna amp should concern you in this regard however [emoji16]
I had a coupling cap go and the valve red plated badly.
Luckily I caught it in time as I could see the valve glowing brightly through the top grill.
It depends on:
why they die.
Typically after several thousand hours emission will go down and power will be reduced. But, a tube that has "flashed over" might experience a short or very low impedance and this might be dangerous for the rest of the amp.
Conclusion : any tube that has flashed should be replaced. Don't make any changes to a "commercial amp" , do no "tuberolling". Observing these simple rules will keep the amp safe .
There are some reasons why parts fail:
1. Sometimes the coupling caps are nicely rated for the operating plate voltage of the driver tube. For example, the driver plate operates normally at 250V, and a 400V coupling cap is employed. But . . . the B+ is 500V, and when the driver tube is cold at power-up, the 500V is applied through the driver plate resistor load, and so 500V is on the 400V cap, and the other end of the cap goes to the output tube's grid resistor to ground (or even worse grid resistor to -45V fixed bias). Use a 600V coupling cap here, right?
2. A Pentode screen, or Beam Power screen is being subjected to more voltage or dissipation than it is rated for. Then it gets so hot that the screen wires move, and shorts to the control grid. Or the hot screen shorts to the suppressor grid/Beam forming plates. If that screen is connected directly to the UL tap, you can see B+ flowing through the transformer primary, and driving the grounded suppressor grid/Beam forming plates. Those are connected to the cathode. The cathode is connected to a low ohms sense resistor, or to a self bias resistor in parallel with a bypass cap (poor cap, and poor resistor).
3. There are many other failure modes.
I try my best to use a fairly simple solution to over-current situations. For example: I use a fast-blow fuse in series with a slow-blow fuse where the mains power comes into the amp, then on to the power switch, and then the power transformer primary. The values of the fuses depends on the amplifier currents. I use solid state diodes for the B+.
At turn on, the cold filaments draw max current, and the B+ caps are at Zero Volts, so the B+ primary draws max current. The fast-blow fuse has to be able to not blow open with this max power-up inrush current. Once the amplifier is warm, the slow-blow fuse has to be able to not blow open with the steady current of a single ended amp, or the max current of a push pull amplifier at maximum power output. An example of one of my amplifiers: A 1.25A fast-blow, and 0.6A (600mA) slow-Blow fuse are in series. Now, if there is a catastrophic short (screen to control grid), the fast-blow may open. Now, if an output tube starts a slow and increasing thermal run-away, the slow-blow may open. Get the idea?
1. Sometimes the coupling caps are nicely rated for the operating plate voltage of the driver tube. For example, the driver plate operates normally at 250V, and a 400V coupling cap is employed. But . . . the B+ is 500V, and when the driver tube is cold at power-up, the 500V is applied through the driver plate resistor load, and so 500V is on the 400V cap, and the other end of the cap goes to the output tube's grid resistor to ground (or even worse grid resistor to -45V fixed bias). Use a 600V coupling cap here, right?
2. A Pentode screen, or Beam Power screen is being subjected to more voltage or dissipation than it is rated for. Then it gets so hot that the screen wires move, and shorts to the control grid. Or the hot screen shorts to the suppressor grid/Beam forming plates. If that screen is connected directly to the UL tap, you can see B+ flowing through the transformer primary, and driving the grounded suppressor grid/Beam forming plates. Those are connected to the cathode. The cathode is connected to a low ohms sense resistor, or to a self bias resistor in parallel with a bypass cap (poor cap, and poor resistor).
3. There are many other failure modes.
I try my best to use a fairly simple solution to over-current situations. For example: I use a fast-blow fuse in series with a slow-blow fuse where the mains power comes into the amp, then on to the power switch, and then the power transformer primary. The values of the fuses depends on the amplifier currents. I use solid state diodes for the B+.
At turn on, the cold filaments draw max current, and the B+ caps are at Zero Volts, so the B+ primary draws max current. The fast-blow fuse has to be able to not blow open with this max power-up inrush current. Once the amplifier is warm, the slow-blow fuse has to be able to not blow open with the steady current of a single ended amp, or the max current of a push pull amplifier at maximum power output. An example of one of my amplifiers: A 1.25A fast-blow, and 0.6A (600mA) slow-Blow fuse are in series. Now, if there is a catastrophic short (screen to control grid), the fast-blow may open. Now, if an output tube starts a slow and increasing thermal run-away, the slow-blow may open. Get the idea?
Why not contact Kevin Deal/Prima Luna directly and find out whether or not there is an issue.
These amps are generally reliable from what I have heard, but you should replace the tubes periodically and perhaps just follow the manufacturer's guidelines. Probably not a bad idea to use their tubes or ones they recommend.
As an owner you should be able to get assistance directly from them.
These amps are generally reliable from what I have heard, but you should replace the tubes periodically and perhaps just follow the manufacturer's guidelines. Probably not a bad idea to use their tubes or ones they recommend.
As an owner you should be able to get assistance directly from them.
If there are resistors in series with the plate, screen, or cathode, sometimes those will be burned
by a bad tube. Fuses also.
Thank you kindly guys for sharing your experiences.
Im not in the US. The amp is working great but sadly Im not getting any support or replies from the shop that I bought it from & this goes for Primaluna themselves as well. Have never open up the amps before perhaps will do so soon.
Thanks again
Im not in the US. The amp is working great but sadly Im not getting any support or replies from the shop that I bought it from & this goes for Primaluna themselves as well. Have never open up the amps before perhaps will do so soon.
Thanks again
I know you're not in the u.s. lol.. I am surprised though that no one at Prima Luna has responded to you.
I design and build all my own tube gear, I've encountered few failures. Keep track of the number of hours and replace the tubes every few thousand hours of operation. I'd recommend getting them matched and burned in - if import duties are not excessive I would purchase from Jim McShane.
I recently replaced an octet of SV6550C that had been in a pair of amps I designed and built for a client back in 1999 that had 18 years of use and probably close to 20K hours on them - they were tired but still working acceptably well.
I design and build all my own tube gear, I've encountered few failures. Keep track of the number of hours and replace the tubes every few thousand hours of operation. I'd recommend getting them matched and burned in - if import duties are not excessive I would purchase from Jim McShane.
I recently replaced an octet of SV6550C that had been in a pair of amps I designed and built for a client back in 1999 that had 18 years of use and probably close to 20K hours on them - they were tired but still working acceptably well.
Thank you kindly again Kevin. Yes Im surprised as well that I didn't get a reply from Primaluna, wrote to them twice. I've done a fair bit of diy mainly on dac & SS amp. Tube stuff Im weary due to it's high voltage, furthers I've gone was a tube preamp that I muck around with years back.
Cheers
Cheers
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.