My experience was less than 2 years ago when I was refurbing a pair of Heath kit monoblocks. Granted, with a 5V4 the turn-on surge pushed them a bit (it's not really a very slow warm-up) and the surge approached 520VDC before the output tubes warmed up, but after a few snap-crackle-pops I'd had enough. I admire the attempt to replicate the can caps of yore, but I wouldn't try to use them at anywhere near their rated voltage.
Those surge voltages are the rule rather than the exception in old tube rectifier and choke input supply gear. I would simply want two 400V caps in series for something that hits 500+. You see 550V-rated boutique caps, but Nichicon’s regular line stops at 450. There is a reason - the aluminum oxide can only do so much.
I've used Epcos 600V snap-ins for four years now with no failures across seven or eight amplifiers (knock wood!), and they often see a surge to 520VDC, depending on what rectifier tube is in use. Nichicon now makes a 600V, 105 degree cap. I'll probably switch to those.
Gee grovegardner,
Back in the day, those original caps didn't like over voltage either. The original capacitor tolerances were %50 tolerance. They were never subjected to high temperatures. That was the entire point of their construction and good designers didn't put them in those situations either.
As for the caps you're talking about, the original parts may be rated for +105°, but that is outside the can, not inside. Also, the C-V product is lower on the new "stuffed" cans, and C-V product can indicate how good a capacitor is. Newer smaller caps are generally not as good as the same exact values in a larger case.
I've used CE caps since I discovered them, and not had a failure. They perform at least as good as the originals, and better than the individual cap stuff jobs I see. Also better than individual capacitors strung everywhere with the wrong ground points as a result.
The only negative? They aren't cheap, but if you look at what the originals used to sell for ('cause I remember) and consider inflation. They are really inexpensive. I'm paying shipping, duties and more taxes than you are.
They area great product, they are not too expensive. They also allow the equipment to operate as intended.
Back in the day, those original caps didn't like over voltage either. The original capacitor tolerances were %50 tolerance. They were never subjected to high temperatures. That was the entire point of their construction and good designers didn't put them in those situations either.
As for the caps you're talking about, the original parts may be rated for +105°, but that is outside the can, not inside. Also, the C-V product is lower on the new "stuffed" cans, and C-V product can indicate how good a capacitor is. Newer smaller caps are generally not as good as the same exact values in a larger case.
I've used CE caps since I discovered them, and not had a failure. They perform at least as good as the originals, and better than the individual cap stuff jobs I see. Also better than individual capacitors strung everywhere with the wrong ground points as a result.
The only negative? They aren't cheap, but if you look at what the originals used to sell for ('cause I remember) and consider inflation. They are really inexpensive. I'm paying shipping, duties and more taxes than you are.
They area great product, they are not too expensive. They also allow the equipment to operate as intended.
For new builds, sure. Snap-ins. For restoring old equipment you shouldn't have any problems. I've been restoring tube product since before FP caps were discontinued, so I have always used them. There was a gap when they weren't available, and those were dark days. Yes, I used snap-in caps for the first filter for sure, but only when I couldn't get the FP style. I have had a couple 500V snap-in capacitors go open on me.
Major brands’ 600V caps may be two complete 400 or 450 V elements in series internally. There is something about the oxide construction that makes it only reliable to about 500V and then it just gets iffy.
FPs were never the most reliable caps in the first place, even in 1955. One would expect modern copies to be somewhat better, but it’s like making a modern 2N3055. I still wouldn’t use one at 100 volts.
That is amusing. I'm seeing 40 ~70 year old FP caps that still function well, and they measure well too. That isn't just capacitance.
Filter capacitors are in a high stress application. Of course, the oxide layers are fragile to some extent. How many fail because people just plug equipment in that has been idle for years? You have got to ramp this stuff up slowly to allow the oxide layers to reform or heal. Same for NOS FP caps. If you don't, they fail (big surprise).
I'm sorry, but I can't say a good quality FP style (twist-loc) capacitor is unreliable in any way.
Filter capacitors are in a high stress application. Of course, the oxide layers are fragile to some extent. How many fail because people just plug equipment in that has been idle for years? You have got to ramp this stuff up slowly to allow the oxide layers to reform or heal. Same for NOS FP caps. If you don't, they fail (big surprise).
I'm sorry, but I can't say a good quality FP style (twist-loc) capacitor is unreliable in any way.
Not necessarily “unreliable”, but just old technology that can’t take the beating of new units meant for SMPS duty. A cap that can take 2A of ripple current at 100 kHZ will have an easy life at 200 mA at 120 Hz. The latter was a challenge in the olden days.
+1. The gung-ho perspective of many when they come across unknown equipment, or that has been in storage, is worth trying to highlight, even for what may be considered 'recent' gear. But even the process of ramping should be preceded by a visual inspection for safety aspects and signs of part distress/failure.
The aa-50 is electrically pretty much the same as the AA-100.
The AA-100 is a solid design but is has some very weak points that result in long term failures.
They are
1) The 7591 output tubes are run hot in this design with fixed bias that can not be adjusted.
There is no way to easily measure the bias current in any one output tube.
This causes problems as the tubes age or in the case they are replaced as due to tube tolerance the bias current in anyone tube can reach destructive levels. The 7591 tube was a "bleeding edge" tube for it's time so running it hot does not help.
2) That all important fixed bias voltage is developed by a selenium rectifier that as it ages becomes less effective. This results in the fixed negative grid voltage on the 7591 tubes slowly becoming smaller with a resulting increase in output tube bias current. The already hot 7591 tubes get hotter still until something dies like the output tubes or output transformer. AA-100 and AA-50 often show burn marks on the case top from this issue.
3) The phenolic PCB that the very hot 7591 are mounted on slowly burns into carbon. This causes insulation breakdown on the output board leading to the bias levels on the output tubes becoming unstable leading again to overheating and tube failure. Tracks become loose and intermittent noises and problems start.
4) The 0.1uF coupling capacitors for the grids of the 7591 are packed in tight to the very hot 7591 tubes and will with time begin to fail and leak. This causes the 7189 tubes to run hotter still and can lead the the fuse blowing, 5AR4 failing, 7591 failure or output and power transformer failure.
5) The grid bias resistor on the 7591 in a AA-50 is 470K and the 7591 data sheet suggest a maximum value of 300K for fixed bias. As the already hot 7591 tubes age and start to develop some gas a increase in grid current results. This leads to a slow increase in the bias current for the 7591 and they get hotter still. The AA-50 needs pretty much a perfect set of 7591 tubes to be reliable.
Unfortunately all of the above almost always leads to a slow death in any AA-50 or AA-100 and the only fix is often to replace to output board.
Luckily there are/were some replacement boards being sold.
My suggestions on trouble shooting your current problems is as follows.
Make sure there is a load resistor of each speaker outputs to protect the output transformers from over voltage.
1) Remove the 5AR4 and connect a bench DC supply to the 5AR4 cathode pin #8. Apply no AC power the the AA-50.
Slowly increase the DC bench supply up to the normal operation DC voltage of the AA-50 of about 425V.
The current should be low and stay that way. Leave it a few minutes to be sure.
If the current rises suspect the bad can filter capacitor first.
Pull the 7591 tubes next to see if the current falls. One may be shorted.
2) When the above tests ok and with the bench supply still connected apply ac power to the AA-50 with the 5AR4 and the 7591 tubes removed.
The current should slowly rise as the small tubes warm to a tens of mA.
If this is OK next measure the voltage on pin #6 of each 7591 tube. You should see about -16 volts on each pin #6.
If they are all low the selenium rectifier has aged out and it should be replaced with a silicon diode.
This is good practice anyway as that selenium rectifier is a time bomb and putting in a silicone diode will make the 7591 outputs run a bit cooler adding a lot of extra life to your AA-50. Running them a bit cooler has almost no effect on distortion.
If the -16 is low or even positive on only some tube's pin #6 suspect a leaking coupling capacitor or insulation failure on the PCB.
To check the coupling capacitor lift one end of the coupling capacitor that goes to a pin where -16V is off.
If -16 is now restored with one end of the cap lifted then that capacitor is leaky.
Replace all 4 capacitor in any case as this is a just a good idea to avoid future failures.
If the voltage is still wrong on pin#6 with the capacitor to that pin lifted it is likely the PCB has insulation failure and needs to be replaced.
If you get past test 1 and 2 without finding a failure it is possible that one or more of your 7591 tubes has failed.
Best to test each one carefully for shorts and especially GAS before putting them back in.
3) Turn every thing off.
Put in ONE pair only of the 7591 tubes. Slowly increase the DC bench supply to about 425 volts.
The current should be very low. Turn down the DC bench supply.
Apply AC power and wait about 1 minute.
Slowly turn up the DC bench supply to about 425 volts. You should see a increase of around 80mA or so draw on the bench supply over what you saw without any outputs in place.
If it is way higher then one or both of the 7591 may have some gas and leaking in the grid circuit.
Repeat test #3 with the other pair of 7591 tubes in the second location on the output board.
If you got this far one of the above tests should have failed and pointed to the exact issue in your AA-50.
If all the above tests went without any failed tests then you are likely looking at intermittent failure.
Most likely cause is a connection issue(s) on the power board due to PCB deterioration from heat and age.
If that is the case a new PCB is almost always the only long term fix.
Good luck getting your AA-50 back in working order.
The AA-100 is a solid design but is has some very weak points that result in long term failures.
They are
1) The 7591 output tubes are run hot in this design with fixed bias that can not be adjusted.
There is no way to easily measure the bias current in any one output tube.
This causes problems as the tubes age or in the case they are replaced as due to tube tolerance the bias current in anyone tube can reach destructive levels. The 7591 tube was a "bleeding edge" tube for it's time so running it hot does not help.
2) That all important fixed bias voltage is developed by a selenium rectifier that as it ages becomes less effective. This results in the fixed negative grid voltage on the 7591 tubes slowly becoming smaller with a resulting increase in output tube bias current. The already hot 7591 tubes get hotter still until something dies like the output tubes or output transformer. AA-100 and AA-50 often show burn marks on the case top from this issue.
3) The phenolic PCB that the very hot 7591 are mounted on slowly burns into carbon. This causes insulation breakdown on the output board leading to the bias levels on the output tubes becoming unstable leading again to overheating and tube failure. Tracks become loose and intermittent noises and problems start.
4) The 0.1uF coupling capacitors for the grids of the 7591 are packed in tight to the very hot 7591 tubes and will with time begin to fail and leak. This causes the 7189 tubes to run hotter still and can lead the the fuse blowing, 5AR4 failing, 7591 failure or output and power transformer failure.
5) The grid bias resistor on the 7591 in a AA-50 is 470K and the 7591 data sheet suggest a maximum value of 300K for fixed bias. As the already hot 7591 tubes age and start to develop some gas a increase in grid current results. This leads to a slow increase in the bias current for the 7591 and they get hotter still. The AA-50 needs pretty much a perfect set of 7591 tubes to be reliable.
Unfortunately all of the above almost always leads to a slow death in any AA-50 or AA-100 and the only fix is often to replace to output board.
Luckily there are/were some replacement boards being sold.
My suggestions on trouble shooting your current problems is as follows.
Make sure there is a load resistor of each speaker outputs to protect the output transformers from over voltage.
1) Remove the 5AR4 and connect a bench DC supply to the 5AR4 cathode pin #8. Apply no AC power the the AA-50.
Slowly increase the DC bench supply up to the normal operation DC voltage of the AA-50 of about 425V.
The current should be low and stay that way. Leave it a few minutes to be sure.
If the current rises suspect the bad can filter capacitor first.
Pull the 7591 tubes next to see if the current falls. One may be shorted.
2) When the above tests ok and with the bench supply still connected apply ac power to the AA-50 with the 5AR4 and the 7591 tubes removed.
The current should slowly rise as the small tubes warm to a tens of mA.
If this is OK next measure the voltage on pin #6 of each 7591 tube. You should see about -16 volts on each pin #6.
If they are all low the selenium rectifier has aged out and it should be replaced with a silicon diode.
This is good practice anyway as that selenium rectifier is a time bomb and putting in a silicone diode will make the 7591 outputs run a bit cooler adding a lot of extra life to your AA-50. Running them a bit cooler has almost no effect on distortion.
If the -16 is low or even positive on only some tube's pin #6 suspect a leaking coupling capacitor or insulation failure on the PCB.
To check the coupling capacitor lift one end of the coupling capacitor that goes to a pin where -16V is off.
If -16 is now restored with one end of the cap lifted then that capacitor is leaky.
Replace all 4 capacitor in any case as this is a just a good idea to avoid future failures.
If the voltage is still wrong on pin#6 with the capacitor to that pin lifted it is likely the PCB has insulation failure and needs to be replaced.
If you get past test 1 and 2 without finding a failure it is possible that one or more of your 7591 tubes has failed.
Best to test each one carefully for shorts and especially GAS before putting them back in.
3) Turn every thing off.
Put in ONE pair only of the 7591 tubes. Slowly increase the DC bench supply to about 425 volts.
The current should be very low. Turn down the DC bench supply.
Apply AC power and wait about 1 minute.
Slowly turn up the DC bench supply to about 425 volts. You should see a increase of around 80mA or so draw on the bench supply over what you saw without any outputs in place.
If it is way higher then one or both of the 7591 may have some gas and leaking in the grid circuit.
Repeat test #3 with the other pair of 7591 tubes in the second location on the output board.
If you got this far one of the above tests should have failed and pointed to the exact issue in your AA-50.
If all the above tests went without any failed tests then you are likely looking at intermittent failure.
Most likely cause is a connection issue(s) on the power board due to PCB deterioration from heat and age.
If that is the case a new PCB is almost always the only long term fix.
Good luck getting your AA-50 back in working order.
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