Hi.
Here's my situation. I have a Cary SLI-50 (Integrated EL-34 based amp) which uses 2 5U4's as rectifiers.
A while ago, the 5U4's started arcing. The amp has a standby setting for heating up the tubes. When it gets turned on, the tubes will spark. Sometimes, its a minimal thing, other times its a bit like fireworks. The other day, a fuse went.
I have recently done a full re-cap as well as put in new switches - both input and power. The amp has a full compliment of new tubes.
Any thoughts?
Thank you!
Per
Here's my situation. I have a Cary SLI-50 (Integrated EL-34 based amp) which uses 2 5U4's as rectifiers.
A while ago, the 5U4's started arcing. The amp has a standby setting for heating up the tubes. When it gets turned on, the tubes will spark. Sometimes, its a minimal thing, other times its a bit like fireworks. The other day, a fuse went.
I have recently done a full re-cap as well as put in new switches - both input and power. The amp has a full compliment of new tubes.
Any thoughts?
Thank you!
Per
Last edited:
Arcing in rectifiers may be by a faulty rectifier(s) itself or a severe overvoltage across it. Take a time to measure it, and try a current limited power supply, using a bulb in series with the device under test during the testing time, because you can damage the transformer and or the rest of tha amplifier. Be careful with high voltages with tube devices.
We definitely need to see the schematic. Quite a few Cary designs badly abuse vacuum rectifiers.
Are you using ST ("coke") bottle 5U4s of either Chinese or Russian origin? Those tubes are utter trash and repeatedly cause arcing trouble under allowable conditions. 😡
First, any design flaw that abuses the vacuum rectifiers with excessive capacitance must be corrected. Second, Russian or Chinese 5U4G "equivalents" get replaced by either U.S. made OS 5U4GBs or current production ElectroHarmonix (EH) 5U4GBs. GEs will cost less than other U.S. made OS and are just fine. A cylindrical envelope may have less eye appeal than ST, but reliability is head and shoulders above cosmetics.
Are you using ST ("coke") bottle 5U4s of either Chinese or Russian origin? Those tubes are utter trash and repeatedly cause arcing trouble under allowable conditions. 😡
First, any design flaw that abuses the vacuum rectifiers with excessive capacitance must be corrected. Second, Russian or Chinese 5U4G "equivalents" get replaced by either U.S. made OS 5U4GBs or current production ElectroHarmonix (EH) 5U4GBs. GEs will cost less than other U.S. made OS and are just fine. A cylindrical envelope may have less eye appeal than ST, but reliability is head and shoulders above cosmetics.
+1.
Check capacitance(s) at the input to the filter. Never more than 40µF total for a 5U4.
Also, check the DCR of the power transformer primary (and multiply that DCR times the ratio of the primary to 1/2 secondary. For example, 120VAC to 360-0360 is 3X primary to 1/2 secondary 0-360).
Then, check the DCR of the power transformer secondary (one end to the center tap).
Add those two numbers and check it versus the 5U4 spec for capacitor input filter.
The required resistance is not a single number, but may be related to the capacitance, plate voltage, and DC current load.
And as already stated, not all 5U4 tubes are created equal.
Then, check the DCR of the power transformer secondary (one end to the center tap).
Add those two numbers and check it versus the 5U4 spec for capacitor input filter.
The required resistance is not a single number, but may be related to the capacitance, plate voltage, and DC current load.
And as already stated, not all 5U4 tubes are created equal.
To everyone discussing the first cap: did you read the quote above?
Without knowing for sure i would guess that in standby the filaments are on and so is the anode AC supply. What is not yet connected is the center tap to ground. The reason for the sparking just cannot be the input cap.
Could it be the new rectifiers are not up to scratch? I would start by making sure all is well with the filament voltage with new 5U4G in the sockets.
Looks like plate bending on current. Time to replace the tubes before transformer damage. Regards.
When you say full complement of new tubes, this includes the rectifiers?
Have you tried putting the old ones back (assuming they were in working condition)? Perhaps the new ones are not up to scratch, or are DOA.
Have you tried putting the old ones back (assuming they were in working condition)? Perhaps the new ones are not up to scratch, or are DOA.
I don't thin anyone abused rectifiers like Quicksilver did. Years ago I did a diode modification which resulted in the pair of 5AR4's lasting in excess of 15 years.
I would replace your rectifiers if needed and install diodes in series before the plates.
I would replace your rectifiers if needed and install diodes in series before the plates.
First action is : Avoid using the standby switch, lock it in "on". That way the
rectifiers and all other tubes will get a softstart. Also make sure you don't
have accidentally increased the can cap's, especially the first one after the rectifier.
You write "5U4" this old tube cannot stand more then 20uF as first cap when used as cap-input.
5U4GB is a more resilient tube, it can accept 40uF in a cap input configuration.
( there is never any reason to use this "feature" )
Last edited:
Suppose the B+ standby switch is the (only) ground return of the B+ secondary center tap.
And suppose the standby switch is in the standby position before power is applied, and as soon as the the main power switch is turned on the arcing occurs (without closing the standby switch).
How about the possibility that over time and usage, the standby switch has shorted shut (like using a quality 250VAC switch for a 350-0-350VAC B+ secondary).
What could possibly happen?
A pair of the best rectifier tubes is usually less expensive than the price of a new power transformer.
"A little bit of hum with music and a small capacitor, is better than a large capacitor with no music at all" - Proverbs For Tube People
And suppose the standby switch is in the standby position before power is applied, and as soon as the the main power switch is turned on the arcing occurs (without closing the standby switch).
How about the possibility that over time and usage, the standby switch has shorted shut (like using a quality 250VAC switch for a 350-0-350VAC B+ secondary).
What could possibly happen?
A pair of the best rectifier tubes is usually less expensive than the price of a new power transformer.
"A little bit of hum with music and a small capacitor, is better than a large capacitor with no music at all" - Proverbs For Tube People
Last edited:
The directly heated rectifiers will apply B+ before the output valves have fully warmed up. Not sure how much this will contribute towards cathode stripping but it cannot be great.
Cathode stripping happens more often during 2 scenarios:
1. Exceeding the ratings of a rectifier (capacitance value, peak secondary voltage; and the minimum DCR series resistance required versus the first two of these).
2. Tubes with uneven oxide coating (quality).
1. Exceeding the ratings of a rectifier (capacitance value, peak secondary voltage; and the minimum DCR series resistance required versus the first two of these).
2. Tubes with uneven oxide coating (quality).
Yes - but they are far better quality than the other Russian 5U4s. FWIW, I've had zero failures with them since they came out a couple years ago.
What i meant was cathode stripping of the output valves as without a delay B+ would come up first.
All my tube equipment uses either GE or RCA 5U4GT tubes, or 5AS4A.
I don't like to play with fire.
I don't like to play with fire.