I have a friend with a MKiii that has a new VTA board and new quad cap. The 5AR4 tube socket has the diode mod. The amp has worked OK for a couple of months. Bias set at .45v. Now he keeps going through JJ 5AR4 rectifiers. Just installed a new JJ 5AR4 and it starts sparking soon as amp warms up. Thinking maybe quad cap was damaged? Would an inrush thermistor help? I suggested a vintage 5AR4 tube or perhaps switching to SS rectification.
3 posters
5AR4 Tubes Keep Failing
rudes- Posts : 61
Join date : 2012-03-07
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
rudes wrote:I have a friend with a MKiii that has a new VTA board and new quad cap. The 5AR4 tube socket has the diode mod. The amp has worked OK for a couple of months. Bias set at .45v. Now he keeps going through JJ 5AR4 rectifiers. Just installed a new JJ 5AR4 and it starts sparking soon as amp warms up. Thinking maybe quad cap was damaged? Would an inrush thermistor help? I suggested a vintage 5AR4 tube or perhaps switching to SS rectification.
Just my opinion but > The JJ 5AR4 does have some "longevity issues". Get a Tung-Sol or Gold Lion 5AR4. Better made and they should last longer. You should check your present line voltage. Original Dynaco amps were made to be used at the line voltages of the 1950's and 1960's which was in the 115 - 117 VAC range. Now the "standard" line voltage is suppose to be 120 volts. Some places in the USA though do have voltages in the 121 to 124 VAC range. Voltage in this range can cause noticeably shorter rectifier life. If your line voltage is 122 or above run the amp off a variac set to 115 - 118 VAC.
NOTE - You can also run a Mark III off a 5U4 rectifier. There are plenty of USA made GE, RCA, Sylvania 5U4's for sale on Ebay at reasonable prices.
Bob
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
MK3 rectifiers should never go bad, you probably have a bad quad cap
quad cap for ST120 or M125 or MK3 amp (550v) $80 each
https://www.tubes4hifi.com/AMP-PARTS.htm
quad cap for ST120 or M125 or MK3 amp (550v) $80 each
https://www.tubes4hifi.com/AMP-PARTS.htm
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