|
|
|
572B's are such a big problem I've decided to post a sample of any common question and answer.
---- Original Message -----
Subject: Stability at VHF with aDentron 160-10L
> I want to thank you for all the info on your web page. It has helped me out alot. I have an old Dentron 160-10L that I picked up for not too much. The amp has been sitting for manys years. I had to rebuild the power supply because I did not trust the capacitors in it and the cheesy diodes. I ran the amp for days with no HV to degas the tubes. I ran well for several weeks and then I started blowing the .5ohm anode fuse resistor. I think the plate on one of the 572B tubes could be arcing??? I was thinking about all you good info at your site and the VHF stability issue. I was wondering if you could tell me what values would be a good choice for 4X572B tubes. I could play with them in Micro-Cap 8. > Answer:
Almost certainly the problem is poor voltage breakdown in the 572B's. When I
did consulting for Ameritron, nearly 60% of all new 572B's failed high-pot
testing!!
All of the import tubes I have seen fail high-pot testing at old normal
standards. Most Chinese tubes are only tested at 1800 volts, when they should
be tested to withstand 8kV or more. The peak anode voltage in normal
operation is twice the DC anode voltage, and if you mistune or have exciter
ALC transients the voltage can be three or more times DC.
I'd add a energy absorbing resistor in series with the HV supply line, like
one or two 10 ohm 10W resistors in series. An RCD pulse rated
resistor like the 175P series would work. This resistor allows the tube to arc
without catastropic failure of other components.
Otherwise you just need to find the tube with voltage breakdown issues.
|
|