Cap Job - Replace diodes too?

[jjasilli]

Should I replace the old SS diodes too?  Full wave: a string of 2 old bullet shaped green diodes on ea leg of the HV secondary.  If yes, is 1X 1N4007 per side OK?

Background:  Still rebuilding my 2X EL84 Traynor Bassmate.  Replaced the filter caps. 

[sluckey]

I'd leave the original diodes in the amp if they're working. If I replaced them with 1N4007s, I'd like the extra comfort zone of using two per leg.

1N4007 is good for 1000v PIV. That's about 350VAC RMS per leg. Here's how to figure...

Let's say you have a 350-0-350 VAC RMS PT. 350 x 1.414 = 495 peak voltage. When you forward bias the diode with the positive 495v peak, it passes that peak to the filter cap, which charges to +495vdc (Let's assume there's no load on the power supply to drain that +495vdc.) Now, 1/2 cycle later the AC input to the diode has reversed polarity. It's now a negative 495v peak. So, at this instant in time, you have a negative 495v peak on the diode's anode and you have positive 495vdc on the diode's cathode. That's 990 volts across the diode.

Using a single 1000v piv diode is cutting it too close for my comfort in this case. Using two diodes gives me a warm feeling.

So, to see how much voltage 'may' appear on your diode, just multiply 2.828 times your ht leg voltage. Then for a safety margin, double that number and choose/stack your diodes accordingly.

I apologize for the rambling. Just didn't feel like my usual yes/no answer tonight.   

[Tiny_Daddy]

I always replace the diodes because diodes are cheap. Repair returns are not.

[LooseChange]

Lots of Traynors around here and never seen those green diodes go bad. Just saying.

[jjasilli]

Sluckey:  Great info.  I didn't know how to spec out the diode rating.
Tiny Daddy & Loosechange:  It's my own amp, so I'll just keep those diodes.

Thanks guys!

[Geezer]

Lots of Traynors around here

That's so wierd....I've been in the music scene for 35+ years here in VA, been in just about every music store & pawn shop in the state (along with NC, SC, GA), & NEVER ONCE have encountered a Traynor!
I would be willing to bet you could count the number of Traynors in VA on your fingers (maybe one hand)!  
Gotta be "regional" thing.....I guess nobody around here sold them.

[LooseChange]

I'm in Vermont... 45 miles from the Canadian border.

[Tiny_Daddy]

If you can get Keith's India Pale Ale it's good stuff.

[jjasilli]

Meanwhile, I'm in the middle of another session of amp hell.  I'm just an amateur and usually everything goes fine. But every once in a while everything goes to hell.  After installing 2K2 screen resistors my bias sense reisitors blew up; and I keep getting a huge over-curent condition, for the 3rd time in the last 3 rebuilds.  Once I didn't connect power tube grid leak resistors (Stromberg Carlson hi-fi amp).  Next the EL84 power tubes decided to go bad in the middle of the rebuild for a friend (Heraton Amp). This time I don't know yet.  OK, so I'm just whining. 

[jjasilli]

OK, I can stop whining now.  It was the old tubes; they died.  Maybe I helped them die by inadvertently suplying only the screens, but not the plates with B+.  Whatever, they're dead.  Long live the new JJ EL84's!

Frustratingly, both the old dead tubes tested Good on my Sencore Mighty Mite tube tester, even for Shorts.  But they must have an internal short:  with the tubes in the sockets I get a screwy resistance reading from Grid Pin to Ground: 68K instead of 119K AND the resistance reading sets-in over about a minute as if affected by capacitance. 

Anyway, problem solved; back to the re-build! 

[Animatic]

I found one today on an island in Thailand, and scooped it up. Go figure not in VA, but yes in Thailand.
This thread is also very useful. I knew coming here I would find all I wanted to know.