Solid state rectifier vs tube rectifier?

I'm sure you all have cooked a few tube rectifiers in the past. I have tried the Weber copper cap rectifiers and they work good but eventually will fail as well. My question is, has anyone just used a couple of diodes instead of a rectifier tube? And if so, has anyone used the UF1N5408 Ultra fast recovery diodes?
If so, please let me know your experience with these.

Hi -- whenever I rebuild a Mark III or a ST-70 I use UF1N4007's in place of the Selenium Rectifier and they seem to work and sound just fine.

I have a version of the SDS Cap Board for the Mark III from Triode Electronics that uses two 1N5408 diodes (rated at 3 amps each) in place of the Tube Rectifier. Also sounds fine.

Every that I have read says that the UF and 1N type diodes are interchangeable and the only thing that need be considered is the current rating of the diode.

The Weber Copper Caps use two diodes as part of their circuitry. There was a forum Thread a few years back that discussed the internal circuitry of the Webers and it seems to me that the higher output Copper Cap used two 1N5408's.

Hope this helps! Have fun.

Analog Man wrote:My question is, has anyone just used a couple of diodes instead of a rectifier tube?

This is what I plan to do when I build the ST120.

Otherwise I recommend Ruby 5AR4 tubes. RAM Labs should be good too.

Sound Advice! 

Analog Man wrote:I'm sure you all have cooked a few tube rectifiers in the past. I have tried the Weber copper cap rectifiers and they work good but eventually will fail as well. My question is, has anyone just used a couple of diodes instead of a rectifier tube? And if so, has anyone used the UF1N5408 Ultra fast recovery diodes?
If so, please let me know your experience with these.

It should work ok.
The drawback is that a tube rectifier will "slowstart" and start working after 15s (or so), thus the B+ voltage will start rising briefly before the otput tubes start drawing current.
A ss rectifier will start rising at once, and in addition has a much lower forward voltage drop,
thus the caps will be exposed to a higher voltage before the tubes starts drawing current.

One solution is to install a "rubli" delay circuit ( cost 25$ ) that delays B+ until the
tubes start drawing current. It also protects the amp for "short power interrupts" since it
will restart it's delay cycle if the tubes stop draw current.

This may be a personal preference I'll leave the real details to the techies here. However, I once retrieved one of my Mark IIIs from a dude who said it was "beyond repair" and when I got it back found that he had rebuilt it (!!??) and was using it for himself. This was when I was not aware of this board and stupid and did not know how much I could do myself. He had broken the glass of the rectifier tube and used the remaining socket to insert a diode of some sort to replace the tube rectifier. It worked OK, I suppose, but learned that there would be an instant and considerable voltage surge when powering up so I just replaced his gizmo and went back to a tube. Just seemed like the right thing to do. The initial surge seems to be the bugaboo, can overload the multicap etc. unless you have upgraded.

You have to watch the ultra-fast recovery diodes in slow commutation circuits like a 50/60Hz power supply.  In my case the fast turn off UF5408s caused diode switching hash noise in a guitar amp.  Drove me crazy and tried all sorts of snubbers that weren't very effective.  Finally replaced them with fast soft recovery diodes (Philips BYM26E) that cleared the issue.   But I use a Weber WZ68 so if the UF5408s are used in it I have no noise issues.

As peterh mentioned inrush will be tremendous so a current limiter and/or delay circuit is highly recommended.

Also in the case of the ST-120 going "diode only" will cause a rise in B+ voltage so consider the working voltages of the quad caps so not to overly stress them.   Should be no problem for the two supplemental caps.

Another item to consider with rising B+ voltage in the "diode only" design is the diode peak inverse voltage rating.  Both the 5408 and 4007 are rated 1000V pk inv which means they can hold up to 1KV reverse biased before breaking down.   Will you ever be near this limit in a ST-120?   Yes, when the B+ caps are charged to over +500V and the negative half cycle of the AC waveform peaks at under -500V.  Two diodes in series on each leg would easily solve this issue but have the potential for more noise when they both turn off.