Re: 31A Leslie amp and operator error

[Ted Thompson <tthompsn@mail.bcpl.lib.md.us>]

Dave Wallace wrote:
> 
> >
> 
> >
> >  This is re- using diodes for two bad 5Z3's.
> >
> >
> >
> >  Rechecked my B+.. Maybe I used the wrong scale  Seems to be sitting around
> > 380v...schematic calls for 375v. That's more like it. Anyone see a reason 
> > for
> > taking out the diodes?
> >
> >
> >       Wild Bill
> Bill,
>   The tube finatics will tell you that if you have to replace tube type
>   rectifiers in a tube amp, you need some type of delay (to simulate the
>   typical 3 or 4 second delay that the tube rectifier has before it actually
>   supplies power) This is to prevent something called "cathode stripping"
>   Cathode stripping can happen to your amp tubes (6550, 12AU7 etc) when B+
>   power is applied too quickly before they too have had a chance to warm up.
>   When that B+ instantaneously hits them from the silicon rectifier while
>   they are still cold, it tries to strip electrons from the cathode before
>   the cathode is hot enough to boil them off willingly. There are varying
>   opinions about this subject. I'm taking this from articles in a publication
>   called "Glass Audio" They had actual articles to add delay timers when
>   solid state rectifiers must be used to power tube amps. I'm not so sure
>   the entire industry follows these guidelines. There's a lot of hybrid
>   tube eq. out there (tube preamps etc.) that probably use solid state P.S.s
>   Maybe they have some built in delay too.
>   I would say it's better to be safe than sorry, if you cant get some 5Z3's
>   or would rather not use them because they are less effecient and make heat.
> 
>   You should try to incorporate some delay time in the solid state rect.
>   E-mail me if you want me to find the article with this fairly simple
>   project.       Dave Wallace    daw@glenqcy.glenayre.com>


Tips for Rectifier replacement...

Sometimes it matters, and sometimes it dosen't, but a tube rectifier has 
a much different voltage drop when running that a silicone rectifier.  
The same goes for Copper Oxide and Selinium types, usually it is a good 
idea to calculate the voltage drop and provide a dropping resistor to 
bring the voltage down.  This is most likly why the owner of this Leslie 
is now in need of a filter cap.  Even if the voltage on silicon diodes is 
within the caps specs. it can still blow it out because the cap can't 
handle it's spec'ed voltage but could handle the voltage the amp would 
have had using tube type rectifier(s).


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