Replacing Filter Caps Traynor YBA-1

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GussyLoveridge

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Mar 15, 2012
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Hey Folks -

Doing a little digging.  Tried to read up on the META:

Capacitors and choosing them (Walt Jung?s famous article):
http://www.capacitors.com/picking_capacitors/pickcap.htm

Link doesn't seem to work anymore.

Replacing caps in a 70's Traynor YBA-1.  No choke.

Wondering if some JJ 50/50 µF - 500V are a good replacement for the 40/40 µF 450V.

I understand that the voltage rating is fine.  I'm not sure about the tolerance of the caps that were in there, they are, I believe, the original Mallory's.

I figured that they would be a fine replacement but then came across this on the Electrical Audio messageboard:

Postby steve on Sun Oct 21, 2007 3:37 pm

Additional note: The choke in the PSU means you can use higher filter cap values than stock, which will lower the ripple even more (which you should do if you increase the gain of the first preamp section as Scott suggests).

You can't arbitrarily increase the size of caps in amps without a choke because the rectifier will blow if the first cap it sees is too large, but with a choke in there you can double the size of the filter caps with impunity.

For reference, the MKII we have here has JJEL34 power tubes, if you're interested in re-making that amp.

Congratulations on the amp score. Those things are the business.

steve albini
Electrical Audio




Thanks for any help you can offer.

Gus.
 
The difference between 40 and 50uF is negligible so I wouldn't worry about it.

Assuming no other mods, that should be it.
 
Thanks dudes! I really appreciate the help and wisdom.

Can anyone speak to this though?

You can't arbitrarily increase the size of caps in amps without a choke because the rectifier will blow if the first cap it sees is too large, but with a choke in there you can double the size of the filter caps with impunity.

Amp is SS rectified, no choke.

Cheers-

Gus.
 
Hi Gus, we did.  It will be fine. 

Long answer is that the inrush current at turn-on for a MUCH larger cap could be too high for the rectifier diodes and cause them to pop.  A choke will saturate and limit the rush of current.
 
The only thing that will make a rectifier blow will be exceeding its' current or voltage ratings.  If you want to see what this looks like in a tube amp power supply, you can model it with the Duncan Power Supply Designer:

http://www.duncanamps.com/psud2/

The PSU designer will allow you to plot current and voltage over time at different points in the PSU.  Really helpful.

When you first power up a circuit, the filter caps have zero charge on them.  This will look like a direct short to the components feeding the capacitor and result in maximum current flow.  As the caps charge, the current flow will slow until the caps fully charge.  The PSU designer will show you this graphically.
 
> You can't arbitrarily increase the size of caps in amps without a choke....

_IF_ the choke comes *before* the first cap!!

In YBA1 the cap comes first. The rectifier "sees" that cap directly.

So the above statement does NOT apply to YBA1. (And not to 99% of all g-amps ever made.)

There is a choke later, but this does not limit rectifier charge current.

Another thing: the real hysteria is vacuum rectifiers. All the YBA1 plans I found show Silicon rects. While under-rated Silicon is common, it isn't quite the issue; if an older Silicon rect *ever* fails, or if working in there anyway, replace with 3X the rating of the original. (In the 1970s we did not know as much about abusing Silicon and many designs are bound to fail after 100, 1,000, 10,000 turn-ons.)

And as said: Pete was buying 40uFd -50%/+100%. He could have got 79uFd, and he knew it. 50uFd +20% is entirely within old parameters.
 

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