Adding more capacitors to an AMP

[f1802llk]

I have a question about adding more capacitors to my MingDa tube (MC368-b90) amplifier. In the power supply, it uses two 330uf/450v caps in series to filter the output from the rectifiers. I'd like to parallel this rail of caps with a rail of two 470uf/450v caps in series to increase the capacitance of the power supply. Is this a safe mod? I understand that this increase in capacitance may result in more peak currents drawn from the rectifiers thus I may have to replace them with ones that have a higher current rating. Besides this, are there any potential problems that I need to watch out for?

Thanks in advance for any helps,

Frank

[Shoog]

What are you hoping to achieve by adding the capacitance?
You may run into problems of ringing in your power transformer. You may also generate more radio frequency interference which will be difficult to get rids of once created.

Are the rectifiers snubbered.

Overall this may prove to be a bad idea. There is a very good case to be made for minimising power supply capacitance. Some sort of choke (even a very small one) might be a better addition to the power supply.

Shoog

[f1802llk]

Thanks for the quick response. I got some Rifa PEH200 470uf/450v caps intending to replace the two stock AeroM 330uf/450v as I think the former are better in terms of lower ESR. I heard that more capacitance in the power supply can help the bass response as it would provide more peak current, so I thought why not just parallel it. This is not a good idea after all?

What did you mean by "snubbered rectifiers"? The stock ones are IN5408x8, and I plan to replace them by the "ultra fast and soft recovery" ones.

Frank

[ray_moth]

I would echo Shoog's caution against using too much capacitance after the rectifiers. You have more to lose than gain, IMHO, for the reasons he states.

The usual snubber used with silicon rectifier diodes is a 0.01uF capacitor of 1,000v AC rating across each diode. It helps a great deal to suppress the spikes associated with most SS rectifiers, even fast-recovery diodes.

[Shoog]

I think the former are better in terms of lower ESR.

Lowering ESR can be beneficial if the caps are really poor, however lowering ESR beyond a certain point may bring penalties as well. It was found with Gainclones that a little bit of ESR helped to damp ringing in the power supply which was clearly audible.

Sometimes we second guess designers who know infinately more than we do as hobbyists.

Shoog

[Salas]

Originally posted by Shoog
You may run into problems of ringing in your power transformer. You may also generate more radio frequency interference which will be difficult to get rids of once created.

Originally posted by Shoog
Sometimes we second guess designers who know infinately more than we do as hobbyists.

I agree. But I am still not sure if the amps in question fulfill such design thoroughness criteria and associated development phase measurements, or they just sport cap values that came handy economically and were in stock. I would just listen and compare if I was in the thread starter's shoes. I would just take out the original ones with care not to destroy them, and they can be fitted back.

[Shoog]

I agree. But I am still not sure if the amps in question fulfill such design thoroughness criteria and associated development phase measurements, or they just sport cap values that came handy economically and were in stock. I would just listen and compare if I was in the thread starter's shoes. I would just take out the original ones with care not to destroy them, and they can be fitted back.

I would totally agree with this statement. Try it and see.

The only thing to be aware of is that if it does introduce ringing, it is very likely on initial switch on things will sound 'better'. This is because the top end will sound more extended and lively, which will probably tighten up the bottom end. Only extended listening will reveal that the sound is more fatiguing and harder to listen to. This is what intermodulation distortion tends to sound like.
Just be careful and don't jump to conclusions.

Shoog

[f1802llk]

Thank you all for the advices. I'll try and see what happen.

Frank

[f1802llk]

Just as I was about to change the stock caps by Rifa caps, I noticed that the Rifa caps that I got (Rifa PEH200 470uF/450VDC) have date code 0209, meaning they were made in September 2002 according to the information given by Rifa web page, which also says that these caps have a shelf life of four years. So the caps that I have are more than one year past their shelf life. Are they still safe to use? What kind of checks that I can do to make sure that they are still usable?

Thanks in advance,

Frank

[ArtG]

Old unused electrolytic caps usually are fine to use provided you test and "reform" them first. Chemical changes take place in the electrolyte when they sit dormant, and this process needs to be reversed, prior to placing them in service.

There are a number of web sites that have detailed explanations of the process and why it is necessary. It's actually quite easy to do, and in fact it's easy to build a simple reforming circuit out of "junk box" parts, a couple meters and a variac. I did this years ago and routinely do this to all electrolytics that I use, old or new, since it's also a testing process.

While your caps may seem old to you, I'd consider these fairly new, since I've reformed dozens of NOS caps with date codes as far back as the 1970s, and most of them come up just fine!

Google the subject, or here's a link to get you started:

http://www.vcomp.co.uk/tech_tips/ref...eform_caps.htm

As always, when working with high voltage, stay alert and work safely!