[Bill Baker]

I have been working on quite a bit of Ming-Da MC-7 preamps lately. The more recent pieces coming in have been using a 150uF cap after the 5Z3 rectifier

 The original design shows a 5Z3 (5u4) into a simple CLCRC (100uF - 5H choke - 470uF - 2K ohm - 1000uF). Now they are using 150uF and sometimes slightly larger off the rectifier which is causing some problems. The original 100uF was about as large as I would like to see after the rectifier and even that a bit too large for my liking. I am thinking maybe a 68uF after the rectifier as well as lowering the value after the choke.
Thoughts?


I think this is what they call a "surplus" product as they seem to be using whatever they have on hand anymore These are such a nice sounding preamp for the money, why can't they stay consistent.

[aerius]

Because everyone knows that bigger is better, right?  Right?

To be somewhat blunt, a lot of companies don't know what in hell they're doing, they're a lot closer to parts swappers than engineers who actually understand the circuit.  If 100uF is good, 150uF must be better, oh wait, I guess they forgot to check the datasheets for whatever rectifier they're using.  Because if they looked at the datasheets, they'd know what the maximum continuous and transient current limits are, and if they knew those numbers, they wouldn't do something stupid like using a large capacitor as the first element of a power supply filter.

I still say my favourite "stupid" is a certain headphone amp whose circuit was likely lifted from a 6DJ8 pre-amp, and then had 6SN7 tubes plopped into it without changing a single parts value.

[Bill Baker]

It's funny when you can look at 10 different pieces of the same model (at least from this manufacturer) and no two are built the same. As you mentioned, if they looked at data sheets, they would know that a 100uF is the largest recommended cap to be used as the first component off the 5U4 rectifier. I also found a small electrolytic that was installed backwards. Since it wasn't in the power supply, it survived much longer.

 I recently came across the same thing within their 805 mono blocs. I have two pair here and the power supplies are completely different. One pair even has differences within the same "set" of amps.

 It's sad that major manufacturers can get away with this. Where do their standards come from?

[gld]

Typically 60uf is a maximum for most popular rectifiers. Anything larger than this causes problems with the
rectifier. You can use a choke, then a larger cap after that without problems.
gary

[Bill Baker]

Typically 60uf is a maximum for most popular rectifiers. Anything larger than this causes problems with the rectifier.

 I agree. Depending on the design (and the rectifier), I usually prefer something along the lines of a 22uF - 47uF if it is the first component off the tube.

[john curl]

Designers should read the data book to find the tube limitations.  The 'RCA Receiving Tube Manual' states for the 5U4:
"When a filter-input condenser larger than 40uf is used, it may be necessary to use more plate supply impedance than the minimum value shown to limit the peak current to the rated value."
In other words, don't use more than 40 uf.

[JoshK]

In addition to the data manuals, there is a free program on duncanamps's site called PSUD2 that simulates your power supply and tells you when you have run past the limits of your rectifier.  Basically they have no excuse.