Kits useable with a M500 300B Monoblock amp

[ThorstenL]

Hi,

Originally Posted by bprager

The pwr supply change is the addition of the 10uf cap before the choke and the move of the 220's? I have that mapped and have the 10uf as a gold Obligatto.

Yes. That is it.

Originally Posted by bprager

Also, I am adding the 47R resistor and the 22uf cap as was circled in green on the first diagram in post #2.

Okay, I think we still have some confusion.

This Green circle Cap is the one I call "WE Connection".

It is a trick originally used by WE in the 30's and was (re)discovered by several guys in parallel, who all called it their pet names. Lynn Olson suggested WE connection instead and I have stuck with it.

The WE connection uses cancellation to improve power supply noise rejection. The critical stuff is that this "supply noise" is not just the hum but also anything that appears in reaction to the operation of the amplifier (in other words distorted signal send back by the powersupply due to teh Amp loading the supply with signal current).

The bad news is that this cancellation relies on a pretty precise ratio of the WE Capacitor and the cathode bypass capacitor. For a 300B used as in the M500 the ratio is 3.2. Note that the ratio depends heavily on the specifics of the Amp design, so even for a 300B the 3.2 ratio is not always right, other valves need very different ratios. Getting the ratio wrong will significantly reduce the benefit of this circuit trick.

So with a 15uF WE cap as shown the cathode bypass should be 47...51uF. With a 22uF WE Cap you need theoretically 70uF, so use a 68uF/160V Capacitor for the cathode.

With "textbook" values the improvement is generally over 20dB, if you tune the WE cap "just so" you can get over 30dB. Either way, the impact is several times that of the differences measured between the cheapest electrolytic capacitors and the best film cap's

So the key is that this WE Cap and the Cathode cap need a specific ratio, so a given value WE cap MUST be used with the matching value cathode cap.

Also, the WE Cap is the one with the biggest sonic impact in the scheme, it should always have the highest quality, followed by the cathode capacitor with the power supply capacitors at the back.

Originally Posted by bprager

I originally ordered .22uf coupling caps. Can/should I still use these, or return them and get .47's? I had picked the copper AN PIO's.

Yes, you can use these. Again, there is a relation (but less rigid than WE Cap vs. Cathode Cap) between the coupling capacitor value, the gridchoke value and the series RC combo connected across the coupling capacitor.

Originally Posted by bprager

The 15uf and snubber components? Do they need very high quality components or are they not as critical?

The 15uF is the same as the 22uF, so see above.

The Snubber can still benefit from good quality parts.

I would use a decent film Cap and a carbon composite resistor.

With your 0.22uF coupling cap you should use 2.2uF in series with 2.2K for the snubber.

The snubber damps the resonance between gridchoke and coupling cap (I previously used a resistor in parallel with the gridchoke - but this way is lower loss and more elegant) AND boosts the short term power delivery of the Amplifier over just having the 0.22uF Cap.

Ciao T

[bprager]

Ok. As they say, once more with feeling :-)

I'm assuming that your 15uf value is based on the 50uf cap labeled C3 that goes across the 25W1K resistor (the one in the huge gold heatsink).
That matches up with the one labeled C4 on the post 2 schematic where the changes are circled in green.

I had taken pics of the amp, and when I looked for C3, it wasn't 50 or even 100. It was 220uf. (You can see it in the one photo I uploaded in the early posts).

So if I understand correctly, I need to change that cap from 220uf to 68uf and the 22uf Obligatto oil cap I have will work.

And the Cap labeled C2 (currently a BG.47uf), gets changed to the Audio Note Copper PIO .22uf.

The 47R resistor (the other component circled in green that was part of the WE mod), connects from the 470V point and is shown on the schematic as a 47R resistor on a circuit line below C3.

I hope that the above means I finally got it and can quit bothering you with more questions!

Last few however, since ordering a cap or resistor from DIY doesn't make sense, I'll get it local.

I was thinking of a 22uf and 47uf Blackgate VK series in parallel which gets me to 69uf and 1 is 160V, the other 350V rated.
And those weird red chinese resistors: I was going to change them to DIY Kiawme's and picked up 1W resistors. I assume thats high enough as I don't know how to read the wattage of the
red guys.
The snubber components would need to be 600V rated on the cap, 1watt on the resistor basing the values on the fact that the current .47 C2 is a 600V cap.

Thanks!!!!!!!!!!!!!!!!!

Bruce Prager

[ThorstenL]

Hi,

Originally Posted by bprager

So if I understand correctly, I need to change that cap from 220uf to 68uf and the 22uf Obligatto oil cap I have will work.

Yes.

Originally Posted by bprager

And the Cap labeled C2 (currently a BG.47uf), gets changed to the Audio Note Copper PIO .22uf.

Yes on C2 becomes .22uF Audio Note, no on "urrently a BG.47uf" it is that Mundorf cap in there.

Originally Posted by bprager

The 47R resistor (the other component circled in green that was part of the WE mod), connects from the 470V point and is shown on the schematic as a 47R resistor on a circuit line below C3.

Yes.

Originally Posted by bprager

I was thinking of a 22uf and 47uf Blackgate VK series in parallel which gets me to 69uf and 1 is 160V, the other 350V rated.

Sure. You can also find 68uF BG's.

Originally Posted by bprager

And those weird red chinese resistors: I was going to change them to DIY Kiawme's and picked up 1W resistors. I assume thats high enough as I don't know how to read the wattage of the red guys.

Those firecrackers are 2W, but they are not neccesarily needed in terms of dissipation. For all the resistors around 6SL7 and on the 300B grid will be fine in 1W, as will be the 47 Ohm resistor in the power supply line.

Originally Posted by bprager

The snubber components would need to be 600V rated on the cap, 1watt on the resistor basing the values on the fact that the current .47 C2 is a 600V cap.

400..450V will likely suffice, but 630V do no harm. The resistor can be 0.5W.

Ciao T

[bprager]

BINGO!!!!!!!!!!!!!!!

Time to get the last components in and have fun modding.

Super help, can't thank you enough!

Best regards,

Bruce