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EWBrown wrote:You new schematic is right-on!

Rather than having a resistor in parallel with the SS rectifiers, perhaps a 10 ohm, 1W resistor in series with each diode would help absorb some of the transients, or just skip the added resistors altogether.

Understood, thanks. Thanks for checking the schematic; I really like this solution!

Chris

EWBrown wrote:Are the 70V bias taps a separate winding, or are they taken off the 400V windings?

Each 400v winding has a 70v bias tap.

Hey, I really appreciate your interest and help with this!

Regards,

Chris

Question: Can two Mark III amps be integrated on a single chassis? Maybe. How about something like this:



15.5 " wide by 14" deep. Why? Because there's only so much shelf space available. After 38 years of marriage, compromise becomes a survival skill. 8-)

As always, comments/critique would be welcome.

Thanks,

Chris

No technical reason it won't work ... just gonna be HEAVY !
My bedroom bureau warped under the weight of my 60 pound
300B amp ... ha !

Very cool - should sound great !

TomMcNally wrote:No technical reason it won't work ... just gonna be HEAVY !
My bedroom bureau warped under the weight of my 60 pound
300B amp ... ha !

Very cool - should sound great !

Yeh, Tom, you're not kidding about the weight! Fortunately, when I built some living room cabinets I stressed some "special places" to support iron. 2x4 and 3/4" plywood all prettied up to look like a built-in.

I'm encouraged by your comment regarding the technical aspects of the layout. Thanks very much...

Chris

No technical reason it won't work ... just gonna be HEAVY !

wouldn't you also likely have less EMF/AC noise interferance with separate mono units? when i first started thinking about amps about a year and a half ago i had a similar question about the monoblock concept

nyazzip wrote:

No technical reason it won't work ... just gonna be HEAVY !

wouldn't you also likely have less EMF/AC noise interferance with separate mono units?

"Less likely"? Good question, made me think.

What you're really asking is: "Aren't monoblocks inherently better because there would be less EMF/AC noise interference between the channels"?

Originally, the Mark III's were manufactured as monoblocks not for reasons of noise suppression, but to make them affordable. In the Sixty's (of which I am a product), a whole generation of excellent and highly successful amps combined two channels onto one chassis. The good ones, I'm sure, were quieter than the Dynacos.

I have a Harman-Kardon Citation II which is dead-quiet. Amazing, considering the circuitry crammed into the chassis! My point is, it can be done but the challenge (and fun) is to do it properly.

With attention to detail including the use of a power toroid, transformer orientation, more generous spacing between components, shielding and build quality, a pair of Mark III's on a single chassis will be quiet. Just imagine two monoblocks sitting next to each other.

Regards,

Chris

I think MK-III's are mono ... because they came out before
stereo was invented ... all you needed was one amp, one speaker.

TomMcNally wrote:I think MK-III's are mono ... because they came out before
stereo was invented ... all you needed was one amp, one speaker.

You're right, Tom.

That's one of the reasons I want to keep to discreet channels. I have a PAM that will be connected to a '78 needle to play lots of Big Band albums that I inherited. But I also have a need for '60's classic rock, so I'm trying for the best of both worlds.

EWBrown wrote:
Are the 70V bias taps a separate winding, or are they taken off the 400V windings?

================

Each 400v winding has a 70v bias tap.

Hey, I really appreciate your interest and help with this!

Regards,

Chris

Just thinking about the two separate 400V secondary windings with the 70V "bias" taps....

With the full wave rectifier bridge (and it doesn't really make any in this case, whether it is all SS or hybrid) that the 70V taps won't readily generate -70V, the voltage will be positive and unbalanced, in reference to ground. I'll have to look this one over, but at a glasnce, it appears that it cannot generate a negative voltage, without some added complexity. Stay tuned...


You may need a separate transformer to generate tthe -70V (adjustable) bias voltage. The current requirement is rather small, most of the DC curent would be consumed by the bias pots and the divider resistors, the grids themselves take very little current, on the order of microamperes at moat.

EWBrown wrote:
Just thinking about the two separate 400V secondary windings with the 70V "bias" taps....
Stay tuned...

Thanks for thinking about that...I wouldn't know what to do. You bet I'll stay tuned!

Chris

EWBrown wrote:

EWBrown wrote: You may need a separate transformer to generate tthe -70V (adjustable) bias voltage..

Instead of 70v, why not 50v which is what the original Mark III power transformer provides.

Might be easiest for me to get something like a Hammond 167G100, fairly small footprint, 100v with CT, so I'd have the stock 50v each side to work with. Not that pricey from Mouser.

Chris

I think Ed must have been referring to the approximately -70V DC bias supply after half wave rectification, filtering, and resistive losses. My Mark III has a ~55V AC tap for the input to the bias supply.

Maybe simple is best. How about this if we're concerned about using the 70v tap from the 400v winding?



Chris

Ty_Bower wrote:I think Ed must have been referring to the approximately -70V DC bias supply after half wave rectification, filtering, and resistive losses. My Mark III has a ~55V AC tap for the input to the bias supply.

I don't think that's it. I have a toroid that has two seperate 400V windings for each B+ supply. Each has a 70v A/C tap for the bias supply, and Ed has some concern that these might not work because of the "Hybrid" rectifier we're using (see earlier post). At least that's what I think is going on, but obviously I'm no expert.

To take the easy way out, I'm suggesting that I add a 100v CT trannie that will supply the two bias supplies needed, one from each side of the CT. That should supply approx. -70V DC similar to what you're suggesting, I think.

Thanks for the help,

Chris