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Old 14th March 2004, 01:55 AM   #11
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Old 14th March 2004, 02:19 AM   #12
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Frank said:
Quote:
As for schematics, the Quad II is on the net, as is most of Frank McIntosh's work.
Could you name a few specific McIntosh models that you think would be good subjects? I have found a couple of schematics, and they looked insanely complex to me.

Quote:
So, unless you have a competent winder as your best friend, you're still stuck...
Come to think of it, I can't think of any DIYer having such an amp on the net...
On the bright side, I vaguely recall at least one source for replacement Mac iron...
So, if you don't mind using the 6L6 family, KT88 and relatives, you should at least have one source of suitable iron for a project.
Would you mind eyeballing these transformers, and telling me which, if any, of your favorite designs they might suit? The last one even talks about the McIntosh design...

Best,

George Ferguson
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Old 14th March 2004, 02:28 AM   #13
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Hi,

Quote:
Care to share any of your thoughts on tweaking MACs?
Sure.
Basically what we did, provided it wasn't a collectors' item, is to upgrade the PS, passive components, selected NOS tubes, wiring etc.

The schematic itself I wouldn't touch.
It's all very much interdependent and utterly clever.

Quote:
Could you name a few specific McIntosh models that you think would be good subjects?
MC240, MC275.

Quote:
I have found a couple of schematics, and they looked insanely complex to me.
They are impressive at first glance indeed.

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Would you mind eyeballing these transformers, and telling me which, if any, of your favorite designs they might suit?
Will do.

Cheers,
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Old 14th March 2004, 02:40 AM   #14
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Default Will do.

Hi,

Done....

Pretty impressive specs as I'd expect from someone as brilliant as Menno VdV....

This is one guy that actually built at least one OTL amp BEFORE he started speccing his OPTs. Recpect that...

If only the price were on a per pair basis, right?...Sigh....

What can I say, if this is your thing and your pockets are deep enough, it's hard to imagine any better quality than this.

Put it another way, if I wouldn't have an OTL already and I could chose, this would by my choice.

Cheers,
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Old 14th March 2004, 03:11 AM   #15
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Default YOUR LUCKY DAY....

Hi,

Hmmm...Seems the Jadis JA 80 is on the net already...

So here goes:

JA 80.

Beware however that all of the Jadis I know of have a falling HF response as from ~ 16 kHz.
This is due to the driver stage, NOT the OPT.

Cheers,
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Old 14th March 2004, 04:35 AM   #16
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Frank said:

Quote:
Put it another way, if I wouldn't have an OTL already and I could chose, this would by my choice.
High praise indeed! And which design would you put it in?

I'm searching for excellent sound at 50+WPC.

Best,

George Ferguson
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Old 14th March 2004, 11:16 PM   #17
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Hey-Hey!!!,
No wonder they have a HF roll off. a 12AX7 driving a pair of UL rigged power valves with each plate?

The Jadis OPT is an expansion of the Acro TO-350 and Dynaco A440/441. The originals have a 16% tertiary winding and Jadis adds another( unknown %-age) so both screen and cathodes have a winding to ride.

I am considering an interim progression with my current amps and use the A441's for pentode rigged 813's with the filaments riding the tertiary winding. B+ to be ~600. All I'll need are a pair of good, low capacitance( to keep the capacitive load on the cathode down...) filament tx's.
regards,
Douglas
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Old 15th March 2004, 01:58 AM   #18
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Hi,

Quote:
No wonder they have a HF roll off. a 12AX7 driving a pair of UL rigged power valves with each plate?
Yeah...I know, I know.
Discussed that with the designers ages ago and they insist on stick with their recipe claiming it's part and parcel of the "magic" of the amp.

I can live with it, for awhile, as these amps make you forget you even HAVE loudspeakers in the room.

Between you and me, I think it must be possible to have both treats...
That would be a real winner and that's what put Audiomat on the map anyway.

Cheers,
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Old 15th March 2004, 04:40 AM   #19
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Not sure if this follows from the thread, but let's discuss CFs.

Since grids should be shaken not stirred it does make sense to use a CF since Zout is very low.

However, a CF is not the best option if the load is hard to drive, as load current seems to severely affect its linearity, particularly as grid current can be significant (up to 25% of plate current) and the CF stage current is usually around 5-15mA. And while setting the B- point of the CF is a convenient way of implementing bias control, slow turn on, and direct coupling the output tube grid, the unity gain is an issue. To stay with just two voltage gain tubes, you need all the gain you can get with both SE and PP. To further complicate this choice, I've found the best operating point for a CF is difficult to find compared to a PLT.

On this basis, I normally go for a grunty triode, such as a 6BX7, running around 30mA in plate loaded configuration. With its 1.3K rp this gives more than adequate drive, and because plate current is high, any grid current is usually not such a problem. The one disadvantage is cap coupling, but this isolates fault conditions and in any case I normally use cathode biasing.

I honestly believe the reason most amps use this topology, in fact, is due to the phenomenon of convergence which SY referred to some time back. Of all the choices available, this one delivers the optimal cost/benefit

Cheers,

Hugh
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Old 15th March 2004, 05:33 PM   #20
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Quote:
Originally posted by AKSA
[B]However, a CF is not the best option if the load is hard to drive[b]
Huh?

Quote:
as load current seems to severely affect its linearity, particularly as grid current can be significant (up to 25% of plate current) and the CF stage current is usually around 5-15mA.
And you suggest driving grids with what??? Transformer coupling where there's NO corrective NFB applied? Maybe not...

Quote:
And while setting the B- point of the CF is a convenient way of implementing bias control, slow turn on, and direct coupling the output tube grid, the unity gain is an issue.
Howso? You need the 100 gain anyway. And now it can be provided by a single 6AV6 rather than a cascaded 6SN7. (If they had more gain (in both sections), a dissimilar dual triode would be perfect.)

Quote:
To stay with just two voltage gain tubes
What's this matter? I'll assume you are talking in ZNFB terms so there's no loop to cause phase shift oscillations in.

Quote:
you need all the gain you can get with both SE and PP. To further complicate this choice, I've found the best operating point for a CF is difficult to find compared to a PLT.
Huh? Oh, more of that... Dude, the best operating point is the one which suits the circuit electrically. You can't hear a CF anyway, remember?

Quote:
On this basis, I normally go for a grunty triode, such as a 6BX7, running around 30mA in plate loaded configuration. With its 1.3K rp this gives more than adequate drive, and because plate current is high, any grid current is usually not such a problem.
Hmm, above you said grid current can easily be up in the 100mA range ... so um, 30mA plate... but... uh. . . hey, I think 70 miliamperes just disappeared. That must be it.

Quote:
The one disadvantage is cap coupling, but this isolates fault conditions and in any case I normally use cathode biasing.
If you mean while direct-coupled to an output tube, this is almost as bad as direct-coupling to the driver tube. With fixed bias, the driver's drift has NO effect, the CF's will shift 1V absolute maximum through its own drift and output tube current can be corrected as it ages.
With cathode bias on the output, you waste 50 useful volts of course, and the CF needs to be fixed at zero volts. But that's impossible, so either you use a negative supply for it (and if you're going to do that you might as well fixed-bias it) or you have to cap-couple it as in the original schematic above. But that defeats the purpose of using a CF in the very first place because you can't drive the output any better than a plate follower 6SN7.

Tim
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