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Old 17th September 2012, 03:33 AM   #21
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Hi Francois,

Yes, you got my question just fine. I, just today, decided that I am going to go with the quad of KT88s or 6550s. If not, I'd have a pile of transformers in the floor for a year

Rationale? Yes, the octet of EL34s might give lower distortion, but it is also twice the circuitry for bias, etc.

I plan to take my time on this one. I have built and tinkered with many amps, and I know that it is a journey and not a destination.

I will receive my screen chokes this week sometime, and then I will drill the holes for them and take a short break while I gather chassis parts and resources. Builds like this get expensive quick!

I will take plenty of images and make sure it is documented. I think there are a bunch of these out there and people trying to decide what to do with them.

Blair
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Old 17th September 2012, 03:37 AM   #22
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Click the image to open in full size.

Here is what I did today. I have the other channel cut as well, but no donor amp yet All in time.

Blair
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Old 17th September 2012, 04:03 AM   #23
dgta is offline dgta  United States
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Still not quite sure where this is going. It looks like you're basically building new amps from scratch using the MO200 iron. If so, you need to look at the OPT primary impedance vs the operating point of whatever tubes you choose, since they will not be 8417s. I would hash out that design detail first before arbitrarily choosing the output tubes. Otherwise you may find yourself in a bind.
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Old 17th September 2012, 04:11 AM   #24
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Location: Champaign, Illinois
Quote:
Originally Posted by deicide67 View Post
Click the image to open in full size.

Here is what I did today. I have the other channel cut as well, but no donor amp yet All in time.

Blair
Blair,

That top plate looks great! You must have access to a laser cutter. I have been dreaming about chasses like yours.

Is your choke at the right bottom as shown in the picture? If so, provided the laminations are 90 degrees from the power trannie, this looks like a good layout to me. What is the small tranformer mounted in the bottom row? Heaters?

I know the purists would like you to turn the two output transformers (if used in stereo) but the crosstalk has been inaudible to me. if used in MO-200 mode it would make no difference, of course. (I think)

Francois
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Old 17th September 2012, 04:11 AM   #25
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I have mentioned it several times that the primaries are 4.5K on the output iron. Looking at both the KT88 and 6550 datasheets, they will work well into that load with 300V on the screens. Yes, I'm not building the bogen amp, or I would have just used the original chassis.

Blair
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Old 17th September 2012, 04:16 AM   #26
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Hi Francois,

Nah, just a redneck and a drill press I've been building amps for about 10 years. Just not too many that aren't proven designs.

In the image, the bottom row is: OPT, OPT, screen transformer, power transformer.

Just above the power transformer is the 193Q, and not drilled and mounted is a 7H choke for my screens. I plan on using a choke regulated input to drive my screens and driver tubes plates.

Blair
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Old 17th September 2012, 04:19 AM   #27
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Location: Champaign, Illinois
Quote:
Originally Posted by dgta View Post
Still not quite sure where this is going. It looks like you're basically building new amps from scratch using the MO200 iron. If so, you need to look at the OPT primary impedance vs the operating point of whatever tubes you choose, since they will not be 8417s. I would hash out that design detail first before arbitrarily choosing the output tubes. Otherwise you may find yourself in a bind.
I do agree, since it is essentially a new amp design, from scratch. That was why I mentioned the different driver requirements - perhaps not explicitly enough.

But I expect our builder Blair was well aware of that.

Francois
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Old 17th September 2012, 04:39 AM   #28
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Yes,

I'm still not sure what I will do for a driver which is why I have the three holes drilled. If you look, it's marked for a potential fourth if needed.

I have to complete the chassis first though.

I have to get the 1" bar stock to mill for the sides.

Blair
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Old 17th September 2012, 04:48 AM   #29
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Location: Champaign, Illinois
Quote:
Originally Posted by deicide67 View Post
Hi Francois,

Nah, just a redneck and a drill press I've been building amps for about 10 years. Just not too many that aren't proven designs.

In the image, the bottom row is: OPT, OPT, screen transformer, power transformer.

Blair
Hi Blair,

That is truely impressive redneck drill press craftmanship! I guess bourbon was another redneck accomplishment, so I should not be surprized. Or was bourbon a Southernly Gentlemen's accomplishment. (Please pardon me if I am treading on sacred ground!)

Again, sorry if I missed previous discussion, but what front-end/phase splitter are you using? As was pointed out that could have more effect or beneficial influence than the 3dB gained in doubling the power by going MO 200 from 100. Unless you have very inefficient speakers 200 vs. 100 watts is probably not an important factor in your ultimate fidelity. FWIW I am planning to have my Bogens as two MO-100's

Best,
Francois
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Old 17th September 2012, 05:03 AM   #30
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Hi Francois,

I have LOTS of practice drilling chassis!

I am looking at Williamson front ends. I plan to use some CCS on the PI at least, but I'm not sure what I will do yet. It really depends on the layout and how crowded things get.

I am using mine in original parallel form because the output impedances are a bit more practical at 8, 3.125 ohms.

My speakers are large passive dipoles that are about 89-90db.
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