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Old 7th March 2012, 04:45 PM   #1
tictac is offline tictac  United States
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Default Want to use El Cheapo P.I. for Scott LK72 rebuild

Howdy,

My first post here...

I'm rebuilding a Scott LK72 and I've seen some ideas here I'd like to use.

I want to use the ****-eyed rectifier to replace the 5AR4. Then I'll use the 5V winding for the preamp filaments.

Then the 50V winding will be solely for the - bias circuit; problem is I would like to use the "El Cheapo" P.I. but I see the CCS needs -180V to operate.

My question is: How can I use the 12AT7 CCS P.I. of the El Cheapo with the
-50V I'll be getting from the negative bias supply?

Thank You for any insight you can provide...

TT
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Old 7th March 2012, 06:16 PM   #2
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We used an LK48 for an El Cheapo variant. We used a different CCS and the -50V worked fine.

dave
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Old 7th March 2012, 06:24 PM   #3
tictac is offline tictac  United States
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Is there a schematic of that anywhere?
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Old 7th March 2012, 06:59 PM   #4
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Quote:
Originally Posted by tictac View Post
I'm rebuilding a Scott LK72 and I've seen some ideas here I'd like to use.
Why in the world would you want to totally destroy the classic and collector value of this amplifier with a lower power output stage. You're not "rebuilding" it, you're bastardizing it. I would understand this only if it were beyond economical repair and you wanted to pick the bones for parts.
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Old 7th March 2012, 07:43 PM   #5
tictac is offline tictac  United States
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If you recall the LK amps were kit amps; put together by novice builders. Some no doubt did a great job. The build of my LK72 kit amp is OK but some of features of the amp like bias balance etc... were not installed as per the schematic, so the amp is not stock to begin with.

Secondly, all the posts I've read about restoring this amp call for the replacement of ALL coupling caps, electrolytics, selinium rectifier, and many of the carbon-comp resistors (like all of the power supply w/ metal films), plus replacing all resistors that have drifted off value.

So basically I'm already changing at least 1/2 of the components of this amp; plus when I look at the guts of this amp I see what looks like a pile of spagetti of a wiring job which makes my head hurt just looking at it.

Tell you what, if you're still offended by my rebuilding this amp I'd be happy to sell it to you and let you restore it to it's original glory. I'd much rather start with a brand new chassis and build an amp from scratch anyway. This is going to be a pain to make right; I don't like the chassis, the layout, and all the superfluous controls but it does have nice iron...

I got the idea from an Audio Express Mag article about a 222 rebuild, er bastardization; so I guess I'm the only one who's been down this road...
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Old 7th March 2012, 07:53 PM   #6
asmith is offline asmith  United States
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Default What do you want for it.

What would you want for it, and where are you located
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Old 7th March 2012, 08:14 PM   #7
tictac is offline tictac  United States
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I'd take 450.00 for it; it's the model with the brown faceplate. I can send pictures if you like.

I'm in Oregon
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Old 7th March 2012, 08:20 PM   #8
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Quote:
Originally Posted by tictac View Post
If you recall the LK amps were kit amps; put together by novice builders. Some no doubt did a great job. The build of my LK72 kit amp is OK but some of features of the amp like bias balance etc... were not installed as per the schematic, so the amp is not stock to begin with.
OK, I forgot about the kit option. If there's no inspector's stamp inside the chassis, it probably was home built. But I still question the choice to replace the output tubes with smaller weaker units. Improving the biasing circuit, which is simple, is a lot easier then replacing sockets. And yes the output iron is good. And no, I have enough classic gear to redo should I decide to.
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Old 7th March 2012, 09:02 PM   #9
tictac is offline tictac  United States
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I didn't say anything about changing the output tubes; I'm going to keep the 7591's. But I do want to redo the P.I. The designer of the El Cheapo, I believe his name is Eli, told another poster that his 12AT7 CCS P.I. in the El Cheapo amp would easily drive a pair of 7591's to full power.

The reason I want to go with a diffrent P.I. than the stock Scott P.I. is I've read several posts about problems with this design; mainly the tubes themselves. Plus I have a decent stock of 12AT7's but none of the 6BL8 tubes called out in the schematic...

I'm just looking for a way to implement the 12AT7 CCS P.I. without having to build an H.V. negative supply...

I've not familiar with using constant current sources which is why I'm asking for help or maybe another design that would work well for driving a PP pair of 7591's.

TT
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Old 7th March 2012, 09:33 PM   #10
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Quote:
Originally Posted by tictac View Post
I'm just looking for a way to implement the 12AT7 CCS P.I. without having to build an H.V. negative supply...
Since the current required for the PI CCS is low, you could "steal" some voltage from one of the 5AR4 plates through a resistor, a diode and a filter cap to provide higher negative voltage then the bias supply provides. Just a simple wave circuit with three parts will do it.
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