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PostPosted: December 24th, 2020, 9:18 pm 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 235
The primary winding on the output transformer isn't connected correctly.
Take a look at the original circuit on page 1.

ratbagp wrote:
Roscoe, could you check this out? I can play with R4 as you suggest.

Image

Playing with PSUD I think I can get pretty close to 400 vdc.

ray


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PostPosted: December 24th, 2020, 10:57 pm 
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Joined: January 14th, 2015, 11:15 pm
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OK thanks.

Just finished the first bottle of champers!

ray


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PostPosted: December 25th, 2020, 6:12 am 
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Don't drink and screen drive...... :obscene-drinkingcheers:

ratbagp wrote:
OK thanks.

Just finished the first bottle of champers!

ray


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PostPosted: January 6th, 2021, 6:57 pm 
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Joined: January 14th, 2015, 11:15 pm
Posts: 460
Our boxes have finally arrived. Now to start building after I find the appropriate parts after unpacking. I never realized how much I missed building until all my parts were packed away.

http://ratbagp.blogspot.com/2021/01/our-boxes-arrive.html

ray


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PostPosted: January 6th, 2021, 8:32 pm 
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I still have some of mine to unpack so I feel your pain. I've started pulling the wires from my audio room to the basement where my equipment rack is located.


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PostPosted: January 6th, 2021, 11:38 pm 
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Location: Parkville, Maryland
ratbagp wrote:
Our boxes have finally arrived. Now to start building after I find the appropriate parts after unpacking. I never realized how much I missed building until all my parts were packed away.

http://ratbagp.blogspot.com/2021/01/our-boxes-arrive.html

ray

And my wife wonders why I would never move. I moved to where I am now 29 years ago from my first house. I look at your pictures and it reminds me of the massive task it takes to relocate one's existence. :shock:

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PostPosted: January 7th, 2021, 1:23 am 
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Some 25 years ago when I started to get interested in building amps, I bought two of these cupboards from Ikea to store parts.

Image

My wife lusted after them and when she went to Ikea to get more they had disappeared, never to return. That lust has continued ever since and as part of the move, I have given them to her. She loves the little drawers.

Getting back to Roscoe's amp, in the course of unpacking some of the boxes I have come across the major parts I will need to build so I should be able to get cracking in the next few days.

ray


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PostPosted: January 7th, 2021, 11:07 am 
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Location: Parkville, Maryland
ratbagp wrote:
Some 25 years ago when I started to get interested in building amps, I bought two of these cupboards from Ikea to store parts.

Image

My wife lusted after them and when she went to Ikea to get more they had disappeared, never to return. That lust has continued ever since and as part of the move, I have given them to her. She loves the little drawers.

Getting back to Roscoe's amp, in the course of unpacking some of the boxes I have come across the major parts I will need to build so I should be able to get cracking in the next few days.

ray

That's a cool cabinet! My wife is into jewelry making and that piece would be perfect. What a shame they stopped selling them.

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PostPosted: January 14th, 2021, 7:51 pm 
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Posts: 460
It's time to start building my version of the Roscoe.

Starting at the power supply, I have an Antek toroid 4TK400A. I plan to wire the primaries in series for the 240 vac here in Oz. I will have a thermistor on the primary.

https://www.antekinc.com/content/AN-4TK400.pdf

I've been playing with PSUD and I can get to about 430 vdc with tube rectification with a simple CLC of 10 uf DC Link, Hammond 193L (5H 300 ma 57 ohms) and 150 uf of DC link. I should be able to play with the input capacitor to lower or raise the final voltage.

I can use either 5AR4 or a couple of 6D22S. Which would you recommend out of the two? I have room for either case in my layout.

ray


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PostPosted: February 25th, 2021, 4:46 pm 
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OK, so I finally got the second one built. I changed a couple resistor and cap values along the way.... Driver plate load is now 3645ohm (3.3k+345R), output tube cathode bias resistor is now 220R, bypass caps for both tubes are now 820uF/35V. Everything else can remain the same, may play with smaller cathode bypass caps at some point in the future.

So, how do they sound? Well, I'm pretty happy with them right out of the box, with no time spent on break-in, and no playing with values/types on the passive components. I also want to play with output tubes which may work better in this topology, used the 6550s since I have a lot of pulls from the Audio Research VT150SEs that Dad used to have, but I'm sure there are better options. Right now, both tubes are operated near their max dissipation, If that bothers you, increasing the driver plate load resistor will drop the dissipation on both tubes, increasing the cathode bias resistor on the output stage (start with 250ohms) will decrease the dissipation on the output tube. Of course, if you're using 6V6s for the drivers, you don't have to worry as much about driver dissipation. Because the amp is direct coupled, changing the driver tube cathode bias resistor, or the driver tube screen grid resistor can cause a huge variation in the output tube current, so make small changes there if you're going to play with those.

Roscoe

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