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Meet Dolly, the Olson clone!

Redboy

Redboy

a few good watts
If you've read any of my past posts you know I've got a soft spot in my heart for a little single-ended EL84 Olson amp I pulled out of a console some time back:

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("Yep," you say, "There's that ugly amp again!") :D

It's an amp I liked so much, in fact, that I decided to clone it.

And so I introduce you to Dolly, the Olson clone:

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It's a stereo amp with EL84/6BQ5 output tubes, a 12AX7 driver, and EZ81 rectification. Volume control is via a cheap RadioShack 100K stereo pot.

I wasn't ever able to find a Sam's folder or a schematic for my Olson, so I mapped out the schematic myself. That worked out pretty well except that I had no good indication of the specs for the output transformers and no knowledge of how to go about measuring them. I opted to try Edcor iron in this project, mostly because it was cheap and got some good press.

Right now I'm getting some oscillation from the amp, which I imagine is from not quite having the OPT's exactly matched to the originals. For every other component in the amp I was able to read or measure a value, and I sourced parts that were an exact match whenever possible. The end result is that Dolly is an exact replica except for her layout and iron. That's what makes me think the oscillation (motorboating?) is in that area.
 
Some underbelly shots. I know it's a spaghetti mess. I'm not particularly proud of how the power supply wiring turned out...

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When I initially plugged it in, I used a lightbulb variac with a 40W bulb in it. I couldn't have been happier when it worked right the first time! With a crummy tuner hooked up to it, it was pure magic when faint classical music started fading in! :music:

I let it play for awhile that way, then pulled the variac out. Powered up and got music for a moment and then a nasty wawawawa - like being buzzed by a lowflying prop plane! :sigh:
 
Looking good Redboy!

You make me one of these and I'll make you a pair of my fullrangers!
 
Don't quote me on this but I thought I read in an RCA tube manual (which I cannot find right now) that if motorboating accurs then swap the leads on the primary side of the OPT. Hopefully a tube guru can verify this, as I am not sure.
 
Great looking stuff, Nate! I admire your ability to figure things out and stick to a plan. You've done something to be proud of, for sure. We proudly announce the birth of Dolly Olson! 5lbs, 2oz, 15"x6"x8" or whatever.

When I get the time and inclination I'll do something with my tube monoblocks from the Roberts 720.

Thanks for the inspiration, Nate.

Rick
 
Redboy said:
When I initially plugged it in, I used a lightbulb variac with a 40W bulb in it. I couldn't have been happier when it worked right the first time! With a crummy tuner hooked up to it, it was pure magic when faint classical music started fading in! :music:

I let it play for awhile that way, then pulled the variac out. Powered up and got music for a moment and then a nasty wawawawa - like being buzzed by a lowflying prop plane! :sigh:
Click to expand...

Bummah!
 
clydeselsor said:
Don't quote me on this but I thought I read in an RCA tube manual (which I cannot find right now) that if motorboating accurs then swap the leads on the primary side of the OPT. Hopefully a tube guru can verify this, as I am not sure.
Click to expand...
There, I've quoted you... :D

Swap leads on both sides? I wonder. C'mon gurus, I need help!
 
Suweet Redboy!

Great looking craftmanship!

btw...didn't Dolly prematurely kick the bucket?

Kidding, kidding, kidding!
 
I like Dolly. Olson does have any ugly transformer, but Dolly looks quite clean on the surface.
 
That's a gassy tube. Some of the pics were taken earlier while I was still test-fitting the components. I didn't want to risk a klutzy move on a good old stock tube, so I plugged in a couple dead tubes.

You weren't supposed to notice that!
 
Redboy said:
There, I've quoted you... :D

Swap leads on both sides? I wonder. C'mon gurus, I need help!
Click to expand...

Yeah, put the B+ on the plate.

I dunno, I never managed to get a single-ended 6BQ5 to motorboat....a push-pull 6BQ5...but not a single ended....

What's the primary impedance for the Edcor OPT?

I think I see feedback to the driver cathodes, maybe try it without feedback. It will have more gain, ratty, noisy, but it may determine why it's motorboating.

The coupling cap from the proceeding stage may too low in value? Though I only thought that was with respect to PP.
 
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