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Author Topic: The Radio Shack Boston "Realistic" amp conversion to God knows what
Digital-
Larry

Posts: 192
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Post The Radio Shack Boston "Realistic" amp conversion to God knows what
on: November 10, 2012, 10:24
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While I wait for a 9-pin socket to come in so I can finish my Webcor single-ended restoration, I'm turning my attention to a little desktop phono/radio amplifier which I've described at my blog:

https://joefriday-lg.com/2010/12/26/my-third-tube-amp-project/

Today I thought I would trace out the existing wiring prior to ripping it all out. Some of these tube amp aficionado sites will give you a free membership if you come up with a schematic that they don't have. Well, they don't have this one.

The top photo at the link above shows the rear panel of the amp. In the middle you can see two little holes in a flush connector that's mounted behind an oddly shaped hole. I have to tell you, this is not stock! I've looked inside and these 2 signals appear to be somebody's attempt to tap off the voltages going from the push pull tube outputs (plates) to the output transformer. Not sure what they would have done with this, but I also noticed that they put a high pass filter (series 0.1 cap, 22k shunt resistor) to the output. One one side, the resistor is shunted to ground. On the other side, the resistor is shunted to the frickin screen power supply, which is probably a few hundred volts! Coming right out the rear panel atcha baby! Holy crud! It looks like the person mistook a lug on the can capacitor for ground. But it really, really is not ground.

The first thing I'm going to do is chop that network out of there. More pictures to come.

joe
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Posts: 224
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Post Re: The Radio Shack Boston "Realistic" amp conversion to God knows what
on: November 10, 2012, 11:10
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I want to HEAR it! 🙂

Digital-
Larry

Posts: 192
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Post Re: The Radio Shack Boston "Realistic" amp conversion to God knows what
on: November 10, 2012, 12:48
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Yeah? Hmmm. Well then it will be time for some experimentation. I will start by finishing up tracing the schematic (I already chopped out the parasitic network of death).

Then I'll put a jack on the current "radio" input so that I can check the existing power amp design/sound.

My modifications are going to be fairly significant after that, because as it stands today, the 6SL7 is used for the phase invertor, while the 6SC7 takes the mic and phono inputs to give them some gain. Following the Fender Deluxe 5C3, I want to use the 6SC7 for the phase invertor and then use the two triode sections of the 6SL7 in series (along with a tone stack). I'll need to record it and compare. The worst thing that could happen is that I decide that the original topology was best, after I've spent hours changing it.

Digital-
Larry

Posts: 192
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Post Re: The Radio Shack Boston "Realistic" amp conversion to God knows what
on: November 11, 2012, 12:28
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I was tracing the circuitry on this and found myself completely stymied by the phase invertor layout. I Google'd "6V6 6SL7 power amp" and got a link with a schematic showing nearly this exact topology!

schematic

Notice how the 6SL7 phase invertor's cathode is biased through the output transformer! That's going to be a tough feedback loop to break. I'd have to add a separate cathode resistor to ground if I wanted to switch out or break that one. The schematic above also shows an "ultralinear" power amp topology where taps on the primary side of the output transformer are used to adjust the screen voltages in the power tubes. My Radio Shack amp does not have the ultralinear transformer.

Digital-
Larry

Posts: 192
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Post Re: The Radio Shack Boston "Realistic" amp conversion to God knows what
on: November 13, 2012, 07:00
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Ooops, posted something in the wrong place... stay tuned...

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