Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tubefiend on February 04, 2016, 12:45:56 pm
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I recently picked up an old early fifties Olson RA-165 hi fi amp, in pitiful shape, and thought it had the basics to make a decent sounding guitar amp. It was built with a 6SC7 preamp, a 6SL7 preamp/phase inverter and two 6V6's in PP, with a 5Y3. I thought, "heck this is practically early Fender here!"
After replacing the filter caps, I ripped all of the old crystal phono cartridge and tuner input resistors and capacitors out, and "Fenderized" it with a 68K input resistor and a 1meg first stage grid bias resistor, added a preamp gain pot between the 6SC7 stages, changed out the plate resistors to something more appropriate and swapped out the coupling caps to .022 uF. I also changed the 6V6 cathode resistor to a 250 ohm 5 watt, and dropped the bypass cap from 125 uF to 50uF. I added a set of Sovtec tubes from AES. I ended up with a pretty cool sounding amp. The volt amp rating of the PT is a little light to go full blown 14 watts plate dissipation, so I settled for 11 watts, with a plate current of 36 ma. The output transformer is very beefy as was the norm on amps like this back then.
I measured the power output at 10 watts even.
Hey, for something I got for practically nothing except for the tubes I replaced, and the small amount spent on resistors and caps, it's pretty awesome! Now I wish my woodworking skills were there to make a cabinet for it. Presently I'm running it through a Weber 10" single speaker extension cabinet, with a Jensen Special Design 40w speaker that sounds great!
It's fun to do stuff like this, resurrecting something that some people would throw out and turning it into something useful! Put a mic in front of that 10" Jensen, and your gigging!
Pete
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:thumbsup:
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Well the great thing was when they USED TO throw them out. Now they want $125 and $50 to ship it.
How many mono 6V6-fired hi-fi amps do you think are out there? Millions and millions, even if 2/3rds are junked.
But good going!
Instead of building a cabinet from scratch, unless you like the piggyback config, my suggestion would be to try and find a dead SS amp that could hold your deal with a 10 or 12" speaker. Maybe you can buy such a thing for $20 and have tolex, metal corners, grill cloth, and a handle all set to go.
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Just a quick question on the input you mentioned. Did you put in two jacks? Normally if it is single jack the input resistor is 33k, and dual is 68k (68k in parallel ends up at 34k so the same as if single).
Sounds like a really interesting amp! I'm looking into making an old gibson and it uses some old tubes too (6SQ7 and 6N7) do you have any audio samples?
~Phil
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I also enjoy turning old junk into a rocking guitar amp.
There can be significant challenges to overcome in the process, but I think that helps grow our learning and understanding.
None of my junk conversions are ever going to look like mscaggs work. :l2:
But the final costs of these things can't be beat.
And I think my conversions sound better to my ears just because of the things I had to go through to make them work. :l2:
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I also enjoy turning old junk into a rocking guitar amp.
There can be significant challenges to overcome in the process, but I think that helps grow our learning and understanding.
None of my junk conversions are ever going to look like mscaggs work. :l2:
But the final costs of these things can't be beat.
And I think my conversions sound better to my ears just because of the things I had to go through to make them work. :l2:
Agreed, mscaggs makes them look like an art form no?
I definitely learn so much with each build I think it's impossible to learn more, and then I do... :)
~Phil
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Just a quick question on the input you mentioned. Did you put in two jacks? Normally if it is single jack the input resistor is 33k, and dual is 68k (68k in parallel ends up at 34k so the same as if single).
Sounds like a really interesting amp! I'm looking into making an old gibson and it uses some old tubes too (6SQ7 and 6N7) do you have any audio samples?
~Phil
I used one jack, with the 68k. I'm going to tack in another in parallel to see the difference. Thanks for the reminder!!
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No worries :)
~Phil