Princeton all Octal Guitar Amp based on old PA system

Discussion in 'Amp Central Station' started by TwistinbythePool, Jun 9, 2021 at 7:05 PM.

  1. TwistinbythePool

    TwistinbythePool TDPRI Member

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    Hi All,

    I'm new here and part of the reason I wanted to join this forum was the project that I'm about to embark on. I've worked as a teacher for 25 years and all this time there was a dusty old PA amp way up in the top corner shelf of my parts supply room.

    I've been reading up on Octal Amps from the 40s and 50s and realized that 1. I have the tubes (I've tested the amp and it still works and I have many other tubes as well -- looks like there used to be radio courses here back in the 60s) 2. I have lots of extra parts -- and some interested students 3. It looks like an old Princeton Amp with a 5V3, 6SC7, 6SL7, and two 6V6s.

    Here are some pics... I'd like to keep this project short and sweet and cheap -- my budget for this year is blown. The circuit board is small and it looks like a lot of the soldering is point to point.

    I've looked at Rob Robinette's site which was very helpful but I'd like to hear your opinions on this project. I've already relocated the output transformer to the top side 'cause I think it looks cool. What do you think?

    https://drive.google.com/file/d/1n2m1Vb6faIaTGh2-TSq0ki_QfBa-AFtl/view?usp=sharing
    https://drive.google.com/file/d/1FAupfuJhibELHRbAll5U78oZd1ko7UKV/view?usp=sharing
    https://drive.google.com/file/d/1OlGnx6ZVGLxyIkYAo0O1uxgxoR1Smq9F/view?usp=sharing
     
  2. 2L man

    2L man Tele-Meister

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    Welcome!

    It might take time and pondering but you should get sound out of it. Perhaps being a PA its gain is low and you need to use a gain pedal between it and guitar? But one 6sl7 and one 6sc7 are enough for guitar pre amp and phase inverter so perhaps only few resistor changes are needed or not?
     
  3. jmp81sc

    jmp81sc Tele-Meister

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    You have everything you need there for an early Deluxe. I would trace out the circuit and make a schematic to evaluate what you have first then convert from there. As 2L man said probably just a few changes would yield a good sounding amp.

    Good luck
    John
     
  4. Peegoo

    Peegoo Poster Extraordinaire

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  5. Mowgli

    Mowgli Tele-Meister

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    You may also want to look at the '52 - '54 Gibson Les Paul model GA-40 schematic; similar tube compliment.
     
  6. TwistinbythePool

    TwistinbythePool TDPRI Member

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    Thanks everyone! I started reading old2new last night... I'll read it again tomorrow and I will trace what I've got. I've ordered the missing filter caps and I'll keep everyone posted. Today was inventory day --our High School Electronics shop did used to teach radio courses. I've been looking at old Navy Freq. Generators and I've stripped out the old circuit board... thinking maybe I'll put the filter Caps on it and put it in the center of the chassis where the output trans used to be... maybe add a ground bar?
    https://drive.google.com/file/d/1iRMub7ZVSGpNTjxJBqBTKA4eavHt8F9f/view?usp=sharing
    https://drive.google.com/file/d/1aLcrOHtLuCmSuBIm0r_bODgKWPudr5vG/view?usp=sharing
    https://drive.google.com/file/d/1CHGmbBt3Hv6ABZY-RN06eyGJnBimR3oL/view?usp=sharing
     
    Last edited: Jun 10, 2021 at 10:27 PM
  7. TwistinbythePool

    TwistinbythePool TDPRI Member

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    Ok update ... I've traced the circuit and it turns out the 6SC7 is not feeding signal into the 6SL7 like I thought, instead its sending its signal to an 6V6. The 6SL7 is connected separately from the SC7 to the microphone and Guitar input. So I'm going to modify this setup to create an all octal 5E3 either with the same tubes or with two SL7s. Btw I've been digging around my supply room and found these. Can I use these as Filter Caps? -- They must be 50 years old by now. IMG_7885.jpeg
     
  8. wrathfuldeity

    wrathfuldeity Tele-Afflicted

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    Interested, however lacking in knowledge, equipment, skills and time. So sent a few old amps for some bench time and delighted with the results.
     
  9. tobyk

    tobyk TDPRI Member

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    You can check with a multimeter if they work, but they should not, being 50 years old..
     
  10. 2L man

    2L man Tele-Meister

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    Many multimeters have capacitance measure but it does not often define leakage very well. For that connect a high value series resistance and connect RC to a high DC voltage and after a while measure voltage over the resistor and if there is voltage capacitor leaks. Some leakage on electrolyts is normal and I can't say what is reject value but there are articles in internet.

    Capacitors also can "heal" when used but only testing will reveal is it worth it.

    However few decades old electrolyts can be still quite good where modern can be bad after ten years of storage. But old electrolyt did release much less peak current than modern electrolyts do but that is fine for mains and audio frequencies where modern electrolyts often need to operate on high frequency "switching" appliances.

    I have used NOS electrolyts on my tube amp builds because often they are axial :)
     
  11. BobbyZ

    BobbyZ Doctor of Teleocity

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    My luck with old electrolytic caps hasn't been great. They look good, test fine with the typical 9 volt meter but either the bass is farty or they just start leaking after some use.
    For trial builds they maybe fine but I wouldn't get real real carried away fastening them down.
    I wouldn't buy NOS E-caps at any price.
     
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  12. TwistinbythePool

    TwistinbythePool TDPRI Member

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    Thanks everyone. I will check those caps in a day or two when I get some time. I've ordered new ones anyway along with new 1 meg pots. The problem with my PA mod is solved thanks to charisjapan whose 2019 all octal Princeton-ish project gave me the dual 6SL7 preamp schematic I was looking for.
     
  13. TwoBear

    TwoBear Tele-Holic

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    Congratulations and nice to see students still interested in Good Old tube technology. I'm pretty sure there is a tutorial on "Mr Carlson's lab" on how to build a tester for caps. That might make a good project for the class.
     
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  14. BobbyZ

    BobbyZ Doctor of Teleocity

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    The good testers test them at working voltages which is normally somewhere north of 400 volts. I believe Sprague made one like that and maybe there was other ones too.
    It's amazing how many leaky bubbled caps will actually read pretty close to their stated capacitance with a 9 volt meter.
    I've never put much stock into testing electrolytic caps other than subbing in fresh caps and listening to the amp. Never left the old caps after doing that.
     
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  15. TwoBear

    TwoBear Tele-Holic

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    After seeing so many Mr Carlson's lab shows I'd be pretty surprised if he didn't have power transistors in there, he's pretty darn thorough.
     
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