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5 Watt Frankenstein Build

Discussion in 'Shock Brother's DIY Amps' started by janglemore, Aug 28, 2020.

  1. janglemore

    janglemore TDPRI Member

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    Hello all. This is my first post in this forum. I have a 5 watt Monoprice amp that is no longer operational. There is a short somewhere in the power circuit and the PCB and miniature components make it too difficult for me to go further than the troubleshooting I’ve already done.

    At this point I’m considering building a new 5 watt amp that reuses the salvageable components. I believe the PT and OT are okay. And I would reuse the cabinet and the 8 ohm Celestion speaker. So I’m thinking that I should be able to get a working amp for (hopefully) minimal cost.

    So what to build? I’m thinking a 5F1 or a 5F2A would be doable. My main unknown is how to work with the PT. The B+ output is lower than what’s typically seen in the Fender circuits. And it does not have a 5 VAC secondary for tube rectification, so I’ll need to do diodes.

    Is it possible to use the power flow circuit from the monoprice and the signal flow circuit from the Fender?
    DA8CE596-2D28-445F-8508-6C6A6DEA37D1.png
     
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  2. WalthamMoosical

    WalthamMoosical Tele-Holic Ad Free Member

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    Welcome! ... can't help you with the amp, sorry ..
     
  3. janglemore

    janglemore TDPRI Member

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    Thanks!
     
  4. hepular

    hepular Tele-Holic

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    look at @robrob's site: once you cobble together a diode rectification solution, you probably could get a usable circuit--there seems to be an ongoing debate about :"what on earth was leo thinking biasing those poor 6v6s so hot" with these circuits and the bf princetons and deluxes, so throwing under 300v at the 6v6 might not get you total champ, but it might be interesting
     
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  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Nothing wrong with ~300V on a 6V6, check out the 5C1 and 5E1 Champ, couple of Princetons from that era, too. :)
     
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  6. Paul-T

    Paul-T Tele-Meister

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    I used the 275V PT tap for my 5F2a, complete with a bigger voltage drop for valve rectification, and it sounds great. SO I would think a 5f1 or 5F2a, with diode rectification, would be a no-brainer.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Absolutely!

    The first capacitor after the diodes (rectifier) is 100uF. You will want to stick pretty close to 100uF. No less than 50uF. This cap will replace the two 16uF caps of the 5F2A schematic.
    From there on just use the 5F2A schematic.
    You may want to change the 8uF to 16uF for extra filtering.

    (Of course you will want to stay with the more modern heater supply.)
     

    Attached Files:

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  8. jvin248

    jvin248 Poster Extraordinaire

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    .

    Look up the youtube channels Uncle Doug and D-lab. They both scratch build amps to go in old cases and radios.

    .
     
  9. janglemore

    janglemore TDPRI Member

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    Thanks to all who responded. I’m definitely a fan of Uncle Doug, Jack, Ollie, Rusty and company. His videos are long but fun. I also enjoy D Lab (lets have a glass of wine). I’ve also spent quite a bit of time on robrob and el34 websites. Robrob’s website is a fantastic resource. Trying to soak in all the great info from those sources and this forum, I’ve learned more than I remember 8^). I feel like a piker compared to you guys.

    I’m feeling better about the PT now. And thanks Lowerleftcoast for the tip on that first filter cap. I was reading another thread about upsizing caps and wondering about that monster.

    I also need a bit of help with the heater circuit. The PT heater secondary measured 7.38 VAC. It has no CT. And there is a PTC thermistor and hum pot in the circuit. Is it advisable to recreate the original heater circuit as shown on the original Monoprice schematic or should I go with an artificial center tap. What about that thermistor? The voltage seems a bit high? I know I’ll need a dropping resistor for the LED, does the thermistor act as a dropping resistor to bring the voltage down to 6.3 for the tubes?

    So much to learn.
     
  10. sds1

    sds1 Tele-Afflicted

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    Is that measured with tubes installed or no? If no, the the voltage will drop when tube are in and the heaters start drawing current. I dunno what the thermistor is for but I reckon it's a protection mechanism. The humdinger is just an adjustable artificial CT.
     
  11. janglemore

    janglemore TDPRI Member

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    Ok, that makes sense. That voltage was measured with no tubes.
     
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  12. printer2

    printer2 Poster Extraordinaire

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    Other than C21 on the plate of the second stage and moving the tone stack I would be fine with the last two stages and power supply. About the tone control used in the Monoprice, how was it? It was first used by Garnet amps, I have seen it but have never tried it out myself. The 220R resistor on the speaker output is a nice touch, in case the speaker jack goes open it gives a minimum load to the transformer giving some protection to the transformer.
     
  13. janglemore

    janglemore TDPRI Member

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    Thanks for the comments. So you would move the tone circuit one stage earlier? I noticed it’s inserted just before the power tube.

    I wish I could tell you how the tone control worked. I didn’t have very much time using the amp before it went tango uniform, less than 10 times total and well less than 5 hrs of use. When I purchased the amp, I was almost exclusively an acoustic player. Got the Monoprice for about $80 just to have something. And then rarely used it over the next few years.

    Thanks for the info on R8. I was wondering why that was there.
     
  14. Treehouse

    Treehouse Tele-Meister

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    Disappointing to hear about amp’s reliability. I’ll take it as a reminder to never buy another PCB/SMT guitar amp!
     
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  15. printer2

    printer2 Poster Extraordinaire

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    No, not saying to move the tone position. I was wondering how it worked in that position. Might have to try it myself.
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    One other thing about the old schematic.

    The bias resistor for the 6V6 is listed as 1 Watt.
    Later schematics upgrade that to 2 Watt. Most of us will use a 5 to 10 Watt in that position. OMMV.
    The bias resistor does get hot. The higher wattage resistors spread the heat through a larger surface area. IMO, 1 Watt is too small.
     
  17. sds1

    sds1 Tele-Afflicted

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    Nothing inherently unreliable about PCB or SMD.

    An amp built to be as cheap as possible will always use PCB because assembly can be automated.

    But it's the "cheap as possible" part that made the amp unreliable.
     
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  18. janglemore

    janglemore TDPRI Member

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    But it’s the PCB that makes me not want to repair this amp. Too jumbled up and parts too small for me to deal with.
     
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  19. sds1

    sds1 Tele-Afflicted

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    Yah that part can be a challenge! You really need some lighted magnifier goggles to work with SMD... just looking at this board for the first time, seems like they put the entire preamp on SMD components.
     
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  20. Jlwctn

    Jlwctn Tele-Meister

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    Consider that it is commonplace to discuss well functioning amps that are 60+ years in age, and that their construction techniques and methods are still proven viable.
    What is to be said then about PCB amps that reach less than half of that age, and the PCBs are conductive and arcing, for example?
    If the components would have been souced in an "as cheap as possible" manner, then why wouldn't the PCB have been, as well?
    These Monoprice amps are little treasure chests of parts, when you consider what is in there: a covered cabinet, speaker, chassis, PT and OT, all at a much lower price than if you had sourced them individually.
    Approaching them as functional mod platforms opens up a world of possibilities for those who are looking to get "their feet wet" in the vacuum tube electronics world.
     
    Last edited: Sep 1, 2020
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