It begins ... but what is it?

Discussion in 'Shock Brother's DIY Amps' started by goonie, Jul 7, 2020.

  1. goonie

    goonie Friend of Leo's

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    Hey guys. long time lurker first time builder.
    I was having some chronic volume spiking issues with my Pro Junior and rather than do the sensible thing and get it fixed, I thought what the heck, why not chuck out the PCB and build a new hand wired amp using the cabinet, chassis, speaker etc..
    Excited to be taking the plunge into amp building! A man needs a hobby and this will be a good substitute for my previous 'outlet' of working on cars ... which my back can't take anymore!
    So I have ditched Netflix and now working my way through Uncle Doug's videos and Robrob's website ... amazing resources.

    The picture tells the story ... I've got a chassis, switches, PT and OT... though I have read the OT should probably get turfed.

    Now keep in mind I'm a first-timer so building something relatively simple would be prudent ... what would you do?

    pic.jpg
     
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  2. galaxiex

    galaxiex Tele-Afflicted

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    I'd build an eyelet card version of the Pro Jr to keep it simple.

    Kinda like I did on my Blues Jr.... except it's a bit more complex than the Pro.... ;)

    100_2443.jpg
     
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  3. wabashslim

    wabashslim Tele-Afflicted

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    Well, first of all, I wouldn't take Uncle Doug as the last word in electronic theory. He's learned a lot and improved his presentations since he started, and he's always fun to watch and has interesting projects but there are limits to his knowledge and sometimes he goes beyond, that's all. Rob, from what I've seen, is much more trustworthy.
     
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  4. goonie

    goonie Friend of Leo's

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    Thanks for the response! I wouldn't mind building a hand-wired Pro Junior as it's a great amp but it also seems quite complicated...i.e. lots of parts compared to one of the 'classic' Fender designs. So I was leaning towards something a little bit less challenging, maybe with just a single power tube.
     
  5. King Fan

    King Fan Poster Extraordinaire

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    Look at a 5F2a (basically tweed Champ with a tone control) or black/silverface Champ (AA764). Two of the most iconic small Fenders, and for sure two of the easiest to build...

    Have a look at this thread, too. Smart folks over at el34world...

    https://el34world.com/Forum/index.php?topic=21975.0
     
  6. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    For us car guys, it is like pondering a gutted Datsun 510. Will it be shaped into a fuel efficient commuter, or a SCCA racer?

    It comes down to... What do you want? Well, what do you want that will fit within the parameters.

    If you would like it to be tweedy or bluesy, then go with a 5F2A or 5E3 cathode biased type amp.
    If you would like it to be a cleaner blackface era(ish) amp, use a fixed bias circuit. A chopped and channeled AB763 or maybe a 5F6A for a Marshall influence.
    (FYI. With the low B+ voltage you may only get a little cleaner than the Pro Jr. The Pro Jr does do clean as you know.)

    I assume you want to skip onboard reverb and vibrato to keep things simple so any of the above mentioned circuits are not too complex.

    My personal preference, if you stay with a Push Pull circuit, would be to change over to octal 6V6s. The 6V6 would be a little easier on the small PT.:D
     
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  7. awasson

    awasson Poster Extraordinaire Gold Supporter

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    Car guy here.... For me it was always about how big a small block can I fit in there and instead of Datsun 510, it was 240z, Vega or Capri. :D
     
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  8. goonie

    goonie Friend of Leo's

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  9. goonie

    goonie Friend of Leo's

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    Alrighty after much pondering and weighing up options, I reminded myself that the primary goals of this amp build are to (a) keep it very simple as it's my first build, while (b) where possible recycling the key parts from the Pro Junior, particularly the PT.

    So I guess the best way to tick both those boxes is build a two knob, single-ended EL84-based amp with solid state rectifier.
     
  10. Nickfl

    Nickfl Tele-Afflicted

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    Perhaps an AC4?
     
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  11. printer2

    printer2 Poster Extraordinaire

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    I am going to disagree. I think you should do a push-pull circuit as you already have the transformer. It does not have to be much harder than a single ended amp. A 12AX7 and a pair of EL84's in Self Split.

    [​IMG]
     
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  12. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    So an OT, eyelet board, and small parts then your ready to build. The OT should have around 5k on the primary so very similar to the 5F2A but with that little something the EL84 is known for. Might be interesting to try more plate resistance on the 12a_7 like so many EL84 amps preceding this one.

    Looking forward to this build. Subscribed.
     
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  13. Jimclarke100

    Jimclarke100 Tele-Afflicted

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    The Pro Junior is a great little amp, just plagued by hiss.
    In your position I’d go for an eyelet board version, I’d wind down the bias on the output tubes though. The originals run hot.
    Personally I wouldn’t bin the OT. Use the original and see what you think. They are easy enough to replace later if you don’t like it.
     
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  14. goonie

    goonie Friend of Leo's

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    Hmm that does look pretty straightforward. Is that a schematic of a build you've done, or have seen?
     
  15. awasson

    awasson Poster Extraordinaire Gold Supporter

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    That sure is a simple looking amp. What’s the downside to that self split design compared to going with a 12ax7 phase splitter.

    I’d absolutely go with this or a similar push pull. All the parts are there for the using.
     
  16. printer2

    printer2 Poster Extraordinaire

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    There are downsides to the design. The way the circuit works sort of like a high power Long Tail Pair inverter. The signal comes in the one tube and modulates the current through the cathode resistor which is used as the signal source for the other tube. Because it needs to see the signal on the cathode resistor we can not put a capacitor across the cathode resistor. This reduces the gain of the output stage as compared to a pair that is being fed by a inverter.

    The second drawback is the circuit can only operate in Class A and not in class AB. Because the second pentode gets its signal from the first pentode the tubes have to be center biased as in a class A amp. So while a single tube amp in Class A (numbers are relative) could make 5W and a pair in class AB can make 15W, the pair in class A P-P could only make 10W. So while the circuit does away with one stage it does come at the cost of a little less gain and less power than could be had from the same output tube set.

    It was used in radios in the early 1940's, maybe the 30's also but I can't remember well enough. People did not have much money and companies looked at all ways to make their radios affordable. There is a little more advanced version of the circuit which allows the uses of a cathode capacitor but it uses more parts. Rather than using the cathode resistor as the source of the signal to the other tube they took a signal off the screen of the first tube and fed it into the second tube. But that might not be the best circuit for a new builder.

    The above circuit also has a different distortion profile than a regular phase inverter fed pair. I seem to recall that the second harmonic does not cancel out, not an issue with us as we do not mind a little distortion. I have built a split phase inverter amp a long time ago but remember little about it other than it works. I am not sure of the above values, the cathode resistor will depend on the voltage from the power supply. The voltage can be higher in a Class AB amp than a Class A (In Class AB the bias is less and when driven each tube id off for part of the signal allowing it to cool down. In Class A it is always biased on (until clipping) and the idle voltage and current has to be kept below the plate's dissipation rating. I put a 1k resistor in front of the first power supply capacitor to reduce the voltage, might have to look at the original amp circuit to see what may need to be done.

    So it is not a difficult circuit as compared to a regular Champ type of amp. The performance is not much better than a single ended amp with a parallel pair of output tubes. The advantage is that the output transformer being a push-pull can be smaller than a single ended one as the bias current for both tubes cancels out. Not a lot of guitar amps use this circuit, if you go for a P-P OT you usually are willing to put in a phase inverter. There is one design that has been done many times by people, the FireFly.

    [​IMG]


    I came across this thread, it has a layout for a Rob Deluxe Micro but with a Self Split OT. It uses a dual triode tube (12AU7) and with a pair of EL84's would need a screen node put in the power supply. A mistake on the board layout shown is the cathode resistor still has a capacitor across it from when the drawing was used for a parallel output configuration. It also has a master volume, might be a worthy addition.

    https://el34world.com/Forum/index.php?topic=21486.msg228694#msg228694

    Just for fun, this thread has a Magnatone schematic of an amp using a screen signal to the second output tube, about 2/3 of the way down.

    https://el34world.com/Forum/index.php?topic=21887.0


    Found this hifi design from way back with 6BQ5's (EL84's), looks like 250V with a 130 ohm cathode resistor.

    http://www.triodeel.com/compact.html

    A High Octane using the Self Split EL84.


    https://monster.partyhat.co/article/precision-electronics-pa-20-conversion/


    An interesting link, seems I have been down this road before. And the Photobucket links work.

    https://ampgarage.com/forum/viewtopic.php?t=22408


    Sadly no information to go with this one but the vid is a RL84 Self-Split with a 12AX7.



    For future reference, someone doing the screen drive version of the Magatone.


    https://www.freestompboxes.org/viewtopic.php?f=2&t=25900


    OK, I got a little carried away here. Enough reading to keep you busy.
     
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  17. goonie

    goonie Friend of Leo's

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    Fantastic info thanks @printer2 I look forward to delving into those links.

    So I guess the key benefit of a 'self split' for me is I can re-use the Pro Jr OT (and single-ended OTs aren't cheap in Australia!). The downside is less efficiency and less power, which I'm fine with as it's going to be a home-use amp.
     
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  18. awasson

    awasson Poster Extraordinaire Gold Supporter

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    I think for your needs the self split design that @printer2 posted is ideal. It’s got a low parts count so good for a first attempt and yes, it uses the PP transformer you already have.

    ...And if at a later date you decided you needed more gain/volume, you could add another 12ax7 for a phase inverter and rewire the outputs to it. Your preamp could stay largely unchanged.
     
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  19. printer2

    printer2 Poster Extraordinaire

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    With Class A the tubes are idling with higher current that Class AB which the Jr was running. So the schematic shows a 1K resistor before the first filter capacitor. I just took a guess on it, I think a 500 ohm might be more appropriate. I would use two 1k 10W resistors here. The other two ps resistors 2W, 1W for the 470R screens. Rather than the 150R cathode resistor, use 120R 5W instead. The 4.7k resistor on the first EL84 grid can be anything between 1.5k to 22k I would think. It and the rest of the resistors can be 1/2W or 1W, I would go 1W on the 100k plate resistors. 1W for the 100R will keep them cool.

    The 22uF bypass caps can be smaller values to tune the bass response. Other values like 10, 4.7, 2.7, 0.68 uF are some ideas. If you are ordering parts might as well get them as they will not cost you much and waste hours of your life trying the different values deciding what you like. The voltage is only a few volts here so anything goes, 10-25V would be more than adequate. I will see if I have time to toss the circuit together to check it out.
     
  20. goonie

    goonie Friend of Leo's

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    You guys are awesome thanks.

    Like a true shock brother, I'm already planning the next build before the current one has started! Someone local is advertising a PR cab/chassis for $250 ($US175) and I'm thinking hmm I should grab it and build myself a handwired PRRI one day.
     

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