First amp build completed

Discussion in 'Amp Tech Center' started by 8urchevy, Feb 14, 2019.

  1. Luthier Vandros

    Luthier Vandros Tele-Holic

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    When I switched to lower efficiency speakers, I was able to get the edge of breakup tones I wanted at *near* bedroom levels. It’s all about getting that overdrive window into your living and playing situation. All else being up to spec, these are where it’s at for me.

    “But but you won’t get great cleans on the stage!”, they’ll tell you. Lies! You’re going to be mic’d 95% of the time. That means you’ll have the tone you want 5% of the time with too much amp. You’ll leave every show dejected and will blow more money on bandaids that may or may not wash out at the next show.

    With the tone OP is going after (and many of us, too), specing the ot/speakers for the proper scenario are game changers! I no longer need 2 pedals that had long standing use on my board: a Barber compressor and a Maxon 808. Well, in the off chance that I end up flying to a gig unable to bring my own amp, I’ll keep those tasty bandaids to apply to the venue’s backline.

    I read a recent comment here and can’t recall the thread title, but an older and more experienced cat slid in a powerful comment that flew under the radar for many (myself included). He was mentioning the importance the controls on the guitar itself had on achieving a particular tone. It made zero sense to me at the time because until then, I never had acccess to a touch-sensitive amp. Now that I do, I have a hard time going back to my old rig that was built to sound the same at all volume levels. With that tone, I had no toneful use for the volume pot on my guitar. It was simply being used as an on off control.

    There’s no going back! The volume and tone controls now paint in shades of grey instead of only black or white.
     
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  2. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Here’s the power amp schem - shows original tube types and voltages your pt is supplying. Something to the tune of 460vp in original form. This will also give you insight into the impedance the ot wants to see on the primary side, so you can possibly substitute tubes for the 5f6a’s classic compliment.

    http://www.makearadio.com/schematics/images/bogen-mx30a-07.jpg

    And the preamp here:
    http://www.makearadio.com/schematics/images/bogen-mx30a-06.jpg

    Edit: I never considered what you said in your opening post about using the original tubes. My primitive lizard brain never connected the dots. You could massage that thing into something that produces a very unique and soulful tone.
     
    Last edited: Feb 17, 2019
  3. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Ok last post, I promise. Have a look at this thread. EL34World is run by Doug Hoffman, a legend in the DIY tube amp world. Lots of entertaining and informative stuff on the topic there.

    A gent found himself in your position with the same amp recently, but you went off in a certain direction already. There’s good dialog about your existing power tubes and what’s possible. The guy that goes by the handle “PRR” has a 45lb brain and is formidable when it comes to electronics - he’s at guru level, so consider his technical advice carefully. Have a look out for “sluckey”, as well. So many helpful hands there with huge reserves of institutional knowledge. Also, huge shout out to our own robrob; his website is a great resource and has gotten my behind out of the frying pan a few times!

    https://el34world.com/Forum/index.php?topic=19649.0

    If you happen to have only modded the preamp to bassman specs then, as he suggests:

    “If you are sure the 7868 are good, leave the whole power stage alone. Bogen output stages make excellent instrument amps. Play with R67 "62K"(?) at 150K, 330K, to "loosen-up" the power stage for less abrupt breakup and more color at medium levels.”

    Tuna town, baby.
     
    Last edited: Feb 17, 2019
  4. 8urchevy

    8urchevy Tele-Meister

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    OH yeah. It gets plenty loud. lol
     
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  5. 8urchevy

    8urchevy Tele-Meister

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    I have the amp completely wired to Bassman 5f6a schematic. The tubes and transformers are the only original Bogen parts left in it. Thank you for all the info and resources. I hope to do the voltage checks tomorrow including the bias. If I manage to do it tomorrow I will post the results.
     
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  6. 8urchevy

    8urchevy Tele-Meister

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    The hum is indeed affected by the volume controls. I put a master volume trainwreck type 3 on it per rob robinette's diagram on his site. It works great by the way. Initially I thought that the pilot light wiring and lamp locations were the culprit as they were right above, well 4in. or so, my input. I removed them and it got just a smidgen better. All the caps, resistors, etc. are brand new and I tested them prior to install and they block all DC. Hopefully tomorrow I can record all the voltages on the tubes and power supply. Perhaps they are a little too high for this circuit. Also, I am going to go through the grounding to see if I can get the hum to clear up.

    The tone controls also seem to have little noticeable affect with the bass control being the most noticeable. I have chopsticked everything and went through the connections several times against the diagrams Rob Robinette has on his site. There is absolutely no popping or crackling on any solder joint and the connections are as shown on the diagrams.

    One other thing to note is that it sounds very compressed, with a hard snappy attack and note bloom. Works great for country tones, but stifles dynamics I so desperately want. Including a light break up. I am going to replace the Celestion 70/80's with some cheap speakers I happen to have on hand to see if that will provide what I am after. They certainly are less efficient. Those Celestions are rated for way more than what this amp puts out.

    You all are such a great help and resource. Thank you very much for taking the time to help me out.
     
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  7. Luthier Vandros

    Luthier Vandros Tele-Holic

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    That’s all those plate volts ganging up on your tone!
     
  8. 8urchevy

    8urchevy Tele-Meister

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    If the plate voltages are too high could I install a wirewound resistor on the B+ off of the rectifier tube pin for the value needed to bring the voltages down where they need to be? That would be the simplest method I can think of, but unsure of what wattage to use. I reckon I would need to add all the wattages of the resistors in the B+ circuit to determine the wattage of resistor to do that? I would really hate to change the resistors between the filter caps if I can avoid it.
     
  9. Luthier Vandros

    Luthier Vandros Tele-Holic

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    I was partially messin with ya, bc I have only a ballpark assumption of your amp’s values based upon an old schematic. All speculation.

    If you’re getting that schematic’s posted 460v on your power tube plates, then the preamp is cooking, too. What’s throwing my mental recipe off is the absence of 5881’s/6l6’s and hard numbers telling me how that Bogen pt is playing with that circuit. I’m also so unfamiliar with the stock Bogen’s power tube compliment. And gah! What was wall voltage way back then?

    The 5f6a has around 430v coming off the choke. If you determine that everything is off by 30v or so (even fender states +/- 20% variance on posted schematic values), then you could bring the whole shebang down globally with a nice zener diode between the pt’s center tap and ground (iirc).

    But if the bassman circuit dials down the pt’s juice and gets you in that 20% window, you can probably get away with focusing on the preamp’s plate supplies. For me the magic number is between 145-160v there.

    If your preamp plates are in the 190v-200v+ range, that’s twin reverb stiff. That’s a huge part of what gets you that baseball bat attack and cannon like report at the pick. I believe the bassman has them around 180v. Still fairly stout for my taste, but that’s just my taste. I find the sweetness of tone in the lower range and the bolder silverface artillery in the higher.

    If you get your plates etc under control, that tweed is gonna sing! I put that same mv into a 35w blackface head I built and it works really well. Also sourced from robrob’s site.
     
    Last edited: Feb 17, 2019
  10. peteb

    peteb Friend of Leo's

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    Good to hear,


    If it’s loud, then it should break up.




    Hopefully the voltages will show what’s up
     
  11. 8urchevy

    8urchevy Tele-Meister

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    It does break up into crunch, but the volume, not the master, has to be around 2 o'clock for it to happen and it is very very tight with hardly any treble. Over driven tones a little past that which is quite fizzy and thin. I am hoping that plate voltages are my nemesis. We shall see soon enough. I should have had the forethought to check the B+ voltages prior to the build and adjusted resistor values accordingly. I guess I got too excited and skipped that crucial step. Now I am paying for it. lol
     
  12. 8urchevy

    8urchevy Tele-Meister

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    Can you explain the zener diode's function to bring down the voltage and how to determine the spec for it to bring the voltage down to where I need it? This is first I have heard of using one for this purpose. Thank you
     
  13. Lynxtrap

    Lynxtrap Tele-Holic

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    Did you follow the grounding principle that Rob uses? That is, the whole circuit should basically be grounded to chassis at only one point.

    In Rob's layout, that point is by the input jack. I would never do that, although there is probably a reason for it. Just seems counter intuitive to have the ground point at the most sensitive point in the whole circuit.

    Anyway, you want to avoid random grounding points to chassis.

    But start with pulling one tube at a time to try and localize the hum.
     
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  14. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Don’t ground the big ripple stuff by the input jack. Keep that as far away from there as possible.

    Ground your power supply filters near the power cord ground either soldered direct to chassis or on a lug off one of the pt’s bolts. Ground your center tap right on the first filter caps’s negative. Preamp grounds can be a’la fender.

    Robrob won’t steer you wrong, but this visual may help.

    http://el34world.com/charts/grounds.htm
     
  15. robrob

    robrob Poster Extraordinaire Ad Free Member

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    For fixed bias amps we have to put the zeners in the B+ supply line. Putting them on the power transformer center tap will mess up the bias voltage.

    More info here: https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage
     
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  16. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Zener diodes come in all sorts of ratings according to voltage. Say you determine you’ve got 40v of excessive voltage being produced by the pt. You’d get a zener rated at 40v. It takes 40 applied volts before that diode will allow voltage to flow across it. That 40 volts, called the breakover voltage, will be consumed by the Zener during its operation, so it will be deducted from whatever amount is passing through. It’s like the rating is the toll the voltage must pay to pass. If youre applying 100v to it, only 60v will make it through to the rest of the circuit in this example.

    The come in handy packaging, whereby it will be mounted to the chassis, which will act as a big ol’ heat sink. I have some spares in storage in various ratings that I can mail to you, but I may not have the exact rating you need.

    That’s just one way to skin the cat, but it’s something that effects the circuit globally. When I found the stiffness too much to bear in one of my builds, I used a combo of larger dropping resistors, plate resistors, and reduced filtering around the phase inverter and 1st stage to reduce the strident tone and hard feel under the fingers. I chose not to use the zener in this instance because the preamp voltages where really close to where I wanted them and I was using a more surgical approach, not that I’m any sort of guru. I do what most others do, tread where the pioneers have made way.

    Another neat plug and play trick is to substitute rectifier tubes to find one that will produce a larger voltage drop. Sometimes you can shed 5-20+ global volts that simply. Next thing you know, the amp has a looser feel with the dynamics and tone you’re after.
     
  17. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Good catch, but I believe he implemented cathode bias on his.
     
    Last edited: Feb 18, 2019
  18. Lynxtrap

    Lynxtrap Tele-Holic

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    A resistor in place of a choke can also soften the response a bit. The resistor needs to have a large enough power rating.
     
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Sorry about that, I don't know why I was thinking fixed bias.
     
  20. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Because we’ve all had too many solder fumes :D
     
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