Blackface Twin Reverb Build (my first from scratch!)

Discussion in 'Shock Brother's DIY Amps' started by JKube, May 24, 2017.

  1. JKube

    JKube TDPRI Member

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    Well that power up didn't get very far.....

    I built a light bulb limiter to be safe. I have a variac plugged into the wall, limiter into the variac, amp into limiter. Tried it on my Bugera, worked exactly as described on Rob's site. Tried it (after waiting for my nerves to settle) on the twin build..... nothing from the bulb. I get a pilot light. Voltages from the OT all seem solid. What gives? I'm not gonna move on until I know that this is acceptable...

    Secondary voltages are:
    High Voltage secondary
    388 VAC
    387 VAC
    Low voltage
    65.3 VAC
    Heaters
    3.7 VAC
    3.7 VAC

    Why oh why is my bulb not responding? Might seem like a dumb question, but I'll ask. The labeling on the mojo backplate is different than my DRRI. I'm assuming up is on for both power and standby. On the DR, it says standby below and "on" above. The twin just says standby above. Wouldn't be the first time this kit threw me for a loop....

    IMG_9095.JPG IMG_9096.JPG
     
  2. Phrygian77

    Phrygian77 Tele-Afflicted

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    Light bulb not lighting up is a good thing... Assuming you built the limiter correctly.
     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You are safe if the bulb doesn't light--worst case is no power tube current is flowing.

    Try the standby switch in both positions.

    Get the variac voltage up and try a guitar and see if you get audio out of the speaker--don't forget the speaker.
     
  4. JKube

    JKube TDPRI Member

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    I haven't even hit the standby switch yet. Just the power to "on", with no bulb reaction. I know a brightly lit bulb is bad, but a completely open circuit? I figured that was just as bad. I'm on step one. No tubes, no standby power. I guess I'll move forward carefully.
     
  5. clintj

    clintj Friend of Leo's

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    No tubes? That sounds like a normal bulb response at this point. There's nothing to pull current once the caps are charged, other than the pilot bulb. Check your bias voltage is around -45V or so, then adjust it to the most negative reading you can reach. Actually biasing the tubes comes later.

    I'd add the power tubes, then do the next test.

    And yes, the standby switch should match the power switch when it's in play mode. :)

    Oh, after looking at your pics again: plug in your speakers before adding the power tubes. The shorting jack should protect the OT, but I don't trust it until it's proven to work. You need to be able to hear any warning signs as you go, too. Loud hums, pops, etc.
     
  6. JKube

    JKube TDPRI Member

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    Well my friends. We had a Blackface Twin Reverb. Ran through Rob's entire power up procesure, and she's singing like a bird. The craziest part, It is so quiet, it would be impossible for me to describe its quietness. I'm talking zero hum, zero hiss, zero presence. I have the volume dimed, a single coil strat in the neck position plugged, and you can't even tell it's on with you face 12 inches away from the speaker. Pluck one string and you're deaf. Unreal.

    The only weird thing that happened was on start up, with all pots at zero, I rolled up the volume and got nothing. I troubleshot for a bit, before finally bringing up the tone pots and voila. Sound. The treble pot acted like a volume pot, but after a few minutes of schematic review, I checked and all of the tone pots are functioning as tone pots. I'm sure there's some logic to the volume not coming up on initial power up with treble at zero. I'll figure it out later, I have some guitarin' to do.

    It sounds as I expect right now. Bias is full cold, so it's kind of empty. I haven't tested vibrato or reverb just yet. I also plan to take some crazy resolution photos before it goes together. More to come, but here she is.

    IMG_9091.JPG IMG_9093.JPG IMG_9097.JPG
     
  7. King Fan

    King Fan Friend of Leo's

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    Yeah, oh yeah. Great!!!!!
     
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  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Congrats on getting it up and running. It's a really great looking build.
     
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  9. theprofessor

    theprofessor Friend of Leo's

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    Dude, you just built a TWIN REVERB! Congratulations!
     
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  10. JKube

    JKube TDPRI Member

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    Thanks guys! I've turned up the bias a bit, still hardly any noise. First issue though, I can only get 19.1ma of plate current with the bias pot maxed. Tried two meters, all 4 tubes, all sockets. 460V on the plates, what gives?
     
  11. moosie

    moosie Doctor of Teleocity Ad Free Member

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    Stop it. You're making me want to build one now. And I do not need another Twin... :)
    Change the 27k resistor on your bias pot for something different. I just can never remember offhand if you want larger or smaller. Smaller, I think. Try a 15k.
     
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  12. clintj

    clintj Friend of Leo's

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    Yes, smaller. Try a 22k, and if it's still too cold try another step down. That's the ground resistor, so lowering it lessens the negative bias voltage. I like the big Fenders at around 55-65%, but that's just my preference.

    Yeah, the blackface tone stack has befuddled folks before. It's straightforward enough, but has a few quirks like you just found. Do the reverb and vibrato circuits work as expected, too?

    Congratulations! You pulled off a major build there. Now finish that bias adjustment and go tick off the whole neighborhood! :D
     
  13. JKube

    JKube TDPRI Member

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    Appreciate it, and let me say for the billionth time, thanks to you and all the other folks here. I hope that my pictures and question (with your answers), helps the next guy to stumble a little less.

    I had a 22k 1W, tried it and got to 22ma. I'm gonna run to Fry's and grab. 10K and a 15K and step it down one at a time. It would be nice to have a bit of range, since I can only get to about 30% right now. I've never biased a 4-OT amp, so I really just wanted to be sure using a single socket bias probe wasn't actually showing me half of the plate current or something.

    I built a shorting jack for the vibrato and hooked up the reverb tank. Both work great! I do wish I could slow the vibrato a bit more. With the pot on 0, it's still sounds at about 2 on my DRRI. The reverb is pretty early too. 2-3 is full echo, not much happens past that. Anything easier than pot swaps to assist that?

    I can't wait to get the bias set and actually open this thing up. I've waited long enough!
     
  14. moosie

    moosie Doctor of Teleocity Ad Free Member

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    You do need to divide current by 2, when biasing a quad.
     
  15. moosie

    moosie Doctor of Teleocity Ad Free Member

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    Swap the .01 ceramic for a .02. It's the one between the two 1M resistors (physically closest to your front panel).
     
  16. JKube

    JKube TDPRI Member

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    Even if I'm biasing a single socket at a time? That would mean that I'm at 9ma each, which seems impossible based on the sound I'm getting. That
    Would be 13.8%, or 4 watts. I'm using a eurotube bias probe inline with the tube, measuring current direct.
     
  17. moosie

    moosie Doctor of Teleocity Ad Free Member

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    I have no idea how those bias probes work, but if single socket, then no, you're right.

    For example, if biasing using the cathode resistor method, no adjustment is required. But I normally bias using Rob's method, measuring plate voltage drop, and the number of tubes on one side of the OT primary must be factored in.
     
  18. RifleSlinger

    RifleSlinger Tele-Meister

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    JKube,

    This thread is incredibly inspirational. I can't wait to hear your Twin (I hope you post a clip).
     
  19. JKube

    JKube TDPRI Member

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    Well, I had some really weird stuff happen with the bias resistor, but we got there. I had to get metal instead of carbon comp (limited options on Sunday) and picked up 15K 1W and 10K 1W, but when I subbed it in I got 0 ma plate current and a low farty distorted tone. I went back to the 27K and still had the bad sound. Played with it for a while and decided to try another 15K. It worked and I got that tube biased to 38ma (58-60%), then pulled the probe to do the other side. When I plugged it in to the second tube, I had garbage tone again. Good power on all legs, full plate current. Just no current reading on the plate and farty sound. Put the carbon comp back in and had it back to normal, but still biased super cold. Decided to try the 10K metal and voila, perfect 10-60 ma range on the bias pot. I took it to 58%, plugged it up, and summoned the ghost of Leo fender. It sounds like every amp should sound. A couple quirks to work out (reverb squeal at high level, vibrato tick), but I could leave like it is forever and be ecstatic. So loud, so clean, headroom is out of this world. I can't stop playing it!

    After I mounted her up in her new curly maple home for the first time.
    IMG_9136.JPG IMG_9137.JPG IMG_9139.JPG
     
  20. Nickfl

    Nickfl Tele-Afflicted

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    That's a great looking build! Really nice work on the lead dress, I'm not surprised it's dead quiet at idle.
     
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