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First Amp Build: Blackvibe

Discussion in 'Shock Brother's DIY Amps' started by Larsenguitars, Jan 18, 2021.

  1. Larsenguitars

    Larsenguitars TDPRI Member

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    I'm nearing the finish line on my first amp build - the Robinette Blackvibe, built in a Princeton Reverb chassis. I built the amp with a type-2 master volume and solid-state reverb and a negative feedback selector. To my great satisfaction, everything works and sounds wonderful! Despite my lack of experience in the tube amp domain, I was able to work through the process fairly smoothly thanks to Rob's website, TDPRI, and others.

    The reverb circuit I used was from the Hot Rod Deluxe. I was unhappy with that circuit at first as the reverb went from off to "GIGANTIC CAVERN" almost instantly. Adding a voltage divider with a 1M resistor and a 1M pot at the input gave me a nice dwell control. Now I can dial back the intensity of the reverb and increase the mix and find the sweet spot. I also switched to a medium decay tank which is still plenty long for me.

    The (almost) finished product:

    I built the box jointed pine cabinet and did the upholstery work as well. There are some major warts in my upholstery work, but it looks good enough from a distance! I'm working on a faceplate and backplate currently. I'd be glad to share more photos and/or discuss the build process if anyone is interested!

    One thing that's been bugging me is the B+ voltage. The power supply is sagging far more than I would have expected. I'm using a Mojo 762EX transformer which should supply 250mA and a GZ34 rectifier. The power tubes are JJ 6L6GC. With no preamp or power amp tubes installed I see about 465V on the plates. With tubes installed and the bias set to a minimum, it drops to about 450 on the plates. As I increase the bias, the B+ drops more than I would expect. Currently, with the power tubes biased about 68%, the plates are sitting at 420V. I would expect some voltage drop as current increases, but this seems excessive to me. Any thoughts or suggestions on the subject would be appreciated.

    Thanks to all who have asked and answered questions here over the years. It made a lot of my questions just a google search away.
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Hi Larsenguitars,

    I get a "no permission" error when I click on your links. You can imbed your images by posting a link using the "insert picture" icon.

    Your voltage drop does seem excessive for a 250ma (Blackface Vibroverb) rated power transformer and a GZ34. I would disconnect the B+ wire and measure the B+ current to see what the amp is actually pulling.
     
  3. Larsenguitars

    Larsenguitars TDPRI Member

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    Hopefully, you can see the pictures now. I had the permissions set wrong.

    I was curious as to whether one of my filter caps might be leaky. A couple of times playing with the preamp gain cranked, I heard some ghost notes (octave down). As soon as I get a chance to open it back up, I will report back with the results. Thanks, Rob.

    -Paul
     
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  4. D'tar

    D'tar Friend of Leo's

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    Beautiful amp......

    We need to see the "fiddly bits" inside! I don't recall who I am quoting there but I have always wanted to say that. LOL

    Ghost notes. May need to reflow your grounding of the filter caps.
     
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  5. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Looks great! What tolex/covering did you use?
     
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  6. Larsenguitars

    Larsenguitars TDPRI Member

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    Thanks for the kind words. I really enjoyed the process, well, 98% of it.

    How could I neglect the fiddly bits!? I have gut shots as well as pics from the building the cab that I will get around to posting.

    It is a marine vinyl that my wife and I found at Joann Fabrics. Its make/model is Spradling Softside. I've never tolexed anything before but this stuff was very stiff and difficult to work with. I probably didn't do myself any favors by using a product that isn't quite "amp certified". It was probably the most stressful part of the entire build, lol.

    That being said, I love the look!
     
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  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The amp looks great. For troubleshooting the filter caps you can alligator clip another 16 to 32uF cap in parallel with the B+1, 2, 3 caps to see if that kills the ghost note.
     
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  8. Larsenguitars

    Larsenguitars TDPRI Member

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    Currently have some 80uF 500V F&T caps in the A filter. I had to get a bit creative to get them to fit and feel secure. To be honest, it looks a bit dumb. I just ordered some 70uF 350's to replace those. When they show up I'm going to open everything back up and troubleshoot.

    Aside from my minor hiccups, this amp is killer....and loud! I already had a friend ask me to build him one. To which I responded: you might want to wait a little while and see if mine explodes first...

    Thanks for the suggestions!
     
  9. Larsenguitars

    Larsenguitars TDPRI Member

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    Finally got around to checking the B+ current. 103mA. Doesn't seem out of whack to me.

    Dummy me swapped the A filter caps before measuring the current. Unfortunately, I don't have a before and after comparison

    Voltages are higher now, about 447 on plates. I thought that was a good sign until I looked at my notes from the last time I set the bias and saw that the line voltage was 119.6 and last night it was over 125. That seems to account for my higher voltages.

    Bottom line is that I see about a 7% sag in B+ when the bias is set near 70% and a total B+ current of 103mA. I don't have enough experience to say what normal is. If this is within the normal range, I'll leave it be. If it's high, I'll continue shooting trouble. Thoughts?
     
    Last edited: Jan 26, 2021
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That sounds pretty normal to me.
     
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  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Just to rule out positive feedback, disconnect the NFB and see if that solves the problem. My 5F6A Bassman had positive feedback and the only symptom was a funky tone at certain volumes.
     
  12. Larsenguitars

    Larsenguitars TDPRI Member

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    I do have a negative feedback switch installed. But I didn't think to note which position the switch was in when I heard the ghost notes. I've only heard the ghost notes when playing with volume and mid/raw on 10 with the master turned down. There is actually a lot of flippin' overdrive available in this setting and maybe I'm dealing with a byproduct of that amount of gain. Under "normal" playing conditions, I've never heard anything strange.
     
  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, the mid/raw adds a lot more gain than a stock Bassman so it is pushing the amp hard.
     
  14. Larsenguitars

    Larsenguitars TDPRI Member

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    Well maybe Murphy isn't out to get me after all. ;)
    Thanks a million for all the valuable information you provide, Rob. Your site considerably lowered the learning curve enough to help my first build go smoothly.

    Here's the fiddly bits:


    And a couple shots of the raw cab before covering:


    A shot of the circuit board with the slightly oversized F&T caps:
     
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