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Whatever happened to that Hohner amp? 5e3 Build Content

Discussion in 'Shock Brother's DIY Amps' started by rocksmoot, Dec 4, 2017.

  1. rocksmoot

    rocksmoot Tele-Meister

    165
    May 6, 2012
    Rogers, Arkansas
    Yeah, I did a full build thread on aguitarforum.com, but I brought it over here to get the rough edges smoothed out. Your notion that there's crud on the terminal is one that occurred to me but I've probed that pin every which way I can imagine, and I've reflowed the solder joint and it looks really nice. I don't think it's a short in the tube unless both tubes have it.

    Both tubes are pretty hot. Once temps reach the "damn, that hurts!" stage I can't really tell the difference.

    I wish (for the hundredth time) I had an oscope. Had some nice ones in the service.
     
    Last edited: Dec 7, 2017

  2. rocksmoot

    rocksmoot Tele-Meister

    165
    May 6, 2012
    Rogers, Arkansas
    Well, I fixed it. I replaced the rectifier socket. It was a cheap ceramic and all the other sockets were Belton sockets. After I did the swap I'm getting 378 volts on pin 3 of V3. Most likely it was a questionable solder joint, but there ya go. Thanks for chiming in on this issue, guys!

    Now I'm waiting on my zeners and VVR parts.
     

  3. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    370
    Feb 2, 2015
    Brick, N.J. USA
    here is definitely the better place for this part of it.
    Ok so you swapped tubes and the problem stayed on the socket pin and did not move with the tube. Check.

    do you have unloaded B+ voltage at v3 socket pin 3 with the tube out?
    At the OT side of the plate resistor?
    Could the socket be burnt / carbon traces and be conducting somewhere else?

    The tube is warm, your cathode voltage is similar to v4, the amp sounds good.
    Something is misleading on this.

    If output tube is pretty hot I figure that as a general go / no go that the tube is conducting or not. I had a cool one and a failed open screen resistor was the culprit.

    I just read on an audio forum if the anode is open and voltage is present on the screen, it will destroy the tube.

    I'm not sure how but I am thinking the only + charge then becomes the screen and the current path is from the cathode through the screen. But wouldnt that still be limited by the screen resistor?
    Or is that the case where NO screen resistor is employed?
     

  4. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    370
    Feb 2, 2015
    Brick, N.J. USA
    I am glad you got it sorted but I dont get that.
    B+ leaves the rectifier socket to the OT CT and comes out both ends to the plate resistors.

    If it were me I would keep rechecking that v3 pin3 reading while you are in the scaling the B+

    I just ordered some zeners for my 5e3 too.
     
    Last edited: Dec 7, 2017

  5. rocksmoot

    rocksmoot Tele-Meister

    165
    May 6, 2012
    Rogers, Arkansas
    I don't get it either. It doesn't make sense to me since the voltages looked good at the B+ anyway. Unless of course things are breaking down under some kind of load but that doesn't make sense either because I measured it at idle. Maybe some smart guy will come along and explain to us how it's all very elementary.
     
    jtcnj likes this.

  6. rocksmoot

    rocksmoot Tele-Meister

    165
    May 6, 2012
    Rogers, Arkansas
    I've finally gotten around to addressing the high plate voltages. Using the 5C3 voltages Fender supplied on their original layout, I'm going for around 350VDC on the plates. It took three 12V zener diodes to get me to 355 so I'm happy with that. I also had to change the cathode resistor to 300ohms to get the bias to around 11 watts per tube, still a little hot but a tiny hair under 100%.

    For anyone playing at home, here's a look at the final voltages with a 12AY7 in V1, 12AX7 in V2. The heaters are a bit high imho, but still in +/- 10% range.

    B+ A 366
    B+ B 333
    B+ C 243

    Filament 6.8VAC

    V1
    1 119
    3 1.8
    6 116
    8 1.9

    V2
    1 119.2
    3 1.3
    6 199
    7 16
    8 44

    V3
    1 & 8 22.7
    3 354
    4 331

    V4
    1 & 8 22.7
    3 355
    4 331
     
    awasson likes this.

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