Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

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

    159
    May 6, 2012
    Rogers, Arkansas
    I posted a thread about three years ago putting in front of the group here an opportunity to help decide the fate of a very old Hohner solid state amp that had been given to me:

    http://www.tdpri.com/threads/hohner-hi-210r-help-me-decide-its-fate.530375/

    Since I posted that, life got in the way as it is wont to do and it languished in my garage until last week when I finally decided to get off my duff and put a 5e3 circuit in the thing. It's built using the cabinet, speakers and chassis (gutted) of the original amp and sourced with parts I bought from various places. It's built with a Hoffman 5e3 turret board and uses his layout.

    I finished the build yesterday It sounds wonderful, all chimey and 3D shimmery when clean. I do have a problem with it though, but I'll pose that question tomorrow (I'm too tired to go much further tonight.) In the mean time, here are some pictures to enjoy, and a cheesy sound clip.


    chassis limiter.jpg

    finished chassis front.jpg
    finished rear.jpg
    finished front.jpg

     
    tubeswell and Nickfl like this.

  2. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    One thing I'll add before calling it a night. It's pretty much a stock 5e3 with the exception of separate cathode circuits for each preamp stage and with .022 coupling caps between the PI and power tubes. I've also installed Rob Robinette's master volume control, which I had disconnected while troubleshooting some nasty oscillation which turned out to be a cold solder joint in the tone stack, so I'll be re-connecting the mod.

    May as well talk about the issue I'm having. I have a pretty high B+ voltage, with 408 VDC on the plates. This is with a Mojo756 PT which puts out 384-0-384. I'm running at about 18 watts per tube with a 250 ohm cathode resistor, the value of which I'd like to keep. The tubes are Chinese, and they're not red-plating but I'd like to have the voltages closer to spec. I have all kinds of headroom and a marvelous clean it is, but very little in the way of breakup. I'm looking for good ways to bring that down under 400. I've got a NOS 5y3 on order to replace the Sovtek 5y3 that's in there, but I suspect I'll have to do more.
     

  3. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    So there are two basic strategies I'm considering. The first is to install a couple of zeners in series on the HV secondary center tap. That seems like a popular approach. The second is to install a VVR to scale the power. I'm actually leaning toward the latter since it's a useful mod in its own right irrespective of the high B+. It's a little unclear to me what voltages I should look for on the power tube plates to get closer to a 5e3's basic characteristics, and I'd love to hear some ideas.
     

  4. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    Gosh, it's quiet in here. Ah well, I enjoy talking to myself too.

    I've got the parts for both the zener approach and the VVR approach on order from mouser. I love having stuff on order because I love anticipating getting little packages in my mailbox. It's like Christmas all the time.
     
    awasson likes this.

  5. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    53
    Nov 18, 2010
    Vancouver
    I’m surprised nobody experienced has popped in to offer some direction too.

    Anyway, when I built my Tiny Terror Clone, I needed to see 325 volts at B+ and in reality, I was seeing 356 volts. I had done some mods to the power supply that resulted in some efficiencies. The solution for mine was a 5 watt resistor to knock B+ down. I’m at 326 volts now and everything from PT to tubes runs just that much cooler.

    I’m certain a 5E3 expert will have some sort of sure fire way to resolve yours. There’s a little chatter on the internet recommending not using Sovtec 5Y3 tubes (not sure what your using) and changing the bias resistor or using Zener diodes. http://music-electronics-forum.com/t36207/
     

  6. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    53
    Nov 18, 2010
    Vancouver

  7. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    Thanks for jumping in. I'm using a Sovtek 5y3. Before that I had a solid state rectifier and the voltages were much higher.
     

  8. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas

  9. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    53
    Nov 18, 2010
    Vancouver
    Interesting that a lot of the chatter about the Sovtek indicate that it will result in significantly higher voltage on the plates. Apparently JJ has a 5Y3 that is more conducive to running in the traditional 5E3 amp's power supply circuit.

    The other thing I read which would be easy to do was to increase the tube cathode resistor to bias the tubes colder. I think it's supposed to be 250 ohms and I've read recommendations from 330 ohm - 470 ohm to cool things down. You should be able to calculate the exact value based on voltage, amperage and the resulting wattage.
     

  10. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    Yes, I've done the numbers. Here they are for the power tubes. Note that the mA numbers assume 2mA for screen current as a rule of thumb. The percent dissipation assumes a 14 watt rating for the no-name chinese tubes I have in there now, an optimistic assumption to be sure.

    Plate: 408
    Cathode: 24.8
    Plt/Cath Volts: 383.2
    mA per Tube: 0.0476
    Watts: 18.24
    Pct Diss: 130.29%


    Changing the cathode resistor value will also change the plate and screen voltages but it's a useful exercise that shows increasing the value to 350 ohms will get me to 91%. It's also interesting to note that with a solid state rectifier I had about 435 vdc on the plates and 152% dissipation.

    My real interest in getting the voltage down is to actually decrease headroom. Our band doesn't play terribly loud but we do love to rock and I'd like to be able to crank the amp a little bit to breakup. With the voltages this high break up is way out of reach, volume wise.
     
    Last edited: Dec 5, 2017
    awasson likes this.

  11. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    I got a Sylvania 5y3GT NOS tube in the mail today. I dropped it in and it brought the plate voltage down to 378. Major step in the right direction but still running at about 110%. A zener will probably get me there. Those parts will be here tomorrow, along with the parts for a VVR which I'm going to install as well regardless of the results the zeners have.

    I may have a bad socket on V3. I can't measure any voltage at all on pin 3. Strange because the amp sounds great. I swapped tubes around and reflowed the solder joint there, still nothing. Wonder if I have a dying/dead OT?
     

  12. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    I pulled the output tubes and was able to get good voltages from the OT so I'm guessing there's something else going on upstream.
     

  13. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    can you post an as-built schematic with the voltages?
     

  14. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    Here's the schematic, voltages to follow. The only variation from the schematic is the PI->power tube coupling caps are .0022uf. The thing is I was able to read both V3 and V4 pin 3 voltages at one point.


    Capture.JPG
     
    Last edited: Dec 7, 2017 at 12:08 AM

  15. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    Voltages

    volts.png

    I appreciate you taking a look!
     

  16. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    I found a burned spot through to the foil on C2, normal channel coupling cap. Changed that out with no real difference in either voltages or quality of sound. I also put the C7 and C8 coupling caps back to stock so the whole amp is now built to the schematic.

    I guess I should have stated this earlier. The tube complement is:

    V1 - Sovtek 12AY7
    V2 - Chinese (no brand) 12AX7. These were cheap but sound great.
    V3/4 - Shuguang 6V6GT
    V5 - Sylvania NOS 5Y3GT
     

  17. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    Is V3 p3 really 0vdc.. Not good:(

    discharge caps amp unplugged, measure resistance from center tap to each plate on v3,v4. Keep these #'s for future ref.
     
    Last edited: Dec 7, 2017 at 11:04 AM

  18. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    It's not really 0 volts the meter won't settle in on a reading. Almost like it's ac voltage. I have measured the resistance between the secondary tabs both are around 210 ohms. When I take the tubes out I measure 576 volts on each tap so I think the OT is good unless it's breaking down under load.
     

  19. rocksmoot

    rocksmoot Tele-Meister

    159
    May 6, 2012
    Rogers, Arkansas
    And I have swapped the power tubes around so it's not a bad tube. I'm starting to wonder if I have either a bad socket or something else going on up stream in the circuit. The strange thing is the amp plays and sounds very nice.
     

  20. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    360
    Feb 2, 2015
    Brick, N.J. USA
    if you've got similar voltages at pin8 and the OT taps for both v3 and v4 chances are your pin3 voltage is probably there and the spot you are probing with your meter has flux crud on it. INternally in the tube if the plate is somehow open and the screen shorted the screen resistor would be flowing a lot of current and I think you would see the (bad) results.

    do both output tubes seem about the same warm / hot?
    If so, and pin8 voltage seems right, v3 is conducting - or thats my take on it but I am no expert.

    EDIT: I just realized you have a build thread for this on another forum. I recognized the pics and story but was having some real strong deja-vu reading this!!
     

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